Journal articles on the topic 'Atmospheric turbulence – Measurement; Troposphere; Wind shear – Measurement'
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Schneider, Andreas, Johannes Wagner, Jens Söder, Michael Gerding, and Franz-Josef Lübken. "Case study of wave breaking with high-resolution turbulence measurements with LITOS and WRF simulations." Atmospheric Chemistry and Physics 17, no. 12 (June 30, 2017): 7941–54. http://dx.doi.org/10.5194/acp-17-7941-2017.
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Abstract. Measurements of turbulent energy dissipation rates obtained from wind fluctuations observed with the balloon-borne instrument LITOS (Leibniz-Institute Turbulence Observations in the Stratosphere) are combined with simulations with the Weather Research and Forecasting (WRF) model to study the breakdown of waves into turbulence. One flight from Kiruna (68° N, 21° E) and two flights from Kühlungsborn (54° N, 12° E) are analysed. Dissipation rates are of the order of 0. 1 mW kg−1 (∼ 0.01 K d−1) in the troposphere and in the stratosphere below 15 km, increasing in distinct layers by about 2 orders of magnitude. For one flight covering the stratosphere up to ∼ 28 km, the measurement shows nearly no turbulence at all above 15 km. Another flight features a patch with highly increased dissipation directly below the tropopause, collocated with strong wind shear and wave filtering conditions. In general, small or even negative Richardson numbers are affirmed to be a sufficient condition for increased dissipation. Conversely, significant turbulence has also been observed in the lower stratosphere under stable conditions. Observed energy dissipation rates are related to wave patterns visible in the modelled vertical winds. In particular, the drop in turbulent fraction at 15 km mentioned above coincides with a drop in amplitude in the wave patterns visible in the WRF. This indicates wave saturation being visible in the LITOS turbulence data.
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Kunkel, Daniel, Peter Hoor, Thorsten Kaluza, Jörn Ungermann, Björn Kluschat, Andreas Giez, Hans-Christoph Lachnitt, Martin Kaufmann, and Martin Riese. "Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves." Atmospheric Chemistry and Physics 19, no. 19 (October 9, 2019): 12607–30. http://dx.doi.org/10.5194/acp-19-12607-2019.
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Abstract. Stratosphere–troposphere exchange within extratropical cyclones provides the potential for anthropogenic and natural surface emissions to rapidly reach the stratosphere as well as for ozone from the stratosphere to penetrate deep into the troposphere, even down into the boundary layer. The efficiency of this process directly influences the surface climate, the chemistry in the stratosphere, the chemical composition of the extratropical transition layer, and surface pollution levels. Here, we present evidence for a mixing process within extratropical cyclones which has gained only a small amount of attention so far and which fosters the transport of tropospheric air masses into the stratosphere in ridges of baroclinic waves. We analyzed airborne measurement data from a research flight of the WISE (Wave-driven ISentropic Exchange) campaign over the North Atlantic in autumn 2017, supported by forecasts from a numerical weather prediction model and trajectory calculations. Further detailed process understanding is obtained from experiments of idealized baroclinic life cycles. The major outcome of this analysis is that air masses mix in the region of the tropopause and potentially enter the stratosphere in ridges of baroclinic waves at the anticyclonic side of the jet without changing their potential temperature drastically. This quasi-isentropic exchange occurs above the outflow of warm conveyor belts, in regions which exhibit enhanced static stability in the lower stratosphere and a Kelvin–Helmholtz instability across the tropopause. The enhanced static stability is related to radiative cooling below the tropopause and the presence of small-scale waves. The Kelvin–Helmholtz instability is related to vertical shear of the horizontal wind associated with small-scale waves at the upper edge of the jet stream. The instability leads to the occurrence of turbulence and consequent mixing of trace gases in the tropopause region. While the overall relevance of this process has yet to be assessed, it has the potential to significantly modify the chemical composition of the extratropical transition layer in the lowermost stratosphere in regions which have previously gained a small amount of attention in terms of mixing in baroclinic waves.
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Li, Qiang, Markus Rapp, Anne Schrön, Andreas Schneider, and Gunter Stober. "Derivation of turbulent energy dissipation rate with the Middle Atmosphere Alomar Radar System (MAARSY) and radiosondes at Andøya, Norway." Annales Geophysicae 34, no. 12 (December 16, 2016): 1209–29. http://dx.doi.org/10.5194/angeo-34-1209-2016.
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Abstract. We present the derivation of turbulent energy dissipation rate ε from a total of 522 days of observations with the Middle Atmosphere Alomar Radar SYstem (MAARSY) mesosphere–stratosphere–troposphere (MST) radar running tropospheric experiments during the period of 2010–2013 as well as with balloon-borne radiosondes based on a campaign in the summer 2013. Spectral widths are converted to ε after the removal of the broadening effects due to the finite beam width of the radar. With the simultaneous in situ measurements of ε with balloon-borne radiosondes at the MAARSY radar site, we compare the ε values derived from both techniques and reach an encouraging agreement between them. Using all the radar data available, we present a preliminary climatology of atmospheric turbulence in the UTLS (upper troposphere and lower stratosphere) region above the MAARSY site showing a variability of more than 5 orders of magnitude inherent in turbulent energy dissipation rates. The derived ε values reveal a log-normal distribution with a negative skewness, and the ε profiles show an increase with height which is also the case for each individual month. Atmospheric turbulence based on our radar measurements reveals a seasonal variation but no clear diurnal variation in the UTLS region. Comparison of ε with the gradient Richardson number Ri shows that only 1.7 % of all the data with turbulence occur under the condition of Ri < 1 and that the values of ε under the condition of Ri < 1 are significantly larger than those under Ri > 1. Further, there is a roughly negative correlation between ε and Ri that is independent of the scale dependence of Ri. Turbulence under active dynamical conditions (velocity of horizontal wind U > 10 m s−1) is significantly stronger than under quiet conditions (U < 10 m s−1). Last but not least, the derived ε values are compared with the corresponding vertical shears of background wind velocity showing a linear relation with a corresponding correlation coefficient r = 58 % well above the 99.9 % significance level. This implies that wind shears play an important role in the turbulence generation in the troposphere and lower stratosphere (through the Kelvin–Helmholtz instability).
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Dutta, G., M. C. Ajay Kumar, P. Vinay Kumar, P. V. Rao, B. Bapiraju, and H. Aleem Basha. "High resolution observations of turbulence in the troposphere and lower stratosphere over Gadanki." Annales Geophysicae 27, no. 6 (June 11, 2009): 2407–15. http://dx.doi.org/10.5194/angeo-27-2407-2009.
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Abstract. High resolution (150 m) wind measurements from 13–17 July 2004 by Mesosphere-Stratosphere-Troposphere (MST) radar and 15–16 July 2004 by Lower Atmospheric Wind Profiler (LAWP) have been used to study the time variation of turbulence intensity. Layers of higher turbulence are observed in the lower stratosphere on 15–16 July which give rise to mixing in the region. Enhancement in short-period gravity wave activity and turbulent layers are observed after 22:00 LT which could be due to a dry convection event that occurred at that time. The breakdown of the convectively generated high frequency waves seems to have given rise to the turbulence layers. Wind shear is found to be high above the easterly jet, but very poor correlation is observed between square of wind shear and turbulence parameters in the region. The heights of the turbulent layers in the lower stratosphere do not correlate with levels of minimum Richardson number. A monochromatic inertia gravity wave could be identified during 13–17 July 2004. A non-linear interaction between the waves of different scales as proposed by Hines (1992) might also be responsible for the breakdown and generation of turbulence layers.
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Jen-La Plante, Imai, Yongfeng Ma, Katarzyna Nurowska, Hermann Gerber, Djamal Khelif, Katarzyna Karpinska, Marta K. Kopec, Wojciech Kumala, and Szymon P. Malinowski. "Physics of Stratocumulus Top (POST): turbulence characteristics." Atmospheric Chemistry and Physics 16, no. 15 (August 2, 2016): 9711–25. http://dx.doi.org/10.5194/acp-16-9711-2016.
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Abstract. Turbulence observed during the Physics of Stratocumulus Top (POST) research campaign is analyzed. Using in-flight measurements of dynamic and thermodynamic variables at the interface between the stratocumulus cloud top and free troposphere, the cloud top region is classified into sublayers, and the thicknesses of these sublayers are estimated. The data are used to calculate turbulence characteristics, including the bulk Richardson number, mean-square velocity fluctuations, turbulence kinetic energy (TKE), TKE dissipation rate, and Corrsin, Ozmidov and Kolmogorov scales. A comparison of these properties among different sublayers indicates that the entrainment interfacial layer consists of two significantly different sublayers: the turbulent inversion sublayer (TISL) and the moist, yet hydrostatically stable, cloud top mixing sublayer (CTMSL). Both sublayers are marginally turbulent, i.e., the bulk Richardson number across the layers is critical. This means that turbulence is produced by shear and damped by buoyancy such that the sublayer thicknesses adapt to temperature and wind variations across them. Turbulence in both sublayers is anisotropic, with Corrsin and Ozmidov scales as small as ∼ 0.3 and ∼ 3 m in the TISL and CTMSL, respectively. These values are ∼ 60 and ∼ 15 times smaller than typical layer depths, indicating flattened large eddies and suggesting no direct mixing of cloud top and free-tropospheric air. Also, small scales of turbulence are different in sublayers as indicated by the corresponding values of Kolmogorov scales and buoyant and shear Reynolds numbers.
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Malinowski, S. P., H. Gerber, I. Jen-La Plante, M. K. Kopec, W. Kumala, K. Nurowska, P. Y. Chuang, D. Khelif, and K. E. Haman. "Physics of Stratocumulus Top (POST): turbulent mixing across capping inversion." Atmospheric Chemistry and Physics 13, no. 24 (December 17, 2013): 12171–86. http://dx.doi.org/10.5194/acp-13-12171-2013.
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Abstract. High spatial resolution measurements of temperature and liquid water content, accompanied by moderate-resolution measurements of humidity and turbulence, collected during the Physics of Stratocumulus Top experiment are analyzed. Two thermodynamically, meteorologically and even optically different cases are investigated. An algorithmic division of the cloud-top region into layers is proposed. Analysis of dynamic stability across these layers leads to the conclusion that the inversion capping the cloud and the cloud-top region is turbulent due to the wind shear, which is strong enough to overcome the high static stability of the inversion. The thickness of this mixing layer adapts to wind and temperature jumps such that the gradient Richardson number stays close to its critical value. Turbulent mixing governs transport across the inversion, but the consequences of this mixing depend on the thermodynamic properties of cloud top and free troposphere. The effects of buoyancy sorting of the mixed parcels in the cloud-top region are different in conditions that permit or prevent cloud-top entrainment instability. Removal of negatively buoyant air from the cloud top is observed in the first case, while buildup of the diluted cloud-top layer is observed in the second one.
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Ghosh, A. K., V. Siva Kumar, K. Kishore Kumar, and A. R. Jain. "VHF radar observation of atmospheric winds, associated shears and <b>C<sup>2</sup></b><b><sub>n</sub></b> at a tropical location: interdependence and seasonal pattern." Annales Geophysicae 19, no. 8 (August 31, 2001): 965–73. http://dx.doi.org/10.5194/angeo-19-965-2001.
