Journal articles on the topic 'Near-surface stratification'
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1
Reverdin, G., S. Morisset, H. Bellenger, J. Boutin, N. Martin, P. Blouch, J. Rolland, F. Gaillard, P. Bouruet-Aubertot, and B. Ward. "Near–Sea Surface Temperature Stratification from SVP Drifters." Journal of Atmospheric and Oceanic Technology 30, no. 8 (August 1, 2013): 1867–83. http://dx.doi.org/10.1175/jtech-d-12-00182.1.
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Abstract This study describes how the hull temperature (Ttop) measurements from multisensor surface velocity program (SVP) drifters can be combined with other measurements to provide quantitative information on near-surface vertical temperature stratification during large daily cycles. First, Ttop is compared to the temperature measured at 17 -cm depth from a float tethered to the SVP drifter. These 2007–12 SVP drifters present a larger daily cycle by 1%–3% for 1°–2°C daily cycle amplitudes, with a maximum difference close to the local noon. The difference could result from flow around the SVP drifter in the presence of temperature stratification in the top 20 cm of the water column but also from a small influence of internal drifter temperature on Ttop. The largest differences were found for small drifters (Technocean) for very large daily cycles, as expected from their shallower measurements. The vertical stratification is estimated by comparing these hull data with the deeper T or conductivity C measurements from Sea-Bird sensors 25 (Pacific Gyre) to 45 cm (MetOcean) below the top temperature sensor. The largest stratification is usually found near local noon and early afternoon. For a daily cycle amplitude of 1°C, these differences with the upper level are in the range of 3%–5% of the daily cycle for the Pacific Gyre drifters and 6%–10% for MetOcean drifters with the largest values occurring when the midday sun elevation is lowest. The relative differences increase for larger daily cycles, and the vertical profiles become less linear. These estimated stratifications are well above the uncertainty on Ttop.
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Santos-Garcia, Andrea, Maria Marta Jacob, and W. Linwood Jones. "SMOS Near-Surface Salinity Stratification Under Rainy Conditions." IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 9, no. 6 (June 2016): 2493–99. http://dx.doi.org/10.1109/jstars.2016.2527038.
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Fischer, Tim, Annette Kock, Damian L. Arévalo-Martínez, Marcus Dengler, Peter Brandt, and Hermann W. Bange. "Gas exchange estimates in the Peruvian upwelling regime biased by multi-day near-surface stratification." Biogeosciences 16, no. 11 (June 5, 2019): 2307–28. http://dx.doi.org/10.5194/bg-16-2307-2019.
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Abstract. The coastal upwelling regime off Peru in December 2012 showed considerable vertical concentration gradients of dissolved nitrous oxide (N2O) across the top few meters of the ocean. The gradients were predominantly downward, i.e., concentrations decreased toward the surface. Ignoring these gradients causes a systematic error in regionally integrated gas exchange estimates, when using observed concentrations at several meters below the surface as input for bulk flux parameterizations – as is routinely practiced. Here we propose that multi-day near-surface stratification events are responsible for the observed near-surface N2O gradients, and that the gradients induce the strongest bias in gas exchange estimates at winds of about 3 to 6 m s−1. Glider hydrographic time series reveal that events of multi-day near-surface stratification are a common feature in the study region. In the same way as shorter events of near-surface stratification (e.g., the diurnal warm layer cycle), they preferentially exist under calm to moderate wind conditions, suppress turbulent mixing, and thus lead to isolation of the top layer from the waters below (surface trapping). Our observational data in combination with a simple gas-transfer model of the surface trapping mechanism show that multi-day near-surface stratification can produce near-surface N2O gradients comparable to observations. They further indicate that N2O gradients created by diurnal or shorter stratification cycles are weaker and do not substantially impact bulk emission estimates. Quantitatively, we estimate that the integrated bias for the entire Peruvian upwelling region in December 2012 represents an overestimation of the total N2O emission by about a third, if concentrations at 5 or 10 m depth are used as surrogate for bulk water N2O concentration. Locally, gradients exist which would lead to emission rates overestimated by a factor of two or more. As the Peruvian upwelling region is an N2O source of global importance, and other strong N2O source regions could tend to develop multi-day near-surface stratification as well, the bias resulting from multi-day near-surface stratification may also impact global oceanic N2O emission estimates.
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Iyer, Suneil, and Kyla Drushka. "Turbulence within Rain-Formed Fresh Lenses during the SPURS-2 Experiment." Journal of Physical Oceanography 51, no. 5 (May 2021): 1705–21. http://dx.doi.org/10.1175/jpo-d-20-0303.1.
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AbstractObservations of salinity, temperature, and turbulent dissipation rate were made in the top meter of the ocean using the ship-towed Surface Salinity Profiler as part of the second Salinity Processes in the Upper Ocean Regional Study (SPURS-2) to assess the relationships between wind, rain, near-surface stratification, and turbulence. A wide range of wind and rain conditions were observed in the eastern tropical Pacific Ocean near 10°N, 125°W in summer–autumn 2016 and 2017. Wind was the primary driver of near-surface turbulence and the mixing of rain-formed fresh lenses, with lenses generally persisting for hours when wind speeds were under 5 m s−1 and mixing away immediately at higher wind speeds. Rain influenced near-surface turbulence primarily through stratification. Near-surface stratification caused by rainfall or diurnal warming suppressed deeper turbulent dissipation rates when wind speeds were under 3 m s−1. In one case with 4–5 m s−1 winds, rain-induced stratification enhanced dissipation rates within the stratified layer. At wind speeds above 7–8 m s−1, strong stratification was not observed in the upper meter during rain, indicating that rain lenses do not form at wind speeds above 8 m s−1. Raindrop impacts enhanced turbulent dissipation rates at these high wind speeds in the absence of near-surface stratification. Measurements of air–sea buoyancy flux, wind speed, and near-surface turbulence can be used to predict the presence of stratified layers. These findings could be used to improve model parameterizations of air–sea interactions and, ultimately, our understanding of the global water cycle.
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Pernica, Patricia, and Mathew Wells. "Frequency of episodic stratification in the near surface of Lake Opeongo and other small lakes." Water Quality Research Journal 47, no. 3-4 (August 1, 2012): 227–37. http://dx.doi.org/10.2166/wqrjc.2012.001.
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Wind-driven mixing in the epilimnion of a deep lake can be suppressed when there is a weak near surface stratification, which occurs frequently during periods of strong solar heating and weak winds. Using data from a vertical chain of fast response thermistors, we analyze the frequency of near surface stratification in the top 2 meters of the epilimnion in Lake Opeongo, Ontario for the periods between May and August in 2009 and 2010. Near surface thermoclines (as defined by dT/dz > 0.2 °C m−1 between 1 and 2 m) occur for 24% of the sampling period in 2009, 37% of the sampling period in 2010 and correspond to periods of high values of gradient Richardson number. During daytime the epilimnion is stratified up to 45% of the time. At night, cooling generally leads to a more isothermal profile, but near surface thermoclines still form at least 20% of the time. Extended periods of near surface stratification (>1 h), account for more than 80% of the stratified period. We compare these findings with previous observations from the Experimental Lakes Area in Northern Ontario, and discuss the biological implications of episodic stratification.
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Li, Yifan, Matthew Marander, Rebecca Mort, Fei Liu, Xin Yong, and Shan Jiang. "Who wins the race near the interface? Stratification of colloids, nano-surfactants, and others." Journal of Applied Physics 132, no. 11 (September 21, 2022): 110901. http://dx.doi.org/10.1063/5.0098710.
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The diffusion of colloids, nanoparticles, and small molecules near the gas–liquid interface presents interesting multiphase transport phenomena and unique opportunities for understanding interactions near the surface and interface. Stratification happens when different species preside over the interfaces in the final dried coating structure. Understanding the principles of stratification can lead to emerging technologies for materials’ fabrication and has the potential to unlock innovative industrial solutions, such as smart coatings and drug formulations for controlled release. However, stratification can be perplexing and unpredictable. It may involve a complicated interplay between particles and interfaces. The surface chemistry and solution conditions are critical in determining the race of particles near the interface. Current theory and simulation cannot fully explain the observations in some experiments, especially the newly developed stratification of nano-surfactants. Here, we summarize the efforts in the experimental work, theory, and simulation of stratification, with an emphasis on bridging the knowledge gap between our understanding of surface adsorption and bulk diffusion. We will also propose new mechanisms of stratification based on recent observations of nano-surfactant stratification. More importantly, the discussions here will lay the groundwork for future studies beyond stratification and nano-surfactants. The results will lead to the fundamental understanding of nanoparticle interactions and transport near interfaces, which can profoundly impact many other research fields, including nanocomposites, self-assembly, colloidal stability, and nanomedicine.
