Journal articles on the topic 'Covariance turbulente'
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1
Foken, T., F. Wimmer, M. Mauder, C. Thomas, and C. Liebethal. "Some aspects of the energy balance closure problem." Atmospheric Chemistry and Physics Discussions 6, no. 2 (April 27, 2006): 3381–402. http://dx.doi.org/10.5194/acpd-6-3381-2006.
Abstract:
Abstract. After briefly discussing several reasons for the energy balance closure problem in the surface layer, the paper focuses on the influence of the low frequency part of the turbulence spectrum on the residual. Changes in the turbulent fluxes in this part of the turbulence spectrum were found to have a significant influence on the changes of the residual. Using the ogive method, it was found that the eddy-covariance method underestimates turbulent fluxes in the case of ogives converging for measuring times longer than the typical averaging interval of 30 min. Additionally, the eddy-covariance method underestimates turbulent fluxes for maximal ogive functions within the averaging interval, both mainly due to advection and non-steady state conditions. This has a considerable influence on the use of the eddy-covariance method.
2
Foken, T., F. Wimmer, M. Mauder, C. Thomas, and C. Liebethal. "Some aspects of the energy balance closure problem." Atmospheric Chemistry and Physics 6, no. 12 (September 28, 2006): 4395–402. http://dx.doi.org/10.5194/acp-6-4395-2006.
Abstract:
Abstract. After briefly discussing several reasons for the energy balance closure problem in the surface layer, the paper focuses on the influence of the low frequency part of the turbulence spectrum on the residual. Changes in the turbulent fluxes in this part of the turbulence spectrum were found to have a significant influence on the changes of the residual. Using the ogive method, it was found that the eddy-covariance method underestimates turbulent fluxes in the case of ogives converging for measuring times longer than the typical averaging interval of 30 min. Additionally, the eddy-covariance method underestimates turbulent fluxes for maximal ogive functions within the averaging interval, both mainly due to advection and non-steady state conditions. This has a considerable influence on the use of the eddy-covariance method.
3
Krenk, Steen, and Randi N. Møller. "Turbulent wind field representation and conditional mean-field simulation." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 475, no. 2223 (March 2019): 20180887. http://dx.doi.org/10.1098/rspa.2018.0887.
Abstract:
The covariance structure of a homogeneous isotropic turbulent wind velocity field is derived in terms of modified Bessel functions for an extended form of the Kàrmàn velocity spectrum, including explicit expressions for the transverse coherence functions. A concept of transformed isotropic turbulence is introduced to account for differences in the axial, transverse and vertical fluctuating wind velocities and length scales in natural wind. A special form of the auto-regressive simulation format is developed for convected turbulence with exponentially increasing intervals to the regression planes. In each step, the wind velocity field in a transverse plane is represented by a conditional mean field and a stochastic contribution determined explicitly by the time–space covariances. Simulation results are presented for a square area of dimension less than the integral length scale, representative of buildings and wind turbines, and a horizontal line of length six times the length scale, representative of a long-span bridge. The simulations demonstrate high accuracy of simulated spectral densities, covariance functions and transverse coherence functions. The simulated results do not show visible dependence on the specific points used for the simulated records. The efficiency and the free simulation point configuration suggest high competitiveness compared to fast Fourier transform-based spectral methods.
4
Lenschow, Donald H., David Gurarie, and Edward G. Patton. "Modeling the diurnal cycle of conserved and reactive species in the convective boundary layer using SOMCRUS." Geoscientific Model Development 9, no. 3 (March 7, 2016): 979–96. http://dx.doi.org/10.5194/gmd-9-979-2016.
Abstract:
Abstract. We have developed a one-dimensional second-order closure numerical model to study the vertical turbulent transport of trace reactive species in the convective (daytime) planetary boundary layer (CBL), which we call the Second-Order Model for Conserved and Reactive Unsteady Scalars (SOMCRUS). The temporal variation of the CBL depth is calculated using a simple mixed-layer model with a constant entrainment coefficient and zero-order discontinuity at the CBL top. We then calculate time-varying continuous profiles of mean concentrations and vertical turbulent fluxes, variances, and covariances of both conserved and chemically reactive scalars in a diurnally varying CBL. The set of reactive species is the O3–NO–NO2 triad. The results for both conserved and reactive species are compared with large-eddy simulations (LES) for the same free-convection case using the same boundary and initial conditions. For the conserved species, we compare three cases with different combinations of surface fluxes, and CBL and free-troposphere concentrations. We find good agreement of SOMCRUS with LES for the mean concentrations and fluxes of both conserved and reactive species except near the CBL top, where SOMCRUS predicts a somewhat shallower depth, and has sharp transitions in both the mean and turbulence variables, in contrast to more smeared-out variations in the LES due to horizontal averaging. Furthermore, SOMCRUS generally underestimates the variances and species–species covariances. SOMCRUS predicts temperature–species covariances similar to LES near the surface, but much smaller magnitude peak values near the CBL top, and a change in sign of the covariances very near the CBL top, while the LES predicts a change in sign of the covariances in the lower half of the CBL. SOMCRUS is also able to estimate the intensity of segregation (the ratio of the species–species covariance to the product of their means), which can alter the rates of second-order chemical reactions; however, for the case considered here, this effect is small. The simplicity and extensibility of SOMCRUS means that it can be utilized for a broad range of turbulence-mixing scenarios and sets of chemical reactions in the planetary boundary layer; it therefore holds great promise as a tool to incorporate these processes within air quality and climate models.
5
Lenschow, D. H., D. Gurarie, and E. G. Patton. "Modeling the diurnal cycle of conserved and reactive species in the convective boundary layer." Geoscientific Model Development Discussions 8, no. 10 (October 29, 2015): 9323–72. http://dx.doi.org/10.5194/gmdd-8-9323-2015.
Abstract:
Abstract. We have developed a one-dimensional second-order closure numerical model to study the vertical turbulent transport of trace reactive species in the convective (daytime) planetary boundary layer (CBL), which we call the Second-Order Model for Conserved and Reactive Unsteady Scalars (SOMCRUS). The temporal variation of the CBL depth is calculated using a simple mixed-layer model with a constant entrainment coefficient and zero-order discontinuity at the CBL top. We then calculate time-varying continuous profiles of mean concentrations and vertical turbulent fluxes, variances, and covariances of both conserved and chemically-reactive scalars in a diurnally-varying CBL. The set of reactive species is the O3–NO–NO2 triad. The results for both conserved and reactive species are compared with large-eddy simulations (LES) for the same free-convection case using the same boundary and initial conditions. For the conserved species, we compare three cases with different combinations of surface fluxes, and CBL and free-troposphere concentrations. We find good agreement of SOMCRUS with LES for the mean concentrations and fluxes of both conserved and reactive species except near the CBL top, where SOMCRUS predicts a somewhat shallower depth, and has sharp transitions in both the mean and turbulence variables, in contrast to more smeared out variations in the LES due to horizontal averaging. Furthermore, SOMCRUS generally underestimates the variances and species-species covariances. SOMCRUS predicts temperature-species covariances similar to LES near the surface, but much smaller magnitude peak values near the CBL top, and a change in sign of the covariances very near the CBL top, while the LES predicts a change in sign of the covariances in the lower half of the CBL. SOMCRUS is also able to estimate the intensity of segregation (the ratio of the species-species covariance to the product of their means), which can alter the rates of second-order chemical reactions; however, for the case considered here, this effect is small. The simplicity and extensibility of SOMCRUS means that it can be utilized for a broad range of turbulence mixing scenarios and sets of chemical reactions in the planetary boundary layer; it therefore holds great promise as a tool to incorporate these processes within air quality and climate models.
6
Guerra, Maricarmen, and Jim Thomson. "Turbulence Measurements from Five-Beam Acoustic Doppler Current Profilers." Journal of Atmospheric and Oceanic Technology 34, no. 6 (June 2017): 1267–84. http://dx.doi.org/10.1175/jtech-d-16-0148.1.
Abstract:
AbstractTwo new five-beam acoustic Doppler current profilers—the Nortek Signature1000 AD2CP and the Teledyne RDI Sentinel V50—are demonstrated to measure turbulence at two energetic tidal channels within Puget Sound, Washington. The quality of the raw data is tested by analyzing the turbulent kinetic energy frequency spectra, the turbulence spatial structure function, the shear in the profiles, and the covariance Reynolds stresses. The five-beam configuration allows for a direct estimation of the Reynolds stresses from along-beam velocity fluctuations. The Nortek’s low Doppler noise and high sampling frequency allow for the observation of the turbulent inertial subrange in both the frequency spectra and the turbulence structure function. The turbulence parameters obtained from the five-beam acoustic Doppler current profilers are validated with turbulence data from simultaneous measurements with acoustic Doppler velocimeters. These combined results are then used to assess a turbulent kinetic energy budget in which depth profiles of the turbulent kinetic energy dissipation and production rates are compared. The associated codes are publicly available on the MATLAB File Exchange website.
7
Santos, Diego Jatobá, Celso Von Randow, and Pablo E. S. Oliveira. "Variabilidade temporal dos fluxos noturnos determinados a partir de duas diferentes metodologias no nível de 325 m acima da floresta Amazônica." Ciência e Natura 42 (August 28, 2020): e14. http://dx.doi.org/10.5902/2179460x45356.
