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Evaluation of Atmosphere-Land Interactions in an LES from the Perspective of Heterogeneity Propagation


doi: 10.1007/s00376-015-5212-6

  • Atmosphere-land interactions simulated by an LES model are evaluated from the perspective of heterogeneity propagation by comparison with airborne measurements. It is found that the footprints of surface heterogeneity, though as 2D patterns can be dissipated quickly due to turbulent mixing, as1D projections can persist and propagate to the top of the atmospheric boundary layer. Direct comparison and length scale analysis show that the simulated heterogeneity patterns are comparable to the observation. The results highlight the model's capability in simulating the complex effects of surface heterogeneity on atmosphere-land interactions.
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  • Albertson J. D., M. B. Parlange, 1999: Surface length scales and shear stress: Implications for land-atmosphere interaction over complex terrain. Water Resour. Res., 35, 2121- 2132.10.1029/1999WR900094c88bf56b-a7ce-4718-a478-c9e4dce512d9cc1733dc53e8827c4df3bd07fc761ce4http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F1999WR900094%2Fpdfrefpaperuri:(3967eef954dc41395c1cf1449057635e)http://onlinelibrary.wiley.com/doi/10.1029/1999WR900094/pdfA large eddy simulation (LES) code of the atmospheric boundary layer (ABL) has been developed and applied to study the effect of spatially variable surface properties on the areally averaged surface shear stress at the land-atmosphere interface. The LES code simulates the space and time evolution of the large-scale turbulent eddies and their transport effects in the ABL. We report here on simulations of flow over spatially variable roughness fields. The dynamics are simulated, and the resulting space-time fields are averaged to explore the effects of the surface variability length scales on the average surface shear stress, as used in large-scale models to estimate scalar fluxes, such as evaporation. We observe asymmetrical response of the smooth-to-rough and rough-to-smooth transitions, such that the effects of the transitions accumulate rather than cancel. It is shown that the presence of abrupt changes in surface roughness and the atmosphere's response to these patches create a marked dependence of the statistical structure of surface shear stress on the length scale of the surface patches. An increase in regionally averaged surface stress for decreasing horizontal patch length scale is found.
    Avissar R., 1991: A statistical-dynamical approach to parameterize subgrid-scale land-surface heterogeneity in climate models. Surveys in Geophysics, 12, 155- 178.10.1007/978-94-009-2155-9_8717f1f2d596f5d51d9884135887b5ee4http%3A%2F%2Flink.springer.com%2F10.1007%2FBF01903417http://link.springer.com/10.1007/BF01903417The potential application of this statistical-dynamical parameterization is illustrated by simulating (i) the development of an agricultural area in an arid region and (ii) the process of deforestation in a tropical region. Both cases emphasize the importance of land-atmosphere interactions on regional hydrologic processes and climate.
    Avissar R., 1992: Conceptual aspects of a statistical-dynamical approach to represent landscape subgrid-scale heterogeneities in atmospheric models. J. Geophys. Res., 97, 2729- 2742.10.1029/91JD017515f14725c405c9a81d1f88881cddc1823http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F91JD01751%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/91JD01751/fullLand-surface characteristics play a key role in the partition of energy received at the Earth's surface and, as a result, have a major impact on the atmosphere. Yet the representation of land-surface processes in atmospheric models is not realistic. Actual state-of-the-art parameterizations of land surfaces do not account for the landscape heterogeneity found on the resolvable scale of these models and are based on a large amount of empirical constants that in practice can be difficult to estimate. An alternative parameterization based on a statistical-dynamical approach is suggested here. With this approach, the most important characteristics of the soil-plant-atmosphere system that affect the partition of energy (e.g., plant stomatal conductance, soil humidity, surface roughness) would be represented by a probability density function (pdf) rather than by a single -渞epresentative- value. Typically, such pdf's are characterized by two to four parameters. A primary simplified version of this parameterization was used to estimate the land-surface energy fluxes that are produced at the grid scale by various distributions of stomatal conductance under a broad range of environmental conditions. To demonstrate the potential of the approach, results were compared with the same fluxes calculated with a big leaf model using the mean stomatal conductance that corresponds to the distributions. Large absolute and relative differences are obtained between the two schemes for many combinations of stomatal conductance pdf's and environmental conditions. These differences are due mainly to the nonlinearity of the processes involved in the redistribution of the energy absorbed at the ground surface. These numerical experiments demonstrate the importance of accounting for landscape heterogeneity in the simulation of land-surface energy fluxes, and demonstrate the potential benefits of adopting a statistical-dynamical approach for the representation of land-surface processes in atmospheric models.