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Mellado, Juan Pedro, Chiel C. van Heerwaarden, and Jade Rachele Garcia. "Near-Surface Effects of Free Atmosphere Stratification in Free Convection." Boundary-Layer Meteorology 159, no. 1 (November 17, 2015): 69–95. http://dx.doi.org/10.1007/s10546-015-0105-x.
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Sutherland, Graig, Louis Marié, Gilles Reverdin, Kai H. Christensen, Göran Broström, and Brian Ward. "Enhanced Turbulence Associated with the Diurnal Jet in the Ocean Surface Boundary Layer." Journal of Physical Oceanography 46, no. 10 (October 2016): 3051–67. http://dx.doi.org/10.1175/jpo-d-15-0172.1.
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AbstractDetailed observations of the diurnal jet, a surface intensification of the wind-driven current associated with the diurnal cycle of sea surface temperature (SST), were obtained during August and September 2012 in the subtropical Atlantic. A diurnal increase in SST of 0.2° to 0.5°C was observed, which corresponded to a diurnal jet of 0.15 m s−1. The increase in near-surface stratification limits the vertical diffusion of the wind stress, which in turn increases the near-surface shear. While the stratification decreased the turbulent dissipation rate ε below the depth of the diurnal jet, there was an observed increase in ε within the diurnal jet. The diurnal jet was observed to increase the near-surface shear by a factor of 5, which coincided with enhanced values of ε. The diurnal evolution of the Richardson number, which is an indicator of shear instability, is less than 1, suggesting that shear instability may contribute to near-surface turbulence. While the increased stratification due to the diurnal heating limits the depth of the momentum flux due to the wind, shear instability provides an additional source of turbulence that interacts with the enhanced shear of the diurnal jet to increase ε within this shallow layer.
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LaCasce, J. H. "Surface Quasigeostrophic Solutions and Baroclinic Modes with Exponential Stratification." Journal of Physical Oceanography 42, no. 4 (April 1, 2012): 569–80. http://dx.doi.org/10.1175/jpo-d-11-0111.1.
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Abstract The author derives baroclinic modes and surface quasigeostrophic (SQG) solutions with exponential stratification and compares the results to those obtained with constant stratification. The SQG solutions with exponential stratification decay more rapidly in the vertical and have weaker near-surface velocities. This then compounds the previously noted problem that SQG underpredicts the velocities associated with a given surface density anomaly. The author also examines how the SQG solutions project onto the baroclinic modes. With constant stratification, SQG waves larger than deformation scale project primarily onto the barotropic mode and to a lesser degree onto the first baroclinic mode. However, with exponential stratification, the largest projection is on the first baroclinic mode. The effect is even more pronounced over rough bottom topography. Therefore, large-scale SQG waves will look like the first baroclinic mode and vice versa, with realistic stratification.
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LAMB, KEVIN G. "A numerical investigation of solitary internal waves with trapped cores formed via shoaling." Journal of Fluid Mechanics 451 (January 25, 2002): 109–44. http://dx.doi.org/10.1017/s002211200100636x.
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The formation of solitary internal waves with trapped cores via shoaling is investigated numerically. For density fields for which the buoyancy frequency increases monotonically towards the surface, sufficiently large solitary waves break as they shoal and form solitary-like waves with trapped fluid cores. Properties of large-amplitude waves are shown to be sensitive to the near-surface stratification. For the monotonic stratifications considered, waves with open streamlines are limited in amplitude by the breaking limit (maximum horizontal velocity equals wave propagation speed). When an exponential density stratification is modified to include a thin surface mixed layer, wave amplitudes are limited by the conjugate flow limit, in which case waves become long and horizontally uniform in the centre. The maximum horizontal velocity in the limiting wave is much less than the wave's propagation speed and as a consequence, waves with trapped cores are not formed in the presence of the surface mixed layer.
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Hall, Rob A., John M. Huthnance, and Richard G. Williams. "Internal Wave Reflection on Shelf Slopes with Depth-Varying Stratification." Journal of Physical Oceanography 43, no. 2 (February 1, 2013): 248–58. http://dx.doi.org/10.1175/jpo-d-11-0192.1.
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Abstract Reflection of internal waves from sloping topography is simple to predict for uniform stratification and linear slope gradients. However, depth-varying stratification presents the complication that regions of the slope may be subcritical and other regions supercritical. Here, a numerical model is used to simulate a mode-1, M2 internal tide approaching a shelf slope with both uniform and depth-varying stratifications. The fractions of incident internal wave energy reflected back offshore and transmitted onto the shelf are diagnosed by calculating the energy flux at the base of slope (with and without topography) and at the shelf break. For the stratifications/topographies considered in this study, the fraction of energy reflected for a given slope criticality is similar for both uniform and depth-varying stratifications. This suggests the fraction reflected is dependent only on maximum slope criticality and independent of the depth of the pycnocline. The majority of the reflected energy flux is in mode 1, with only minor contributions from higher modes due to topographic scattering. The fraction of energy transmitted is dependent on the depth-structure of the stratification and cannot be predicted from maximum slope criticality. If near-surface stratification is weak, transmitted internal waves may not reach the shelf break because of decreased horizontal wavelength and group velocity.
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Metzger, M., B. J. McKeon, and H. Holmes. "The near-neutral atmospheric surface layer: turbulence and non-stationarity." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 365, no. 1852 (January 16, 2007): 859–76. http://dx.doi.org/10.1098/rsta.2006.1946.
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The neutrally stable atmospheric surface layer is used as a physical model of a very high Reynolds number, canonical turbulent boundary layer. Challenges and limitations with this model are addressed in detail, including the inherent thermal stratification, surface roughness and non-stationarity of the atmosphere. Concurrent hot-wire and sonic anemometry data acquired in Utah's western desert provide insight to Reynolds number trends in the axial velocity statistics and spectra.
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Alcayaga, Leonardo, Gunner Chr Larsen, Mark Kelly, and Jakob Mann. "Identification of large-scale atmospheric structures under different stability conditions using Dynamic Mode Decomposition." Journal of Physics: Conference Series 2265, no. 2 (May 1, 2022): 022006. http://dx.doi.org/10.1088/1742-6596/2265/2/022006.
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Abstract We investigate the characteristic of large-scale coherent motions over a large horizontal domain using the Dynamic Mode Decomposition (DMD) spectral analysis algorithm applied on measurements from two long-range pulsed lidars. We show the results and advantages of this methodology on six cases representative of three thermal stratification conditions at two heights relevant for wind energy: near-neutral, unstable and stable stratification at 50m and 200m above ground level. For these cases the DMD algorithm show three types of structures: streaks near the surface for near-neutral for neutral stratification, large-scale convective rolls for the unstable cases and sheet-like rotational patches for stable conditions. The DMD algorithm also shows the stationary effects of the terrain on the flow at 50m above ground level, within the atmospheric surface layer. The possibility of isolating terrain effects from coherent motions makes DMD attractive for studying complex atmospheric flow phenomena as well as to have more realistic input for wind farm flow simulations.
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Boutin, J., Y. Chao, W. E. Asher, T. Delcroix, R. Drucker, K. Drushka, N. Kolodziejczyk, et al. "Satellite and In Situ Salinity: Understanding Near-Surface Stratification and Subfootprint Variability." Bulletin of the American Meteorological Society 97, no. 8 (August 1, 2016): 1391–407. http://dx.doi.org/10.1175/bams-d-15-00032.1.
