Calculation of basin-scale surface fluxes by combining remotely sensed data and atmospheric properties in a semiarid landscape

Optical remote sensing data collected during the Monsoon '90 experiment in the Walnut Gulch Experimental Watershed in southern Arizona were used to estimate basin-scale surface temperature, net radiation (R _n) and soil heat flux (G). These were combined with several atmospheric boundary-layer (ABL) models to allow computation of basin-scale surface fluxes of sensible (H) and latent heat (LE). The calculated fluxes were compared to averages from a network of surface flux stations. One ABL model calculatedH using a bulk similarity approach for wind and temperature with remotely sensed surface temperature as the lower boundary condition. With basin-scale estimates ofR _n andG, LE was solved as a residual. The other ABL model applied atmospheric profiles from a series of soundings in the conservation equations of temperature and humidity in the mixed layer to computeH andLE directly. By combining theseH values withR _n andG, calculation ofLE by residual also was performed. The ABL-derivedH values differed from the averages from the surface network by roughly 20 and 30% for the bulk similarity and conservation approaches, respectively. ForLE, these same differences were around 10 and 70%. The disparity was reduced to nearly 30% for the conservation approach whenLE was solved as a residual. Days with significant spatial variation in surface soil moisture and/or cloud cover were associated with most of the disagreement between the ABL-derived and surface-based values. This was particularly true for conservation estimates ofLE. The bulk similarity method appeared less sensitive non-ideal environmental conditions. This may in part be due to the use of remotely sensed information, which provided a lower boundary value of surface temperature and estimates ofR _n andG over the study area, thereby allowing for residual calculations ofLE. Such information clearly has utility for assessing the surface energy and water balance at basin scale.     

Intra-field variability of scalar flux densities across a transition between a desert and an irrigated potato field

This paper reports on measurements of sensible and latent heat and CO₂ fluxes made over an irrigated potato field, growing next to a patch of desert. The study was conducted using two eddy correlation systems. One measurement system was located within the equilibrium boundary layer 800 m downwind from the edge of the potato field. The other measurement system was mobile and was placed at various downwind positions to probe the horizontal transition of vertical scalar fluxes. Latent (LE) and sensible (H) heat fluxes, measured at 4 m above the surface, exhibited marked variations with downwind distance over the field. Only after the fetch to height ratio exceeded 75 to 1 didLE andH become invariant with downwind distance. When latent and sensible heat fluxes were measured upwind of this threshold, significant advection of humidity-deficit occurred, causing a vertical flux divergence ofH andLE.The measured fluxes of momentum, heat, and moisture were compared with predictions from a second-order closure two-dimensional atmospheric boundary layer model. There is good agreement between measurements and model predictions. A soil-plant-atmosphere model was used to examine nonlinear feedbacks between humidity-deficits, stomatal conductance and evaporation. Data interpretation with this model revealed that the advection of hot dry air did not enhance surface evaporation rates near the upwind edge of the potato field, because of negative feedbacks among stomatal conductance, humidity-deficits, andLE. This finding is consistent with results from several recent studies.     

简单生物圈模式 (SiB2) 中湍流能量通量对近地层两类阻抗系数的敏感性研究

陆—气之间的能量交换是通过近地层湍流热量和水汽通量来实现的.以往的研究表明近地层阻抗对湍流能量通量有着不同程度的影响,但是关于陆面模式中阻抗系数的取值范围却始终没有统一的标准.为了深入了解简单生物圈模式 (SiB₂) 中近地层阻抗系数取值变化对湍流能量通量的影响,我们以那曲站为例,分别采用传统的逐个因子分析法和考虑参数间相互作用的部分因子分析法定量地研究了夏季该观测站近地层湍流能量通量分别对冠层阻抗系数C₁和地表阻抗系数C₂的敏感性响应.结果表明,感热通量对地表阻抗系数C₂更为敏感,而潜热通量则对冠层阻抗系数C₁较为敏感;感热通量随C₁增加而增大,随C₂增加而减小,而潜热通量则随C₁或C₂的增加而减小;不管是感热通量还是潜热通量,它们对阻抗系数的敏感度随阻抗系数的增大而减小,而对阻抗系数的相对敏感度则随阻抗系数的增大而增大.最后,结合那曲站夏季下垫面稀疏短草的分布特点分析了造成感热通量和潜热通量敏感变化各异的原因.     