    Avissar R., R. A. Pielke, 1989: A parameterization of heterogeneous land surfaces for atmospheric numerical models and its impact on regional meteorology. Mon. Wea. Rev., 117, 2113- 2136.10.1175/1520-0493(1989)117<2113:APOHLS>2.0.CO;224f77dac58a0144ac3fd33c023aa4fbahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1989MWRv..117.2113Ahttp://adsabs.harvard.edu/abs/1989MWRv..117.2113AAbstract Natural land surfaces are usually heterogeneous over the resolvable scales considered in atmospheric numerical models. Therefore, model surface parameterizations that assume surface homogeneity may fail to represent the surface forcing accurately. In this paper, a parameterization of the subgrid-scale forcing of heterogeneous land surfaces for atmospheric numerical models is suggested. In each surface grid element of the numerical model similar homogeneous land patches located at different places within the element are regrouped into subgrid classes. Then, for each one of the subgrid classes, a sophisticated micrometeorological model of the soil-plant-atmosphere system is applied to assess the surface temperature, humidity, and fluxes to the atmosphere. The global fluxes of energy between the grid and the atmosphere are obtained by averaging according to the distribution of the subgrid classes. In addition to the surface forcing, detailed micrometeorological conditions of the patches are assessed for the domain simulated by the atmospheric model. This parameterization was incorporated into a mesoscale numerical model to test the impact of subgrid-scale land surface heterogeneities on the development of local circulations. Where strong contrasts in total sensible heat flux are generated by land surface heterogeneities, circulations as strong as sea breezes may develop.
    Avissar R., T. Schmidt, 1998: An evaluation of the scale at which ground-surface heat flux patchiness affects the convective boundary layer using large-eddy simulations. J. Atmos. Sci., 55, 2666- 2689.10.1175/1520-0469(1998)055<2666:AEOTSA>2.0.CO;20f47a267548bfbfeec4667420f86978bhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1998JAtS...55.2666Ahttp://adsabs.harvard.edu/abs/1998JAtS...55.2666APresents a study which investigated the effects of surface heterogeneities produced by surface sensible heat flux waves, with different means, amplitudes and wavelengths on the convective boundary layer (CBL). Examination of the scale surface in which heterogeneity starts to significantly affect the heat fluxes in the CBL; Use of the large-eddy simulation option of the Regional Atmospheric Modeling System; Where the system was developed.
    Brunsell N. A., D. B. Mechem, and M. C. Anderson, 2011: Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning. Atmos. Chem. Phys., 11, 3403- 3416.10.5194/acp-11-3403-2011471a09abc12130a352bd5f16f4e5ff39http%3A%2F%2Fwww.oalib.com%2Fpaper%2F1371035http://www.oalib.com/paper/1371035The role of land-atmosphere interactions under heterogeneous surface conditions is investigated in order to identify mechanisms responsible for altering surface heat and moisture fluxes. Twelve coupled land surface - large eddy simulation scenarios with four different length scales of surface variability under three different horizontal wind speeds are used in the analysis. The base case uses Landsat ETM imagery over the Cloud Land Surface Interaction Campaign (CLASIC) field site for 3 June 2007. Using wavelets the surface fields are band-pass filtered in order to maintain the spatial mean and variances to length scales of 200m, 1600m, and 12.8km as lower boundary conditions to the model (approximately 0.25, 1.2 and 9.5 times boundary layer height). The simulations exhibit little variation in net radiation. Rather, there is a pronounced change in the partitioning of the surface energy between sensible and latent heat flux. The sensible heat flux is dominant for intermediate surface length scales. For smaller and larger scales of surface heterogeneity, which can be viewed as being more homogeneous, the latent heat flux becomes increasingly important. The simulations showed approximately 50Wm-2 difference in the spatially averaged latent heat flux. The results reflect a general decrease of the Bowen ratio as the surface conditions transition from heterogeneous to homogeneous. Air temperature is less sensitive to variations in surface heterogeneity than water vapor, which implies that the role of surface heterogeneity may be to maximize convective heat fluxes through modifying and maintaining local temperature gradients. More homogeneous surface conditions (i.e. smaller length scales), on the other hand, tend to maximize latent heat flux. The intermediate scale (1600 m) this does not hold, and is a more complicated interaction of scales. Scalar vertical profiles respond predictably to the partitioning of surface energy. Fourier spectra of the vertical wind speed, air temperature and specific humidity (?~, ?~ and ?~) and associated cospectra (?~ ?~, ?~ ?~ and ?~?~ ), however, are insensitive to the length scale of surface heterogeneity, but the near surface spectra are sensitive to the mean wind speed.
    Chen F., J. Dudhia, 2001: Coupling an advanced land surface-hydrology model with the Penn State-NCAR MM5 modeling system. Part I: Model implementation and sensitivity. Mon. Wea. Rev., 129, 569- 585.10.1175/1520-0493(2001)129<0569:CAALSH>2.0.CO;2609cbe5b-5261-4d8f-a70c-f188da93725a274bd04e07139af864f5aef6dc50c54ahttp://www.researchgate.net/publication/51996388_Coupling_an_Advanced_Land_SurfaceHydrology_Model_with_the_Penn_StateNCAR_MM5_Modeling_System._Part_I_Model_Implementation_and_Sensitivityhttp://www.researchgate.net/publication/51996388_Coupling_an_Advanced_Land_SurfaceHydrology_Model_with_the_Penn_StateNCAR_MM5_Modeling_System._Part_I_Model_Implementation_and_SensitivityPart I. Focuses on issues related to the implementation of an advanced land surface-hydrology model in the Pennsylvania State University-National Center for Atmospheric Research fifth-generation Mesoscale Model. Selection of the land surface model (LSM); Brief description of the soil thermodynamics and soil hydrology of the LSM; Initialization of soil moisture.