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Abstract. The turbulence refractivity structure constant (C2n ) is an important parameter of the atmosphere. VHF radars have been used extensively for the measurements of C2n. Presently, most of such observations are from mid and high latitudes and only very limited observations are available for equatorial and tropical latitudes. Indian MST radar is an excellent tool for making high-resolution measurements of atmospheric winds, associated shears and turbulence refractivity structure constant (C2n). This radar is located at Gadanki (13.45° N, 79.18° E), a tropical station in India. The objective of this paper is to bring out the height structure of C2n for different seasons using the long series of data (September 1995 – August 1999) from Indian MST radar. An attempt is also made to understand such changes in the height structure of C2n in relation to background atmospheric parameters such as horizontal winds and associated shears. The height structure of C2n, during the summer monsoon and post-monsoon season, shows specific height features that are found to be related to Tropical Easterly Jet (TEJ) winds. It is important to examine the nature of the radar back-scatterers and also to understand the causative mechanism of such scatterers. Aspect sensitivity of the received radar echo is examined for this purpose. It is observed that radar back-scatterers at the upper tropospheric and lower stratospheric heights are more anisotropic, with horizontal correlation length of 10–20 m, as compared to those observed at lower and middle tropospheric heights.Key words. Meteorology and atmospheric dynamics (climatology; tropical meteorology; turbulence)
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Kirkwood, S., M. Mihalikova, T. N. Rao, and K. Satheesan. "Turbulence associated with mountain waves over Northern Scandinavia – a case study using the ESRAD VHF radar and the WRF mesoscale model." Atmospheric Chemistry and Physics 10, no. 8 (April 16, 2010): 3583–99. http://dx.doi.org/10.5194/acp-10-3583-2010.
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Abstract. We use measurements by the 52 MHz wind-profiling radar ESRAD, situated near Kiruna in Arctic Sweden, and simulations using the Advanced Research and Weather Forecasting model, WRF, to study vertical winds and turbulence in the troposphere in mountain-wave conditions on 23, 24 and 25 January 2003. We find that WRF can accurately match the vertical wind signatures at the radar site when the spatial resolution for the simulations is 1 km. The horizontal and vertical wavelengths of the dominating mountain-waves are ~10–20 km and the amplitudes in vertical wind 1–2 m/s. Turbulence below 5500 m height, is seen by ESRAD about 40% of the time. This is a much higher rate than WRF predictions for conditions of Richardson number (Ri) <1 but similar to WRF predictions of Ri<2. WRF predicts that air crossing the 100 km wide model domain centred on ESRAD has a ~10% chance of encountering convective instabilities (Ri<0) somewhere along the path. The cause of low Ri is a combination of wind-shear at synoptic upper-level fronts and perturbations in static stability due to the mountain-waves. Comparison with radiosondes suggests that WRF underestimates wind-shear and the occurrence of thin layers with very low static stability, so that vertical mixing by turbulence associated with mountain waves may be significantly more than suggested by the model.
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Singh, Narendra, R. R. Joshi, H. Y. Chun, G. B. Pant, S. H. Damle, and R. D. Vashishtha. "Seasonal, annual and inter-annual features of turbulence parameters over the tropical station Pune (18°32' N, 73°51' E) observed with UHF wind profiler." Annales Geophysicae 26, no. 12 (November 24, 2008): 3677–92. http://dx.doi.org/10.5194/angeo-26-3677-2008.
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Abstract. The present study is specifically focused on the seasonal, annual and inter-annual variations of the refractive index structure parameter (Cn2) using three years of radar observations. Energy dissipation rates (ε) during different seasons for a particular year are also computed over a tropical station, Pune. Doppler spectral width measurements made by the Wind Profiler, under various atmospheric conditions, are utilized to estimate the turbulence parameters. The refractive index structure parameter varies from 10−17.5 to 10−13 m−2/3 under clear air to precipitation conditions in the height region of 1.05 to 10.35 km. During the monsoon months, observed Cn2 values are up to 1–2 orders of magnitude higher than those during pre-monsoon and post-monsoon seasons. Spectral width correction for various non-turbulent spectral broadenings such as beam broadening and shear broadening are made in the observed spectral width for reliable estimation of ε under non-precipitating conditions. It is found that in the lower tropospheric height region, values of ε are in the range of 10−6 to 10−3 m2 s−3. In summer and monsoon seasons the observed values of ε are larger than those in post-monsoon and winter seasons in the lower troposphere. A comparison of Cn2 observed with the wind profiler and that estimated using Radio Sonde/Radio Wind (RS/RW) data of nearby Met station Chikalthana has been made for the month of July 2003.
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Hirono, M., H. Luce, M. Yamamoto, and S. Fukao. "Horizontal maps of echo power in the lower stratosphere using the MU radar." Annales Geophysicae 22, no. 3 (March 19, 2004): 717–24. http://dx.doi.org/10.5194/angeo-22-717-2004.
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Abstract. In recent works, zenithal and azimuthal angle variations of echo power measured by VHF Stratosphere-Troposphere (ST) radars have been analyzed in detail using different radar multi-beam configurations. It was found that the azimuthal angle corresponding to maximum echo power is closely related to the direction of the horizontal wind shear. These properties indicate that local wind shear affects the tilt of the scatterers. Moreover, horizontal maps of echo power collected using a large set of beams steered pulse-to-pulse up to 40 degrees off zenith revealed that the power distribution pattern in the troposphere is often skewed. In this work, a three-dimensional description of echo power variations up to 24 degrees off zenith is shown for measurements in the lower stratosphere (i.e. up to approximately 20km) using a "sequential multi-beam" (SMB) configuration. Such a description was not possible above the tropopause with classical multi-beam configurations because of the loss of radar sensitivity due to the limited integration time by the use of a large number of beams. This work attempts to complete previous descriptions of the phenomenon by some observations in the lower stratosphere discussed in association with complementary balloon measurements. Key words. Meteorology and atmospheric dynamics (turbulence) – Radio Science (remote sensing)
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Newton, Richard, and William Randel. "Observations of Upper-Tropospheric Temperature Inversions in the Indian Monsoon and Their Links to Convectively Forced Quasi-Stationary Kelvin Waves." Journal of the Atmospheric Sciences 77, no. 8 (July 27, 2020): 2835–46. http://dx.doi.org/10.1175/jas-d-20-0042.1.
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Abstract High-vertical-resolution temperature measurements from GPS radio occultation data show frequent upper-tropospheric inversions over the equatorial Indian Ocean during the summer monsoon season. Each year, around 30% of profiles in this region have temperature inversions near 15 km during the monsoon season, peaking during July–September. This work describes the space–time behavior of these inversions, and their links to transient deep convection. The Indian Ocean inversions occur episodically several times each summer, with a time scale of 1–2 weeks, and are quasi stationary or slowly eastward moving. Strong inversions are characterized by cold anomalies in the upper-troposphere (12–15 km), warm anomalies in the tropopause layer (16–18 km), and strong zonal wind anomalies that are coherent with temperature anomalies. Temperature and wind anomalies are centered over the equator and show a characteristic eastward phase tilt with height with a vertical wavelength near 5 km, consistent with a Kelvin wave structure. Composites of outgoing longwave radiation (OLR) show that strong inversions are linked to enhanced deep convection over the equatorial Indian Ocean, preceding the inversions by ~2–6 days. These characteristics suggest that the inversions are linked to convectively forced Kelvin waves, which are Doppler shifted by the easterly monsoonal winds such that they remain quasi stationary in the equatorial Indian Ocean. These large-scale waves influence circulation on the equatorial side of the Indian monsoon anticyclone; they may provide a positive feedback to the underlying convection, and are possibly linked with regions of shear-induced turbulence.
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Wroblewski, Donald E., Owen R. Coté, Jorg M. Hacker, and Ron J. Dobosy. "Cliff–Ramp Patterns and Kelvin–Helmholtz Billows in Stably Stratified Shear Flow in the Upper Troposphere: Analysis of Aircraft Measurements." Journal of the Atmospheric Sciences 64, no. 7 (July 1, 2007): 2521–39. http://dx.doi.org/10.1175/jas3956.1.
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Abstract Cliff–ramp patterns (CR) are a common feature of scalar turbulence, characterized by a sharp temperature increase (cliff) followed by a more gradual temperature decrease (ramp). Aircraft measurements obtained from NOAA best aircraft turbulence probes (BAT) were used to characterize and compare CR patterns observed under stably stratified conditions in the upper troposphere, a region for which there are few such studies. Experimental data were analyzed for three locations, one over Wales and two over southern Australia, the latter in correspondence with the Southern Hemisphere winter subtropical jet stream. Comparison of observed CR patterns with published direct numerical simulations (DNS) revealed that they were likely signatures of Kelvin–Helmholtz (KH) billows, with the ramps associated with the well-mixed billows and the cliffs marking the highly stretched braids. Strong correlation between potential temperature and horizontal velocity supported the KH link, though expected correlations with vertical velocity were not observed. The temperature fronts associated with the cliffs were oriented in a direction approximately normal to the mean wind direction. Locally high values of temperature structure constant near these fronts were associated with steep temperature gradients across the fronts; this may be misleading in the context of electromagnetic propagation, suggesting a false positive indication of high levels of small-scale turbulence that would not correspond to scintillation effects. Billow aspect ratios, braid angles, and length scales were estimated from the data and comparisons with published DNS provided a means for assessing the stage of evolution of the KH billows and the initial Richardson number of the layer.
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Ueda, Hiromasa, Tetsuo Fukui, Mizuo Kajino, Mitsuaki Horiguchi, Hiroyuki Hashiguchi, and Shoichiro Fukao. "Eddy Diffusivities for Momentum and Heat in the Upper Troposphere and Lower Stratosphere Measured by MU Radar and RASS, and a Comparison of Turbulence Model Predictions." Journal of the Atmospheric Sciences 69, no. 1 (January 1, 2012): 323–37. http://dx.doi.org/10.1175/jas-d-11-023.1.
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Abstract Recently, middle- and upper-atmosphere Doppler radar (MU radar) has enabled the measurement of middle-atmosphere turbulence from radar backscatter Doppler spectra. In this work, eddy diffusivities for momentum Km in the upper troposphere and lower stratosphere during clear-air conditions were derived from direct measurements of the Reynolds stress and vertical gradient of mean wind velocity measured by MU radar. Eddy diffusivity for heat Kh below 8 km was determined from measurements of temperature fluctuations by the Radio Acoustic Sounding System (RASS) attached to the MU radar. The eddy diffusivity for momentum was on the order of 10 m2 s−1 in the upper troposphere and decreased gradually in the stratosphere by an order of magnitude or more. The eddy diffusivity for heat was almost of the same order of magnitude as Km. Estimates of eddy diffusivity from the radar echo power spectral width give fairly good values compared with the direct measurement of Km. Applicability of three turbulence models—the spectral width method, the k–ɛ model modified for stratified flows, and the algebraic stress model—were also examined, using radar observation values of turbulent kinetic energy k and turbulent energy dissipation rate ɛ together with atmospheric stability observations from rawinsonde data. It is concluded that the algebraic stress model shows the best fit with the direct measurement of Km, even in the free atmosphere above the atmospheric boundary layer once k and ɛ values are obtained from observations or a model.
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Fiorino, Steven, Santasri Bose-Pillai, and Kevin Keefer. "Re-Visiting Acoustic Sounding to Advance the Measurement of Optical Turbulence." Applied Sciences 11, no. 16 (August 20, 2021): 7658. http://dx.doi.org/10.3390/app11167658.