Abstract:
The present study aimed to analyze and compare the temporal variability of the nocturnal fluxes of CO2, sensitive and latent heat, calculated from two different methodologies: one with a 5-minute temporal window (using the eddy covariance technique), and another with 109 minutes (from multiresolution decomposition). For this, night series of 25 nights were used between October and November 2015. The analyzes were made for two groups of distinct turbulence patterns: one with intermittent regime and the other with homogeneous turbulence. The results showed that the fluxes obtained by the classical method of eddy covariance were dependent on the intensity of the turbulence. On the other hand, the fluxes calculated from the multiresolution decomposition technique showed significant fluctuations in the temporal evolution of all scalars analyzed, with the largest percentage differences between the two approaches occurring in the homogeneous turbulence regime group, which was characterized by predominantly weak turbulent activity throughout the night. In the comparison made, the methodology employed in the 109-minute window showed greater efficiency in the estimates of exchanges at 325 m in the ATTO tower, especially during conditions of low turbulent activity.
8
Chen, J., Y. Hu, Y. Yu, and S. Lü. "Ergodicity test of the eddy-covariance technique." Atmospheric Chemistry and Physics 15, no. 17 (September 4, 2015): 9929–44. http://dx.doi.org/10.5194/acp-15-9929-2015.
Abstract:
Abstract. The ergodic hypothesis is a basic hypothesis typically invoked in atmospheric surface layer (ASL) experiments. The ergodic theorem of stationary random processes is introduced to analyse and verify the ergodicity of atmospheric turbulence measured using the eddy-covariance technique with two sets of field observational data. The results show that the ergodicity of atmospheric turbulence in atmospheric boundary layer (ABL) is relative not only to the atmospheric stratification but also to the eddy scale of atmospheric turbulence. The eddies of atmospheric turbulence, of which the scale is smaller than the scale of the ABL (i.e. the spatial scale is less than 1000 m and temporal scale is shorter than 10 min), effectively satisfy the ergodic theorems. Under these restrictions, a finite time average can be used as a substitute for the ensemble average of atmospheric turbulence, whereas eddies that are larger than ABL scale dissatisfy the mean ergodic theorem. Consequently, when a finite time average is used to substitute for the ensemble average, the eddy-covariance technique incurs large errors due to the loss of low-frequency information associated with larger eddies. A multi-station observation is compared with a single-station observation, and then the scope that satisfies the ergodic theorem is extended from scales smaller than the ABL, approximately 1000 m to scales greater than about 2000 m. Therefore, substituting the finite time average for the ensemble average of atmospheric turbulence is more faithfully approximate the actual values. Regardless of vertical velocity or temperature, the variance of eddies at different scales follows Monin–Obukhov similarity theory (MOST) better if the ergodic theorem can be satisfied; if not it deviates from MOST. The exploration of ergodicity in atmospheric turbulence is doubtlessly helpful in understanding the issues in atmospheric turbulent observations and provides a theoretical basis for overcoming related difficulties.
9
RISTORCELLI, J. R. "A pseudo-sound constitutive relationship for the dilatational covariances in compressible turbulence." Journal of Fluid Mechanics 347 (September 25, 1997): 37–70. http://dx.doi.org/10.1017/s0022112097006083.
Abstract:
The mathematical consequences of a few simple scaling assumptions regarding the effects of compressibility are explored using a singular perturbation idea and the methods of statistical fluid mechanics. Representations for the pressure–dilatation and dilatational dissipation appearing in single-point moment closures for compressible turbulence are obtained. The results obtained, in as much as they come from the same underlying procedure, represent a unified development for both dilatational covariances. While the results are expressed in the context of a statistical turbulence closure they provide, with very few phenomenological assumptions, an interesting and clear mathematical model for the ‘scalar’ effects of compressibility. For homogeneous turbulence with quasi-normal large scales the expressions derived are – in the small turbulent Mach number squared isotropic limit – exact. The expressions obtained contain constants that have a precise physical significance and are defined in terms of integrals of the longitudinal velocity correlation. The pressure–dilatation covariance is found to be a non-equilibrium phenomenon related to the time rate of change of the kinetic energy and internal energy of the turbulence; it is seen to scale with α2M2t εs [Pk/ε−1] (Sk/εs)2. Implicit in the scaling is a dependence on the square of a gradient Mach number, S[lscr ]/c. A new feature indicated by the analysis is the appearance of the Kolmogorov scaling coefficient, α, suggesting that large-scale quantities embodied in the well-established ε∼u˜3/[lscr ] relationship provide a link to the structural dependence of the effects of compressibility. The expressions for the dilatational dissipation are found to depend on the turbulent Reynolds number and scale as M4t (Sk/εs)4R−1t. The scalings for the pressure–dilatation are found to produce an excellent collapse of the pressure–dilatation data from direct numerical simulation.
10
Liu, Lei, Yu Shi, and Fei Hu. "Characteristics of intrinsic non-stationarity and its effect on eddy-covariance measurements of CO<sub>2</sub> fluxes." Nonlinear Processes in Geophysics 29, no. 1 (March 24, 2022): 123–31. http://dx.doi.org/10.5194/npg-29-123-2022.
Abstract:
Abstract. Stationarity is a critical assumption in the eddy-covariance method that is widely used to calculate turbulent fluxes. Many methods have been proposed to diagnose non-stationarity attributed to external non-turbulent flows. In this paper, we focus on intrinsic non-stationarity (IN) attributed to turbulence randomness. The detrended fluctuation analysis is used to quantify IN of CO2 turbulent fluxes in the downtown of Beijing. Results show that the IN is common in CO2 turbulent fluxes and is a small-scale phenomenon related to the inertial sub-range turbulence. The small-scale IN of CO2 turbulent fluxes can be simulated by the Ornstein–Uhlenbeck (OU) process as a first approximation. Based on the simulation results, we find that the flux-averaging time should be greater than 27 s to avoid the effects of IN. Besides, the non-stationarity diagnosis methods that do not take into account IN would possibly make a wrong diagnosis with some parameters.
11
HŒPFFNER, JÉRÔME, YOSHITSUGU NAKA, and KOJI FUKAGATA. "Realizing turbulent statistics." Journal of Fluid Mechanics 676 (April 18, 2011): 54–80. http://dx.doi.org/10.1017/jfm.2011.32.
Abstract:
How to design an artificial inflow condition in simulations of the Navier–Stokes equation that is already fully turbulent? This is the turbulent inflow problem. This first question is followed by: How much of the true turbulence must be reproduced at the inflow? We present a technique able to produce a random field with the exact two-point two-time covariance of a given reference turbulent flow. It is obtained as the output of a linear filter fed with white noise. The method is illustrated on the simulation of a turbulent free shear layer. The filter coefficients are obtained from the solution of the Yule–Walker equation, and the computation can be performed efficiently using a recursive solution procedure. The method should also be useful in the study of flow receptivity, when the processes of transition to turbulence are sensitive to the perturbation environment.
12
Tresso, Riccardo, and David R. Munoz. "Homogeneous, Isotropic Flow in Grid Generated Turbulence." Journal of Fluids Engineering 122, no. 1 (November 30, 1999): 51–56. http://dx.doi.org/10.1115/1.483226.
Abstract:
Detailed grid generated turbulent analysis has been completed using a three-dimensional hot-wire anemometer and traversing mechanism to identify a homogeneous, isotropic flow region downstream of a square mesh. The three-dimensional fluctuating velocity measurements were recorded along the centerline of a wind tunnel test section and spatially over the entire wind tunnel cross section downstream of the square mesh. Turbulent intensities for various grid sizes and Reynolds numbers ranged from a minimum of 0.2 percent to a maximum of 2.2 percent in each of the three principal velocity directions. Spatial homogeneity and isotropy were determined for several turbulent flow conditions and downstream positions using the method of covariances. Covariances, in theory, should approach zero asymptotically; however, in practice, this was not achievable. A subjective judgment is required to determine downstream location where the variance of the three covariances reaches a value close to zero. The average standard deviation provides an estimate for defining the limit or subjective threshold needed to determine the onset of homogeneous, isotropic flow. Implementing this threshold, a quantitative method was developed for predicting the streamwise location for the onset of the homogeneous, isotropic flow region downstream of a 25.4 mm square grid as a function of Reynolds number. A comparison of skewness, determined from one-dimensional hot wire anemometer measurements, and covariances, determined from three dimensional hot wire anemometer measurements, indicates a need for caution when relying solely on one-dimensional measurements for determination of turbulence isotropy. The comprehensive three-dimensional characterization also provides an improved understanding of spatial distribution of fundamental turbulence quantities generated by the grid within a low-speed wind tunnel. [S0098-2202(00)02501-3]
13
CONNELL, R. J., D. KULASIRI, J. LENNON, and D. F. HILL. "COMPUTATIONAL MODELING OF TURBULENT VELOCITY STRUCTURES FOR AN OPEN CHANNEL FLOW USING KARHUNEN–LOÉVE EXPANSION." International Journal of Computational Methods 04, no. 03 (September 2007): 493–519. http://dx.doi.org/10.1142/s0219876207001242.