    Deardorff J. W., 1970: A numerical study of three-dimensional turbulent channel flow at large Reynolds numbers. J. Fluid Mech., 41, 453- 480.10.1017/S0022112070000691b2e1bc881b640fad882d8d90085bfb87http%3A%2F%2Fjournals.cambridge.org%2Fabstract_S0022112070000691http://journals.cambridge.org/abstract_S0022112070000691The three-dimensional, primitive equations of motion have been integrated numerically in time for the case of turbulent, plane Poiseuille flow at very large Reynolds numbers. A total of 6720 uniform grid intervals were used, with sub-grid scale effects simulated with eddy coefficients proportional to the local velocity deformation. The agreement of calculated statistics against those measured by Laufer ranges from good to marginal. The eddy shapes are examined, and only the u-component, longitudinal eddies are found to be elongated in the downstream direction. However, the lateral v eddies have distinct downstream tilts. The turbulence energy balance is examined, including the separate effects of vertical diffusion of pressure and local kinetic energy.
    Deardorff J. W., 1972: Numerical investigation of neutral and unstable planetary boundary layers. J. Atmos. Sci., 29, 91- 115.10.1175/1520-0469(1972)029<0091:NIONAU>2.0.CO;2a2b9a0ad27eb22f387be259c842e0869http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1972jats...29...91dhttp://adsabs.harvard.edu/abs/1972jats...29...91dAbstract Results of numerical integrations are presented for a neutrally stratified planetary boundary layer containing a passive scalar, and for three unstable cases with upward heat flux. The air is assumed unsaturated. A total of either 16,000 or 32,000 grid points was used in a three-dimensional region with length and width several times the height of the boundary layer. A key result is the irrelevance of the neutral height scale, u * / f , and its replacement by the height z i of the inversion base which confines the convective mixing when &minus z i / L is as small as 1.5 ( L is the Monin-Obukhoy length). Shapes of the eddies are examined for &minus z i / L =0, 1.5, 4.5 and 45; and only for the two slightly unstable cases were the vertical velocity eddies distinctly elongated as in Ekman-layer theories. At large instabilities it is shown how the friction velocity u / * loses its influence upon the turbulence intensifies and a convective velocity wale becomes important. Vertical profiles of mean wind, potential temperature, momentum flux, gross eddy coefficients, flux correlation coefficients, turbulence intensifies, temperature variance and pressure fluctuations are presented and compared, when possible, with measurements. Comparison is hindered by the lack of observations of z i and L in almost all field studies. Various terms in the turbulence kinetic energy equation, which are difficult to measure, are discussed quantitatively. The rate at which particles released near the surface are transported vertically by the calculated turbulence is found from Lagrangian integrations to be up to two orders of magnitude greater in unstable cases than in a typical neutral case.
    Giorgi F., R. Avissar, 1997: Representation of heterogeneity effects in earth system modeling: Experience from land surface modeling. Rev. Geophys., 35, 413- 438.10.1029/97RG017548135a64e8ad407df4625ef0bd0e95721http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F97RG01754%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1029/97RG01754/citedbyABSTRACT The land surface is characterized by pro- nounced spatial heterogeneity that spans a wide range of scales. This heterogeneity affects the surface energy and water budgets, as well as the land-atmosphere exchanges of momentum, heat, water and other constituents, through a number of highly nonlinear processes. The resolution of present-day Earth (or climate) system models is still too coarse to explicitly capture the effects of surface heterogeneity, which therefore needs to be parameterized within the framework of complex and nonlinear land surface process schemes. The effects of surface heterogeneity are here grouped in two catego- ries, which we define as "aggregation" and "dynamical" effects. Models of aggregation effects attempt to calcu- late the contribution of different subgrid scale surface types to the grid box average energy and water budgets and surface-atmosphere exchanges. Such models have been based on discrete approaches, whereby heteroge- neity is described in terms of a finite number of subgrid "tiles" or "patches," and on continuous approaches, in which heterogeneity is described in terms of probability density functions. Subgrid scale aggregation has been shown to especially affect the surface latent and sensible heat fluxes, the simulation of snow, and the dynamics of soil moisture and runoff. Dynamical heterogeneity ef- fects are associated with microscale and mesoscale cir- culations induced by heterogeneous surfaces. These cir- culations can influence boundary layer structure, cloud formation, precipitation, and vertical transfer of mo- mentum, energy, and water up to the midtroposphere. In the last decade or so, the importance of land surface heterogeneity representation has been increasingly rec- ognized in a large number of new studies. This paper reviews and critically discusses different approaches that have been proposed to represent aggregation and dy- namical effects of surface heterogeneity and their incor- poration in land surface process schemes. Some of the methodologies discussed in this paper are of general nature and therefore can be of interest for problems of subgrid scale process description in other geophysical disciplines.