Dissimilarity of Scalar Transport in the Convective Boundary Layer in Inhomogeneous Landscapes

A land-surface model (LSM) is coupled with a large-eddy simulation (LES) model to investigate the vegetation-atmosphere exchange of heat, water vapour, and carbon dioxide (CO₂) in heterogeneous landscapes. The dissimilarity of scalar transport in the lower convective boundary layer is quantified in several ways: eddy diffusivity, spatial structure of the scalar fields, and spatial and temporal variations in the surface fluxes of these scalars. The results show that eddy diffusivities differ among the three scalars, by up to 10–12%, in the surface layer; the difference is partly attributed to the influence of top-down diffusion. The turbulence-organized structures of CO₂ bear more resemblance to those of water vapour than those of the potential temperature. The surface fluxes when coupled with the flow aloft show large spatial variations even with perfectly homogeneous surface conditions and constant solar radiation forcing across the horizontal simulation domain. In general, the surface sensible heat flux shows the greatest spatial and temporal variations, and the CO₂ flux the least. Furthermore, our results show that the one-dimensional land-surface model scheme underestimates the surface heat flux by 3–8% and overestimates the water vapour and CO₂ fluxes by 2–8% and 1–9%, respectively, as compared to the flux simulated with the coupled LES-LSM.     

Source areas for scalars and scalar fluxes

The spatial resolution of meteorological observations of scalars (such as concentrations or temperature) and scalar fluxes (e.g., water-vapour flux, sensible heat flux) above inhomogeneous surfaces is in general not known. It is determined by the surface area of influence orsource area of the sensor, which for sensors of quantities that are subject to turbulent diffusion, depends on the flow and turbulence conditions.Functions describing the relationship between the spatial distribution of surface sources (or sinks) and a measured signal at height in the surface layer have been termed thefootprint function or thesource weight function. In this paper, the source area of levelP is defined as the integral of the source weight function over the smallest possible domain comprising the fractionP of the total surface influence reflected in the measured signal. Source area models for scalar concentration and for passive scalar fluxes are presented. The results of the models are presented as characteristic dimensions of theP=50% source areas (i.e., the area responsible for 50% of the surface influence): the maximum source location (i.e., the upwind distance of the surface element with the maximum-weight influence), the near and the far end of the source area, and its maximal lateral extension. These numerical model results are related directly to non-dimensional surface-layer scaling variables by a non-linear least squares method in a parameterized model which provides a user-friendly estimate of the surface area responsible for measured concentrations or fluxes. The source area models presented here allow conclusions to be made about the spatial representativeness and the localness (these terms are defined in the text) of flux and concentration measurements.     

On the Temperature and Humidity Dissimilarity in the Marine Surface Layer

The dissimilarity of temperature and humidity transfer in the marine surface layer (MSL) is investigated through the relative transport efficiency and correlation coefficient of these two scalars. We examine their variability and relationship with mean values, as well as spectral characteristics. It is shown that the dissimilarity between these two scalars in the MSL is a function of stability, the boundary-layer depth, and flow steadiness. In general the temperature and humidity are less correlated in shallow marine boundary layers compared to deep marine boundary layers, due to the stronger impact of the boundary-layer scale in breaking the “same source, same sink” assumption for scalar similarity. This is supported by the combination of our spectral analysis of scalar fluxes and corresponding measured and modelled boundary-layer depth. This assumption is also broken in near-neutral conditions, when there is an efficient latent heat transfer but negligible sensible heat transfer. Our data suggest that parametrization of humidity fluxes via similarity theory could still be reliable when the correlation coefficient $>$ 0.5, and in near-neutral conditions the humidity flux can be estimated without use of the sensible heat flux.     