Abstract:
This paper develops an analytical two-part covariance kernel from velocity correlations across a two-dimensional (vertical and flow directions) Particle Image Velocimetry (PIV) flow field. This will form the basis of a model of wave action in flood water that includes the underlying turbulence velocity field using Karhunen–Loéve (KL) expansion. The PIV data was from the supercritical flow area immediately upstream of an undular hydraulic jump. This paper derives a one-dimensional solution for the associated integral equations. It also discusses a relationship between the two parts of the kernel and the turbulent production and turbulent dissipation.
14
Hartmann, Jörg, Martin Gehrmann, Katrin Kohnert, Stefan Metzger, and Torsten Sachs. "New calibration procedures for airborne turbulence measurements and accuracy of the methane fluxes during the AirMeth campaigns." Atmospheric Measurement Techniques 11, no. 7 (July 31, 2018): 4567–81. http://dx.doi.org/10.5194/amt-11-4567-2018.
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Abstract. Low-level flights over tundra wetlands in Alaska and Canada have been conducted during the Airborne Measurements of Methane Emissions (AirMeth) campaigns to measure turbulent methane fluxes in the atmosphere. In this paper we describe the instrumentation and new calibration procedures for the essential pressure parameters required for turbulence sensing by aircraft that exploit suitable regular measurement flight legs without the need for dedicated calibration patterns. We estimate the accuracy of the mean wind and the turbulence measurements. We show that airborne measurements of turbulent fluxes of methane and carbon dioxide using cavity ring-down spectroscopy trace gas analysers together with established turbulence equipment achieve a relative accuracy similar to that of measurements of sensible heat flux if applied during low-level flights over natural area sources. The inertial subrange of the trace gas fluctuations cannot be resolved due to insufficient high-frequency precision of the analyser, but, since this scatter is uncorrelated with the vertical wind velocity, the covariance and thus the flux are reproduced correctly. In the covariance spectra the -7/3 drop-off in the inertial subrange can be reproduced if sufficient data are available for averaging. For convective conditions and flight legs of several tens of kilometres we estimate the flux detection limit to be about 4 mg m−2 d−1 for w′CH4′‾, 1.4 g m−2 d−1 for w′CO2′‾ and 4.2 W m−2 for the sensible heat flux.
15
Ren, Yan, Hongsheng Zhang, Wei Wei, Bingui Wu, Xuhui Cai, and Yu Song. "Effects of turbulence structure and urbanization on the heavy haze pollution process." Atmospheric Chemistry and Physics 19, no. 2 (January 28, 2019): 1041–57. http://dx.doi.org/10.5194/acp-19-1041-2019.
Abstract:
Abstract. In this paper, an automated algorithm is developed, which is used to identify the spectral gap during the heavy haze pollution process, reconstruct acquired data, and obtain pure turbulence data. Comparisons of the reconstructed turbulent flux and eddy covariance (EC) flux show that there are overestimations regarding the exchange between the surface and the atmosphere during heavy haze pollution episodes. After reconstruction via the automated algorithm, pure turbulence data can be obtained. We introduce a definition to characterize the local intermittent strength of turbulence (LIST). The trend in the LIST during pollution episodes shows that when pollution is more intense, the LIST is smaller, and intermittency is stronger; when pollution is weaker, the LIST is larger, and intermittency is weaker. At the same time, the LIST at the city site is greater than at the suburban site, which means that intermittency over the complex city area is weaker than over the flat terrain area. Urbanization seems to reduce intermittency during heavy haze pollution episodes, which means that urbanization reduces the degree of weakening in turbulent exchange during pollution episodes. This result is confirmed by comparing the average diurnal variations in turbulent fluxes at urban and suburban sites during polluted and clean periods. The sensible heat flux, latent heat flux, momentum flux, and turbulent kinetic energy (TKE) in urban and suburban areas are all affected when pollution occurs. Material and energy exchanges between the surface and the atmosphere are inhibited. Moreover, the impact of the pollution process on suburban areas is much greater than on urban areas. The turbulent effects caused by urbanization seem to help reduce the consequences of pollution under the same weather and pollution source condition, because the turbulence intermittency is weaker, and the reduction in turbulence exchange is smaller over the urban underlying surface.
16
Xiao, X., H. C. Zuo, Q. D. Yang, S. J. Wang, L. J. Wang, J. W. Chen, B. L. Chen, and B. D. Zhang. "On the factors influencing surface-layer energy balance closure and their seasonal variability over semi-arid loess plateau of Northwest China." Hydrology and Earth System Sciences Discussions 8, no. 1 (January 19, 2011): 555–84. http://dx.doi.org/10.5194/hessd-8-555-2011.
Abstract:
Abstract. In the context of September 2006–August 2010 eddy covariance data of the Semi-Arid Climate Change and Environment Observatory, Lanzhou University (SACOL) as a platform of the Key Laboratory of Semi-Arid Climate Change, Ministry of Education, an intensive study is performed of the SACOL data quality and energy balance closure (EBC) characteristics on a seasonal basis, the EBC impacts of the flux contribution from the target source zone, the low-frequency part of the turbulence spectra, turbulent mixing intensity and diverse schemes for surface soil heat fluxes. Evidence suggests that (1) through the steady state test (SST) and integral turbulence characteristics (ITC) tests as well as analysis of flux contribution from target area to the EBC, it is found that most of the eddy covariance data are within a domain of effective quality. The valid data account for 77.5, 75.4, 68.3 and 61.6% of seasonal total for spring, summer, autumn and winter, respectively; (2) the EBC shows its appreciable seasonal variability, with the energy residual making up 19.0, 14.8, 11.6 and 7.7% of net radiation in winter, summer, autumn and spring, respectively; (3) (i) Flux contribution from the target zone has greater EBC impact and as the flux contribution in percentage increases, EBC is correspondingly improved. Even the percentage reaches 100%, the energy balance fails to be closed entirely. (ii) The Ogive function analysis shows that the EBC suffers the effect of relatively small (maximum) low-frequency turbulent flux in spring and summer (winter). (iii) Turbulence intensity exerts noticeable impact on the EBC; when turbulent mixing arrives at certain intensity, the EBC is in an optimal state and stabilized. (iv) Different schemes of surface soil thermal flux have significant effect on the EBC.
17
Lilley, Geoffrey M. "The Acoustic Spectrum in the Sound Field of Isotropic Turbulence." International Journal of Aeroacoustics 4, no. 1-2 (January 2005): 11–19. http://dx.doi.org/10.1260/1475472053730011.
Abstract:
Lighthill's theory of Aerodynamic Noise uses an acoustic analogy by which the equivalent acoustic sources in a turbulent flow have first to be calculated, measured or modeled from a time-accurate evaluation of the unsteady flow. Proudman considered the application to the noise radiated from weakly compressible isotropic turbulence, by evaluating the fourth-order space-time covariance of Lighthill's stress tensor. The resulting acoustic power per unit volume of turbulence was found to follow the eighth power law of velocity and showed the efficiency of conversion of turbulent kinetic energy into sound was extremely small, being proportional to the fifth power of the turbulent Mach number. These results are of particular interest today, since, all turbulent shear flows were shown by Kolmogorov to become locally isotropic near the dissipation range. Hence the high frequency asymptotic far-field radiated noise spectrum generated from eddies in the inertial subrange of all turbulent shear flows should obey a universal frequency law as obtained from isotropic turbulence, namely f−7/ 2, a result first found by Meecham and Ford. From studies of the fluctuations in this nonlinear energy transfer cascade from large to small eddies it is possible to gain a better insight as to the physical process, and the reason for its weak efficiency, whereby sound is generated and radiated as a broadband spectrum. A key factor is shown to be the weak fluctuations in the rate of dilatation in the compressible turbulent flow, which explains the reason for the efficiency of sound conversion obtained by Lighthill.
18
Gibert, Fabien, Grady J. Koch, Jeffrey Y. Beyon, Timothy W. Hilton, Kenneth J. Davis, Arlyn Andrews, Pierre H. Flamant, and Upendra N. Singh. "Can CO2 Turbulent Flux Be Measured by Lidar? A Preliminary Study." Journal of Atmospheric and Oceanic Technology 28, no. 3 (March 1, 2011): 365–77. http://dx.doi.org/10.1175/2010jtecha1446.1.
Abstract:
Abstract The vertical profiling of CO2 turbulent fluxes in the atmospheric boundary layer (ABL) is investigated using a coherent differential absorption lidar (CDIAL) operated nearby a tall tower in Wisconsin during June 2007. A CDIAL can perform simultaneous range-resolved CO2 DIAL and velocity measurements. The lidar eddy covariance technique is presented. The aims of the study are (i) an assessment of performance and current limitation of available CDIAL for CO2 turbulent fluxes and (ii) the derivation of instrument specifications to build a future CDIAL to perform accurate range-resolved CO2 fluxes. Experimental lidar CO2 mixing ratio and vertical velocity profiles are successfully compared with in situ sensors measurements. Time and space integral scales of turbulence in the ABL are addressed that result in limitation for time averaging and range accumulation. A first attempt to infer CO2 fluxes using an eddy covariance technique with currently available 2-μm CDIAL dataset is reported.