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Abstract Remote sensing of salinity using satellite-mounted microwave radiometers provides new perspectives for studying ocean dynamics and the global hydrological cycle. Calibration and validation of these measurements is challenging because satellite and in situ methods measure salinity differently. Microwave radiometers measure the salinity in the top few centimeters of the ocean, whereas most in situ observations are reported below a depth of a few meters. Additionally, satellites measure salinity as a spatial average over an area of about 100 × 100 km2. In contrast, in situ sensors provide pointwise measurements at the location of the sensor. Thus, the presence of vertical gradients in, and horizontal variability of, sea surface salinity complicates comparison of satellite and in situ measurements. This paper synthesizes present knowledge of the magnitude and the processes that contribute to the formation and evolution of vertical and horizontal variability in near-surface salinity. Rainfall, freshwater plumes, and evaporation can generate vertical gradients of salinity, and in some cases these gradients can be large enough to affect validation of satellite measurements. Similarly, mesoscale to submesoscale processes can lead to horizontal variability that can also affect comparisons of satellite data to in situ data. Comparisons between satellite and in situ salinity measurements must take into account both vertical stratification and horizontal variability.
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Lucas, Natasha S., Alan L. M. Grant, Tom P. Rippeth, Jeff A. Polton, Matthew R. Palmer, Liam Brannigan, and Stephen E. Belcher. "Evolution of Oceanic Near-Surface Stratification in Response to an Autumn Storm." Journal of Physical Oceanography 49, no. 11 (November 2019): 2961–78. http://dx.doi.org/10.1175/jpo-d-19-0007.1.
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AbstractUnderstanding the processes that control the evolution of the ocean surface boundary layer (OSBL) is a prerequisite for obtaining accurate simulations of air–sea fluxes of heat and trace gases. Observations of the rate of dissipation of turbulent kinetic energy (ε), temperature, salinity, current structure, and wave field over a period of 9.5 days in the northeast Atlantic during the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) are presented. The focus of this study is a storm that passed over the observational area during this period. The profiles of ε in the OSBL are consistent with profiles from large-eddy simulation (LES) of Langmuir turbulence. In the transition layer (TL), at the base of the OSBL, ε was found to vary periodically at the local inertial frequency. A simple bulk model of the OSBL and a parameterization of shear driven turbulence in the TL are developed. The parameterization of ε is based on assumptions about the momentum balance of the OSBL and shear across the TL. The predicted rate of deepening, heat budget, and the inertial currents in the OSBL were in good agreement with the observations, as is the agreement between the observed value of ε and that predicted using the parameterization. A previous study reported spikes of elevated dissipation related to enhanced wind shear alignment at the base of the OSBL after this storm. The spikes in dissipation are not predicted by this new parameterization, implying that they are not an important source of dissipation during the storm.
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Gibbs, M. T., M. J. Bowman, and D. E. Dietrich. "Maintenance of Near-Surface Stratification in Doubtful Sound, a New Zealand Fjord." Estuarine, Coastal and Shelf Science 51, no. 6 (December 2000): 683–704. http://dx.doi.org/10.1006/ecss.2000.0716.
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Koszalka, I., L. Ceballos, and A. Bracco. "Vertical mixing and coherent anticyclones in the ocean: the role of stratification." Nonlinear Processes in Geophysics 17, no. 1 (January 22, 2010): 37–47. http://dx.doi.org/10.5194/npg-17-37-2010.
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Abstract. The role played by wind-forced anticyclones in the vertical transport and mixing at the ocean mesoscale is investigated with a primitive-equation numerical model in an idealized configuration. The focus of this work is to determine how the stratification impacts such transport. The flows, forced only at the surface by an idealized wind forcing, are predominantly horizontal and, on average, quasigeostrophic. Inside vortex cores and intense filaments, however, the dynamics is strongly ageostrophic. Mesoscale anticyclones appear as "islands" of increased penetration of wind energy into the ocean interior and they represent the maxima of available potential energy. The amount of available potential energy is directly correlated with the degree of stratification. The wind energy injected at the surface is transferred at depth through the generation and subsequent straining effect of Vortex Rossby Waves (VRWs), and through near-inertial internal oscillations trapped inside anticyclonic vortices. Both these mechanisms are affected by stratification. Stronger transfer but larger confinement close to the surface is found when the stratification is stronger. For weaker stratification, vertical mixing close to the surface is less intense but below about 150 m attains substantially higher values due to an increased contribution of both VRWs, whose time scale is on the order of few days, and of near-inertial motions, with a time scale of few hours.
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Liu, Xinchun, Yongde Kang, Hongna Chen, and Hui Lu. "Comparison of surface wind speed and wind speed profiles in the Taklimakan Desert." PeerJ 10 (April 1, 2022): e13001. http://dx.doi.org/10.7717/peerj.13001.
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Near-surface (10 m) wind speed (NWS) plays a crucial role in many areas, including the hydrological cycle, wind energy production, and the dispersion of air pollution. Based on wind speed data from Tazhong and the northern margins of the Taklimakan Desert in Xiaotang in spring, summer, autumn, and winter of 2014 and 2015, statistical methods were applied to determine the characteristics of the diurnal changes in wind speed near the ground and the differences in the wind speed profiles between the two sites. The average wind speed on a sunny day increased slowly with height during the day and rapidly at night. At heights below 4 m the wind speed during the day was higher than at night, whereas at 10 m the wind speed was lower during the day than at night. The semi-empirical theory and Monin–Obukhov (M–O) similarity theory were used to fit the NWS profile in the hinterland of the Tazhong Desert. A logarithmic law was applied to the neutral stratification wind speed profile, and an exponential fitting correlation was used for non-neutral stratification. The more unstable the stratification, the smaller the n. Using M–O similarity theory, the “linear to tens of” law was applied to the near-neutral stratification. According to the measured data, the distribution of φM with stability was obtained. The γm was obtained when the near-surface stratum was stable in the hinterland of Tazhong Desert and the βm was obtained when it was unstable. In summer, γm and βm were 5.84 and 15.1, respectively, while in winter, γm and βm were 1.9 and 27.1, respectively.
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Johnson, Leah, Craig M. Lee, Eric A. D’Asaro, Jacob O. Wenegrat, and Leif N. Thomas. "Restratification at a California Current Upwelling Front. Part II: Dynamics." Journal of Physical Oceanography 50, no. 5 (May 2020): 1473–87. http://dx.doi.org/10.1175/jpo-d-19-0204.1.
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AbstractA coordinated multiplatform campaign collected detailed measurements of a restratifying surface intensified upwelling front within the California Current System. A companion paper outlined the evolution of the front, revealing the importance of lateral advection at tilting isopycnals and increasing stratification in the surface boundary layer with a buoyancy flux equivalent to 2000 W m−2. Here, observations were compared with idealized models to explore the dynamics contributing to the stratification. A 2D model combined with a reduced form of the horizontal momentum equations highlight the importance of transient Ekman dynamics, turbulence, and thermal wind imbalance at modulating shear in the boundary layer. Specifically, unsteady frictional adjustment to the rapid decrease in wind stress created vertically sheared currents that advected horizontal gradients to increase vertical stratification on superinertial time scales. The magnitude of stratification depended on the strength of the horizontal buoyancy gradient. This enhanced stratification due to horizontal advection inhibited nighttime mixing that would have otherwise eroded stratification from the diurnal warm layer. This underscores the importance of near-surface lateral restratification for the upper ocean buoyancy budget on diel time scales.
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Stiperski, Ivana, Marcelo Chamecki, and Marc Calaf. "Anisotropy of Unstably Stratified Near-Surface Turbulence." Boundary-Layer Meteorology 180, no. 3 (June 15, 2021): 363–84. http://dx.doi.org/10.1007/s10546-021-00634-0.