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Optical turbulence, as determined by the widely accepted practice of profiling the temperature structure constant, CT2, via the measurement of ambient atmospheric temperature gradients, can be found to differ quite significantly when characterizing such gradients via thermal-couple differential temperature sensors as compared to doing so with acoustic probes such as those commonly used in sonic anemometry. Similar inconsistencies are observed when comparing optical turbulence strength derived via CT2 as compared to those through direct optical or imaging measurements of small fluctuations of the index of refraction of air (i.e., scintillation). These irregularities are especially apparent in stable atmospheric layers and during diurnal quiescent periods. Our research demonstrates that when care is taken to properly remove large-scale index of refraction gradients, the sonic anemometer-derived velocity structure constant, Cv2, coupled with the similarly derived turbulence-driven index of refraction and vertical wind shear gradients, provides a refractive index structure constant, Cn2, that can more closely match the optical turbulence strengths inferred by more direct means such as scintillometers or differential image motion techniques. The research also illustrates the utility and robustness of quantifying Cn2 from CT2 at a point using a single sonic anemometer and establishes a clear set of equations to calculate volumetric Cn2 data using instrumentation that measures wind velocities with more spatial/temporal fidelity than temperature.
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Cantero, Elena, Javier Sanz, Fernando Borbón, Daniel Paredes, and Almudena García. "On the measurement of stability parameter over complex mountainous terrain." Wind Energy Science 7, no. 1 (February 2, 2022): 221–35. http://dx.doi.org/10.5194/wes-7-221-2022.
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Abstract. Atmospheric stability has a significant effect on wind shear and turbulence intensity, and these variables, in turn, have a direct impact on wind power production and loads on wind turbines. It is therefore important to know how to characterise atmospheric stability in order to make better energy yield estimation in a wind farm. Based on the research-grade meteorological mast at Alaiz (CENER's test site in Navarre, Spain) named MP5, this work compares and evaluates different instrument set-ups and methodologies for stability characterisation, namely the Obukhov parameter, measured with a sonic anemometer, and the bulk Richardson number based on two temperature and one wind speed measurement. The methods are examined considering their theoretical background, implementation complexity, instrumentation requirements, and practical use in connection to wind energy applications. The sonic method provides a more precise local measurement of stability while the bulk Richardson is a simpler, robust and cost-effective technique to implement in wind assessment campaigns. Using the sonic method as a benchmark, it is shown that to obtain reliable bulk Richardson measurements in onshore sites it is necessary to install one of the temperature sensors close to the ground where the temperature gradient is stronger.
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Wang, Binbin, Qian Liao, Jianen Xiao, and Harvey A. Bootsma. "A Free-Floating PIV System: Measurements of Small-Scale Turbulence under the Wind Wave Surface." Journal of Atmospheric and Oceanic Technology 30, no. 7 (July 1, 2013): 1494–510. http://dx.doi.org/10.1175/jtech-d-12-00092.1.
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Abstract An in situ free-floating underwater miniature particle image velocimetry (UWMPIV) system is developed and applied to measure the structure of turbulence in the aqueous side of the wind wave surface boundary layer. The UWMPIV system provides a direct way to measure the aqueous side turbulence dissipation rate and vortex structures immediately below the air–water interface, which are important parameters that determine the gas exchange rate across the air–water interface subjected to a low-to-moderate wind shear. The impact of platform motion on the measurement of small-scale turbulence is discussed and found to be insignificant. A series of field experiments under a near “zero-fetch” wind wave condition and one open water experiment under a low wind condition were conducted on Lake Michigan to demonstrate the capabilities of the free-floating particle image velocimetry (PIV) system. The dissipation rate estimated with a “direct method” and with a “spectra fitting” method are compared. Vertical profiles of the turbulence dissipation rate suggest a power-law dependency with depth below the water surface. Surface shear velocities estimated through the aqueous side Reynolds stress distribution agreed well with wind stresses estimated by the classic drag law for zero-fetch wind wave conditions, where the primary source of turbulence was wind shear. For the open water experiment under a very low wind condition, a high dissipation rate was observed near the water surface, suggesting a high turbulence production rate by surface waves, and the profile of dissipation rate showed a slower decay rate with depth in the presence of waves.
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Eigenmann, R., S. Metzger, and T. Foken. "Generation of free convection due to changes of the local circulation system." Atmospheric Chemistry and Physics 9, no. 21 (November 12, 2009): 8587–600. http://dx.doi.org/10.5194/acp-9-8587-2009.
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Abstract. Eddy-covariance and Sodar/RASS experimental measurement data of the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007 are used to investigate the generation of near-ground free convection conditions (FCCs) in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed that FCCs are initiated near the ground in situations where moderate to high buoyancy fluxes and a simultaneously occurring drop of the wind speed were present. The minimum in wind speed – observable by the Sodar measurements through the whole vertical extension of the valley atmosphere – is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyancy then dominates over shear within the production of turbulence kinetic energy near the ground. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) when the level of free convection, which starts above the Obukhov length, drops below that of the sonic anemometer. An analysis of the scales of turbulent motions during FCCs using wavelet transform shows the occurrence of large-scale turbulence structures. Regarding the entire COPS measurement period, FCCs in the morning hours occur on about 50% of all days. Enhanced surface fluxes of latent and sensible heat are found on these days.
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Unterstrasser, S., R. Paoli, I. Sölch, C. Kühnlein, and T. Gerz. "Dimension of aircraft exhaust plumes at cruise conditions: effect of wake vortices." Atmospheric Chemistry and Physics 14, no. 5 (March 14, 2014): 2713–33. http://dx.doi.org/10.5194/acp-14-2713-2014.
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Abstract. The dispersion of aircraft emissions during the vortex phase is studied using a 3-D LES model with Lagrangian particle tracking. The simulations start with a fully rolled-up vortex pair of a type B747/A340 airplane and the tracer centred around the vortex cores. The tracer dilution and plume extent is studied for a variety of ambient and aircraft parameters until aircraft-induced effects have ceased. For typical upper tropospheric conditions, the impact of stratification is more dominant compared to turbulence intensity or vertical wind shear. Moreover, the sensitivity to the initial tracer distribution was found to be weak. Along the transverse direction, the tracer concentrations can be well approximated by a Gaussian distribution, along the vertical a superposition of three Gaussian distributions is adequate. For the studied parameter range, the vertical plume expansion ranges from 400 m to 550 m and cross-sectional area from 4.0 × 104 m2 to 6 × 104 m2 after six minutes. For validation, selected simulations were compared to an alternative LES model and to in-situ NO-measurements.
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Kwong, K. M., Max H. Y. Wong, James N. K. Liu, and P. W. Chan. "An Artificial Neural Network with Chaotic Oscillator for Wind Shear Alerting." Journal of Atmospheric and Oceanic Technology 29, no. 10 (October 1, 2012): 1518–31. http://dx.doi.org/10.1175/2011jtecha1501.1.
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Abstract Current research based on various approaches including the use of numerical weather prediction models, statistical models, and machine learning models have provided some encouraging results in the area of long-term weather forecasting. But at the level of mesoscale and even microscale severe weather phenomena (involving very short-term chaotic perturbations) such as turbulence and wind shear phenomena, these approaches have not been so successful. This research focuses on the use of chaotic oscillatory-based neural networks for the study of a mesoscale weather phenomenon, namely, wind shear, a challenging and complex meteorological problem that has a vital impact on aviation safety. Using lidar data collected at the Hong Kong International Airport via the Hong Kong Observatory, it is possible to forecast the Doppler velocities with satisfactory accuracy and validate the prediction model with the potential to generate the wind shear alert. Experimental results are found to be comparable to the actual measurement. Moreover, the selected testing cases and results show that the value of correlation coefficient between the predicted and lidar-measured wind velocities exceeds 0.9 with various window sizes ranging from 1 to 3 h. These provide areas for further research of the proposed model and lidar technology for turbulence and wind shear forecasts.
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Abraham, Carsten, and Adam H. Monahan. "Climatological Features of the Weakly and Very Stably Stratified Nocturnal Boundary Layers. Part I: State Variables Containing Information about Regime Occupation." Journal of the Atmospheric Sciences 76, no. 11 (November 1, 2019): 3455–84. http://dx.doi.org/10.1175/jas-d-18-0261.1.
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Abstract The atmospheric nocturnal stable boundary layer (SBL) can be classified into two distinct regimes: the weakly SBL (wSBL) with sustained turbulence and the very SBL (vSBL) with weak and intermittent turbulence. A hidden Markov model (HMM) analysis of the three-dimensional state-variable space of Reynolds-averaged mean dry static stability, mean wind speed, and wind speed shear is used to classify the SBL into these two regimes at nine different tower sites, in order to study long-term regime occupation and transition statistics. Both Reynolds-averaged mean data and measures of turbulence intensity (eddy variances) are separated in a physically meaningful way. In particular, fluctuations of the vertical wind component are found to be much smaller in the vSBL than in the wSBL. HMM analyses of these data using more than two SBL regimes do not result in robust results across measurement locations. To identify which meteorological state variables carry the information about regime occupation, the HMM analyses are repeated using different state-variable subsets. Reynolds-averaged measures of turbulence intensity (such as turbulence kinetic energy) at any observed altitude hold almost the same information as the original set, without adding any additional information. In contrast, both stratification and shear depend on surface information to capture regime transitions accurately. Use of information only in the bottom 10 m of the atmosphere is sufficient for HMM analyses to capture important information about regime occupation and transition statistics. It follows that the commonly measured 10-m wind speed is potentially a good indicator of regime occupation.
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Wulfmeyer, Volker, David D. Turner, B. Baker, R. Banta, A. Behrendt, T. Bonin, W. A. Brewer, et al. "A New Research Approach for Observing and Characterizing Land–Atmosphere Feedback." Bulletin of the American Meteorological Society 99, no. 8 (August 2018): 1639–67. http://dx.doi.org/10.1175/bams-d-17-0009.1.
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AbstractForecast errors with respect to wind, temperature, moisture, clouds, and precipitation largely correspond to the limited capability of current Earth system models to capture and simulate land–atmosphere feedback. To facilitate its realistic simulation in next-generation models, an improved process understanding of the related complex interactions is essential. To this end, accurate 3D observations of key variables in the land–atmosphere (L–A) system with high vertical and temporal resolution from the surface to the free troposphere are indispensable.Recently, we developed a synergy of innovative ground-based, scanning active remote sensing systems for 2D to 3D measurements of wind, temperature, and water vapor from the surface to the lower troposphere that is able to provide comprehensive datasets for characterizing L–A feedback independently of any model input. Several new applications are introduced, such as the mapping of surface momentum, sensible heat, and latent heat fluxes in heterogeneous terrain; the testing of Monin–Obukhov similarity theory and turbulence parameterizations; the direct measurement of entrainment fluxes; and the development of new flux-gradient relationships. An experimental design taking advantage of the sensors’ synergy and advanced capabilities was realized for the first time during the Land Atmosphere Feedback Experiment (LAFE), conducted at the Atmospheric Radiation Measurement Program Southern Great Plains site in August 2017. The scientific goals and the strategy of achieving them with the LAFE dataset are introduced. We envision the initiation of innovative L–A feedback studies in different climate regions to improve weather forecast, climate, and Earth system models worldwide.