Comparison of areally averaged turbulent fluxes over non-homogeneous terrain: Results from the EFEDA-field experiment

In June 1991 the EFEDA-field experiment (ECHIVAL Field Experiment in a Desertification-Threatened Area) was carried out in the Spanish province Castilla-La Mancha, to improve the understanding of the interactions between the soil, the vegetation and the atmosphere.Here results of energy balance studies at the Barrax site are given, one of the three intensively studied experimental sites within Castilla-La Mancha. This area is characterized by a large fraction of irrigated fields (40%) while the remaining 60% was fallow land at the end of June 1991. The energy balances over these two characteristic land-use classes totally differ. While for the irrigated fields the evapotranspiration is dominant, for the non-irrigated fields the sensible and the soil heat fluxes dominate and the latent heat flux is nearly negligible.In order to achieve areally averaged turbulent fluxes, surface, SODAR and aircraft data have been used. Comparing the surface fluxes from all three facilities, it can be found that:The sensible heat flux estimation from SODAR (_w-method) gives reliable results when a calibration of _w is done with another independent system (e.g. aircraft).Aircraft measurements in conjunction with energy budget methods yield surface fluxes of sensible heat, which are about 20% lower than the areally averaged values calculated by the surface measurements. The areally averaged latent heat fluxes from aircraft and surface measurements agree better than the sensible heat fluxes.     

Footprints in Homogeneously and Heterogeneously Driven Boundary Layers Derived from a Lagrangian Stochastic Particle Model Embedded into Large-Eddy Simulation

A Lagrangian stochastic (LS) model, which is embedded into a parallelised large-eddy simulation (LES) model, is used for dispersion and footprint evaluations. For the first time an online coupling between LES and LS models is applied. The new model reproduces concentration patterns, which were obtained in prior studies, provided that subgrid-scale turbulence is included in the LS model. Comparisons with prior studies show that the model evaluates footprints successfully. Streamwise dispersion leads to footprint maxima that are situated less far upstream than previously reported. Negative flux footprints are detected in the convective boundary layer (CBL). The wide range of applicability of the model is shown by applying it under neutral and stable stratification. It is pointed out that the turning of the wind direction with height leads to a considerable dependency of source areas on height. First results of an application to a heterogeneously heated CBL are presented, which emphasize that footprints are severely affected by the inhomogeneity.     

Land surface temperature shaped by urban fractions in megacity region

The influence of spatial scales on surface fluxes is an interesting but not fully investigated question. This paper presents an analysis on the influence of spatial scales on surface fluxes in the north Tibetan Plateau based on eddy covariance (EC) and large aperture scintillometer (LAS) data at site Nagqu/BJ, combined with the land surface temperature (LST) and normalized difference vegetation index (NDVI) of moderate-resolution imaging spectroradiometer (MODIS). The analysis shows that sensible heat fluxes calculated with LAS data (H_LAS) agree reasonably well with sensible heat fluxes calculated with EC data (H_EC) in the rain and dry seasons. The difference in their footprints due to the wind direction is an important reason for the differences in H_EC and H_LAS. The H_LAS are statistically more consistent with H_EC when their footprints overlap than when their footprints do not. A detailed analysis on H_EC and H_LAS changes with net radiation and wind direction in rain and dry season indicates that the spatial heterogeneity in net radiation created by clouds contributes greatly to the differences in H_EC and H_LAS in short-term variations. A significant relationship between the difference in footprint-weighted averages of LST and difference in H_EC and H_LAS suggests that the spatial heterogeneity in LST at two spatial scales is a reason for the differences in H_EC and H_LAS and that LST has a positive correlation with the differences in H_EC and H_LAS. A significant relationship between the footprint-weighted averages of NDVI and the ratio of sensible heat fluxes at two spatial scales to net radiation (H/Rn) in the rain season supports the analysis that the spatial heterogeneity in canopy at two spatial scales is another reason for differences in H_EC and H_LAS and that canopy has a negative correlation with (H/Rn). An analysis on the influence of the difference in aerodynamic roughness lengths at two spatial scales on sensible heat fluxes shows that the influence is greater in the dry season and smaller in the rain season because the ratio of z_{0m_LAS} to z_{0m_EC} is big in the dry season and is close to 1.0 in the rain season. This study on spatial scales on surface fluxes in the Tibetan Plateau will be helpful in analyzing and understanding its influence on climate.     