19
Tian, Yifeng, Farhad A. Jaberi, Zhaorui Li, and Daniel Livescu. "Numerical study of variable density turbulence interaction with a normal shock wave." Journal of Fluid Mechanics 829 (September 22, 2017): 551–88. http://dx.doi.org/10.1017/jfm.2017.542.
Abstract:
Accurate numerical simulations of shock–turbulence interaction (STI) are conducted with a hybrid monotonicity-preserving–compact-finite-difference scheme for a detailed study of STI in variable density flows. Theoretical and numerical assessments of data confirm that all turbulence scales as well as the STI are well captured by the computational method. Linear interaction approximation (LIA) convergence tests conducted with the shock-capturing simulations exhibit a similar trend of converging to LIA predictions to shock-resolving direct numerical simulations (DNS). The effects of density variations on STI are studied by comparing the results corresponding to an upstream multi-fluid mixture with the single-fluid case. The results show that for the current parameter ranges, the turbulence amplification by the normal shock wave is much higher and the reduction in turbulence length scales is more significant when strong density variations exist. Turbulent mixing enhancement by the shock is also increased and stronger mixing asymmetry in the postshock region is observed when there is significant density variation. The turbulence structure is strongly modified by the shock wave, with a differential distribution of turbulent statistics in regions having different densities. The dominant mechanisms behind the variable density STI are identified by analysing the transport equations for the Reynolds stresses, vorticity, normalized mass flux and density specific volume covariance.
20
Rannik, Ü., P. Keronen, I. Mammarella, and T. Vesala. "Vertical advection and nocturnal deposition of ozone over a boreal pine forest." Atmospheric Chemistry and Physics Discussions 8, no. 5 (October 23, 2008): 18437–55. http://dx.doi.org/10.5194/acpd-8-18437-2008.
Abstract:
Abstract. Night-time ozone deposition for a Scots pine forest in Southern Finland was studied at the SMEAR II measurement station by evaluating the turbulent eddy covariance (EC), storage change and vertical advection fluxes. Similarly to night-time carbon dioxide flux, the eddy-covariance flux of ozone was decreasing with turbulence intensity (friction velocity), and storage change of the compound did not compensate the reduction (well-known night-time measurement problem). Accounting for vertical advection resulted in invariance of ozone deposition rate on turbulence intensity. This was also demonstrated for carbon dioxide, verified by independent measurements of NEE by chamber systems. The result highlights the importance of advection when considering the exchange measurements of any scalar. Analysis of aerodynamic and laminar boundary layer resistances by the model approach indicated that the surface resistance and/or chemical sink strength was limiting ozone deposition. The possible aerial ozone sink by known fast chemical reactions with sesquiterpenes and NO explain only a minor fraction of ozone sink. Thus the deposition is controlled either by stomatal uptake or surface reactions or both of them, the mechanisms not affected by turbulence intensity. Therefore invariance of deposition flux on turbulence intensity is expected also from resistance and chemical sink analysis.
21
Rannik, Ü., I. Mammarella, P. Keronen, and T. Vesala. "Vertical advection and nocturnal deposition of ozone over a boreal pine forest." Atmospheric Chemistry and Physics 9, no. 6 (March 23, 2009): 2089–95. http://dx.doi.org/10.5194/acp-9-2089-2009.
Abstract:
Abstract. Night-time ozone deposition for a Scots pine forest in Southern Finland was studied at the SMEAR II measurement station by evaluating the turbulent eddy covariance (EC), storage change and vertical advection fluxes. Similarly to night-time carbon dioxide flux, the eddy-covariance flux of ozone was decreasing with turbulence intensity (friction velocity), and storage change of the compound did not compensate the reduction (well-known night-time measurement problem). Accounting for vertical advection resulted in invariance of ozone deposition rate on turbulence intensity. This was also demonstrated for carbon dioxide, verified by independent measurements of NEE by chamber systems. The result highlights the importance of advection when considering the exchange measurements of any scalar. Analysis of aerodynamic and laminar boundary layer resistances by the model approach indicated that the surface resistance and/or chemical sink strength was limiting ozone deposition. The possible aerial ozone sink by known fast chemical reactions with sesquiterpenes and NO explain only a minor fraction of ozone sink. Thus the deposition is controlled either by stomatal uptake or surface reactions or both of them, the mechanisms not affected by turbulence intensity. Therefore invariance of deposition flux on turbulence intensity is expected also from resistance and chemical sink analysis.
22
Durden, D. J., C. J. Nappo, M. Y. Leclerc, H. F. Duarte, G. Zhang, M. J. Parker, and R. J. Kurzeja. "On the impact of wave-like disturbances on turbulent fluxes and turbulence statistics in nighttime conditions: a case study." Biogeosciences 10, no. 12 (December 23, 2013): 8433–43. http://dx.doi.org/10.5194/bg-10-8433-2013.
Abstract:
Abstract. The interpretation of flux measurements in nocturnal conditions is typically fraught with challenges. This paper reports on how the presence of wave-like disturbances in a time series, can lead to an overestimation of turbulence statistics, errors when calculating the stability parameter, erroneous estimation of the friction velocity u* used to screen flux data, and errors in turbulent flux calculations. Using time series of the pressure signal from a microbarograph, wave-like disturbances at an AmeriFlux site are identified. The wave-like disturbances are removed during the calculation of turbulence statistics and turbulent fluxes. Our findings suggest that filtering eddy-covariance data in the presence of wave-like events prevents both an~overestimation of turbulence statistics and errors in turbulent flux calculations. Results show that large-amplitude wave-like events, events surpassing three standard deviations, occurred on 18% of the nights considered in the present study. Remarkably, on flux towers located in a very stably stratified boundary-layer regime, the presence of a gravity wave can enhance turbulence statistics more than 50%. In addition, the presence of the disturbance modulates the calculated turbulent fluxes of CO2 resulting in erroneous turbulent flux calculations of the order of 10% depending on averaging time and pressure perturbation threshold criteria. Furthermore, the friction velocity u* was affected by the presence of the wave, and in at least one case, a 10% increase caused u* to exceed the arbitrary 0.25 m s−1 threshold used in many studies. This results in an unintended bias in the data selected for analysis in the flux calculations. The impact of different averaging periods was also examined and found to be variable specific. These early case study results provide an insight into errors introduced when calculating "purely" turbulent fluxes. These results could contribute to improving modeling efforts by providing more accurate inputs of both turbulent kinetic energy, and isolating the turbulent component of u* for flux selection in the stable nocturnal boundary layer.
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Dixit, T. "Vorticity covariance in turbulence." Astrophysics and Space Science 158, no. 1 (1989): 141–44. http://dx.doi.org/10.1007/bf00637449.
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Schaller, Carsten, Mathias Göckede, and Thomas Foken. "Flux calculation of short turbulent events – comparison of three methods." Atmospheric Measurement Techniques 10, no. 3 (March 9, 2017): 869–80. http://dx.doi.org/10.5194/amt-10-869-2017.
Abstract:
Abstract. The eddy covariance method is commonly used to calculate vertical turbulent exchange fluxes between ecosystems and the atmosphere. Besides other assumptions, it requires steady-state flow conditions. If this requirement is not fulfilled over the averaging interval of, for example, 30 min, the fluxes might be miscalculated. Here two further calculation methods, conditional sampling and wavelet analysis, which do not need the steady-state assumption, were implemented and compared to eddy covariance. All fluxes were calculated for 30 min averaging periods, while the wavelet method – using both the Mexican hat and the Morlet wavelet – additionally allowed us to obtain a 1 min averaged flux. The results of all three methods were compared against each other for times with best steady-state conditions and well-developed turbulence. An excellent agreement of the wavelet results to the eddy covariance reference was found, where the deviations to eddy covariance were of the order of < 2 % for Morlet as well as < 7 % for Mexican hat and thus within the typical error range of eddy covariance measurements. The conditional sampling flux also showed a very good agreement to the eddy covariance reference, but the occurrence of outliers and the necessary condition of a zero mean vertical wind velocity reduced its general reliability. Using the Mexican hat wavelet flux in a case study, it was possible to locate a nightly short time turbulent event exactly in time, while the Morlet wavelet gave a trustworthy flux over a longer period, e.g. 30 min, under consideration of this short-time event. At a glance, the Mexican hat wavelet flux offers the possibility of a detailed analysis of non-stationary times, where the classical eddy covariance method fails. Additionally, the Morlet wavelet should be used to provide a trustworthy flux in those 30 min periods where the eddy covariance method provides low-quality data due to instationarities.
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Prabha, Thara V., Monique Y. Leclerc, Anandakumar Karipot, and David Y. Hollinger. "Low-Frequency Effects on Eddy Covariance Fluxes under the Influence of a Low-Level Jet." Journal of Applied Meteorology and Climatology 46, no. 3 (March 1, 2007): 338–52. http://dx.doi.org/10.1175/jam2461.1.