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AbstractClassic Monin–Obukov similarity scaling states that in a stationary, horizontally homogeneous flow, in the absence of subsidence, turbulence is dictated by the balance between shear production and buoyancy production/destruction, whose ratio is characterized by a single universal scaling parameter. An evident breakdown in scaling is observed though, through large scatter in traditional scaling relations for the horizontal velocity variances under unstable stratification, or more generally in complex flow conditions. This breakdown suggests the existence of processes other than local shear and buoyancy that modulate near-surface turbulence. Recent studies on the role of anisotropy in similarity scaling have shown that anisotropy, even if calculated locally, may encode the information about these missing processes. We therefore examine the possible processes that govern the degree of anisotropy in convective conditions. We first use the reduced turbulence-kinetic-energy budget to show that anisotropy in convective conditions cannot be uniquely described by a balance of buoyancy and shear production and dissipation, but that other terms in the budget play an important role. Subsequently, we identify a ratio of local time scales that acts as a proxy for the anisotropic state of convective turbulence. This ratio can be used to formulate a new non-dimensional group. Results show that building on this approach the role of anisotropy in scaling relations over complex terrain can be placed into a more generalized framework.
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Mahadevan, Amala, Eric D’Asaro, Craig Lee, and Mary Jane Perry. "Eddy-Driven Stratification Initiates North Atlantic Spring Phytoplankton Blooms." Science 337, no. 6090 (July 5, 2012): 54–58. http://dx.doi.org/10.1126/science.1218740.
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Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.
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Hughes, Kenneth G., James N. Moum, and Emily L. Shroyer. "Evolution of the Velocity Structure in the Diurnal Warm Layer." Journal of Physical Oceanography 50, no. 3 (March 2020): 615–31. http://dx.doi.org/10.1175/jpo-d-19-0207.1.
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AbstractThe daily formation of near-surface ocean stratification caused by penetrating solar radiation modifies heat fluxes through the air–sea interface, turbulence dissipation in the mixed layer, and the vertical profile of lateral transport. The transport is altered because momentum from wind is trapped in a thin near-surface layer, the diurnal warm layer. We investigate the dynamics of this layer, with particular attention to the vertical shear of horizontal velocity. We first develop a quantitative link between the near-surface shear components that relates the crosswind component to the inertial turning of the along-wind component. Three days of high-resolution velocity observations confirm this relation. Clear colocation of shear and stratification with Richardson numbers near 0.25 indicate marginal instability. Idealized numerical modeling is then invoked to extrapolate below the observed wind speeds. This modeling, together with a simple energetic scaling analysis, provides a rule of thumb that the diurnal shear evolves differently above and below a 2 m s−1 wind speed, with limited sensitivity of this threshold to latitude and mean net surface heat flux. Only above this wind speed is the energy input sufficient to overcome the stabilizing buoyancy flux and thereby induce marginal instability. The differing shear regimes explain differences in the timing and magnitude of diurnal sea surface temperature anomalies.
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Randelhoff, Achim, Ilker Fer, and Arild Sundfjord. "Turbulent Upper-Ocean Mixing Affected by Meltwater Layers during Arctic Summer." Journal of Physical Oceanography 47, no. 4 (April 2017): 835–53. http://dx.doi.org/10.1175/jpo-d-16-0200.1.
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AbstractEvery summer, intense sea ice melt around the margins of the Arctic pack ice leads to a stratified surface layer, potentially without a traditional surface mixed layer. The associated strengthening of near-surface stratification has important consequences for the redistribution of near-inertial energy, ice–ocean heat fluxes, and vertical replenishment of nutrients required for biological growth. The authors describe the vertical structure of meltwater layers and quantify their seasonal evolution and their effect on turbulent mixing in the oceanic boundary layer by analyzing more than 450 vertical profiles of velocity microstructure in the seasonal ice zone north of Svalbard. The vertical structure of the density profiles can be summarized by an equivalent mixed layer depth hBD, which scales with the depth of the seasonal stratification. As the season progresses and melt rates increase, hBD shoals following a robust pattern, implying stronger vertical stratification, weaker vertical eddy diffusivity, and reduced vertical extent of the mixing layer, which is bounded by hBD. Through most of the seasonal pycnocline, the vertical eddy diffusivity scales inversely with buoyancy frequency (Kρ ∝ N−1). The presence of mobile sea ice alters the magnitude and vertical structure of turbulent mixing primarily through stronger and shallower stratification, and thus vertical eddy diffusivity is greatly reduced under sea ice. This study uses these results to develop a quantitative model of surface layer turbulent mixing during Arctic summer and discuss the impacts of a changing sea ice cover.
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Cronin, Meghan F., and William S. Kessler. "Near-Surface Shear Flow in the Tropical Pacific Cold Tongue Front*." Journal of Physical Oceanography 39, no. 5 (May 1, 2009): 1200–1215. http://dx.doi.org/10.1175/2008jpo4064.1.
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Abstract Near-surface shear in the Pacific cold tongue front at 2°N, 140°W was measured using a set of five moored current meters between 5 and 25 m for nine months during 2004–05. Mean near-surface currents were strongly westward and only weakly northward (∼3 cm s−1). Mean near-surface shear was primarily westward and, thus, oriented to the left of the southeasterly trades. When the southwestward geostrophic shear was subtracted from the observed shear, the residual ageostrophic currents relative to 25 m were northward and had an Ekman-like spiral, in qualitative agreement with an Ekman model modified for regions with a vertically uniform front. According to this “frontal Ekman” model, the ageostrophic Ekman spiral is forced by the portion of the wind stress that is not balanced by the surface geostrophic shear. Analysis of a composite tropical instability wave (TIW) confirms that ageostrophic shear is minimized when winds blow along the front, and strengthens when winds blow oblique to the front. Furthermore, the magnitude of the near-surface shear, both in the TIW and diurnal composites, was sensitive to near-surface stratification and mixing. A diurnal jet was observed that was on average 12 cm s−1 stronger at 5 m than at 25 m, even though daytime stratification was weak. The resulting Richardson number indicates that turbulent viscosity is larger at night than daytime and decreases with depth. A “generalized Ekman” model is also developed that assumes that viscosity becomes zero below a defined frictional layer. The generalized model reproduces many of the features of the observed mean shear and is valid both in frontal regions and at the equator.
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Liu, Yongqiang, Ali Mamtimin, Wen Huo, Xinghua Yang, Xinchun Liu, Fan Yang, and Qing He. "Nondimensional Wind and Temperature Profiles in the Atmospheric Surface Layer over the Hinterland of the Taklimakan Desert in China." Advances in Meteorology 2016 (2016): 1–8. http://dx.doi.org/10.1155/2016/9325953.
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Observed turbulent fluxes, wind, and temperature profiles at Tazhong station over the hinterland of the Taklimakan Desert in China have been analyzed to evaluate empirical parameters used in the profile functions of desert surface layer. The von Kármán constant derived from our observations is about 0.396 in near-neutral stratification, which is in good agreement with many other studies for different underlying surface. In our analysis, the turbulent Prandtl number is about 0.75 in near-neutral conditions. For unstable range, the nondimensional wind and temperature profile functions are best fitted by the exponents of −1/4 and −1/2, respectively. The linear relations still hold for stable stratification in this extremely arid desert. However, the parameters used in their profile functions need to be revised to be applicable for desert surfaces.
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Johnson, Leah, Craig M. Lee, Eric A. D’Asaro, Leif Thomas, and Andrey Shcherbina. "Restratification at a California Current Upwelling Front. Part I: Observations." Journal of Physical Oceanography 50, no. 5 (May 2020): 1455–72. http://dx.doi.org/10.1175/jpo-d-19-0203.1.
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AbstractA coordinated survey between a subsurface Lagrangian float and a ship-towed Triaxus profiler obtained detailed measurements of a restratifying surface intensified front (above 30 m) within the California Current System. The survey began as downfront winds incited mixing in the boundary layer. As winds relaxed and mixing subsided, the system entered a different dynamical regime as the front developed an overturning circulation with large vertical velocities that tilted isopycnals and stratified the upper ocean within a day. The horizontal buoyancy gradient was 1.5 × 10−6 s−2 and associated with vorticity, divergence, and strain that approached the Coriolis frequency. Estimates of vertical velocity from the Lagrangian float reached 1.2 × 10−3 m s−1. These horizontal gradients and vertical velocities were consistent with submesoscale dynamics that are distinct from the classic quasigeostrophic framework used to describe larger-scale flows. Vertical and horizontal gradients of velocity and buoyancy in the vicinity of the float revealed that sheared currents differentially advected the horizontal buoyancy gradient to increase vertical stratification. This was supported by analyses of temperature and salinity gradients that composed the horizontal and vertical stratification. Potential vorticity was conserved during restratification at 16 m, consistent with adiabatic processes. Conversely, potential vorticity near the surface (8 m) increased, highlighting the role of friction in modulating near-surface stratification. The observed increase in stratification due to these submesoscale processes was equivalent to a heat flux of 2000 W m−2, which is an order-of-magnitude larger than the average observed surface heat flux of 100 W m−2.