    Hechtel L. M., R. B. Stull, and C.-H. Moeng, 1990: The effects of nonhomogeneous surface fluxes on the convective boundary layer: A case study using large-eddy simulation. J. Atmos. Sci., 47, 1721- 1741.10.1175/1520-0469(1990)047<1721:TEONSF>2.0.CO;2a65b5fcf459e52e35047392d1dec5373http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1990JAtS...47.1721Hhttp://adsabs.harvard.edu/abs/1990JAtS...47.1721HAbstract Most land surfaces are quasi-randomly nonhomogeneous, yet most boundary-layer studies assume homogeneous or simply varying surface conditions. In this study, nonhomogeneous surface fluxes of realistic scale and amplitude are applied to a large-eddy-simulation (LES) model. The boundary conditions are representative of conditions observed near Chickasha, Oklahoma during the Boundary Layer Experiment 1983 (BLX83), while the model is a modified version of Moeng's LES model. The afternoon of 28 May 1983, which had light winds and cloud-free skies, is simulated by using data collected during BLX83 to provide initial and boundary conditions; the simulation verifies fairly well against observations. A second simulation of the case-study afternoon uses horizontally homogeneous surface fluxes. The results of the two runs are compared to see what effect the quasi-random nonhomogeneous conditions have on mixed-layer development. The inclusion of realistic size (450-900 m) and amplitude (2 (C) 2 variance in surface temperature) non-homogeneities in a model of this resolution (50 m in the vertical, 150 m in the horizontal) does not in this case change the development of the mixed layer. The two runs show no significant differences in area-averaged statistics, possibly because of the nonzero wind which was present during the simulated afternoon. In the nonhomogeneous case, there is no evidence that thermals are anchored to or are preferentially forming over certain surface features, and there is no change in the thermal structure as evidenced by the power spectra of temperature, moisture, or vertical velocity. Until our understanding of the physical processes involved in ground-to-atmosphere heat and moisture transfer improve, and until LES models are made more sensitive to such transfer, it appears that the use of horizontally homogeneous bottom boundary conditions is sufficient to adequately simulate the development of the boundary layer during combined free and forced convection.
    Huang H.-Y., S. A. Margulis, 2010: Evaluation of a fully coupled large-eddy simulation-land surface model and its diagnosis of land-atmosphere feedbacks. Water Resour. Res., 46,W06512, doi: 10.1029/2009WR008232.f5ee2cdcfd5466e08c988e466f9b71f4http%3A%2F%2Fd.scholar.cnki.net%2Fdetail%2FSJWCTEMP_U%2FSJWC12111501072264http://d.scholar.cnki.net/detail/SJWCTEMP_U/SJWC12111501072264
    Huang H.-Y., B. Stevens, and S. A. Margulis, 2008: Application of dynamic subgrid-scale models for large-eddy simulation of the daytime convective boundary layer over heterogeneous surfaces. Bound.-Layer Meteor., 126, 327- 348.10.1007/s10546-007-9239-98c884adceb98150aa656a4fc354f1fb5http%3A%2F%2Flink.springer.com%2F10.1007%2Fs10546-007-9239-9http://link.springer.com/10.1007/s10546-007-9239-9The sensitivity of large-eddy simulation (LES) to the representation of subgrid-scale (SGS) processes is explored for the case of the convective boundary layer (CBL) developing over surfaces with varying degrees of spatial heterogeneity. Three representations of SGS processes are explored: the traditional constant Smagorinsky-illy model and two other dynamic models with Lagrangian averaging approaches to calculate the Smagorinsky coefficient ( C S ) and SGS Prandtl number ( Pr ). With initial data based roughly on the observed meteorology, simulations of daytime CBL growth are performed over surfaces with characteristics (i.e. fluxes and roughness) ranging from homogeneous, to striped heterogeneity, to a realistic representation of heterogeneity as derived from a recent field study. In both idealized tests and the realistic case, SGS sensitivities are mostly manifest near the surface and entrainment zone. However, unlike simulations over complex domains or under neutral or stable conditions, these differences for the CBL simulation, where large eddies dominate, are not significant enough to distinguish the performance of the different SGS models, irrespective of surface heterogeneity.
    Huang J. P., X. Lee, and E. G. Patton, 2009: Dissimilarity of scalar transport in the convective boundary layer in inhomogeneous landscapes. Bound.-Layer Meteor., 130, 327- 345.10.1007/s10546-009-9356-8c344037022acfc153dc574fd0280e4dfhttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs10546-009-9356-8http://link.springer.com/10.1007/s10546-009-9356-8A 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 1009.12%, 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 309.8% and overestimates the water vapour and CO2 fluxes by 209.8% and 109.9%, respectively, as compared to the flux simulated with the coupled LES-LSM.