Urban-scale variations of turbulence parameters and fluxes

Sensible heat (H) and latent heat (LE) fluxes and turbulence statistics in St. Louis, Missouri and the surrounding region are presented. The urban-scale analyses were derived from a series of aircraft transects at 150 m above ground across the metropolitan area during the afternoon convective period. The results revealed that H varied by a factor of two to four in the region; the largest values were associated with the urban heat island. LE varied across the urban area by about a factor of four, but low values of LE overlaid the urban heat island. Consequently, the Bowen ratio (H/LE) exhibited large spatial variability, with a maximum value greater than 1.5 over the city and values less than 0.2 in nonurban areas. The areas along the Mississippi River and adjacent low lying marshland northeast of the downtown area displayed significantly smaller H and Bowen ratio. The derived surface heat storage term (G) for this area as well as for the urban area exceeded either H or LE.The spatial patterns for the standard deviations of the three velocity components ( _u,v,w ), temperature ( _T ), and absolute humidity ( _q ), are also presented. The patterns of _u,v,w were similar to the pattern of H. the highest values associated with the urban heat island. The correlation coefficient between the vertical velocity and temperature fluctuations was highest over the city, and a noteworthy minimum was observed in the upwind area over the river and marshland in association with low H. The convective similarity relationships for _u,v,w appeared to be approximately valid spatially, as variations were typically less than 10% from theory over the urban area and nonurban region, except for a 40% anomaly in the lowland around the river northeast of the city.Measurements of H from 30-m towers within various land-use areas were contrasted with the aircraft data. Land-use differences in H at the surface were at least as large as those observed at 150 m across the city. This was primarily because of the measurement requirement that the minimum resolvable fetch increases with measurement height.     

Heat flux apportionment to heterogeneous surfaces using flux footprint analysis

Heat flux data collected from the Baiyangdian Heterogeneous Field Experiment were analyzed using the footprint method. High resolution （25 m） Landsat-5 satellite imaging was used to determine the land cover as one of four surface types： farmland, lake, wetland, or village. Data from two observation sites in September 2005 were used. One site （Wangjiazhai） was characterized by highly heterogeneous surfaces in the central area of the Baiyangdian： lake/wetland. The other site （Xiongxian） was on land with more uniform surface cover. An improved Eulerian analytical flux footprint model was used to determine “source areas” of the heat fluxes measured at towers located at each site from surrounding landscapes of mixed surface types. In relative terms results show that wetland and lake areas generally contributed most to the observed heat flux at Wangjiazhai, while farmland contributed most at Xiongxian. Given the areal distribution of surface type contributions, calculations were made to obtain the magnitudes of the heat flux from lake, wetland and farmland to the total observed flux and apportioned contributions of each surface type to the sensible and latent heat fluxes. Results show that on average the sensible heat flux from wetland and farmland were comparable over the diurnal cycle, while the latent heat flux from farmland was somewhat larger by about 30-50 W m-2 during daytime. The latent and sensible fluxes from the lake source in daytime were about 50 W m-2 and 100 W m-2 less, respectively, than from wetland and farmland. The results are judged reasonable and serve to demonstrate the potential for flux apportionment over heterogeneous surfaces.     