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Schlosser, Tamara L., Nicole L. Jones, Ruth C. Musgrave, Cynthia E. Bluteau, Gregory N. Ivey, and Andrew J. Lucas. "Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field." Journal of Physical Oceanography 49, no. 7 (July 2019): 1973–94. http://dx.doi.org/10.1175/jpo-d-18-0194.1.
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AbstractUsing 18 days of field observations, we investigate the diurnal (D1) frequency wave dynamics on the Tasmanian eastern continental shelf. At this latitude, the D1 frequency is subinertial and separable from the highly energetic near-inertial motion. We use a linear coastal-trapped wave (CTW) solution with the observed background current, stratification, and shelf bathymetry to determine the modal structure of the first three resonant CTWs. We associate the observed D1 velocity with a superimposed mode-zero and mode-one CTW, with mode one dominating mode zero. Both the observed and mode-one D1 velocity was intensified near the thermocline, with stronger velocities occurring when the thermocline stratification was stronger and/or the thermocline was deeper (up to the shelfbreak depth). The CTW modal structure and amplitude varied with the background stratification and alongshore current, with no spring–neap relationship evident for the observed 18 days. Within the surface and bottom Ekman layers on the shelf, the observed velocity phase changed in the cross-shelf and/or vertical directions, inconsistent with an alongshore propagating CTW. In the near-surface and near-bottom regions, the linear CTW solution also did not match the observed velocity, particularly within the bottom Ekman layer. Boundary layer processes were likely causing this observed inconsistency with linear CTW theory. As linear CTW solutions have an idealized representation of boundary dynamics, they should be cautiously applied on the shelf.
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Shrira, Victor I., and Philippe Forget. "On the Nature of Near-Inertial Oscillations in the Uppermost Part of the Ocean and a Possible Route toward HF Radar Probing of Stratification." Journal of Physical Oceanography 45, no. 10 (October 2015): 2660–78. http://dx.doi.org/10.1175/jpo-d-14-0247.1.
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AbstractInertial band response of the upper ocean to changing wind is studied both theoretically and by analysis of observations in the northwestern Mediterranean. On the nontraditional f plane, because of the horizontal component of the earth’s rotation for waves of inertial band with frequencies slightly below the local inertial frequency f, there is a waveguide in the mixed layer confined from below by the pycnocline. It is argued that when the stratification is shallow these waves are most easily and strongly excited by varying winds as near-inertial oscillations (NIOs). These motions have been overlooked in previous studies because they are absent under the traditional approximation. The observations that employed buoys with thermistors, ADCPs, and two 16.3-MHz Wellen Radar (WERA) HF radars were carried out in the Gulf of Lion in April–June 2006. The observations support the theoretical picture: a pronounced inertial band response occurs only in the presence of shallow stratification and is confined to the mixed layer, and the NIO penetration below the stratified layer is weak. NIO surface magnitude and vertical localization are strongly affected by the presence of even weak density stratification in the upper 10 m. The NIO surface signatures are easily captured by HF radars. Continuous 1.8-yr HF observations near the Porquerolles Island confirm that shallow stratification is indeed the precondition for a strong NIO response. The response sensitivity to stratification provides a foundation for developing HF radar probing of stratification and, indirectly, vertical mixing, including spotting dramatic mixing events and spikes of vertical heat, mass, and momentum exchange.
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Tang, Wenqing, Simon H. Yueh, Akiko Hayashi, Alexander G. Fore, W. Linwood Jones, Andrea Santos-Garcia, and Maria Marta Jacob. "Rain-Induced Near Surface Salinity Stratification and Rain Roughness Correction for Aquarius SSS Retrieval." IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing 8, no. 12 (December 2015): 5474–84. http://dx.doi.org/10.1109/jstars.2015.2463768.
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Sengupta, Debasis, G. N. Bharath Raj, M. Ravichandran, J. Sree Lekha, and Fabrice Papa. "Near-surface salinity and stratification in the north Bay of Bengal from moored observations." Geophysical Research Letters 43, no. 9 (May 7, 2016): 4448–56. http://dx.doi.org/10.1002/2016gl068339.
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Hershberger, P. K., J. E. Rensel, A. L. Matter, and F. B. Taub. "Vertical distribution of the chloromonad flagellate Heterosigma carterae in columns: implications for bloom development." Canadian Journal of Fisheries and Aquatic Sciences 54, no. 10 (October 1, 1997): 2228–34. http://dx.doi.org/10.1139/f97-131.
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Blooms of the marine flagellate Heterosigma carterae have been associated with catastrophic fish kills at mariculture facilities around the world. The precise cause(s) of the sudden appearance and disappearance of Heterosigma surface blooms has not been completely described or understood. Environmental data from prior studies of blooms indicate the presence of vertical stratification of the water column that is often induced by freshwater runoff. We report the relatively rapid concentration of Heterosigma cells at the surface of tubes shortly after the addition of distilled water to the surface. This phenomenon of cell concentration in surface waters may partially explain the sudden appearance of Heterosigma cells near the surface and subsequent disappearance when vertical stratification is lost due to turbulence or mixing. The results may lead to bloom mitigation techniques for net pen aquaculture.
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Taylor, John R., and Sutanu Sarkar. "Stratification Effects in a Bottom Ekman Layer." Journal of Physical Oceanography 38, no. 11 (November 1, 2008): 2535–55. http://dx.doi.org/10.1175/2008jpo3942.1.
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Abstract A stratified bottom Ekman layer over a nonsloping, rough surface is studied using a three-dimensional unsteady large eddy simulation to examine the effects of an outer layer stratification on the boundary layer structure. When the flow field is initialized with a linear temperature profile, a three-layer structure develops with a mixed layer near the wall separated from a uniformly stratified outer layer by a pycnocline. With the free-stream velocity fixed, the wall stress increases slightly with the imposed stratification, but the primary role of stratification is to limit the boundary layer height. Ekman transport is generally confined to the mixed layer, which leads to larger cross-stream velocities and a larger surface veering angle when the flow is stratified. The rate of turning in the mixed layer is nearly independent of stratification, so that when stratification is large and the boundary layer thickness is reduced, the rate of veering in the pycnocline becomes very large. In the pycnocline, the mean shear is larger than observed in an unstratified boundary layer, which is explained using a buoyancy length scale, u*/N(z). This length scale leads to an explicit buoyancy-related modification to the log law for the mean velocity profile. A new method for deducing the wall stress based on observed mean velocity and density profiles is proposed and shows significant improvement compared to the standard profile method. A streamwise jet is observed near the center of the pycnocline, and the shear at the top of the jet leads to local shear instabilities and enhanced mixing in that region, despite the fact that the Richardson number formed using the mean density and shear profiles is larger than unity.
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Deng, Yuan Chang, and Zhen Cao Zou. "The Effects of Vertical Stratification and Land Use Data on Numerical Simulation of Wind Energy Resources." Applied Mechanics and Materials 535 (February 2014): 135–40. http://dx.doi.org/10.4028/www.scientific.net/amm.535.135.
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By adjusting the distribution of vertical layers and increasing its number in WRF model, this paper mainly studies the effects of vertical stratification on the near surface wind field and vertical profile simulation. The test outcomes show that moderately increasing vertical layers can effectively improve the near surface wind field simulation results, while it has little influence on the numeral and changing trend of high vertical wind profile. Considering both accuracy and efficiency, it is recommended to set 10~15 layers below 300m. On the basis of this research, instead of USGS data by using the MODIS_30S data, the data underlying surface land in Shenzhen and HK area are updated. Comparative results between the two schemes, due to the roughness and drag coefficient of difference types of surface are not identical; the surface data has a significant impact on wind field and wind profile simulation. Using the MODIS land use data which is more consistent with the actual situation can improve the accuracy of numerical simulation.