    Letzel M. O., S. Raasch, 2003: Large eddy simulation of thermally induced oscillations in the convective boundary layer. J. Atmos. Sci., 60, 2328- 2341.a438434ea9ae728a00238f8ce739934ahttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2003JAtS...60.2328L/s?wd=paperuri%3A%283cc84596a8da7578f618e13b066d90b1%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2003JAtS...60.2328L&ie=utf-8
    Liu S. F., Y. P. Shao, 2013: Soil-layer configuration requirement for large-eddy atmosphere and land surface coupled modeling. Atmospheric Science Letters, 14, 112- 117.10.1002/asl2.42645215850f4280aa6244222c69078f54fhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fasl2.426%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1002/asl2.426/citedbyABSTRACT To investigate the soil-layer configuration requirement for numerical studying large-eddy scale atmosphere and land surface interactions, we conduct two experiments with an atmosphere and land surface coupled large-eddy model, one with the &lsquo;usual&rsquo; soil-layer configuration and the other with a finer one. It is found that the former configuration prevents the soil to respond to atmospheric large eddies and the soil acts solely as a fixed external condition to the atmospheric turbulence, and is thus inadequate for large-eddy atmosphere and land surface coupled modeling. It is shown that soil-layer configuration has a profound impact on the simulated atmospheric boundary layer dynamics. Copyright 漏 2013 Royal Meteorological Society
    Liu S. F., Y. P. Shao, M. Hintz, and S. Lennartz-Sassinek, 2015: Multiscale decomposition for heterogeneous land-atmosphere systems. J. Geophys. Res., 120, 917- 930.7375b834-2916-479c-a50d-2bae0f508dd1bb0d94202f7431565e8260cb4cce6de1http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2014JD022258%2Fabstract/s?wd=paperuri%3A%288dc6671375d0050c169556c32e5c1d77%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2F2014jd022258%2Fabstract&ie=utf-8
    Mahrt L., J. L. Sun, D. Vickers, J. I. MacPherson, J. R. Pederson, and R. L. Desjardins, 1994: Observations of fluxes and inland breezes over a heterogeneous surface. J. Atmos. Sci., 51, 2484- 2499.10.1175/1520-0469(1994)0512.0.CO;2213640f770c2ec7b3f6716c7f5d20130http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1994JAtS...51.2484Mhttp://adsabs.harvard.edu/abs/1994JAtS...51.2484MRepeated aircraft runs at about 33 m over heterogeneous terrain are analyzed to study the spatial variability of the mesoscale flow and turbulent fluxes. An irrigated area, about 12 km across, generates a relatively cool moist inland breeze. As this air flows out over the warmer, drier surrounding land surface, an internal boundary layer develops within the inland breeze, which then terminates at a well-defined inland breeze front located about 1陆 km downstream from the change of surface conditions. This front is defined by horizontal convergence, rising motion, and sharp spatial change of moisture, carbon dioxide, and ozone.Both a scale analysis and the observations suggest that the overall vertical motion associated with the inland breeze is weak. However, the observations indicate that this vertical motion and attendant vertical transport are important in the immediate vicinity of the front, and the inland breeze does lead to significant modification of the turbulent flux. In the inland breeze downstream from the surface wetness discontinuity, strong horizontal advection of moisture is associated with a rapid increase of the turbulent moisture flux with height. This large moisture flux appears to be partly due to mixing between the thin moist inland breeze and overlying drier air.As a consequence of the strong vertical divergence of the flux in the transition regions, the fluxes measured even as low as a few tens of meters are not representative of the surface fluxes. The spatial variability of the fluxes is also interpreted within the footprint format. Attempts are made to reconcile predictions by footprint and internal boundary-layer approaches.
    Moeng C.-H., 1984: A large-eddy-simulation model for the study of planetary boundary-layer turbulence. J. Atmos. Sci., 41, 2052- 2062.87dce47ec4dd4de863420f2313fe403chttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1984jats...41.2052m/s?wd=paperuri%3A%28ddc909f763bf59be9f0020bacb73fe6b%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1984jats...41.2052m&ie=utf-8
    Moeng C.-H., J. Dudhia, J. Klemp, and P. Sullivan, 2007: Examining two-way grid nesting for large eddy simulation of the PBL using the WRF model. Mon. Wea. Rev., 135, 2295- 2311.10.1175/MWR3406.153c2a6ec92dfc4a6f8c51ca6fb7dacafhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2007MWRv..135.2295Mhttp://adsabs.harvard.edu/abs/2007MWRv..135.2295MThe performance of two-way nesting for large eddy simulation (LES) of PBL turbulence is investigated using the Weather Research and Forecasting model framework. A pair of LES-within-LES experiments are performed where a finer-grid LES covering a smaller horizontal domain is nested inside a coarser-grid LES covering a larger horizontal domain. Both LESs are driven under the same environmental conditions, allowed to interact with each other, and expected to behave the same statistically. The first experiment of the free-convective PBL reveals a mean temperature bias between the two LES domains, which generates a nonzero mean vertical velocity in the nest domain while the mean vertical velocity averaged over the outer domain remains zero. The problem occurs when the horizontal extent of the nest domain is too small to capture an adequate sample of energy-containing eddies; this problem can be alleviated using a nest domain that is at least 5 times the PBL depth in both x and y. The second experiment of the neutral PBL exposes a bias in the prediction of the surface stress between the two LES domains, which is found due to the grid dependence of the Smagorinsky-type subgrid-scale (SGS) model. A new two-part SGS model is developed to solve this problem.