Abstract:
Abstract Turbulent bursts observed over a tall forest canopy during the initiation of a nocturnal low-level jet (LLJ) are studied with the help of wavelet analysis. The burst of turbulence is observed in response to a shear instability associated with the initiation of LLJ. Turbulent kinetic energy, momentum, and CO2-rich cold air are transferred downward by large eddies with length scales that are higher than the LLJ height. Microfronts are observed over the canopy as a secondary instability that enhances the mixing processes within and above the canopy. The scale-dependent wavelet correlation analysis reveals that countergradient fluxes result from low frequencies, whereas cogradient flux is associated with high-frequency turbulent motions. The countergradient flux is initially noted at low frequencies, and, through coherent motions, it is transferred to smaller scales with a nearly 20-min delay. The countergradient flux dominates at the initiation of the event and reduces net flux, whereas enhanced cogradient flux at the decay of the event increases the net flux. The wavelet correlation coefficient corresponding to cogradient and countergradient fluxes is applied to segregate three regions of the spectra corresponding to “turbulent,” “coherent,” and “noncoherent” large scales. These findings are used to examine the implications on eddy covariance flux measurements.
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Durden, D. J., C. J. Nappo, M. Y. Leclerc, H. F. Duarte, G. Zhang, L. B. M. Pires, M. J. Parker, and R. J. Kurzeja. "On the impact of atmospheric waves on fluxes and turbulence statistics during nighttime conditions: a case study." Biogeosciences Discussions 10, no. 3 (March 14, 2013): 5149–73. http://dx.doi.org/10.5194/bgd-10-5149-2013.
Abstract:
Abstract. The interpretation of flux measurements in the nocturnal stable boundary layer is typically fraught with difficulties. This paper reports on how the presence of waves in a time series leads to an overestimation of turbulence statistics and errors in turbulent flux calculations. Using time series of the pressure signal from a microbarograph, the presence of waves at a flux measurement site near Aiken, SC is identified and removed. Our findings suggest that filtering of eddy-covariance data in the presence of wave events prevents both an overestimation of turbulence statistics and errors in turbulent flux calculations. The results showed that large amplitude wave-like events occurred on 31% of the nights considered in the present study. Remarkably, in low-turbulence environments, the presence of a gravity wave can enhance turbulence statistics more than 50%. The presence of the wave modulates the calculated turbulent fluxes of CO2, resulting in erroneous flux calculations of the order of 10% depending on the averaging time and pressure perturbation threshold criteria. In addition, u∗ was affected by the presence of the wave, and in at least one case, a 10% increase caused u∗ to exceed the arbitrary 0.25 ms–1 threshold used in many studies. These preliminary results suggest that biases due to nocturnal atmospheric phenomena can easily creep unnoticed into flux data. The impact of different averaging periods was found to depend on the choice of the variables. This is a product of the width of the averaging window in relation to the wave cycle and dealt with the phase relationship of the variables being analyzed; hence, these errors are primarily introduced through our processing methods. These results provide a novel insight into errors introduced in turbulent fluxes. By contributing more accurate inputs of both turbulent kinetic energy and u∗, these results could be invaluable in improving modeling efforts applied to nocturnal exchange.
27
Revil-Baudard, T., J. Chauchat, D. Hurther, and O. Eiff. "Turbulence modifications induced by the bed mobility in intense sediment-laden flows." Journal of Fluid Mechanics 808 (November 2, 2016): 469–84. http://dx.doi.org/10.1017/jfm.2016.671.
Abstract:
An experimental dataset of high-resolution velocity and concentration measurements is obtained under intense sediment transport regimes to provide new insights into the modification of turbulence induced by the presence of a mobile sediment bed. The physical interpretation of the zero-plane level in the law of the wall is linked to the bed-level variability induced by large-scale turbulent flow structures. The comparison between intrinsic and superficial Reynolds shear stresses shows that the observed strong bed-level variability results in an increased covariance between wall-normal ($w^{\prime }$) and streamwise ($u^{\prime }$) velocity fluctuations. This appears as an additional Reynolds shear stress in the near-wall region. It is also observed that the mobile sediment bed induces an increase of turbulence kinetic energy (TKE) across the boundary layer. However, the increased contribution of interaction events ($u^{\prime }w^{\prime }>0$, i.e. quadrants I and III in the ($u^{\prime },w^{\prime }$) plane) induces a decrease of the turbulent momentum diffusion and an increase of the turbulent concentration diffusion in the suspension region. This result provides an explanation for the modification of the von Kármán parameter and the turbulent Schmidt number observed in the literature for intense sediment transport.
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Majda, Andrew J. "Statistical energy conservation principle for inhomogeneous turbulent dynamical systems." Proceedings of the National Academy of Sciences 112, no. 29 (July 6, 2015): 8937–41. http://dx.doi.org/10.1073/pnas.1510465112.
Abstract:
Understanding the complexity of anisotropic turbulent processes over a wide range of spatiotemporal scales in engineering shear turbulence as well as climate atmosphere ocean science is a grand challenge of contemporary science with important societal impact. In such inhomogeneous turbulent dynamical systems there is a large dimensional phase space with a large dimension of unstable directions where a large-scale ensemble mean and the turbulent fluctuations exchange energy and strongly influence each other. These complex features strongly impact practical prediction and uncertainty quantification. A systematic energy conservation principle is developed here in a Theorem that precisely accounts for the statistical energy exchange between the mean flow and the related turbulent fluctuations. This statistical energy is a sum of the energy in the mean and the trace of the covariance of the fluctuating turbulence. This result applies to general inhomogeneous turbulent dynamical systems including the above applications. The Theorem involves an assessment of statistical symmetries for the nonlinear interactions and a self-contained treatment is presented below. Corollary 1 and Corollary 2 illustrate the power of the method with general closed differential equalities for the statistical energy in time either exactly or with upper and lower bounds, provided that the negative symmetric dissipation matrix is diagonal in a suitable basis. Implications of the energy principle for low-order closure modeling and automatic estimates for the single point variance are discussed below.
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Jocher, Georg, Alexander Schulz, Christoph Ritter, Roland Neuber, Klaus Dethloff, and Thomas Foken. "The Sensible Heat Flux in the Course of the Year at Ny-Ålesund, Svalbard: Characteristics of Eddy Covariance Data and Corresponding Model Results." Advances in Meteorology 2015 (2015): 1–16. http://dx.doi.org/10.1155/2015/852108.
Abstract:
In this paper we present one year of meteorological and flux measurements obtained near Ny-Ålesund, Spitsbergen. Fluxes are derived by the eddy covariance method and by a hydrodynamic model approach (HMA) as well. Both methods are compared and analyzed with respect to season and mean wind direction. Concerning the wind field we find a clear distinction between 3 prevailing regimes (which have influence on the flux behavior) mainly caused by the topography at the measurement site. Concerning the fluxes we find a good agreement between the HMA and the eddy covariance method in cases of turbulent mixing in summer but deviations at stable conditions, when the HMA almost always shows negative fluxes. Part of the deviation is based on a dependence of HMA fluxes on friction velocity and the influence of the molecular boundary layer. Moreover, the flagging system of the eddy covariance software package TK3 is briefly revised. A new quality criterion for the use of fluxes obtained by the eddy covariance method, which is based on integral turbulence characteristics, is proposed.
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Raul, R., and P. S. Bernard. "A Numerical Investigation of the Turbulent Flow Field Generated by a Stationary Cube." Journal of Fluids Engineering 113, no. 2 (June 1, 1991): 216–22. http://dx.doi.org/10.1115/1.2909483.
Abstract:
The turbulent flow field generated by a stationary cube at Reynolds numbers 2000 and 14,000 is investigated numerically. A vorticity-vector potential formulation of the equations of motion is employed. Turbulence effects are accounted for through the use of a vorticity transport closure scheme in which dynamical equations for vorticity mean and covariance are supplemented by a kinematic equation for turbulent kinetic energy. Semi-implicit finite difference approximations to the equations of motion are solved iteratively by a vectorizable 8-color SOR algorithm. The numerical mesh is designed so that the turbulent flow field can be computed down to solid surfaces without the use of wall functions. The properties of the computed flow field, including drag, axial velocity, separation points, and three-dimensional flow structure show good agreement with experimental observations of similar bluff body flows.
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Rosman, Johanna H., and Gregory P. Gerbi. "Interpreting Fixed-Location Observations of Turbulence Advected by Waves: Insights from Spectral Models." Journal of Physical Oceanography 47, no. 4 (April 2017): 909–31. http://dx.doi.org/10.1175/jpo-d-15-0249.1.
Abstract:
AbstractAssigning a physical interpretation to turbulent fluctuations beneath waves is complex because eddies are advected by unsteady wave orbital motion. Here, the kinematic effects of wave orbital motion on turbulent fluctuations at a fixed location were investigated using model turbulence spatial spectra (κ spectra) together with a general form of the frozen turbulence approximation. Model autospectra and cospectra included an inertial subrange, a rolloff at energy-containing scales (L = 2π/κ0), and a dissipation range. Turbulence was advected by a background flow composed of waves (rms orbital velocity σw, peak frequency ωw, and spectral width Δωw) propagating parallel to a current uc. Expressions were derived for turbulence frequency spectra (ω spectra), and parameters were varied across ranges typical in the coastal ocean. Except close to the wave band, the ω-spectrum shape collapses with two dimensionless parameters, a velocity ratio σw/uc, and a time-scale ratio ucκ0/ωw, which can be used to diagnose the effects of wave advection on turbulence spectra. As σw/uc increases, less variance and covariance appear at low frequencies (ω < ucκ0) and more appear at high frequencies (ω > ucκ0). If σw/uc > 2, wave advection must be taken into account when estimating turbulence length scales and integral quantities (e.g., Reynolds stress) from the low-frequency portion of spectra. The offset of the −5/3 region due to waves is unaffected by the rolloff or dissipation range; therefore, previously proposed methods for estimating dissipation rate from wave-affected −5/3 spectra are robust. Although idealized, the results inform the interpretation of turbulence ω spectra beneath waves and guide the estimation of turbulence properties from those spectra.