Simulated Airborne Flux Measurements in a LES generated Convective Boundary Layer

One aim of past boundary-layer experiments with aircraft was the determination of areally averaged heat fluxes. In spite ofsophisticated instrumentation the measured fluxes extrapolated to the ground differed significantly from fluxes measured directly at ground stations. This studypresents simulated sensible heat flux measurements with aircraft flightsthrough a synthetic convective boundary layer created by a401 × 401 × 42 cubic-grid large eddy simulation (LES) with agrid spacing of 50 m. After some considerations with respect to necessary measurement lengths using results ofLenschow and Stankov (1986 – J. Atmos. Sci. 43, 1198–1209), simulated measurementcampaigns were carried out in three modelruns. During each model run five sets ofmeasurement runs were carried out successively.During each set of runs 10 aircraftflew at 10 altitudes with a ground speedof 100 m s^-1 simultaneously throughtime and space. In total, 150 legs were carried out, 15 at each flight level. The resulting`measured' heat fluxes were compared withthose of the `true' flux profiles obtaineddirectly from the ensemble-averagedLES-generated data. No significant systematic error between `measured' and `true' profiles was observed. Furthermore, the comparison of the resulting relative error with the theory ofLenschow and Stankov showed a good agreement at allmeasurement levels.     

Validation of the “Lokal-Modell” over heterogeneous land surfaces using aircraft-based measurements of the REEEFA experiment and comparison with micro-scale simulations

Summary An aircraft-based experimental investigation of the atmospheric boundary layer (ABL) structure and of the energy exchange processes over heterogeneous land surfaces is presented. The measurements are used for the validation of the mesoscale atmospheric model “Lokal-Modell” (LM) of the German Weather Service with 2.8 km resolution. In addition, high-resolution simulations using the non-hydrostatic model FOOT3DK with 250 m resolution are performed in order to resolve detailed surface heterogeneities. Two special observation periods in May 1999 show comparable convective boundary layer (CBL) conditions. For one case study vertical profiles and area averages of meteorological quantities and energy fluxes are investigated in detail. The measured net radiation is highly dependent on surface albedo, and the latent heat flux exhibits a strong temporal variability in the investigation area. A reduction of this variability is possible by aggregation of multiple flight patterns. To calculate surface fluxes from aircraft measurements at low altitude, turbulent energy fluxes were extrapolated to the ground by the budget method, which turned out to be well applicable for the sensible heat flux, but not for the latent flux. The development of the ABL is well captured by the LM simulation. The comparison of spatiotemporal averages shows an underestimation of the observed net radiation, which is mainly caused by thin low-level clouds in the LM compared to observed scattered CBL clouds. The sensible heat flux is reproduced very well, while the latent flux is highly overestimated especially above forests. The realistic representation of surface heterogeneities in the investigation area in the FOOT3DK simulations leads to improvements for the energy fluxes, but an overestimation of the latent heat flux still persists. A study of upscaling effects yields more structures than the LM fields when averaged to the same scale, which are partly caused by the non-linear effects of parameter aggregation on the LM scale.     

Heat Flux Apportionment to Heterogeneous Surfaces Using Flux Footprint Analysis

Heat flux data collected from the Baiyangdian Heterogeneous Field Experiment were analyzed using the footprint method. High resolution (25 m) Landsat-5 satellite imaging was used to determine the land cover as one of four surface types: farmland, lake, wetland, or village. Data from two observation sites in September 2005 were used. One site (Wangjiazhai) was characterized by highly heterogeneous surfaces in the central area of the Baiyangdian: lake/wetland. The other site (Xiongxian) was on land with more uniform surface cover. An improved Eulerian analytical flux footprint model was used to determine "source areas" of the heat fluxes measured at towers located at each site from surrounding landscapes of mixed surface types.In relative terms results show that wetland and lake areas generally contributed most to the observed heat flux at Wangjiazhai, while farmland contributed most at Xiongxian. Given the areal distribution of surface type contributions, calculations were made to obtain the magnitudes of the heat flux from lake, wetland and farmland to the total observed flux and apportioned contributions of each surface type to the sensible and latent heat fluxes. Results show that on average the sensible heat flux from wetland and farmland were comparable over the diurnal cycle, while the latent heat flux from farmland was somewhat larger by about 30-50 W m-2 during daytime. The latent and sensible fluxes from the lake source in daytime were about 50 W m-2 and 100 W m-2 less, respectively, than from wetland and farmland. The results are judged reasonable and serve to demonstrate the potential for flux apportionment over heterogeneous surfaces.     