    Neininger B., W. Fuchs, M. Bäeumle, A. Volz-Thomas, A. S. H. Prévôt, and J. Dommen, 2001: A small aircraft for more than just ozone: MetAir's `Dimona' after ten years of evolving development. Proc.,The 11th Symposium on Meteorological Observations and Instrumentation, American Meteorology Society, Albuquerque, N. M.c977393052ecb44cfd604ac0eea70622http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F240641437_A_SMALL_AIRCRAFT_FOR_MORE_THAN_JUST_OZONE_METAIR%27S_%27DIMONA%27_AFTER_TEN_YEARS_OF_EVOLVING_DEVELOPMENThttp://www.researchgate.net/publication/240641437_A_SMALL_AIRCRAFT_FOR_MORE_THAN_JUST_OZONE_METAIR'S_'DIMONA'_AFTER_TEN_YEARS_OF_EVOLVING_DEVELOPMENTAs Crawford et al. (in this issue) are proposing, more and more environmental parameters can be cap- tured by compact sensors, enabling small aircraft as suitable carriers for state-of-the-art sensors (concept SERA: The Small Environmental Research Aircraft).MetAir has started in 1990 to equip advanced, self launching double-seated motorgliders with long endur- ance with a variety of meteorological and chemical sensors.It was possible to equip and certify two aircraft** with large underwing pods. In each of the two pods, 50 kg of equipment can be flown in a basically undisturbed environment. Up to another 30 kg can be carried in the fuselage.
    Patton E. G., P. P. Sullivan, and C.-H. Moeng, 2005: The influence of idealized heterogeneity on wet and dry planetary boundary layers coupled to the land surface. J. Atmos. Sci., 62, 2078- 2097.10.1175/JAS3465.1a4991a50090ff57a1bed004db64bae81http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005JAtS...62.2078Phttp://adsabs.harvard.edu/abs/2005JAtS...62.2078PAbstract This manuscript describes numerical experiments investigating the influence of 2–30-km striplike heterogeneity on wet and dry convective boundary layers coupled to the land surface. The striplike heterogeneity is shown to dramatically alter the structure of the convective boundary layer by inducing significant organized circulations that modify turbulent statistics. The impact, strength, and extent of the organized motions depend critically on the scale of the heterogeneity λ relative to the boundary layer height z i . The coupling with the land surface modifies the surface fluxes and hence the circulations resulting in some differences compared to previous studies using fixed surface forcing. Because of the coupling, surface fluxes in the middle of the patches are small compared to the patch edges. At large heterogeneity scales ( λ / z i 6518) horizontal surface-flux gradients within each patch are strong enough to counter the surface-flux gradients between wet and dry patches allowing the formation of small cells within the patch coexisting with the large-scale patch-induced circulations. The strongest patch-induced motions occur in cases with 4 < λ / z i < 9 because of strong horizontal pressure gradients across the wet and dry patches. Total boundary layer turbulence kinetic energy increases significantly for surface heterogeneity at scales between λ / z i = 4 and 9; however, entrainment rates for all cases are largely unaffected by the striplike heterogeneity. Velocity and scalar fields respond differently to variations of heterogeneity scale. The patch-induced motions have little influence on total vertical scalar flux, but the relative contribution to the flux from organized motions compared to background turbulence varies with heterogeneity scale. Patch-induced motions are shown to dramatically impact point measurements in a free-convective boundary layer. The magnitude and sign of this impact depends on the location of the measurement within the region of heterogeneity.
    Raasch S., G. Harbusch, 2001: An analysis of secondary circulations and their effects caused by small-scale surface inhomogeneities using large-eddy simulation. Bound.-Layer Meteor., 101, 31- 59.10.1023/A:10192975041092547c7b4e522af23a4d1fd34a200057ahttp%3A%2F%2Flink.springer.com%2F10.1023%2FA%3A1019297504109http://link.springer.com/10.1023/A:1019297504109From previous findings it has often been stated that moderate backgroundwinds of 5 m s 611 eliminate all impacts of surface inhomogeneitiesthat could potentially be produced in realistic landscapes. However, this studyshows that the effects caused by increasing the wind speed stronglydepend on the wind direction relative to the orientation of theinhomogeneities. Secondary circulations remain strong, even for abackground wind of 7.5 m s 611 , when the wind direction is orientatedalong one of the two diagonals of the chessboard pattern. On the otherhand, the effects of inhomogeneities are considerably reduced, even undera modest background wind of 2.5 m s 611 , if the wind direction isturned by 45°. Mechanisms for the different flow regimesare discussed.