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Ohashi, Yoshihiko, Shigeru Aoki, Yoshimasa Matsumura, Shin Sugiyama, Naoya Kanna, and Daiki Sakakibara. "Vertical distribution of water mass properties under the influence of subglacial discharge in Bowdoin Fjord, northwestern Greenland." Ocean Science 16, no. 3 (May 4, 2020): 545–64. http://dx.doi.org/10.5194/os-16-545-2020.
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Abstract. Subglacial discharge has significant impacts on water circulation, material transport, and biological productivity in proglacial fjords of Greenland. To help clarify the fjord water properties and the effect of subglacial discharge, we investigated the properties of vertical water mass profiles of Bowdoin Fjord in northwestern Greenland based on summer hydrographic observations, including turbidity, in 2014 and 2016. We estimated the fraction of subglacial discharge from the observational data and interpreted the observed differences in subglacial plume behavior between two summer seasons with the numerical model results. At a depth of 15–40 m, where the most turbid water was observed, the maximum subglacial discharge fractions near the ice front were estimated to be ∼6 % in 2014 and ∼4 % in 2016. The higher discharge fraction in 2014 was likely due to stronger stratification, as suggested by the numerical experiments performed with different initial stratifications. Turbidity near the surface was higher in 2016 than in 2014, suggesting a stronger influence of turbid subglacial discharge. The higher turbidity in 2016 could primarily be attributed to a greater amount of subglacial discharge, as inferred from the numerical experiments forced by different amounts of discharge. This study suggests that both fjord stratification and the amount of discharge are important factors in controlling the vertical distribution of freshwater outflow.
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MacKinnon, J. A., and M. C. Gregg. "Near-Inertial Waves on the New England Shelf: The Role of Evolving Stratification, Turbulent Dissipation, and Bottom Drag." Journal of Physical Oceanography 35, no. 12 (December 1, 2005): 2408–24. http://dx.doi.org/10.1175/jpo2822.1.
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Abstract Energetic variable near-inertial internal waves were observed on the springtime New England shelf as part of the Coastal Mixing and Optics (CMO) project. Surface warming and freshwater advection tripled the average stratification during a 3-week observational period in April/May 1997. The wave field was dominated by near-inertial internal waves generated by passing storms. Wave evolution was controlled by a balance among wind stress, bottom drag, and turbulent dissipation. As the stratification evolved, the vertical structure of these near-inertial waves switched from mode 1 to mode 2 with associated changes in the magnitude and location of wave shear. The growth of mode-2 waves was attributable to a combination of changing wind stress forcing and a nonlinear coupling between the first and second vertical modes through quadratic bottom stress. To explore both forcing mechanisms, an open-ocean mixed layer model is adapted to the continental shelf. In this model, surface wind stress and bottom stress are distributed over the surface and bottom mixed layers and then projected onto orthogonal vertical modes. The model replicates the correct magnitude and evolving modal distribution of the internal waves and confirms that bottom stress can act to transfer energy between internal wave modes.
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Bettigole, Charles, Juliana Hanle, Daniel A. Kane, Zoe Pagliaro, Shaylan Kolodney, Sylvana Szuhay, Miles Chandler, et al. "Optimizing Sampling Strategies for Near-Surface Soil Carbon Inventory: One Size Doesn’t Fit All." Soil Systems 7, no. 1 (March 17, 2023): 27. http://dx.doi.org/10.3390/soilsystems7010027.
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Soils comprise the largest pool of terrestrial carbon yet have lost significant stocks due to human activity. Changes to land management in cropland and grazing systems present opportunities to sequester carbon in soils at large scales. Uncertainty in the magnitude of this potential impact is largely driven by the difficulties and costs associated with measuring near-surface (0–30 cm) soil carbon concentrations; a key component of soil carbon stock assessments. Many techniques exist to optimize sampling, yet few studies have compared these techniques at varying sample intensities. In this study, we performed ex-ante, high-intensity sampling for soil carbon concentrations at four farms in the eastern United States. We used post hoc Monte-Carlo bootstrapping to investigate the most efficient sampling approaches for soil carbon inventory: K-means stratification, Conditioned Latin Hypercube Sampling (cLHS), simple random, and regular grid. No two study sites displayed similar patterns across all sampling techniques, although cLHS and grid emerged as the most efficient sampling schemes across all sites and strata sizes. The number of strata chosen when using K-means stratification can have a significant impact on sample efficiency, and we caution future inventories from using small strata n, while avoiding even allocation of sample between strata. Our findings reinforce the need for adaptive sampling methodologies where initial site inventory can inform primary, robust inventory with site-specific sampling techniques.
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Cummins, Patrick F., Laurence Armi, and Svein Vagle. "Upstream Internal Hydraulic Jumps." Journal of Physical Oceanography 36, no. 5 (May 1, 2006): 753–69. http://dx.doi.org/10.1175/jpo2894.1.
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Abstract In stratified tidal flow over a sill, the character of the upstream response is determined by a Froude number Fs based on the stratification near the surface. This is distinguished from the Froude number governing the response in the neighborhood of the sill crest, which is based on the weak density step associated with a flow bifurcation. For moderate values of Fs, the upstream response consists of nonlinear waves or a weak undular bore. For larger values of Fs, a strong, quasi-stationary, internal hydraulic jump dominates the upstream response. At sufficiently large values of Fs, the upstream bore is swept downstream and lost. Acoustic backscatter and velocity data are presented for the case of a strong internal bore or gravity current in a tidally modulated sill flow. Numerical simulations with varying near-surface stratification are presented to illustrate the upstream responses at different values of Fs. The theory of two-layer hydraulic flows is invoked to account for the development of the upstream jump.
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Panchishkina, I. N., G. G. Petrova, A. I. Petrov, and O. G. Chkhetiani. "Conduction current density profile transformation near the earth’s surface, in connection with atmospheric stratification change." Journal of Physics: Conference Series 1604 (July 2020): 012002. http://dx.doi.org/10.1088/1742-6596/1604/1/012002.
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Zilitinkevich, S. S., V. L. Perov, and J. C. King. "Near-surface turbulent fluxes in stable stratification: Calculation techniques for use in general-circulation models." Quarterly Journal of the Royal Meteorological Society 128, no. 583 (July 2002): 1571–87. http://dx.doi.org/10.1002/qj.200212858309.
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Bolaños, Rodolfo, Jennifer M. Brown, Laurent O. Amoudry, and Alejandro J. Souza. "Tidal, Riverine, and Wind Influences on the Circulation of a Macrotidal Estuary." Journal of Physical Oceanography 43, no. 1 (January 1, 2013): 29–50. http://dx.doi.org/10.1175/jpo-d-11-0156.1.
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Abstract The effect of tides, river, wind and Earth’s rotation on the three-dimensional circulation in the Dee, a macrotidal estuary, are investigated using a fine-resolution model. The interactions of the large tidal amplitude, currents, river, and wind-generated circulation require baroclinic and unsteady studies to properly understand the estuarine dynamics. Assessment of the model skill has been carried out by model–observation comparisons for salinity, which is the main control for density, surface elevation, current, and turbulence. Stationary nondimensional numbers were only partially able to characterize the dynamics in this (real) complex macrotidal estuary. At low water, tidal straining and constrained river flow cause stratification. Large spatial variability occurs in the current and residual patterns, with flood-dominated maximum values occurring within the tidal channels. The tides control residual circulation by modulating stratification through tidal straining and bathymetric constraint on river flow. Tide–stratification–river interaction causes an unsteady pattern of residual circulation and tidal pulses. River-induced pulses are enhanced near low tide–inducing density-driven circulation. Wind effects are concentrated near the surface, mainly occurring at high tide because of increased fetch. Even though Coriolis has, overall, a small contribution it produces tidal pulses modifying the current and salinity distribution.
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van de Poll, W. H., G. Kulk, K. R. Timmermans, C. P. D. Brussaard, H. J. van der Woerd, M. J. Kehoe, K. D. A. Mojica, R. J. W. Visser, P. D. Rozema, and A. G. J. Buma. "Phytoplankton chlorophyll <i>a</i> biomass, composition, and productivity along a temperature and stratification gradient in the northeast Atlantic Ocean." Biogeosciences 10, no. 6 (June 25, 2013): 4227–40. http://dx.doi.org/10.5194/bg-10-4227-2013.