On the consideration of mesoscale transports in climate modelling

Summary The dynamical effect of land surface heterogeneity on heat fluxes in the atmospheric boundary layer (ABL) is investigated using numerical simulations with a non-hydrostatic model over a wide range of grid resolutions. It is commonly assumed that mesoscale or dynamical fluxes associated with mesoscale and convective circulations simulated by a high-resolution model (subgrid (SG) model) on the subgrid scale of a climate model (large-scale (LS) model) represent additional processes in the ABL, which are not considered by the turbulence scheme of the LS-model, and which can be parameterized using the SG-model. The present study investigates the usefulness of this methodology for small-scale and large-scale idealized heterogeneities using a SG-model resolving mesoscale or even microscale circulations to compute the mesoscale fluxes on the scale of the LS-model. It is shown that the dynamical transports as derived from the SG-model should not be used to correct the parameterized turbulent fluxes of the LS-model. The reason is that the subgrid circulations simulated by the SG-model interact with the fields of wind and scalars in the ABL, which results in reduced turbulent fluxes in the ABL. Thus the methodology of previous studies to use mesoscale/dynamical fluxes for the correction of flux profiles simulated by climate models seems to be questionable.     

The Effect of Surface Heterogeneity on the Structure Parameters of Temperature and Specific Humidity: A Large-Eddy Simulation Case Study for the LITFASS-2003 Experiment

We conduct a high-resolution large-eddy simulation (LES) case study in order to investigate the effects of surface heterogeneity on the (local) structure parameters of potential temperature \(C_T^2\) and specific humidity \(C_q^2\) in the convective boundary layer (CBL). The kilometre-scale heterogeneous land-use distribution as observed during the LITFASS-2003 experiment was prescribed at the surface of the LES model in order to simulate a realistic CBL development from the early morning until early afternoon. The surface patches are irregularly distributed and represent different land-use types that exhibit different roughness conditions as well as near-surface fluxes of sensible and latent heat. In the analysis, particular attention is given to the Monin–Obukhov similarity theory (MOST) relationships and local free convection (LFC) scaling for structure parameters in the surface layer, relating \(C_T^2\) and \(C_q^2\) to the surface fluxes of sensible and latent heat, respectively. Moreover we study possible effects of surface heterogeneity on scintillometer measurements that are usually performed in the surface layer. The LES data show that the local structure parameters reflect the surface heterogeneity pattern up to heights of 100–200 m. The assumption of a blending height, i.e. the height above the surface where the surface heterogeneity pattern is no longer visible in the structure parameters, is studied by means of a two-dimensional correlation analysis. We show that no such blending height is found at typical heights of scintillometer measurements for the studied case. Moreover, \(C_q^2\) does not follow MOST, which is ascribed to the entrainment of dry air at the top of the boundary layer. The application of MOST and LFC scaling to elevated \(C_T^2\) data still gives reliable estimates of the surface sensible heat flux. We show, however, that this flux, derived from scintillometer data, is only representative of the footprint area of the scintillometer, whose size depends strongly on the synoptic conditions.     

Impact of surface heterogeneity on a buoyancy-driven convective boundary layer in light winds

Land-use practices such as deforestation or agricultural management may affect regional climate, ecosystems and water resources. The present study investigates the impact of surface heterogeneity on the behaviour of the atmospheric boundary layer (ABL), at a typical spatial scale of 1 km. Large-eddy simulations, using an interactive soil–vegetation–atmosphere surface scheme, are performed to document the structure of the three-dimensional flow, as driven by buoyancy forces, over patchy terrain with different surface characteristics (roughness, soil moisture, temperature) on each individual patch. The patchy terrain consists of striped and chessboard patterns. The results show that the ABL strongly responds to the spatial configuration of surface heterogeneities. The stripe configuration made of two patches with different soil moisture contents generates the development of a quasi- two-dimensional inland breeze, whereas a three-dimensional divergent flow is induced by chessboard patterns. The feedback of such small-scale atmospheric circulations on the surface fluxes appears to be highly non-linear. The surface sensible and latent heat fluxes averaged over the 25-km² domain may vary by 5% with respect to the patch arrangement.     