    Shao Y. P., S. F. Liu, J. H. Schween, and S. Crewell, 2013: Large-eddy atmosphere-land-surface modelling over heterogeneous surfaces: Model development and comparison with measurements. Bound.-Layer Meteor., 148, 333- 356.10.1007/s10546-013-9823-07307246a-9207-4106-ae24-caab097acf136e8e0be0223c3d73e15af3966fe997dbhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs10546-013-9823-0refpaperuri:(5a8125d746f4d387544f80b2f8267ca7)http://link.springer.com/article/10.1007/s10546-013-9823-0ABSTRACT A model is developed for the large-eddy simulation (LES) of heterogeneous atmosphere and land-surface processes. This couples a LES model with a land-surface scheme. New developments are made to the land-surface scheme to ensure the adequate representation of atmosphere&ndash;land-surface transfers on the large-eddy scale. These include, (1) a multi-layer canopy scheme; (2) a method for flux estimates consistent with the large-eddy subgrid closure; and (3) an appropriate soil-layer configuration. The model is then applied to a heterogeneous region with 60-m horizontal resolution and the results are compared with ground-based and airborne measurements. The simulated sensible and latent heat fluxes are found to agree well with the eddy-correlation measurements. Good agreement is also found in the modelled and observed net radiation, ground heat flux, soil temperature and moisture. Based on the model results, we study the patterns of the sensible and latent heat fluxes, how such patterns come into existence, and how large eddies propagate and destroy land-surface signals in the atmosphere. Near the surface, the flux and land-use patterns are found to be closely correlated. In the lower boundary layer, small eddies bearing land-surface signals organize and develop into larger eddies, which carry the signals to considerably higher levels. As a result, the instantaneous flux patterns appear to be unrelated to the land-use patterns, but on average, the correlation between them is significant and persistent up to about 650 m. For a given land-surface type, the scatter of the fluxes amounts to several hundred W $\text{ m }^{-2}$ , due to (1) large-eddy randomness; (2) rapid large-eddy and surface feedback; and (3) local advection related to surface heterogeneity.
    Skamarock, W. C., Coauthors, 2008: A description of the advanced research WRF version 3. NCAR Tech. Note NCAR/TN-475+STR,113 pp.133fdf5edd3fc85654e5fe959ecf2a0ahttp%3A%2F%2Fntis.library.gatech.edu%2Fhandle%2F123456789%2F2086http://ntis.library.gatech.edu/handle/123456789/2086The development of the Weather Research and Forecasting (WRF) modeling system is a multi-agency effort intended to provide a next-generation mesoscale forecast model and data assimilation system that will advance both the understanding and prediction of mesoscale weather and accelerate the transfer of research advances into operations. The model is being developed as a collaborative effort among the NCAR Mesoscale and Microscale Meteorology (MMM) Division, the National Oceanic and Atmospheric Administration's (NOAA) National Centers for Environmental Prediction (NCEP) and Forecast System Laboratory (FSL), the Department of Defense's Air Force Weather Agency (AFWA) and Naval Research Laboratory (NRL), the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma, and the Federal Aviation Administration (FAA), along with the participation of a number of university scientists. The WRF model is designed to be a flexible, state-of-the-art, portable code that is efficient in a massively parallel computing environment. A modular single-source code is maintained that can be configured for both research and operations. It offers numerous physics options, thus tapping into the experience of the broad modeling community. Advanced data assimilation systems are being developed and tested in tandem with the model. WRF is maintained and supported as a community model to facilitate wide use, particularly for research and teaching, in the university community. It is suitable for use in a broad spectrum of applications across scales ranging from meters to thousands of kilometers. Such applications include research and operational numerical weather prediction (NWP), data assimilation and parameterized-physics research, downscaling climate simulations, driving air quality models, atmosphere-ocean coupling, and idealized simulations (e.g boundary-layer eddies, convection, baroclinic waves). With WRF as a common tool in the university and operational centers, closer ties will be promoted between these communities, and research advances will have a direct path to operations. These hallmarks make the WRF modeling system unique in the history of NWP in the United States.