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Xu, Manman, Shiyong Shao, Ningquan Weng, Liangping Zhou, Qing Liu, and Yuefeng Zhao. "Atmospheric Optical Turbulence Characteristics over the Ocean Relevant to Astronomy and Atmospheric Physics." Applied Sciences 11, no. 22 (November 9, 2021): 10548. http://dx.doi.org/10.3390/app112210548.
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Due to the space and time constraints of turbulence measurement equipment and the experiment scene, it is difficult to obtain the atmosphere refractive index structure constant over the ocean. In this paper, the characteristics of atmospheric optical turbulence in offshore and open ocean conditions are summarized by analyzing the meteorological data obtained from two ocean atmospheric optical parameter field experiments. Because of the influence of land undersurface, the turbulence strength in offshore conditions is roughly the same as that on land and presents different characteristics in open ocean. Compared with the offshore area, the turbulence strength over the open ocean near-surface decreases during the day and increases at night, and the diurnal variation characteristics weaken. The turbulence strength profiles over the offshore area show different characteristics at different times, where the turbulence strength in the morning is higher than that in the evening. By retrieving the meteorological factors affecting the turbulence, it is found that the temperature gradient and wind shear are in good agreement with turbulence strength in both offshore and open ocean areas. Furthermore, the integrated parameters for astronomy and optical telecommunication are derived from profiles over the offshore and open ocean areas. It is of great significance to research the turbulent characteristics of ocean atmosphere for optical transmission and astronomical observations.
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Davis, S. R., R. Talbot, and H. Mao. "Transport and outflow to the North Atlantic in the lower marine troposphere during ICARTT 2004." Atmospheric Chemistry and Physics Discussions 12, no. 1 (January 25, 2012): 2395–434. http://dx.doi.org/10.5194/acpd-12-2395-2012.
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Abstract. An analysis of pollution plumes emitted from sources in the Northeastern US was based on observations from the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) 2004 field campaign. Particular attention was given to the relation of these plumes to coastal transport patterns in lower tropospheric layers throughout the Gulf of Maine (GOM) and their contribution to large-scale pollution outflow from the North American continent. Using measurements obtained during a series of flights of the NOAA WP-3D and the NASA DC-8, a unique quasi-lagrangian case study was conducted for a freshly emitted plume emanating from the New York City source region in late July 2004. The initial development of this plume stemmed from the accumulation of boundary layer pollutants within a coastal residual layer where weak synoptic forcing triggered its advection by mean southwesterly flow. As the plume tracked into the GOM, analysis showed that the plume layer vertical structure evolved into an internal boundary layer form, with signatures of steep vertical gradients in temperature, moisture and wind speed often resulting in periodic turbulence. This structure remained well-defined during the plume study, allowing for the detachment of the plume layer from the surface and thus minimal deposition and plume-sea surface exchange. In contrast, lateral mixing with other low-level plumes was significant during its transit and facilitated in part by persistent shear driven turbulence which further contributed to the high spatial variability in trace gas mixing ratios. The impact of the plume inland was assessed using observations from the AIRMAP air quality network. This impact was noticeably detected as a contribution to poor surface ozone conditions and significant elevations of other major pollutants to levels equaling the highest observed that summer. Further contributions to larger-scale outflow across the North Atlantic was also observed and analyzed.
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VOLKOV, V. V., M. A. STRUNIN, and A. M. STRUNIN. "DETERMINATION OF WIND SHEAR AND TURBULENCE INTENSITY ACCORDING TO YAK42-D “ROSHYDROMET” RESEARCH AIRCRAFT DATA." Meteorologiya i Gidrologiya, no. 9 (2021): 117–29. http://dx.doi.org/10.52002/0130-2906-2021-9-117-129.
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The results of the development and comparative analysis of methods for determining wind shear in the atmosphere (regression and difference ones) based on research aircraft data are presented. It is shown that shear calculation by the regression method gives the error of 0.002-0.006 (m/s)/km (depending on the length of the measurement sections) for horizontal shears and 0.04-0.12 (m/s)/100 m for vertical shears; the respective error of the difference method is 0.007 (m/s)/km and 0.07 (m/s)/100 m. Based on the Yak-42D “Roshydromet” research aircraft data, the values of shears of two horizontal components of wind speed in three directions (two horizontal and vertical) were calculated. According to the data of two research aircraft flights, the maximum values of the horizontal shear of wind speed components were reached above the boundary layer and were equal to 0.2 (m/s)/km, and the vertical shear was 1.2 (m/s)/100 m. The energy profiles of horizontal and vertical turbulent pulsations are constructed, it is shown that intense turbulence smooths wind shears in the convective atmospheric boundary layer.
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Wildmann, Norman, Ramona Eckert, Andreas Dörnbrack, Sonja Gisinger, Markus Rapp, Klaus Ohlmann, and Annelize van Niekerk. "In Situ Measurements of Wind and Turbulence by a Motor Glider in the Andes." Journal of Atmospheric and Oceanic Technology 38, no. 4 (April 2021): 921–35. http://dx.doi.org/10.1175/jtech-d-20-0137.1.
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AbstractA Stemme S10-VT motor glider was equipped with a newly developed sensor suite consisting of a five-hole probe, an inertial navigation and global navigation satellite system, two temperature sensors, and a humidity sensor. By design, the system provides three-dimensional wind vector data that enable the analysis of atmospheric motion scales up to a temporal resolution of 10 Hz. We give a description of components and installation of the system, its calibration, and its performance. The accuracy for the measurement of the wind vector is estimated to be on the order of 0.5 m s−1. As part of the Southern Hemisphere Transport, Dynamics, and Chemistry (SouthTRAC) field campaign, 30 research flights were performed from September 2019 to January 2020. We present statistical analysis of the observations, discriminating pure motor flights from soaring flights in the lee waves of the Andes. We present histograms of flight altitude, airspeed, wind speed and direction, temperature, and relative humidity to document the atmospheric conditions. Probability density functions of vertical air velocity, turbulence kinetic energy (TKE), and dissipation rate complete the statistical analysis. Altogether, 41% of the flights are in weak, 14% in moderate, and 0.4% in strong mountain wave conditions according to thresholds for the measured vertical air velocity. As an exemplary case study, we compare measurements on 11 September 2019 to a high-resolution numerical weather prediction model. The case study provides a meaningful example of how data from soaring flights might be utilized for model validation on the mesoscale and within the troposphere.
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Mahrt, L., Jielun Sun, S. P. Oncley, and T. W. Horst. "Transient Cold Air Drainage down a Shallow Valley." Journal of the Atmospheric Sciences 71, no. 7 (June 20, 2014): 2534–44. http://dx.doi.org/10.1175/jas-d-14-0010.1.
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Abstract Drainage of cold air down a small valley and associated near-surface wind maxima are examined from 20 stations with sonic anemometers at 1 m and from a 20-m tower that includes six sonic anemometers in the lowest 5 m, deployed in the Shallow Cold Pool Experiment (SCP). The small valley is about 270 m wide and 12 m deep with a downvalley slope of 2%–3%. The momentum budget indicates that the flow is driven by the buoyancy deficit of the flow and opposed primarily by the stress divergence while the remaining terms are estimated to be at least an order of magnitude smaller. This analysis also reveals major difficulties in quantifying such a budget due to uncertainties in the measurements, sensitivity to choice of averaging time, and sensitivity to measurement heights. Wind maxima occur as low as 0.5 m in the downvalley drainage flow—the lowest observational level. The downvalley cold air drainage and wind maxima are frequently disrupted by transient modes that sometimes lead to significant vertical mixing. On average, the downvalley drainage of cold air occurs with particularly weak turbulence with stronger turbulence above the drainage flow. The momentum flux profile responds to the shear reversal at the wind maximum on a vertical scale of 1 m or less, suggesting the important role of finescale turbulent diffusion.
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Chakraborty, Sudip, Kathleen A. Schiro, Rong Fu, and J. David Neelin. "On the role of aerosols, humidity, and vertical wind shear in the transition of shallow-to-deep convection at the Green Ocean Amazon 2014/5 site." Atmospheric Chemistry and Physics 18, no. 15 (August 10, 2018): 11135–48. http://dx.doi.org/10.5194/acp-18-11135-2018.
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Abstract. The preconditioning of the atmosphere for a shallow-to-deep convective transition during the dry-to-wet season transition period (August–November) is investigated using Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) GoAmazon2014/5 campaign data from March 2014 to November 2015 in Manacapuru, Brazil. In comparison to conditions observed prior to shallow convection, anomalously high humidity in the free troposphere and boundary layer is observed prior to a shallow-to-deep convection transition. An entraining plume model, which captures this leading dependence on lower tropospheric moisture, is employed to study indirect thermodynamic effects associated with vertical wind shear (VWS) and cloud condensation nuclei (CCN) concentration on preconvective conditions. The shallow-to-deep convective transition primarily depends on humidity, especially that from the free troposphere, which tends to increase plume buoyancy. Conditions preceding deep convection are associated with high relative humidity, and low-to-moderate CCN concentration (less than the 67th percentile, 1274 cm−3). VWS, however, shows little relation to moisture and plume buoyancy. Buoyancy estimates suggest that the latent heat release due to freezing is important to deep convective growth under all conditions analyzed, consistent with potential pathways for aerosol effects, even in the presence of a strong entrainment. Shallow-only convective growth, however, shows an association with a strong (weak) low (deep) level VWS and with higher CCN concentration.
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Wu, Songhua, Kangwen Sun, Guangyao Dai, Xiaoye Wang, Xiaoying Liu, Bingyi Liu, Xiaoquan Song, et al. "Inter-comparison of wind measurements in the atmospheric boundary layer and the lower troposphere with Aeolus and a ground-based coherent Doppler lidar network over China." Atmospheric Measurement Techniques 15, no. 1 (January 5, 2022): 131–48. http://dx.doi.org/10.5194/amt-15-131-2022.
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Abstract. After the successful launch of Aeolus, which is the first spaceborne wind lidar developed by the European Space Agency (ESA), on 22 August 2018, we deployed several ground-based coherent Doppler wind lidars (CDLs) to verify the wind observations from Aeolus. By the simultaneous wind measurements with CDLs at 17 stations over China, the Rayleigh-clear and Mie-cloudy horizontal-line-of-sight (HLOS) wind velocities from Aeolus in the atmospheric boundary layer and the lower troposphere are compared with those from CDLs. To ensure the quality of the measurement data from CDLs and Aeolus, strict quality controls are applied in this study. Overall, 52 simultaneous Mie-cloudy comparison pairs and 387 Rayleigh-clear comparison pairs from this campaign are acquired. All of the Aeolus-produced Level 2B (L2B) Mie-cloudy HLOS wind and Rayleigh-clear HLOS wind and CDL-produced HLOS wind are compared individually. For the inter-comparison result of Mie-cloudy HLOS wind and CDL-produced HLOS wind, the correlation coefficient, the standard deviation, the scaled mean absolute deviation (MAD) and the bias are 0.83, 3.15 m s−1, 2.64 m s−1 and −0.25 m s−1, respectively, while the y=ax slope, the y=ax+b slope and the y=ax+b intercept are 0.93, 0.92 and −0.33 m s−1. For the Rayleigh-clear HLOS wind, the correlation coefficient, the standard deviation, the scaled MAD and the bias are 0.62, 7.07 m s−1, 5.77 m s−1 and −1.15 m s−1, respectively, while the y=ax slope, the y=ax+b slope and the y=ax+b intercept are 1.00, 0.96 and −1.2 m s−1. It is found that the standard deviation, the scaled MAD and the bias on ascending tracks are lower than those on descending tracks. Moreover, to evaluate the accuracy of Aeolus HLOS wind measurements under different product baselines, the Aeolus L2B Mie-cloudy HLOS wind data and L2B Rayleigh-clear HLOS wind data under Baselines 07 and 08, Baselines 09 and 10, and Baseline 11 are compared against the CDL-retrieved HLOS wind data separately. From the comparison results, marked misfits between the wind data from Aeolus Baselines 07 and 08 and wind data from CDLs in the atmospheric boundary layer and the lower troposphere are found. With the continuous calibration and validation and product processor updates, the performances of Aeolus wind measurements under Baselines 09 and 10 and Baseline 11 are improved significantly. Considering the influence of turbulence and convection in the atmospheric boundary layers and the lower troposphere, higher values for the vertical velocity are common in this region. Hence, as a special note, the vertical velocity could impact the HLOS wind velocity retrieval from Aeolus.