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Zieliński, Mariusz, Krzysztof Fortuniak, Włodzimierz Pawlak, and Mariusz Siedlecki. "Turbulent sensible heat flux in Łódź, Central Poland, obtained from scintillometer and eddy covariance measurements." Meteorologische Zeitschrift 22, no. 5 (October 1, 2013): 603–13. http://dx.doi.org/10.1127/0941-2948/2013/0448.
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Fitzgerald, Joseph G., and Brian F. Farrell. "Statistical state dynamics of vertically sheared horizontal flows in two-dimensional stratified turbulence." Journal of Fluid Mechanics 854 (September 12, 2018): 544–90. http://dx.doi.org/10.1017/jfm.2018.560.
Abstract:
Simulations of strongly stratified turbulence often exhibit coherent large-scale structures called vertically sheared horizontal flows (VSHFs). VSHFs emerge in both two-dimensional (2D) and three-dimensional (3D) stratified turbulence with similar vertical structure. The mechanism responsible for VSHF formation is not fully understood. In this work, the formation and equilibration of VSHFs in a 2D Boussinesq model of stratified turbulence is studied using statistical state dynamics (SSD). In SSD, equations of motion are expressed directly in the statistical variables of the turbulent state. Restriction to 2D turbulence facilitates application of an analytically and computationally attractive implementation of SSD referred to as S3T, in which the SSD is expressed by coupling the equation for the horizontal mean structure with the equation for the ensemble mean perturbation covariance. This second-order SSD produces accurate statistics, through second order, when compared with fully nonlinear simulations. In particular, S3T captures the spontaneous emergence of the VSHF and associated density layers seen in simulations of turbulence maintained by homogeneous large-scale stochastic excitation. An advantage of the S3T system is that the VSHF formation mechanism, which is wave–mean flow interaction between the emergent VSHF and the stochastically excited large-scale gravity waves, is analytically understood in the S3T system. Comparison with fully nonlinear simulations verifies that S3T solutions accurately predict the scale selection, dependence on stochastic excitation strength, and nonlinear equilibrium structure of the VSHF. These results constitute a theory for VSHF formation applicable to interpreting simulations and observations of geophysical examples of turbulent jets such as the ocean’s equatorial deep jets.
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Litt, Maxime, Jean-Emmanuel Sicart, Delphine Six, Patrick Wagnon, and Warren D. Helgason. "Surface-layer turbulence, energy balance and links to atmospheric circulations over a mountain glacier in the French Alps." Cryosphere 11, no. 2 (April 18, 2017): 971–87. http://dx.doi.org/10.5194/tc-11-971-2017.
Abstract:
Abstract. Over Saint-Sorlin Glacier in the French Alps (45° N, 6.1° E; ∼ 3 km2) in summer, we study the atmospheric surface-layer dynamics, turbulent fluxes, their uncertainties and their impact on surface energy balance (SEB) melt estimates. Results are classified with regard to large-scale forcing. We use high-frequency eddy-covariance data and mean air-temperature and wind-speed vertical profiles, collected in 2006 and 2009 in the glacier's atmospheric surface layer. We evaluate the turbulent fluxes with the eddy-covariance (sonic) and the profile method, and random errors and parametric uncertainties are evaluated by including different stability corrections and assuming different values for surface roughness lengths. For weak synoptic forcing, local thermal effects dominate the wind circulation. On the glacier, weak katabatic flows with a wind-speed maximum at low height (2–3 m) are detected 71 % of the time and are generally associated with small turbulent kinetic energy (TKE) and small net turbulent fluxes. Radiative fluxes dominate the SEB. When the large-scale forcing is strong, the wind in the valley aligns with the glacier flow, intense downslope flows are observed, no wind-speed maximum is visible below 5 m, and TKE and net turbulent fluxes are often intense. The net turbulent fluxes contribute significantly to the SEB. The surface-layer turbulence production is probably not at equilibrium with dissipation because of interactions of large-scale orographic disturbances with the flow when the forcing is strong or low-frequency oscillations of the katabatic flow when the forcing is weak. In weak forcing when TKE is low, all turbulent fluxes calculation methods provide similar fluxes. In strong forcing when TKE is large, the choice of roughness lengths impacts strongly the net turbulent fluxes from the profile method fluxes and their uncertainties. However, the uncertainty on the total SEB remains too high with regard to the net observed melt to be able to recommend one turbulent flux calculation method over another.
35
Xiao, X., H. C. Zuo, Q. D. Yang, S. J. Wang, L. J. Wang, J. W. Chen, B. L. Chen, and B. D. Zhang. "On the factors influencing surface-layer energy closure and their seasonal variability over the semi-arid Loess Plateau of Northwest China." Hydrology and Earth System Sciences 16, no. 3 (March 20, 2012): 893–910. http://dx.doi.org/10.5194/hess-16-893-2012.
Abstract:
Abstract. The energy observed in the surface layer, when using eddy-covariance techniques to measure turbulent fluxes, is not balanced. Important progress has been made in recent years in identifying potential reasons for this lack of closure in the energy balance, but the problem is not yet resolved. In this paper, long-term data that include output of tower, radiation, surface turbulence flux and soil measurement collected from September 2006 to August 2010 in the Semi-Arid Climate Change and Environment Observatory, Lanzhou University, in the semi-arid Loess Plateau of Northwest China, were analysed, focusing on the seasonal characteristics of the surface energy and the factors that have impact on the energy balance closure (EBC). The analysis shows that (1) the long-term observations are successful; the interaction between the land and the atmosphere in semi-arid climates can be represented by the turbulent transport of energy. In addition, even though the residual is obvious, this suggests that the factors that impact the EBC are stable, and their seasonal variations are identical. The analysis also shows that (2) four factors have obvious impact on the EBC: the diverse schemes for surface soil heat flux, the flux contribution from the target source area, the low-frequency part of the turbulence spectra, and the strength of atmospheric turbulence motion. The impact of these four factors on the EBC are similar in all seasons. Lastly, the results indicate that (3) atmospheric turbulence intensity is a very important factor in terms of its impact on the EBC. The relative turbulence intensity, RIw, characterises the strength of atmospheric turbulence motion, and is found to exert a noticeable impact on the EBC; in all seasons, the EBC is increased when the relative turbulence intensity is enlarged.
36
Gallego-Castillo, C., M. Elagamy, A. Cuerva-Tejero, O. Lopez-Garcia, and S. Avila. "Synthesis of realistic non-homogeneous non-Gaussian turbulent wind fields." Journal of Physics: Conference Series 2767, no. 5 (June 1, 2024): 052019. http://dx.doi.org/10.1088/1742-6596/2767/5/052019.
Abstract:
Abstract This paper presents a sequential method for generating synthetic non-homogeneous non-Gaussian turbulent wind fields with a prescribed time-space covariance structure. The proposed methodology is based on the optimisation of restricted multivariate autoregressive (VAR) models, and the quantile-to-quantile transform between statistical distributions. The considered case study is a non-homogeneous non-Gaussian turbulent wind field over the roof of a high rise building simulated with LES. Results show a notably good matching in terms of the reproduced wind statistical distributions, Covariance Matrix Function (CMF) and Cross Power Spectral Density Matrix (CPSDM). In addition, the synthetic wind field reproduced accurately the recirculation bubble close to the roof. The main advantages of the proposed method are that, once the VAR model is computed, the synthesis of several realisations is computationally very cheap, which is useful for performing several aeroelastic simulations of the same analysis case, as suggested by the standards. The critical point is that, to characterise the statistical features for a specific case study (such as wind turbine wakes or turbulence due to obstacles), an LES simulation of the wind field is required as input. The software employed in this work is open source and it is available on GitHub.
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Clement, Robert J., and John B. Moncrieff. "A Functional Approach to Vertical Turbulent Transport of Scalars in the Atmospheric Surface Layer." Boundary-Layer Meteorology 173, no. 3 (September 10, 2019): 373–408. http://dx.doi.org/10.1007/s10546-019-00474-z.
Abstract:
Abstract Eddy covariance has been the de facto method of analyzing scalar turbulent transport data. To refine the information available from these data, we derive a simplified version of the turbulent scalar-transport equation for the surface layer, which employs a more explicit form of signal decomposition and dispenses with Reynolds averaging in favour of an averaging operator based on the relevant scalar-flux driving variables. The resulting method, termed functional covariance, provides five areas of improvement in flux estimation: (i) Better representation of surface fluxes through closer correspondence of turbulent exchange with variations in the driving variables. (ii) An approximate 25% reduction in flux uncertainty resulting from improved independence of turbulent-flux samples. (iii) Improved data retention through less onerous quality control (stationarity) testing. (iv) Improved estimation of low-frequency flux contributions through reduced uncertainty and avoidance of driving-variable nonstationarity. (v) Potential elimination of flux-storage estimation when state driving-variables are used to define the functional-covariance flux averaging. We describe the important considerations required for application of functional covariance, apply both functional- and eddy-covariance methods to an example dataset, compare the resulting eddy- and functional-covariance estimates, and demonstrate the aforementioned benefits of functional covariance.