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Abstract. Relationships between sea surface temperature (SST, > 10 m) and vertical density stratification, nutrient concentrations, and phytoplankton biomass, composition, and chlorophyll a (Chl a) specific absorption were assessed in spring and summer from latitudes 29 to 63° N in the northeast Atlantic Ocean. The goal of this study was to identify relationships between phytoplankton and abiotic factors in an existing SST and stratification gradient. Furthermore, a bio-optical model was used to estimate productivity for five phytoplankton groups. Nutrient concentration (integrated from 0 to 125 m) was inversely correlated with SST in spring and summer. SST was also inversely correlated with near-surface (0–50 m) Chl a and productivity for stratified stations. Near-surface Chl a and productivity showed exponential relationships with SST. Chl a specific absorption and excess light experiments indicated photoacclimation to lower irradiance in spring as compared to summer. In addition, Chl a specific absorption suggested that phytoplankton size decreased in summer. The contribution of cyanobacteria to water column productivity of stratified stations correlated positively with SST and inversely with nutrient concentration. This suggests that a rise in SST (over a 13–23 °C range) stimulates productivity by cyanobacteria at the expense of haptophytes, which showed an inverse relationship to SST. At higher latitudes, where rising SST may prolong the stratified season, haptophyte productivity may expand at the expense of diatom productivity. Depth-integrated Chl a (0–410 m) was greatest in the spring at higher latitudes, where stratification in the upper 200 m was weakest. This suggests that stronger stratification does not necessarily result in higher phytoplankton biomass standing stock in this region.
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Koue, Jinichi, Hikari Shimadera, Tomohito Matsuo, and Akira Kondo. "Numerical Analysis of Sensitivity of Structure of the Stratification in Lake Biwa, Japan by Changing Meteorological Elements." Water 10, no. 10 (October 22, 2018): 1492. http://dx.doi.org/10.3390/w10101492.
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Climatic factors such as air temperature and wind speed can affect the structure of stratification in Lake Biwa. In general, the rise in air temperature and the decrease in wind speed weaken the vertical mixing and strengthen the structure of the stratification, which interrupts the transport of the substances. However, how much the change of each climate element can influence the structure of the stratification is not clarified. Therefore, it is important to evaluate the effects of each element on the stratification quantitatively. In the present study, we investigated the effect of the change in air temperature and wind speed on the seasonal change of stratification in Lake Biwa by using a three-dimensional hydrodynamic model. Numerical simulations were carried out for a baseline case using realistic meteorological data from 2007 to 2012 and hypothetical cases using meteorological data with modified air temperature or wind speed for sensitivity analysis. The analysis showed that the increase and decrease in air temperature changed the vertical water temperature uniformly in almost all layers. Thus, the strength of the stratification is hardly changed. The increase and decrease in wind speed, however, altered the water temperature near the surface of the lake, so that it significantly influenced the stratification. The increase in wind speed made the water parcels of the surface layer well mixed, and the decrease in wind speed made the mixed layer thinner.
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MacKinnon, J. A., and M. C. Gregg. "Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf." Journal of Physical Oceanography 35, no. 12 (December 1, 2005): 2425–43. http://dx.doi.org/10.1175/jpo2821.1.
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Abstract Integrated observations are presented of water property evolution and turbulent microstructure during the spring restratification period of April and May 1997 on the New England continental shelf. Turbulence is shown to be related to surface mixed layer entrainment and shear from low-mode near-inertial internal waves. The largest turbulent diapycnal diffusivity and associated buoyancy fluxes were found at the bottom of an actively entraining and highly variable wind-driven surface mixed layer. Away from surface and bottom boundary layers, turbulence was systematically correlated with internal wave shear, though the nature of that relationship underwent a regime shift as the stratification strengthened. During the first week, while stratification was weak, the largest turbulent dissipation away from boundaries was coincident with shear from mode-1 near-inertial waves generated by passing storms. Wave-induced Richardson numbers well below 0.25 and density overturning scales of several meters were observed. Turbulent dissipation rates in the region of peak shear were consistent in magnitude with several dimensional scalings. The associated average diapycnal diffusivity exceeded 10−3 m2 s−1. As stratification tripled, Richardson numbers from low-mode internal waves were no longer critical, though turbulence was still consistently elevated in patches of wave shear. Kinematically, dissipation during this period was consistent with the turbulence parameterization proposed by MacKinnon and Gregg, based on a reinterpretation of wave–wave interaction theory. The observed growth of temperature gradients was, in turn, consistent with a simple one-dimensional model that vertically distributed surface heat fluxes commensurate with calculated turbulent diffusivities.
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Schneider, Tapio, and Christopher C. Walker. "Scaling Laws and Regime Transitions of Macroturbulence in Dry Atmospheres." Journal of the Atmospheric Sciences 65, no. 7 (July 1, 2008): 2153–73. http://dx.doi.org/10.1175/2007jas2616.1.
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Abstract In simulations of a wide range of circulations with an idealized general circulation model, clear scaling laws of dry atmospheric macroturbulence emerge that are consistent with nonlinear eddy–eddy interactions being weak. The simulations span several decades of eddy energies and include Earth-like circulations and circulations with multiple jets and belts of surface westerlies in each hemisphere. In the simulations, the eddy available potential energy and the barotropic and baroclinic eddy kinetic energy scale linearly with each other, with the ratio of the baroclinic eddy kinetic energy to the barotropic eddy kinetic energy and eddy available potential energy decreasing with increasing planetary radius and rotation rate. Mean values of the meridional eddy flux of surface potential temperature and of the vertically integrated convergence of the meridional eddy flux of zonal momentum generally scale with functions of the eddy energies and the energy-containing eddy length scale, with a few exceptions in simulations with statically near-neutral or neutral extratropical thermal stratifications. Eddy energies scale with the mean available potential energy and with a function of the supercriticality, a measure of the near-surface slope of isentropes. Strongly baroclinic circulations form an extended regime in which eddy energies scale linearly with the mean available potential energy. Mean values of the eddy flux of surface potential temperature and of the vertically integrated eddy momentum flux convergence scale similarly with the mean available potential energy and other mean fields. The scaling laws for the dependence of eddy fields on mean fields exhibit a regime transition between a regime in which the extratropical thermal stratification and tropopause height are controlled by radiation and convection and a regime in which baroclinic entropy fluxes modify the extratropical thermal stratification and tropopause height. At the regime transition, for example, the dependence of the eddy flux of surface potential temperature and the dependence of the vertically integrated eddy momentum flux convergence on mean fields changes—a result with implications for climate stability and for the general circulation of an atmosphere, including its tropical Hadley circulation.
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Luo, Tao, Renmin Yuan, Zhien Wang, and Damao Zhang. "Quantifying the Hygroscopic Growth of Marine Boundary Layer Aerosols by Satellite-Based and Buoy Observations." Journal of the Atmospheric Sciences 72, no. 3 (February 24, 2015): 1063–74. http://dx.doi.org/10.1175/jas-d-14-0170.1.
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Abstract In this study, collocated satellite and buoy observations as well as satellite observations over an extended region during 2006–10 were used to quantify the humidity effects on marine boundary layer (MBL) aerosols. Although the near-surface aerosol size increases with increasing near-surface relative humidity (RH), the influence of RH decreases with increasing height and is mainly limited to the lower well-mixed layer. In addition, the size changes of MBL aerosols with RH are different for low and high surface wind () conditions as revealed by observations and Mie scattering calculations, which may be related to different dominant processes (i.e., the hygroscopic growth process during low wind and the evaporation process during sea salt production during high wind). These different hygroscopic processes under the different conditions, together with the MBL processes, control the behaviors of the MBL aerosol optical depth () with RH. In particular, under high conditions, the MBL stratifications effects can overwhelm the humidity effects, resulting in a weak relationship of MBL on RH. Under low conditions, the stronger hygroscopic growth can overwhelm the MBL stratification effects and enhance the MBL with increasing RH. These results are important to evaluate and to improve MBL aerosols simulations in climate models.