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Hu, W., F. Letson, R. J. Barthelmie, and S. C. Pryor. "Wind Gust Characterization at Wind Turbine Relevant Heights in Moderately Complex Terrain." Journal of Applied Meteorology and Climatology 57, no. 7 (July 2018): 1459–76. http://dx.doi.org/10.1175/jamc-d-18-0040.1.
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AbstractImproved understanding of wind gusts in complex terrain is critically important to wind engineering and specifically the wind energy industry. Observational data from 3D sonic anemometers deployed at 3 and 65 m at a site in moderately complex terrain within the northeastern United States are used to calculate 10 descriptors of wind gusts and to determine the parent distributions that best describe these parameters. It is shown that the parent distributions exhibit consistency across different descriptors of the gust climate. Specifically, the parameters that describe the gust intensity (gust amplitude, rise magnitude, and lapse magnitude; i.e., properties that have units of length per time) fit the two-parameter Weibull distribution, those that are unitless ratios (gust factor and peak factor) are described by log-logistic distributions, and all other properties (peak gust, rise and lapse times, gust asymmetric factor, and gust length scale) are lognormally distributed. It is also shown that gust factors scale with turbulence intensity, but gusts are distinguishable in power spectra of the longitudinal wind component (i.e., they have demonstrably different length scales than the average eddy length scale). Gust periods at the lower measurement height (3 m) are consistent with shear production, whereas at 65 m they are not. At this site, there is only a weak directional dependence of gust properties on site terrain and land cover variability along sectorial transects, but large gust length scales and gust factors are more likely to be observed in unstable atmospheric conditions.
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Connell, James R., and David C. Powell. "Comparison of Measured and Modeled Turbulence Spectra for a Point Rotating as on a Horizontal-Axis Wind Turbine." Journal of Solar Energy Engineering 111, no. 4 (November 1, 1989): 268–77. http://dx.doi.org/10.1115/1.3268321.
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A theoretical model of the spectrum of rotationally sampled wind speed is tested against corresponding measured wind spectra. The ability to generate such a spectrum, independent of measurement, is important in wind turbine design and testing. The measured spectra were selected from a number of cases measured with a vertical plane array of anemometers. The array was set up to correspond to the tip circle of a nearby MOD-OA wind turbine rotor blade. For testing purposes, the turbulence and mean wind shear were treated separately. In each case the test was based on two correspondence criteria, each involving comparison of results from the theoretical model and results from real data analysis. These criteria were rotational spectra and their integration by harmonic band into energy, particularly the latter. The measured spectra were selected to represent different atmospheric stability conditions. The theory is shown to have respectable accuracy except when applied to the stable atmosphere. Recommendations were made for selection of the values of atmospheric parameters to be used for computing the theoretical result. The importance of the time length of a measured turbulence record to be used in estimating several parameters for input to the theoretical model applied to real wind conditions at a selected turbine site is discussed. The VAX 11/780 computer time required to generate a model spectrum is usually less than one minute.
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Wang, Yansen, Cheryl L. Klipp, Dennis M. Garvey, David A. Ligon, Chatt C. Williamson, Sam S. Chang, Rob K. Newsom, and Ronald Calhoun. "Nocturnal Low-Level-Jet-Dominated Atmospheric Boundary Layer Observed by a Doppler Lidar over Oklahoma City during JU2003." Journal of Applied Meteorology and Climatology 46, no. 12 (December 1, 2007): 2098–109. http://dx.doi.org/10.1175/2006jamc1283.1.
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Abstract Boundary layer wind data observed by a Doppler lidar and sonic anemometers during the mornings of three intensive observational periods (IOP2, IOP3, and IOP7) of the Joint Urban 2003 (JU2003) field experiment are analyzed to extract the mean and turbulent characteristics of airflow over Oklahoma City, Oklahoma. A strong nocturnal low-level jet (LLJ) dominated the flow in the boundary layer over the measurement domain from midnight to the morning hours. Lidar scans through the LLJ taken after sunrise indicate that the LLJ elevation shows a gradual increase of 25–100 m over the urban area relative to that over the upstream suburban area. The mean wind speed beneath the jet over the urban area is about 10%–15% slower than that over the suburban area. Sonic anemometer observations combined with Doppler lidar observations in the urban and suburban areas are also analyzed to investigate the boundary layer turbulence production in the LLJ-dominated atmospheric boundary layer. The turbulence kinetic energy was higher over the urban domain mainly because of the shear production of building surfaces and building wakes. Direct transport of turbulent momentum flux from the LLJ to the urban street level was very small because of the relatively high elevation of the jet. However, since the LLJ dominated the mean wind in the boundary layer, the turbulence kinetic energy in the urban domain is correlated directly with the LLJ maximum speed and inversely with its height. The results indicate that the jet Richardson number is a reasonably good indicator for turbulent kinetic energy over the urban domain in the LLJ-dominated atmospheric boundary layer.
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Robey, Rachel, and Julie K. Lundquist. "Behavior and mechanisms of Doppler wind lidar error in varying stability regimes." Atmospheric Measurement Techniques 15, no. 15 (August 15, 2022): 4585–622. http://dx.doi.org/10.5194/amt-15-4585-2022.
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Abstract. Wind lidars are widespread and important tools in atmospheric observations. An intrinsic part of lidar measurement error is due to atmospheric variability in the remote-sensing scan volume. This study describes and quantifies the distribution of measurement error due to turbulence in varying atmospheric stability. While the lidar error model is general, we demonstrate the approach using large ensembles of virtual WindCube V2 lidar performing a profiling Doppler-beam-swinging scan in quasi-stationary large-eddy simulations (LESs) of convective and stable boundary layers. Error trends vary with the stability regime, time averaging of results, and observation height. A systematic analysis of the observation error explains dominant mechanisms and supports the findings of the empirical results. Treating the error under a random variable framework allows for informed predictions about the effect of different configurations or conditions on lidar performance. Convective conditions are most prone to large errors (up to 1.5 m s−1 in 1 Hz wind speed in strong convection), driven by the large vertical velocity variances in convective conditions and the high elevation angle of the scanning beams (62∘). Range-gate weighting induces a negative bias into the horizontal wind speeds near the surface shear layer (−0.2 m s−1 in the stable test case). Errors in the horizontal wind speed and direction computed from the wind components are sensitive to the background wind speed but have negligible dependence on the relative orientation of the instrument. Especially during low winds and in the presence of large errors in the horizontal velocity estimates, the reported wind speed is subject to a systematic positive bias (up to 0.4 m s−1 in 1 Hz measurements in strong convection). Vector time-averaged measurements can improve the behavior of the error distributions (reducing the 10 min wind speed error standard deviation to <0.3 m s−1 and the bias to <0.1 m s−1 in strong convection) with a predictable effectiveness related to the number of decorrelated samples in the time window. Hybrid schemes weighting the 10 min scalar- and vector-averaged lidar measurements are shown to be effective at reducing the wind speed biases compared to cup measurements in most of the simulated conditions, with time averages longer than 10 min recommended for best use in some unstable conditions. The approach in decomposing the error mechanisms with the help of the LES flow field could be extended to more complex measurement scenarios and scans.
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Shupe, Matthew D., Pavlos Kollias, Michael Poellot, and Edwin Eloranta. "On Deriving Vertical Air Motions from Cloud Radar Doppler Spectra." Journal of Atmospheric and Oceanic Technology 25, no. 4 (April 1, 2008): 547–57. http://dx.doi.org/10.1175/2007jtecha1007.1.
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Abstract A method for deriving vertical air motions from cloud radar Doppler spectrum measurements is introduced. The method is applicable to cloud volumes containing small particles, in this case liquid droplets, which are assumed to trace vertical air motions because of their limited size. The presence of liquid droplets is confirmed using multiple ground-based remote sensors. Corrections for Doppler spectrum broadening due to turbulence, wind shear, and radar beamwidth are applied. As a result of the turbulence broadening correction, the turbulent dissipation rate can also be estimated. This retrieval is demonstrated using measurements from the Department of Energy (DOE) Atmospheric Radiation Measurement Program’s (ARM) site in Barrow, Alaska, during the Mixed-Phase Arctic Cloud Experiment (MPACE) of autumn 2004. Comparisons of the retrievals with measurements by research aircraft near Barrow indicate that, on the whole, the retrievals perform well. A small bias in vertical velocity between the retrievals and aircraft measurements is found, based on a statistical comparison of four cases comprising nearly 6 h of data. Turbulent dissipation rate comparisons suggest that the radar-retrieved vertical velocity might be slightly underestimated because of an underestimate of the turbulence broadening correction. However, large uncertainties in aircraft vertical velocity measurements likely impact the comparison.
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Pichugina, Yelena L., and Robert M. Banta. "Stable Boundary Layer Depth from High-Resolution Measurements of the Mean Wind Profile." Journal of Applied Meteorology and Climatology 49, no. 1 (January 1, 2010): 20–35. http://dx.doi.org/10.1175/2009jamc2168.1.
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Abstract The depth h of the stable boundary layer (SBL) has long been an elusive measurement. In this diagnostic study the use of high-quality, high-resolution (Δz = 10 m) vertical profile data of the mean wind U(z) and streamwise variance σu2(z) is investigated to see whether mean-profile features alone can be equated with h. Three mean-profile diagnostics are identified: hJ, the height of maximum low-level-jet (LLJ) wind speed U in the SBL; h1, the height of the first zero crossing or minimum absolute value of the magnitude of the shear ∂U/∂z profile above the surface; and h2, the minimum in the curvature ∂2U/∂z2 profile. Boundary layer BL here is defined as the surface-based layer of significant turbulence, so the top of the BL was determined as the first significant minimum in the σu2(z) profile, designated as hσ. The height hσ was taken as a reference against which the three mean-profile diagnostics were tested. Mean-wind profiles smooth enough to calculate second derivatives were obtained by averaging high-resolution Doppler lidar profile data, taken during two nighttime field programs in the Great Plains, over 10-min intervals. Nights are chosen for study when the maximum wind speed in the lowest 200 m exceeded 5 m s−1 (i.e., weak-wind, very stable BLs were excluded). To evaluate the three diagnostics, data from the 14-night sample were divided into three profile shapes: Type I, a traditional LLJ structure with a distinct maximum or “nose,” Type II, a “flat” structure with constant wind speed over a significant depth, and Type III, having a layered structure to the shear and turbulence in the lower levels. For Type I profiles, the height of the jet nose hJ, which coincided with h1 and h2 in this case, agreed with the reference SBL depth to within 5%. The study had two major results: 1) among the mean-profile diagnostics for h, the curvature depth h2 gave the best results; for the entire sample, h2 agreed with hσ to within 12%; 2) considering the profile shapes, the layered Type III profiles gave the most problems. When these profiles could be identified and eliminated from the sample, regression and error statistics improved significantly: mean relative errors of 8% for hJ and h1, and errors of <5% for h2, were found for the sample of only Type I and II profiles.