38
Rautenberg, Alexander, Jonas Allgeier, Saskia Jung, and Jens Bange. "Calibration Procedure and Accuracy of Wind and Turbulence Measurements with Five-Hole Probes on Fixed-Wing Unmanned Aircraft in the Atmospheric Boundary Layer and Wind Turbine Wakes." Atmosphere 10, no. 3 (March 7, 2019): 124. http://dx.doi.org/10.3390/atmos10030124.
Abstract:
For research in the atmospheric boundary layer and in the vicinity of wind turbines, the turbulent 3D wind vector can be measured from fixed-wing unmanned aerial systems (UAS) with a five-hole probe and an inertial navigation system. Since non-zero vertical wind and varying horizontal wind causes variations in the airspeed of the UAS, and since it is desirable to sample with a flexible cruising airspeed to match a broad range of operational requirements, the influence of airspeed variations on mean values and turbulence statistics is investigated. Three calibrations of the five-hole probe at three different airspeeds are applied to the data of three flight experiments. Mean values and statistical moments of second order, calculated from horizontal straight level flights are compared between flights in a stably stratified polar boundary layer and flights over complex terrain in high turbulence. Mean values are robust against airspeed variations, but the turbulent kinetic energy, variances and especially covariances, and the integral length scale are strongly influenced. Furthermore, a transect through the wake of a wind turbine and a tip vortex is analyzed, showing the instantaneous influence of the intense variations of the airspeed on the measurement of the turbulent 3D wind vector. For turbulence statistics, flux calculations, and quantitative analysis of turbine wake characteristics, an independent measurement of the true airspeed with a pitot tube and the interpolation of calibration polynomials at different Reynolds numbers of the probe’s tip onto the Reynolds number during the measurement, reducing the uncertainty significantly.
39
Njuki, Sammy M., Chris M. Mannaerts, and Zhongbo Su. "Accounting for Turbulence-Induced Canopy Heat Transfer in the Simulation of Sensible Heat Flux in SEBS Model." Remote Sensing 15, no. 6 (March 14, 2023): 1578. http://dx.doi.org/10.3390/rs15061578.
Abstract:
Surface turbulent heat fluxes are crucial for monitoring drought, heat waves, urban heat islands, agricultural water management, and other hydrological applications. Energy Balance Models (EBMs) are widely used to simulate surface heat fluxes from a combination of remote sensing-derived variables and meteorological data. Single-source EBMs, in particular, are preferred in mapping surface turbulent heat fluxes due to their relative simplicity. However, most single-source EBMs suffer from uncertainties inherent to the parameter kB−1, which is used to account for differences in the source of heat and the sink of momentum when representing aerodynamic resistance in single-source EBMs. For instance, the parameterization of kB−1 in the commonly used single-source Surface Energy Balance System (SEBS) model uses a constant value of the foliage heat transfer coefficient (Ct), in the parameterization of the vegetation component of kB−1 (kBv−1). Thus, SEBS ignores the effect of turbulence on canopy heat transfer. As a result, SEBS has been found to greatly underestimate sensible heat flux in tall forest canopies, where turbulence is a key contributor to canopy heat transfer. This study presents a revised parameterization of kBv−1 for the SEBS model. A physically based formulation of Ct, which considers the effect of turbulence on Ct, is used in deriving the revised parameterization. Simulation results across 15 eddy covariance (EC) flux tower sites show that the revised parameterization significantly reduces the underestimation of sensible heat flux compared to the original parameterization under tall forest canopies. The revised parameterization is relatively simple and does not require additional information on canopy structure compared to some more complex parameterizations proposed in the literature. As such, the revised parameterization is suitable for mapping surface turbulent heat fluxes, especially under tall forest canopies.
40
Flügge, Martin, Mostafa Bakhoday Paskyabi, Joachim Reuder, James B. Edson, and Albert J. Plueddemann. "Comparison of Direct Covariance Flux Measurements from an Offshore Tower and a Buoy." Journal of Atmospheric and Oceanic Technology 33, no. 5 (May 2016): 873–90. http://dx.doi.org/10.1175/jtech-d-15-0109.1.
Abstract:
AbstractDirect covariance flux (DCF) measurements taken from floating platforms are contaminated by wave-induced platform motions that need to be removed before computation of the turbulent fluxes. Several correction algorithms have been developed and successfully applied in earlier studies from research vessels and, most recently, by the use of moored buoys. The validation of those correction algorithms has so far been limited to short-duration comparisons against other floating platforms. Although these comparisons show in general a good agreement, there is still a lack of a rigorous validation of the method, required to understand the strengths and weaknesses of the existing motion-correction algorithms. This paper attempts to provide such a validation by a comparison of flux estimates from two DCF systems, one mounted on a moored buoy and one on the Air–Sea Interaction Tower (ASIT) at the Martha’s Vineyard Coastal Observatory, Massachusetts. The ASIT was specifically designed to minimize flow distortion over a wide range of wind directions from the open ocean for flux measurements. The flow measurements from the buoy system are corrected for wave-induced platform motions before computation of the turbulent heat and momentum fluxes. Flux estimates and cospectra of the corrected buoy data are found to be in very good agreement with those obtained from the ASIT. The comparison is also used to optimize the filter constants used in the motion-correction algorithm. The quantitative agreement between the buoy data and the ASIT demonstrates that the DCF method is applicable for turbulence measurements from small moving platforms, such as buoys.
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Andreas, Edgar L. "Two Experiments on Using a Scintillometer to Infer the Surface Fluxes of Momentum and Sensible Heat." Journal of Applied Meteorology and Climatology 51, no. 9 (September 2012): 1685–701. http://dx.doi.org/10.1175/jamc-d-11-0248.1.
Abstract:
AbstractA traditional use of scintillometry is to infer path-averaged values of the turbulent surface fluxes of sensible heat Hs and momentum τ (, where ρ is air density and u* is the friction velocity). Many scintillometer setups, however, measure only the path-averaged refractive-index structure parameter ; the wind information necessary for inferring u* and Hs comes from point measurements or is absent. The Scintec AG SLS20 surface-layer scintillometer system, however, measures both and the inner scale of turbulence ℓ0, where ℓ0 is related to the dissipation rate of turbulent kinetic energy ɛ. The SLS20 is thus presumed to provide path-averaged estimates of both u* and Hs. This paper describes comparisons between SLS20-derived estimates of u* and Hs and simultaneous eddy-covariance measurements of these quantities during two experiments: one, over Arctic sea ice; and a second, over a midlatitude land site during spring. For both experiments, the correlation between scintillometer and eddy-covariance fluxes is reasonable: correlation coefficients are typically above 0.7 for the better-quality data. For both experiments, though, the scintillometer usually underestimates u* and underestimates the magnitude of Hs when compared with the corresponding eddy-covariance values. The data also tend to be more scattered when < 10−14 m−2/3: the signal-to-noise ratio for scintillometer-derived fluxes decreases as decreases. An essential question that arises during these comparisons is what similarity functions to use for inferring fluxes from the scintillometer and ℓ0 measurements. The paper thus closes by evaluating whether any of four candidate sets of similarity functions is consistent with the scintillometer data.
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Chen, J., Y. Hu, Y. Yu, and S. Lü. "Ergodicity test of the eddy correlation method." Atmospheric Chemistry and Physics Discussions 14, no. 12 (July 10, 2014): 18207–54. http://dx.doi.org/10.5194/acpd-14-18207-2014.
Abstract:
Abstract. The turbulent flux observation in the near-surface layer is a scientific issue which researchers in the fields of atmospheric science, ecology, geography science, etc. are commonly interested in. For eddy correlation measurement in the atmospheric surface layer, the ergodicity of turbulence is a basic assumption of the Monin–Obukhov (M–O) similarity theory, which is confined to steady turbulent flow and homogenous surface; this conflicts with turbulent flow under the conditions of complex terrain and unsteady, long observational period, which the study of modern turbulent flux tends to focus on. In this paper, two sets of data from the Nagqu Station of Plateau Climate and Environment (NaPlaCE) and the cooperative atmosphere–surface exchange study 1999 (CASE99) were used to analyze and verify the ergodicity of turbulence measured by the eddy covariance system. Through verification by observational data, the vortex of atmospheric turbulence, which is smaller than the scale of the atmospheric boundary layer (i.e., its spatial scale is less than 1000 m and temporal scale is shorter than 10 min) can effectively meet the conditions of the average ergodic theorem, and belong to a wide sense stationary random processes. Meanwhile, the vortex, of which the spatial scale is larger than the scale of the boundary layer, cannot meet the conditions of the average ergodic theorem, and thus it involves non-ergodic stationary random processes. Therefore, if the finite time average is used to substitute for the ensemble average to calculate the average random variable of the atmospheric turbulence, then the stationary random process of the vortex, of which spatial scale was less than 1000 m and thus below the scale of the boundary layer, was possibly captured. However, the non-ergodic random process of the vortex, of which the spatial scale was larger than that of the boundary layer, could not be completely captured. Consequently, when the finite time average was used to substitute for the ensemble average, a large rate of error would occur with use of the eddy correction method due to losing the low frequency component information of the larger vortex. When the multi-station observation was compared with the single-station observation, the wide sense of stationary random process originating from the multi-station observation expanded from a vortex which was about 1000 m smaller than a boundary layer scale to the turbulent vortex, which was larger than the boundary layer scale of 2000 m. Therefore, the calculation of the turbulence average or variance and turbulent flux could effectively meet the ergodic assumption, and the results would be approximate to the actual values. Regardless of vertical velocity and temperature, if the ergodic stationary random processes could be met, then the variance of the vortexes in the different temporal scales could follow M–O similarity theory; in the case of the non-ergodic random process, its vortex variance deviated from the M–O similarity relations. The exploration of ergodicity in the atmospheric turbulence measurements is doubtlessly helpful to understanding the issues in atmospheric turbulent flux observation, and provides a theoretical basis for overcoming related difficulties.