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Alam, Md, Richard Bell, Nazmus Salahin, Shahab Pathan, A. T. M. A. I. Mondol, M. J. Alam, M. H. Rashid, P. L. C. Paul, M. I. Hossain, and N. C. Shil. "Banding of Fertilizer Improves Phosphorus Acquisition and Yield of Zero Tillage Maize by Concentrating Phosphorus in Surface Soil." Sustainability 10, no. 9 (September 10, 2018): 3234. http://dx.doi.org/10.3390/su10093234.
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Zero tillage increases stratification of immobile nutrients such as P. However, it is unclear whether near-surface stratification of soil P eases or hampers P uptake by maize (Zea mays L.) which needs an optimum P supply at/before six–leaf–stage to achieve potential grain yield. The aim of the three-year study was to determine whether P stratification, under zero tillage, impaired yield of maize and which P placement methods could improve P uptake on an Aeric Albaquept soil subgroup. Phosphorus fertilizer was placed by: (a) broadcasting before final tillage and sowing of seeds; (b) surface banding beside the row; and (c) deep banding beside the row (both the band placements were done at three–four leaf stage) Phosphorus treatments were repeated for 3 years along with three tillage practices viz.: (a) zero tillage (ZT); (b) conventional tillage (12 cm; CT); and (c) deep tillage (25 cm; DT). In the third year, all the tillage practices gave similar yield of Bangladesh Agricultural Research Institute (BARI) hybrid maize–5, but the highest grain yield was obtained by surface band P placement. After three years of tillage and P placements, the root mass density (RMD) at 0–6 cm depth increased significantly from 1.40 mg cm−3 in DT under deep band placement to 1.98 mg cm−3 in ZT under surface band placement, but not at the other depths. The combination of ZT practices, with broadcast or surface band placement methods, produced the highest available, and total P, content in soil at 0–6 cm depth after harvesting of maize. Accordingly, a significant increase in P uptake by maize was also found with surface banding of P alone and also in combination with ZT. Organic carbon, and total N, also increased significantly at depths of 0–6 cm after three years in ZT treatments with P placed in bands. By contrast, CT and DT practices, under all placement methods, resulted in an even distribution of P up to 24 cm depth. Phosphorus application, by surface banding at the three–four leaf stage, led to increased P uptake at early growth and silking stages, which resulted in highest yield regardless of tillage type through increased extractable P in the soil. Even though ZT increased P stratification near the soil surface, and it increased plant available water content (PAWC) and RMD in the 0–6 cm depth, as did surface banding, it did not improve maize grain yield. Further research is needed to understanding the contrasting maize grain yield responses to P stratification.
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Alvarez, Alberto. "A model for the Arctic mixed layer circulation under a summertime lead: implications for the near-surface temperature maximum formation." Cryosphere 17, no. 8 (August 21, 2023): 3343–61. http://dx.doi.org/10.5194/tc-17-3343-2023.
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Abstract. Leads in sea ice cover have been studied extensively because of the climatic relevance of the intense ocean–atmosphere heat exchange that occurs during winter. Leads are also preferential locations of heat exchange and melting in early summer, but their oceanography and climate relevance, if any, remains largely unexplored during summertime. In particular, the development of a near-surface temperature maximum (NSTM) layer typically 10–30 m deep under different Arctic basins has been observationally related to the penetration of solar radiation through the leads. These observations reveal that the concatenation of calm and wind events in the leads could facilitate the development of the NSTM layer. Using numerical modeling and an idealized framework, this study investigates the formation of the NSTM layer under a summer lead exposed to a combination of calm and moderate wind periods. During the calm period, solar heat accumulates in the upper layers under the lead. Near-surface convection cells are generated daily, extending from the lead sides to its center. Convection cells affect the heat storage in the mixed layer under the lead and the adjacent ice cap. A subsequent wind event (and corresponding ice drift) mixes and spreads fresh and cold meltwater into the warm layers near the surface. Surface mixing results in temperatures in the near-surface layers that are lower than in the deeper layers, where the impact of the surface stresses is weaker. Additionally, the warm waters initially located under the lead surface stretch and spread horizontally. Thus, an NSTM layer is formed. The study analyzes the sensitivity of the depth and temperature of the NSTM layer to buoyancy forcing, wind intensity, ice drift, stratification, and lead geometry. Numerical results suggest that the NSTM layer appears with moderate wind and ice drift and disappears when the wind intensity is higher than 9 m s−1. Depending on the background stratification, the calm period reinforces or becomes critical in NSTM layer formation. According to the results, ice drift is key to the development of the NSTM layer.
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Zhang, Y., J. Ma, and Z. Cao. "The von Kármán constant retrieved from CASE-97 dataset using a variational method." Atmospheric Chemistry and Physics Discussions 8, no. 4 (July 17, 2008): 13667–88. http://dx.doi.org/10.5194/acpd-8-13667-2008.
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Abstract. A variational method is developed to retrieve the von Kármán constant κ from the CASES-97 dataset, collected near Wichita, Kansas, the United States from 6 April to 24 May 1997. In the variational method, a cost function is defined to measure the difference between observed and computed gradients of wind speed, air temperature and specific humidity. An optimal estimated von Kármán constant is obtained by minimizing the cost function through adjusting values of the von Kármán constant. Under neutral stratification, the variational analysis confirms the conventional value of κ(=0.40). For non-neutral stratification, however, κ varies with stability. The computational results show that the κ decreases monotonously from stable to unstable stratification. The variational calculated mean value of the von Kármán constant is 0.390 when the atmospheric stratification is taken into consideration. Relations between κ and surface momentum and heat flux are also examined.
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Zhang, Y., J. Ma, and Z. Cao. "The Von Kármán constant retrieved from CASES-97 dataset using a variational method." Atmospheric Chemistry and Physics 8, no. 23 (December 5, 2008): 7045–53. http://dx.doi.org/10.5194/acp-8-7045-2008.
Full textAbstract:
Abstract. A variational method is developed to retrieve the von Kármán constant κ from the CASES-97 dataset, collected near Wichita, Kansas, the United States from 6 April to 24 May 1997. In the variational method, a cost function is defined to measure the difference between observed and computed gradients of wind speed, air temperature and specific humidity. An optimal estimated von Kármán constant is obtained by minimizing the cost function through adjusting values of the von Kármán constant. Under neutral stratification, the variational analysis confirms the conventional value of κ (=0.40). For non-neutral stratification, however, κ varies with stability. The computational results show that the κ decreases monotonously from stable to unstable stratification. The variational calculated mean value of the von Kármán constant is 0.383~0.390 when the atmospheric stratification is taken into consideration. Relations between κ and surface momentum and heat flux are also examined.
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Portalakis, Panagiotis, Maria Tombrou, John Kalogiros, Aggeliki Dandou, and Qing Wang. "Investigation of Air-Sea Turbulent Momentum Flux over the Aegean Sea with a Wind-Wave Coupling Model." Atmosphere 12, no. 9 (September 16, 2021): 1208. http://dx.doi.org/10.3390/atmos12091208.
Full textAbstract:
Near surface turbulent momentum flux estimates are performed over the Aegean Sea, using two different approaches regarding the drag coefficient formulation, a wave boundary layer model (referred here as KCM) and the most commonly used Coupled Ocean–Atmosphere Response Experiment (COARE) algorithm. The KCM model incorporates modifications in the energy-containing wave spectrum to account for the wave conditions of the Aegean Sea, and surface similarity to account for the stratification effects. Airborne turbulence data during an Etesian outbreak over Aegean Sea, Greece are processed to evaluate the simulations. KCM estimates found up to 10% higher than COARE ones, indicating that the wave-induced momentum flux may be insufficiently parameterized in COARE. Turbulent fluxes measured at about 150 m, and reduced to their surface values accounting for the vertical flux divergence, are consistently lower than the estimates. Under unstable atmospheric stratification and low to moderate wind conditions, the residuals between estimates and measurements are less than 40%. On the other hand, under stable stratification and strong winds, the majority of the residuals are more than 40%. This discrepancy is associated with the relatively high measurement level, shallow boundary layer, and the presence of a low level jet.