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Schertzer, D., and E. Falgarone. "MFGA-IDT2 workshop: Astrophysical and geophysical fluid mechanics: the impact of data on turbulence theories." Nonlinear Processes in Geophysics 3, no. 4 (December 31, 1996): 229–30. http://dx.doi.org/10.5194/npg-3-229-1996.
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Abstract. 1 Facts about the Workshop This workshop was convened on November 13-15 1995 by E. Falgarone and D. Schertzer within the framework of the Groupe de Recherche Mecanique des Fluides Geophysiques et Astrophysiques (GdR MFGA, Research Group of Geophysical and Astrophysical Fluid Mechanics) of Centre National de la Recherche Scientifique (CNRS, (French) National Center for Scientific Research). This Research Group is chaired by A. Babiano and the meeting was held at Ecole Normale Superieure, Paris, by courtesy of its Director E. Guyon. More than sixty attendees participated to this workshop, they came from a large number of institutions and countries from Europe, Canada and USA. There were twenty-five oral presentations as well as a dozen posters. A copy of the corresponding book of abstracts can be requested to the conveners. The theme of this meeting is somewhat related to the series of Nonlinear Variability in Geophysics conferences (NVAG1, Montreal, Aug. 1986; NVAG2, Paris, June 1988; NVAG3, Cargese (Corsica), September, 1993), as well as seven consecutive annual sessions at EGS general assemblies and two consecutive spring AGU meeting sessions devoted to similar topics. One may note that NVAG3 was a joint American Geophysical Union Chapman and European Geophysical Society Richardson Memorial conference, the first topical conference jointly sponsored by the two organizations. The corresponding proceedings were published in a special NPG issue (Nonlinear Processes in Geophysics 1, 2/3, 1994). In comparison with these previous meetings, MFGA-IDT2 is at the same time specialized to fluid turbulence and its intermittency, and an extension to the fields of astrophysics. Let us add that Nonlinear Processes in Geophysics was readily chosen as the appropriate journal for publication of these proceedings since this journal was founded in order to develop interdisciplinary fundamental research and corresponding innovative nonlinear methodologies in Geophysics. It had an appropriate editorial structure, in particular a large number of editors covering a wide range of methodologies, expertises and schools. At least two of its sections (Scaling and Multifractals, Turbulence and Diffusion) were directly related to the topics of the workshop, in any case contributors were invited to choose their editor freely. 2 Goals of the Workshop The objective of this meeting was to enhance the confrontation between turbulence theories and empirical data from geophysics and astrophysics fluids with very high Reynolds numbers. The importance of these data seems to have often been underestimated for the evaluation of theories of fully developed turbulence, presumably due to the fact that turbulence does not appear as pure as in laboratory experiments. However, they have the great advantage of giving access not only to very high Reynolds numbers (e.g. 1012 for atmospheric data), but also to very large data sets. It was intended to: (i) provide an overview of the diversity of potentially available data, as well as the necessary theoretical and statistical developments for a better use of these data (e.g. treatment of anisotropy, role of processes which induce other nonlinearities such as thermal instability, effect of magnetic field and compressibility ... ), (ii) evaluate the means of discriminating between different theories (e.g. multifractal intermittency models) or to better appreciate the relevance of different notions (e.g. Self-Organized Criticality) or phenomenology (e.g. filaments, structures), (iii) emphasise the different obstacles, such as the ubiquity of catastrophic events, which could be overcome in the various concerned disciplines, thanks to theoretical advances achieved. 3 Outlines of the Workshop During the two days of the workshop, the series of presentations covered many manifestations of turbulence in geophysics, including: oceans, troposphere, stratosphere, very high atmosphere, solar wind, giant planets, interstellar clouds... up to the very large scale of the Universe. The presentations and the round table at the end of the workshop pointed out the following: - the necessity of this type of confrontation which makes intervene numerical simulations, laboratory experiments, phenomenology as well as a very large diversity of geophysical and astrophysical data, - presumably a relative need for new geophysical data, whereas there have been recent astrophysical experiments which yield interesting data and exciting questions; - the need to develop a closer intercomparison between various intermittency models (in particular Log-Poisson /Log Levy models). Two main questions were underlined, in particular during the round table: - the behaviour of the extremes of intermittency, in particular the question of divergence or convergence of the highest statistical moments (equivalently, do the probability distributions have algebraic or more rapid falloffs?); - the extension of scaling ranges; in other words do we need to divide geophysics and astrophysics in many small (nearly) isotropic subranges or is it sufficient to use anisotropic scaling notions over wider ranges? 4 The contributions in this special issue Recalling that some of the most useful insights into the nature of turbulence in fluids have come from observations of geophysical flows, Van Atta gives a review of the impacts of geophysical turbulence data into theories. His paper starts from Taylor's inference of the nearly isotropy of atmospheric turbulence and the corresponding elegant theoretical developments by von Karman of the theory of isotropic turbulence, up to underline the fact that the observed extremely large intermittency in geophysical turbulence also raised new fundamental questions for turbulence theory. The paper discusses the potential contribution to theoretical development from the available or currently being made geophysical turbulence measurements, as well as from some recent laboratory measurements and direct numerical simulations of stably stratified turbulent shear flows. Seuront et al. consider scaling and multiscaling properties of scalar fields (temperature and phytoplankton concentration) advected by oceanic turbulence in both Eulerian and Lagrangian frameworks. Despite the apparent complexity linked to a multifractal background, temperature and fluorescence (i.e. phytoplankton biomass surrogate) fields are expressed over a wide range of scale by only three universal multifractal parameters, H, \\alpha and C_l. On scales smaller than the characteristic scale of the ship, sampling is rather Eulerian. On larger scales, the drifting platform being advected by turbulent motions, sampling may be rather considered as Lagrangian. Observed Eulerian and Lagrangian universal multifractal properties of the physical and biological fields are discussed. Whereas theoretical models provide different scaling laws for fluid and MHD turbulent flows, no attempt has been done up to now to experimentally support evidence for these differences. Carbone et al. use measurements from the solar wind turbulence and from turbulence in ordinary fluid flows, in order to assess these differences. They show that the so-called Extended Self-Similarity (ESS) is evident in the solar wind turbulence up to a certain scale. Furthermore, up to a given order of the velocity structure functions, the scaling laws of MHD and fluids flows axe experimentally indistinguishable. However, differences can be observed for higher orders and the authors speculate on their origin. Dudok de Wit and Krasnosel'skikh present analysis of strong plasma turbulence in the vicinity of the Earth's bow shock with the help of magnetometer data from the AMPTE UKS satellite. They demonstrate that there is a departure from Gaussianity which could be a signature of multifractality. However, they point out that the complexity of plasma turbulence precludes a more quantitative understanding. Finally, the authors emphasise the fact that the duration of records prevents to obtain any reliable estimate of structure functions beyond the fourth order. Sylos Labini and Pietronero discuss the problem of galaxy correlations. They conclude from all the recently available three dimensional catalogues that the distribution of galaxies and clusters is fractal with dimension D ~ 2 up to the present observational limits without any tendency towards homogenization. This result is discussed in contrast to angular data analysis. Furthermore, they point out that the galaxy-cluster mismatch disappears when considering a multifractal distribution of matter. They emphasise that a new picture emerges which changes the standard ideas about the properties of the universe and requires a corresponding change in the related theoretical concepts. Chilla et al. investigate with the help of a laboratory experiment the possible influence of the presence of a large scale structure on the intermittency of small scale structures. They study a flow between coaxial co-rotating disks generating a strong axial vortex over a turbulent background. They show that the cascade process is preserved although strongly modified and they discuss the relevance of parameters developed for the description of intermittency in homogeneous turbulence to evaluate this modification.
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Wenz, Florian, Judith Langner, Thorsten Lutz, and Ewald Krämer. "Impact of the wind field at the complex-terrain site Perdigão on the surface pressure fluctuations of a wind turbine." Wind Energy Science 7, no. 3 (June 28, 2022): 1321–40. http://dx.doi.org/10.5194/wes-7-1321-2022.
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Abstract. The surface pressure fluctuations, which are a source of low-frequency noise emissions, are numerically investigated on a 2 MW wind turbine under different inflow conditions. In order to evaluate the impact of a complex-terrain flow, a computational setup is presented that is aimed at reproducing a realistic flow field in the complex terrain in Perdigão, Portugal. A precursor simulation with the steady-state atmospheric computational fluid dynamics (CFD) code E-Wind is used, which was calibrated with meteorological (met) mast data to generate a site- and situation-specific inflow for a high-resolution delayed detached-eddy simulation (DDES) with FLOWer. A validation with lidar and met mast data reveals a good agreement of the flow field in the vicinity of the turbine in terms of mean wind speed and wind direction, whereas the turbulence intensity is slightly underestimated. Further downstream in the valley and on the second ridge, the deviations between simulation and measurement become significantly larger. The geometrically resolved turbine is coupled to the structural solver SIMPACK and simulated both in the complex terrain and in flat terrain with simpler inflows as reference. The surface pressure fluctuations are evaluated on the tower and blades. It is found that the periodic pressure fluctuations at the tower sides and back are dominated by vortex shedding, which strongly depends on the inflow and is reduced by inflow turbulence. However, the dominant pressure fluctuations on the upper part of the tower, which are caused by the blade–tower interaction, remain almost unchanged by the different inflows. The predominant pressure fluctuations on the blades occur with the rotation frequency. They are caused by a combination of rotor tilt, vertical wind shear and inclined flow and are thus strongly dependent on the inflow and the surrounding terrain. The inflow turbulence masks fluctuations at higher harmonics of the blade–tower interaction with its broadband characteristic caused by the interaction of the leading edge and the inflow turbulence.
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Pedersen, Mads Mølgaard, Torben Juul Larsen, Helge Aagaard Madsen, and Gunner Christian Larsen. "More accurate aeroelastic wind-turbine load simulations using detailed inflow information." Wind Energy Science 4, no. 2 (May 28, 2019): 303–23. http://dx.doi.org/10.5194/wes-4-303-2019.
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Abstract. In this paper, inflow information is extracted from a measurement database and used for aeroelastic simulations to investigate if using more accurate inflow descriptions improves the accuracy of the simulated wind-turbine fatigue loads. The inflow information is extracted from nearby meteorological masts (met masts) and a blade-mounted five-hole pitot tube. The met masts provide measurements of the inflow at fixed positions some distance away from the turbine, whereas the pitot tube measures the inflow while rotating with the rotor. The met mast measures the free-inflow velocity; however the measured turbulence may evolve on its way to the turbine, pass beside the turbine or the mast may be in the wake of the turbine. The inflow measured by the pitot tube, in comparison, is very representative of the wind that acts on the turbine, as it is measured close to the blades and also includes variations within the rotor plane. Nevertheless, this inflow is affected by the presence of the turbine; therefore, an aerodynamic model is used to estimate the free-inflow velocities that would have occurred at the same time and position without the presence of the turbine. The inflow information used for the simulations includes the mean wind speed field and trend, the turbulence intensity, the wind-speed shear profile, atmospheric stability-dependent turbulence parameters, and the azimuthal variations within the rotor plane. In addition, instantaneously measured wind speeds are used to constrain the turbulence. It is concluded that the period-specific turbulence intensity must be used in the aeroelastic simulations to make the range of the simulated fatigue loads representative for the range of the measured fatigue loads. Furthermore, it is found that the one-to-one correspondence between the measured and simulated fatigue loads is improved considerably by using inflow characteristics extracted from the pitot tube instead of using the met-mast-based sensors as input for the simulations. Finally, the use of pitot-tube-recorded wind speeds to constrain the inflow turbulence is found to significantly decrease the variation of the simulated loads due to different turbulence realizations (seeds), whereby the need for multiple simulations is reduced.