A Case Study of CO<Subscript>2</Subscript>, CO and Particles Content Evolution in the Suburban Atmospheric Boundary Layer Using a 2-μm Doppler DIAL,a 1-μm Backscatter Lidar and an Array of In-situ Sensors

A network of remote and in-situ sensors was deployed in a Paris suburb in order to evaluate the mesoscale evolution of the daily cycle of CO₂ and related tracers in the atmospheric boundary layer (ABL) and its relation to ABL dynamics and nearby natural and anthropogenic sources and sinks. A 2-μm heterodyne Doppler differential absorption lidar, which combines measurements of, (1) structure of the atmosphere, (2) radial velocity, and (3) CO₂ differential absorption was a particularly unique element of the observational array. We analyse the differences in the diurnal cycle of CO, CO₂, lidar reflectivity (a proxy for aerosol content) and H₂O using the lidar, airborne measurements in the free troposphere and ground-based measurements made at two sites located few kilometres apart. We demonstrate that vertical mixing dominates the early morning drawdown of CO and aerosol content trapped in the former nocturnal layer but not the H₂O and CO₂ mixing ratio variations. Surface fluxes, vertical mixing and advection all contribute to the ABL CO₂ mixing ratio decrease during the morning transition, with the relative importance depending on the rate and timing of ABL rise. We also show evidence that when the ABL is stable, small-scale (0.1-km vertical and 1-km horizontal) gradients of CO₂ and CO are large. The results illustrate the complexity of inferring surface fluxes of CO₂ from atmospheric budgets in the stable boundary layer.     

Subfilter-Scale Modelling Using Transport Equations: Large-Eddy Simulation of the Moderately Convective Atmospheric Boundary Layer

We perform large-eddy simulation (LES) of a moderately convective atmospheric boundary layer (ABL) using a prognostic subfilter-scale (SFS) model obtained by truncating the full conservation equations for the SFS stresses and fluxes. The truncated conservation equations contain production mechanisms that are absent in eddy-diffusivity closures and, thus, have the potential to better parametrize the SFS stresses and fluxes. To study the performance of the conservation-equation-based SFS closure, we compare LES results from the surface layer with observations from the Horizontal Array Turbulence Study (HATS) experiment. For comparison, we also show LES results obtained using an eddy-diffusivity closure. Following past studies, we plot various statistics versus the non-dimensional parameter, Λ _w /Δ, where Λ _w is the wavelength corresponding to the peak in the vertical velocity spectrum and Δ is the filter width. The LES runs are designed using different domain sizes, filter widths and surface fluxes, in order to replicate partly the conditions in the HATS experiment. Our results show that statistics from the different LES runs collapse reasonably and exhibit clear trends when plotted against Λ _w /Δ. The trends exhibited by the production terms in the modelled SFS conservation equations are qualitatively similar to those seen in the HATS data with the exception of SFS buoyant production, which is underpredicted. The dominant production terms in the modelled SFS stress and flux budgets obtained from LES are found to approach asymptotically constant values at low Λ _w /Δ. For the SFS stress budgets, we show that several of these asymptotes are in good agreement with their corresponding theoretical values in the limit Λ _w /Δ → 0. The modelled SFS conservation equations yield trends in the mean values and fluctuations of the SFS stresses and fluxes that agree better with the HATS data than do those obtained using an eddy-diffusivity closure. They, however, underpredict considerably the level of SFS anisotropy near the wall when compared to observations, which could be a consequence of the shortcomings in the model used for the pressure destruction terms. Finally, we address the computational cost incurred due to the use of additional prognostic equations.