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Chinita, Maria J., Georgios Matheou, and João Teixeira. "A Joint Probability Density–Based Decomposition of Turbulence in the Atmospheric Boundary Layer." Monthly Weather Review 146, no. 2 (February 1, 2018): 503–23. http://dx.doi.org/10.1175/mwr-d-17-0166.1.
Abstract:
Abstract In convective flows, vertical turbulent fluxes, covariances between vertical velocity and scalar thermodynamic variables, include contributions from local mixing and large-scale coherent motions, such as updrafts and downdrafts. The relative contribution of these motions to the covariance is important in turbulence parameterizations. However, the flux partition is challenging, especially in regions without convective cloud. A method to decompose the vertical flux based on the corresponding joint probability density function (JPD) is introduced. The JPD-based method partitions the full JPD into a joint Gaussian part and the complement, which represent the local mixing and the large-scale coherent motions, respectively. The coherent part can be further divided into updraft and downdraft parts based on the sign of vertical velocity. The flow decomposition is independent of water condensate (cloud) and can be applied in cloud-free convection, the subcloud layer, and stratiform cloud regions. The method is applied to large-eddy simulation model data of three boundary layers. The results are compared with traditional cloud and cloud-core decompositions and a decaying scalar conditional sampling method. The JPD-based method includes a single free parameter and sensitivity tests show weak dependence on the parameter values. The results of the JPD-based method are somewhat similar to the cloud-core and conditional sampling methods. However, differences in the relative magnitude of the flux decomposition terms suggest that an objective definition of the flow regions is subtle and diagnosed flow properties like updraft characteristics depend on the sampling method. Moreover, the flux decomposition depends on the thermodynamic variable and convection characteristics.
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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 Discussions 9, no. 3 (May 7, 2009): 11367–411. http://dx.doi.org/10.5194/acpd-9-11367-2009.
Abstract:
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 events in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed free convection to be induced in situations where high buoyancy fluxes and a simultaneously occurring wind speed collapse 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. Buoyant forces then dominate over shear forces within turbulence production. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) calculated from directly measured turbulent fluxes. An analysis of the scales of turbulent motions during the free convection event using wavelet transform confirms the large-eddy scale character of the detected plume-like coherent structures. Regarding the entire COPS measurement period, free convection events (FCEs) in the morning hours occur on about 50% of all days.
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Román-Cascón, C., C. Yagüe, L. Mahrt, M. Sastre, G. J. Steeneveld, E. Pardyjak, A. van de Boer, and O. Hartogensis. "Interactions among drainage flows, gravity waves and turbulence: a BLLAST case study." Atmospheric Chemistry and Physics 15, no. 15 (August 14, 2015): 9031–47. http://dx.doi.org/10.5194/acp-15-9031-2015.
Abstract:
Abstract. The interactions among several stable-boundary-layer (SBL) processes occurring just after the evening transition of 2 July 2011 have been analysed using data from instruments deployed over the area of Lannemezan (France) during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. The near-calm situation of the afternoon was followed by the formation of local shallow drainage flows (SDFs) of less than 10 m depth at different locations. The SDF stage ended with the arrival of a stronger wind over a deeper layer more associated with the mountain-plain circulation, which caused mixing and destruction of the SDFs. Several gravity-wave-related oscillations were also observed on different time series. Wavelet analyses and wave parameters were calculated from high resolution and accurate surface pressure data of an array of microbarometers. These waves propagated relatively long distances within the SBL. The effects of these phenomena on turbulent parameters (friction velocity and kinematic heat flux) have been studied through multi-resolution flux decomposition methods performed on high frequency data from sonic anemometers deployed at different heights and locations. With this method, we were able to detect the different time-scales involved in each turbulent parameter and separate them from wave contributions, which becomes very important when choosing averaging-windows for surface flux computations using eddy covariance methods. The extensive instrumentation allowed us to highlight in detail the peculiarities of the surface turbulent parameters in the SBL, where several of the noted processes were interacting and producing important variations in turbulence with height and between sites along the sloping terrain.
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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.
Abstract:
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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HERLINA and G. H. JIRKA. "Experiments on gas transfer at the air–water interface induced by oscillating grid turbulence." Journal of Fluid Mechanics 594 (December 14, 2007): 183–208. http://dx.doi.org/10.1017/s0022112007008968.
Abstract:
The gas transfer process across the air–water interface in a turbulent flow environment, with the turbulence generated in the water phase far away from the surface, was experimentally investigated for varying turbulent Reynolds numbers ReT ranging between 260 and 780. The experiments were performed in a grid-stirred tank using a combined particle image velocimetry – laser induced fluorescence (PIV-LIF) technique, which enables synoptic measurements of two-dimensional velocity and dissolved gas concentration fields. The visualization of the velocity and concentration fields provided direct insight into the gas transfer mechanisms. The high data resolution allowed detailed quantification of the gas concentration distribution (i.e. mean and turbulent fluctuation characteristics) within the thin aqueous boundary layer as well as revealing the near-surface hydrodynamics. The normalized concentration profiles show that as ReT increases, the rate of concentration decay into the bulk becomes slower. Independent benchmark data for the transfer velocity KL were obtained and their normalized values (KLSc0.5/uHT) depend on ReT with exponent −0.25. The spectra of the covariance term c′ w′ indicate that the contribution of c′ w′ is larger in the lower-frequency region for cases with small ReT, whereas for the other cases with higher ReT, the contribution of c′ w′ appears to be larger in the higher-frequency region (small eddies). These interrelated facts suggest that the gas transfer process is controlled by a spectrum of different eddy sizes and the gas transfer at different turbulence levels can be associated with certain dominant eddy sizes. The normalized mean turbulent flux $\overline{c^\prime w^\prime}$ profiles increase from around 0 at the interface to about 1 within a depth of approximately 2δe, where δe is the thickness of the gas boundary layer. The measured turbulent flux (c′ w′) is of the same order as the total flux (j), which shows that the contribution of c′ w′ to the total flux is significant.
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MASSERONI, DANIELE, CHIARA CORBARI, and MARCO MANCINI. "Limitations and improvements of the energy balance closure with reference to experimental data measured over a maize field." Atmósfera 27, no. 4 (January 13, 2015): 335–52. http://dx.doi.org/10.20937/atm.2014.27.04.01.
Abstract:
The use of energy fluxes data to validate land surface models requires that energy balance closure conservationis satisfied, but usually this condition is not verified when the available energy is bigger than the sumof turbulent vertical fluxes. In this work, a comprehensive evaluation of energy balance closure problems isperformed on a 2012 data set from Livraga obtained by a micrometeorological eddy covariance station locatedin a maize field in the Po Valley. Energy balance closure is calculated by statistical regression of turbulentenergy fluxes and soil heat flux against available energy. Generally, the results indicate a lack of closure witha mean imbalance in the order of 20%. Storage terms are the main reason for the unclosed energy balance butalso the turbulent mixing conditions play a fundamental role in reliable turbulent flux estimations. Recentlyintroduced in literature, the energy balance problem has been studied as a scale problem. A representativesource area for each flux of the energy balance has been analyzed and the closure has been performed infunction of turbulent flux footprint areas. Surface heterogeneity and seasonality effects have been studied to understand the influence of canopy growth on the energy balance closure. High frequency data have beenused to calculate co-spectral and ogive functions, which suggest that an averaging period of 30 min may misstemporal scales that contribute to the turbulent fluxes. Finally, latent and sensible heat random error estimationsare computed to give information about the measurement system and turbulence transport deficiencies
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Ristorcelli, J. R., and A. C. Poje. "Lagrangian Covariance Analysis of β-Plane Turbulence." Theoretical and Computational Fluid Dynamics 14, no. 1 (May 1, 2000): 1–20. http://dx.doi.org/10.1007/s001620050122.
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Frehlich, Rod. "Atmospheric turbulence component of the innovation covariance." Quarterly Journal of the Royal Meteorological Society 134, no. 633 (April 2008): 931–40. http://dx.doi.org/10.1002/qj.263.