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Pedruzo-Bagazgoitia, Xabier, Stephan R. de Roode, Bianca Adler, Karmen Babić, Cheikh Dione, Norbert Kalthoff, Fabienne Lohou, Marie Lothon, and Jordi Vilà-Guerau de Arellano. "The diurnal stratocumulus-to-cumulus transition over land in southern West Africa." Atmospheric Chemistry and Physics 20, no. 5 (March 5, 2020): 2735–54. http://dx.doi.org/10.5194/acp-20-2735-2020.
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Abstract. The misrepresentation of the diurnal cycle of boundary layer clouds by large-scale models strongly impacts the modeled regional energy balance in southern West Africa. In particular, recognizing the processes involved in the maintenance and transition of the nighttime stratocumulus to diurnal shallow cumulus over land remains a challenge. This is due to the fact that over vegetation, surface fluxes exhibit a much larger magnitude and variability than on the more researched marine stratocumulus transitions. An improved understanding of the interactions between surface and atmosphere is thus necessary to improve its representation. To this end, the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) measurement campaign gathered a unique dataset of observations of the frequent stratocumulus-to-cumulus transition in southern West Africa. Inspired and constrained by these observations, we perform a series of numerical experiments using large eddy simulation. The experiments include interactive radiation and surface schemes where we explicitly resolve, quantify and describe the physical processes driving such transition. Focusing on the local processes, we quantify the transition in terms of dynamics, radiation, cloud properties, surface processes and the evolution of dynamically relevant layers such as subcloud layer, cloud layer and inversion layer. We further quantify the processes driving the stratocumulus thinning and the subsequent transition initiation by using a liquid water path budget. Finally, we study the impact of mean wind and wind shear at the cloud top through two additional numerical experiments. We find that the sequence starts with a nighttime well-mixed layer from the surface to the cloud top, in terms of temperature and humidity, and transitions to a prototypical convective boundary layer by the afternoon. We identify radiative cooling as the largest factor for the maintenance leading to a net thickening of the cloud layer of about 18 g m−2 h−1 before sunrise. Four hours after sunrise, the cloud layer decouples from the surface through a growing negative buoyancy flux at the cloud base. After sunrise, the increasing impact of entrainment leads to a progressive thinning of the cloud layer. While the effect of wind on the stratocumulus layer during nighttime is limited, after sunrise we find shear at the cloud top to have the largest impact: the local turbulence generated by shear enhances the boundary layer growth and entrainment aided by the increased surface fluxes. As a consequence, wind shear at the cloud top accelerates the breakup and transition by about 2 h. The quantification of the transition and its driving factors presented here sets the path for an improved representation by larger-scale models.
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Nilsson, Erik, Fabienne Lohou, Marie Lothon, Eric Pardyjak, Larry Mahrt, and Clara Darbieu. "Turbulence kinetic energy budget during the afternoon transition – Part 1: Observed surface TKE budget and boundary layer description for 10 intensive observation period days." Atmospheric Chemistry and Physics 16, no. 14 (July 19, 2016): 8849–72. http://dx.doi.org/10.5194/acp-16-8849-2016.
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Abstract. The decay of turbulence kinetic energy (TKE) and its budget in the afternoon period from midday until zero-buoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign for 10 intensive observation period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics and mesoscale situation at the site are also described based upon taller tower measurements, radiosoundings and remote sensing instrumentation. Analysis of the TKE budget during the afternoon transition reveals a variety of different surface layer dynamics in terms of TKE and TKE decay. This is largely attributed to variations in the 8 m wind speed, which is responsible for different amounts of near-surface shear production on different afternoons and variations within some of the afternoon periods. The partitioning of near-surface production into local dissipation and transport in neutral and unstably stratified conditions was investigated. Although variations exist both between and within afternoons, as a rule of thumb, our results suggest that about 50 % of the near-surface production of TKE is compensated for by local dissipation near the surface, leaving about 50 % available for transport. This result indicates that it is important to also consider TKE transport as a factor influencing the near-surface TKE decay rate, which in many earlier studies has mainly been linked with the production terms of TKE by buoyancy and wind shear. We also conclude that the TKE tendency is smaller than the other budget terms, indicating a quasi-stationary evolution of TKE in the afternoon transition. Even though the TKE tendency was observed to be small, a strong correlation to mean buoyancy production of −0.69 was found for the afternoon period. For comparison with previous results, the TKE budget terms are normalized with friction velocity and measurement height and discussed in the framework of Monin–Obukhov similarity theory. Empirically fitted expressions are presented. Alternatively, we also suggest a non-local parametrization of dissipation using a TKE–length scale model which takes into account the boundary layer depth in addition to distance above the ground. The non-local formulation is shown to give a better description of dissipation compared to a local parametrization.
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Premaratne, Pavithra, Wei Tian, and Hui Hu. "A Proper-Orthogonal-Decomposition (POD) Study of the Wake Characteristics behind a Wind Turbine Model." Energies 15, no. 10 (May 13, 2022): 3596. http://dx.doi.org/10.3390/en15103596.
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A comprehensive study was performed to analyze turbine wake characteristics by using a Proper-Orthogonal-Decomposition (POD) method to identify the dominant flow features from a comprehensive experimental database. The wake flow characteristics behind a typical three-bladed horizontal-axis wind turbine (HAWT) were measured in a large-scale wind tunnel with a scaled turbine model placed in a typical offshore Atmospheric Boundary Layer (ABL) wind under a neutral stability condition. A high-resolution Particle Image Velocimetry (PIV) system was used to achieve detailed flow field measurements to characterize the turbulent flows and wake vortex structures behind the turbine model. Statistically averaged measurements revealed the presence of the characteristic helical-tip vortex filament along with a unique secondary vortex filament emanating from 60% of the blade span measured from the hub. Both filaments breakup in the near-wake region (~0.6 rotor diameter downstream) to form shear layers, contrary to previous computational and experimental observations in which vortex filaments break up in the far wake. A Proper-Orthogonal-Decomposition (POD) analysis, based on both velocity and vorticity-based formulations, was used to extract the coherent flow structures, predominantly comprised of tip and midspan vortex elements. The reconstructions showed coherence in the flow field prior to the vortex breakup which subsequently degraded in the turbulent shear layer. The accuracy of the POD reconstructions was validated qualitatively by comparing the prediction results between the velocity and vorticity-based formulations as well as the phase-averaged PIV measurement results. This early vortex breakup was attributed to the reduced pitch between consecutive helical turns, the proximity between midspan filaments and blade tips as well as the turbulence intensity of the incoming boundary layer wind.
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Wilson, Richard, Clara Pitois, Aurélien Podglajen, Albert Hertzog, Milena Corcos, and Riwal Plougonven. "Detection of turbulence occurrences from temperature, pressure, and position measurements under superpressure balloons." Atmospheric Measurement Techniques 16, no. 2 (January 20, 2023): 311–30. http://dx.doi.org/10.5194/amt-16-311-2023.
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Abstract. This article deals with the detection of small-scale turbulence from in situ meteorological measurements performed under superpressure balloons (SPBs). These balloons allow long-duration flights (several months) at a prerequisite height level. The data set is gathered from the Strateole-2 probationary campaign during which eights SPBs flew in the tropical tropopause layer at altitudes of around 19 and 20.5 km from November 2019 to March 2020. Turbulence is not directly measured by the instrument set onboard the SPBs. Nonetheless, there is the potential to derive information about the occurrence of turbulence from the temporally well-resolved measurements of pressure, temperature, and position. It constitutes a challenge to extract the aforementioned information from a measurement set that was not designed for quantifying turbulence, and the paper explains the methodology developed to overcome this difficulty. It is observed that SPBs oscillate quasi-periodically around their equilibrium positions. The oscillation periods, which are 220 s on average with a range of 130 to 500 s, are close to but noticeably smaller than the Brunt–Väisälä period (∼300 s). The amplitude of these vertical motions is ∼±15 m, inducing large fluctuations in all quantities, whether measured (e.g., pressure, temperature and position) or inferred (e.g., density and potential temperature). The relationships between the changes in these quantities and the vertical displacements of the balloons are used to infer properties of the flow in which the SPBs drift. In the case of active turbulence, the vertical stratification as well as the wind shear are likely to be reduced by mixing. Hence, the increments of potential temperature, δθ, and of the vertical displacements of the balloon, δzB, are expected to be uncorrelated because ∂θ/∂z→0. Moreover, the local vertical gradients of measured quantities, temperature (T) and horizontal velocities (u and v), are estimated from the covariance of the increments of the considered quantity with δzB. The Richardson number of the flow is deduced. Several binary indexes (true or false) to describe the state of the flow, laminar or turbulent, are evaluated. These turbulence indexes, based either on correlations between δθ and δzB or on estimates of the local Richardson number, are found to be consistent, as they differ in less than 3 % of cases. The flow is observed to be turbulent for about 5 % of the time, with strong inhomogeneities along the longitude.
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Pryor, S. C., R. J. Barthelmie, L. L. Sørensen, J. G. McGrath, P. Hopke, and T. Petäjä. "Spatial and vertical extent of nucleation events in the Midwestern USA: insights from the Nucleation In ForesTs (NIFTy) experiment." Atmospheric Chemistry and Physics 11, no. 4 (February 21, 2011): 1641–57. http://dx.doi.org/10.5194/acp-11-1641-2011.
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Abstract. Measurements of aerosol particle physical and chemical properties, gas phase concentrations and meteorological parameters were made along a transect in southern Indiana during the Nucleation In ForesTs (NIFTy) experiment conducted in May 2008. These measurements indicate nucleation was observed at all three measurement sites on almost half of all sampling days. The intensity of the nucleation events, as measured by the increase in ≥10 nm aerosol particle number concentrations of approximately 2×104 cm−3 over a layer of at least 300 m depth, is in good agreement with recent model results for the Midwestern USA derived using PMCAMx-UF. During the hour after termination of nucleation approximately half of the number concentration reduction is due to coagulation, while the remainder is due in equal parts to dry deposition and entrainment of relatively ultra-fine aerosol particle free troposphere air. Clear nucleation with continuous subsequent growth is only observed on days when the morning fractional cloud cover was less than 30%. It is associated with a clear transition from a strongly stratified atmosphere with low turbulence intensity and weak vertical velocities, to much a weaker vertical gradient of wind speed, increased turbulence intensity and stronger downwards vertical velocities, consistent with growth of the mixed layer and entrainment of air from the residual layer. Nucleation intensity is not very strongly determined by the prevailing condensational sink. However, there is a strong correlation between both a modified version of the Nucleation Parameter from Boy and Kulmala (2002) and ultrafine aerosol particle number concentrations, and mean morning H2SO4 concentrations and ultrafine aerosol particle number concentrations. Five A-class event days during NIFTy were characterized by values of the dimensionless nucleation parameter of Kuang et al. (2010) that are below 0.3, further indicating the applicability of their postulate that nucleation is favored by LΓ values below 0.7. Based on aerosol particle composition measurements it appears that aerosol particle formation and initial growth to approximately 30 nm diameter is dominated by ammonium and sulfate. Conservative estimates of the percent contribution of H2SO4 to aerosol particle growth (for sub-30 nm aerosol particles) on five A-class event days ranged from 23 to 85%.
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Luce, H., M. Crochet, and F. Dalaudier. "Temperature sheets and aspect sensitive radar echoes." Annales Geophysicae 19, no. 8 (August 31, 2001): 899–920. http://dx.doi.org/10.5194/angeo-19-899-2001.
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Abstract. here have been years of discussion and controversy about the existence of very thin and stable temperature sheets and their relationship to the VHF radar aspect sensitivity. It is only recently that very high-resolution in situ temperature observations have brought credence to the reality and ubiquity of these structures in the free atmosphere and to their contribution to radar echo enhancements along the vertical. Indeed, measurements with very high-resolution sensors are still extremely rare and rather difficult to obtain outside of the planetary boundary layer. They have only been carried out up to the lower stratosphere by Service d’A´ eronomie (CNRS, France) for about 10 years. The controversy also persisted due to the volume resolution of the (Mesosphere)-Stratosphere-Troposphere VHF radars which is coarse with respect to sheet thickness, although widely sufficient for meteorological or mesoscale investigations. The contribution within the range gate of many of these structures, which are advected by the wind, and decay and grow at different instants and could be distorted either by internal gravity waves or turbulence fields, could lead to radar echoes with statistical properties similar to those produced by anisotropic turbulence. Some questions thus remain regarding the manner in which temperature sheets contribute to VHF radar echoes. In particular, the zenithal and azimuthal angular dependence of the echo power may not only be produced by diffuse reflection on stable distorted or corrugated sheets, but also by extra contributions from anisotropic turbulence occurring in the stratified atmosphere. Thus, for several years, efforts have been put forth to improve the radar height resolution in order to better describe thin structures. Frequency interferometric techniques are widely used and have been recently further developed with the implementation of high-resolution data processings. We begin by reviewing briefly some characteristics of the ST radar echoes with a particular emphasis on recent works. Their possible coupling with stable sheets is then presented and their known characteristics are described with some hypotheses concerning their generation mechanisms. Finally, measurement campaigns that took recently place or will be carried out in the near future for improving our knowledge of these small-scale structures are presented briefly.Key words. Meteorology and atmospheric dynamics (turbulence; instruments and techniques) – Radio Science (remote sensing)
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França, G. B., M. V. de Almeida, and A. C. Rosette. "Self-Nowcast Model of extreme precipitation events for operational meteorology." Atmospheric Measurement Techniques Discussions 8, no. 10 (October 16, 2015): 10635–61. http://dx.doi.org/10.5194/amtd-8-10635-2015.
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Abstract. Nowadays many social activities require short-term (one to two hours) and local area forecasts of extreme weather. In particular, air traffic systems have been studying how to minimize the impact of meteorological events, such as turbulence, wind shear, ice, and heavy rain, which are related to the presence of convective systems during all flight phases. This paper presents an alternative self-nowcast model, based on neural network techniques, to produce short-term and local-specific forecasts of extreme meteorological events in the area of the landing and take-off region of Galeão, the principal airport in Rio de Janeiro, Brazil. Twelve years of data were used for neural network training and validation. Data are originally from four sources: (1) hourly meteorological observations from surface meteorological stations at five airports distributed around the study area, (2) atmospheric profiles collected twice a day at the meteorological station at Galeão Airport, (3) rain rate data collected from a network of twenty-nine rain gauges in the study area; and (4) lightning data regularly collected by national detection networks. An investigation was done about the capability of a neural network to produce early warning signs – or as a nowcasting tool – for extreme meteorological events. The self-nowcast model was validated using results from six categorical statistics, indicated in parentheses for forecasts of the first, second, and third hours, respectively, namely: proportion correct (0.98, 0.96, and 0.94), bias (1.37, 1.48, and 1.83), probability of detection (0.84, 0.80, and 0.76), false-alarm ratio (0.38, 0.46, and 0.58), and threat score (0.54, 0.47, and 0.37). Possible sources of error related to the validation procedure are discussed. Two key points have been identified in which there is a possibility of error: i.e., subjectivity on the part of the meteorologist making the observation, and a rain gauge measurement error of about 20 % depending on wind speed. The latter was better demonstrated when lightning data were included in the validation. The validation showed that the proposed model's performance was quite encouraging for the first and second hours.
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Schubiger, Alain, Sarah Barber, and Henrik Nordborg. "Evaluation of the lattice Boltzmann method for wind modelling in complex terrain." Wind Energy Science 5, no. 4 (November 6, 2020): 1507–19. http://dx.doi.org/10.5194/wes-5-1507-2020.
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Abstract. The worldwide expansion of wind energy is making the choice of potential wind farm locations more and more difficult. This results in an increased number of wind farms being located in complex terrain, which is characterised by flow separation, turbulence and high shear. Accurate modelling of these flow features is key for wind resource assessment in the planning phase, as the exact positioning of the wind turbines has a large effect on their energy production and lifetime. Wind modelling for wind resource assessments is usually carried out with the linear model Wind Atlas Analysis and Application Program (WAsP), unless the terrain is complex, in which case Reynolds-averaged Navier–Stokes (RANS) solvers such as WindSim and Ansys Fluent are usually applied. Recent research has shown the potential advantages of large-eddy simulation (LES) for modelling the atmospheric boundary layer and thermal effects; however, LES is far too computationally expensive to be applied outside the research environment. Another promising approach is the lattice Boltzmann method (LBM), a computational fluid technique based on the Boltzmann transport equation. It is generally used to study complex phenomena such as turbulence, because it describes motion at the mesoscopic level in contrast to the macroscopic level of conventional computational fluid dynamics (CFD) approaches, which solve the Navier–Stokes (N–S) equations. Other advantages of the LBM include its efficiency; near-ideal scalability on high-performance computers (HPCs); and ability to easily automate the geometry, the mesh generation and the post-processing. However, the LBM has been applied very little to wind modelling in complex terrain for wind energy applications, mainly due to the lack of availability of easy-to-use tools as well as the lack of experience with this technique. In this paper, the capabilities of the LBM to model wind flow around complex terrain are investigated using the Palabos framework and data from a measurement campaign from the Bolund Hill experiment in Denmark. Detached-eddy simulation (DES) and LES in Ansys Fluent are used as a numerical comparison. The results show that there is in general a good agreement between simulation and experimental data, and the LBM performs better than RANS and DES. Some deviations can be observed near the ground, close to the top of the cliff and on the lee side of the hill. The computational costs of the three techniques are compared, and it has been shown that the LBM can perform up to 5 times faster than DES, even though the set-up was not optimised in this initial study. It can be summarised that the LBM has a very high potential for modelling wind flow over complex terrain accurately and at relatively low costs, compared to solving N–S equations conventionally. Further studies on other sites are ongoing.
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Tusch, Matthieu, Christian Masson, and Pierre Héraud. "Modeling of Turbulent Atmospheric Flow Around Tubular and Lattice Meteorological Masts." Journal of Solar Energy Engineering 133, no. 1 (February 1, 2011). http://dx.doi.org/10.1115/1.4003293.
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This paper presents a numerical study of turbulent atmospheric flow around tubular and lattice meteorological masts and a wind speed and energy uncertainty calculation method based on the numerical results. The flow is described by the Reynolds averaged Navier–Stokes equations, complemented by the shear stress transport turbulence model, with modified constants and source terms added to maintain turbulence properties. ANSYS-CFX 11.0 is used to solve the computational model. The numerical results have been post-processed to account for the wind direction changes during the 10-min-measurement-period, and have been validated with mast data. From the numerical results, a wind speed and energy uncertainty calculation method that takes the wind rose into account is proposed. This technique provides a means to detect incorrectly mounted booms according to the local wind conditions. Most importantly, it produces uncertainty more conservatively than both the International Energy Agency (IEA) recommendations and the IEC-61400-121 (International Electrotechnical Commission) annex G norm. These differences stem from the use of a turbulence model in this paper, which predicts higher flow distortions due to the presence of the mast.
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Guzman, Oscar, and Haiyan Jiang. "Heavier Inner-core Rainfall of Major Hurricanes in the North Atlantic Basin than Other Global Basins." Journal of Climate, April 9, 2021, 1–35. http://dx.doi.org/10.1175/jcli-d-20-0668.1.
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AbstractBased on 19 years of precipitation data collected by the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measurement (GPM) mission, a comparison of the rainfall produced by tropical cyclones (TCs) in different global basins is presented. A total of 1789 TCs were examined in the period from 1998 to 2016 by taking advantage of more than 47,737 observations of TRMM/GPM 3B42 multi-satellite derived rainfall amounts. The axisymmetric component of the TC rainfall is analyzed in all TC-prone basins. The resulting radial profiles show that major hurricanes in the Atlantic basin exhibit significantly heavier inner-core rainfall rates than those in any other basins. To explain the possible causes of this difference, rainfall distributions for major hurricanes are stratified according to different TC intensity and environmental variables. Based on the examination of these parameters, we found that the stronger rainfall rates in the Atlantic major hurricanes are associated with higher values of convective available potential energy, drier relative humidity in the low to middle troposphere, colder air temperature at 250hPa, and stronger vertical wind shear than other basins. These results have important implications in the refining of our understanding of the mechanisms of TC rainfall.
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Caputi, Dani J., Justin Trousdell, Shobhit Mehrotra, Stephen Conley, G. Aaron Alexander, Holly J. Oldroyd, and Ian Faloona. "Entrainment Rates and Their Synoptic Dependence on Wind Speed Aloft in California's Central Valley." Boundary-Layer Meteorology, December 22, 2022. http://dx.doi.org/10.1007/s10546-022-00770-1.
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AbstractDaytime atmospheric boundary layer (ABL) dynamics—including potential temperature budgets, water vapour budgets, and entrainment rates—are presented from in situ flight data taken on six afternoons near Fresno in the San Joaquin Valley (SJV) of California during July/August 2016. The flights took place as a part of the California Baseline Ozone Transport Study aimed at investigating transport pathways of air entering the Central Valley from offshore and mixing down to the surface. Midday entrainment velocity estimates ranged from 0.8 to 5.4 cm s−1 and were derived from a combination of continuously determined ABL heights during each flight and model-derived subsidence rates, which averaged -2.0 cm s−1 in the flight region. A strong correlation was found between entrainment velocity (normalized by the convective velocity scale) and an inverse bulk ABL Richardson number, suggesting that wind shear at the ABL top plays a significant role in driving entrainment. Similarly, we found a strong correlation between the entrainment efficiency (the ratio of entrainment to surface heat fluxes with an average of 0.23 ± 0.15) and the wind speed at the ABL top. We explore the synoptic conditions that generate higher winds near the ABL top and propose that warm anomalies in the southern Sierra Nevada mountains promote increased entrainment. Additionally, a method is outlined to estimate turbulence kinetic energy, convective velocity scale (w*), and the surface sensible heat flux in the ABL from a slow, airborne wind measurement system using mixed-layer similarity theory.