    Smagorinsky J., 1963: General circulation experiments with the primitive equations. Part I: The basic experiment. Mon. Wea. Rev., 91, 99- 164.2be9befd-2335-44f9-bc14-9a6324b4f80902139718c887652643cddd7a4930009dhttp%3A%2F%2Fwww.bioone.org%2Fservlet%2Flinkout%3Fsuffix%3Di1551-5036-68-sp1-89-bibr037%26dbid%3D16%26doi%3D10.2112%252FSI68-012.1%26key%3D10.1175%252F1520-0493%281963%290912.3.CO%253B2refpaperuri:(f63a984bf86ce07f235b9c7d1e814f60)/s?wd=paperuri%3A%28f63a984bf86ce07f235b9c7d1e814f60%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fwww.bioone.org%2Fservlet%2Flinkout%3Fsuffix%3Di1551-5036-68-sp1-89-bibr037%26dbid%3D16%26doi%3D10.2112%252FSI68-012.1%26key%3D10.1175%252F1520-0493%281963%290912.3.CO%253B2&ie=utf-8
    Sullivan P. P., J. C. McWilliams, and C.-H. Moeng, 1994: A subgrid-scale model for large-eddy simulation of planetary boundary-layer flows. Bound.-Layer Meteor., 71, 247- 276.10.1007/BF0071374151ae389bea9a1094d3668079bf014cbfhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2FBF00713741http://link.springer.com/article/10.1007/BF00713741A long-standing problem in large-eddy simulations (LES) of the planetary boundary layer (PBL) is that the mean wind and temperature profiles differ from the Monin-Obukhov similarity forms in the surface layer. This shortcoming of LES has been attributed to poor grid resolution and inadequate sub-grid-scale (SGS) modeling. We study this deficiency in PBL LES solutions calculated over a range of shear and buoyancy forcing conditions. The discrepancy from similarity forms becomes larger with increasing shear and smaller buoyancy forcing, and persists even with substantial horizontal grid refinement. With strong buoyancy forcing, however, the error is negligible. In order to achieve better agreement between LES and similarity forms in the surface layer, a two-part SGS eddy-viscosity model is proposed. The model preserves the usual SGS turbulent kinetic energy formulation for the SGS eddy viscosity, but it explicitly includes a contribution from the mean flow and a reduction of the contributions from the turbulent fluctuations near the surface. Solutions with the new model yield increased fluctuation amplitudes near the surface and better correspondence with similarity forms out to a distance of 0.1-0.2 times the PBL depth, i.e., a typical surface-layer depth. These results are also found to be independent of grid anisotropy. The new model is simple to implement and computationally inexpensive.
    Vereecken H., S. Kollet, and C. Simmer, 2010: Patterns in soil-vegetation-atmosphere systems: Monitoring, modeling, and data assimilation. Vadose Zone Journal, 9, 821- 827.10.2136/vzj2010.012255b6d79f-24c1-46e7-8432-c5852720e1cc4861d86c453d07cf682a1e0eaa892e89http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20113046342.htmlrefpaperuri:(4076de696beddc2134e64cba6a6dffac)http://www.cabdirect.org/abstracts/20113046342.htmlIn this special issue, we present recent scientific work that analyzes the role of patterns in soil-vegetation-atmosphere (SVA) systems over a wide range of scales ranging from the pore scale up to mesoscale catchments. Specific attention is given to the development of novel data assimilation methods, noninvasive measurement techniques that allow mapping spatial patterns of state variables and ...
    Wood N., P. J. Mason, 1991: The influence of static stability on the effective roughness lengths for momentum and heat transfer. Quart. J. Roy. Meteor. Soc., 117, 1025- 1056.10.1002/qj.497117501086c9503984ac18db7796fecef50b1ded2http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.49711750108%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/qj.49711750108/fullABSTRACT The area-averaged properties of the planetary boundary layer over heterogeneous terrain are considered. Previous studies which dealt with the average momentum transfer properties in neutral static stability conditions are extended to include the influence of stratification and also area-averaged properties for heat transfer. Results from numerical simulations demonstrate the utility of values of effective roughness length for both momentum and heat transfer. A heuristic model is found to show good agreement with the numerical simulations and to provide a simple method for estimating these roughness lengths for use in boundary-layer parametrization.Significant anomalies in the surface heat fluxes, particularly those of sensible heat, accompanied the decrease in the sea ice concentration. Substantial atmospheric warming was simulated over and in the vicinity of areas in which leads were considered. In all but one experiment there were anomalous easterlies between about 40 and 60S with westerly anomalies further to the south. The surface pressure at high latitudes appears to change in a consistent fashion with the fraction of open water, with the largest changes occurring in the Weddell and near the Ross Seas.Some of the feedbacks which may enhance the responses here, but which are not included in our model, are discussed.
    Zacharias S., M. Reyers, J. G. Pinto, J. H. Schween, S. Crewell, and M. Kerschgens, 2012: Heat and moisture budgets from airborne measurements and high-resolution model simulations. Meteor. Atmos. Phys., 117, 47- 61.10.1007/s00703-012-0188-6d399e78a154b7d053606eaf2bbb9d542http%3A%2F%2Fwww.springerlink.com%2Fcontent%2Fe67j80294105l474%2Fhttp://www.springerlink.com/content/e67j80294105l474/High-resolution simulations with a mesoscale model are performed to estimate heat and moisture budgets of a well-mixed boundary layer. The model budgets are validated against energy budgets obtained from airborne measurements over heterogeneous terrain in Western Germany. Time rate of change, vertical divergence, and horizontal advection for an atmospheric column of air are estimated. Results show that the time trend of specific humidity exhibits some deficiencies, while the potential temperature trend is matched accurately. Furthermore, the simulated turbulent surface fluxes of sensible and latent heat are comparable to the measured fluxes, leading to similar values of the vertical divergence. The analysis of different horizontal model resolutions exhibits improved surface fluxes with increased resolution, a fact attributed to a reduced aggregation effect. Scale-interaction effects could be identified: while time trends and advection are strongly influenced by mesoscale forcing, the turbulent surface fluxes are mainly controlled by microscale processes.
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Manuscript received: 23 September 2015
Manuscript revised: 03 November 2015
Manuscript accepted: 13 November 2015
通讯作者: 陈斌, bchen63@163.com
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Evaluation of Atmosphere-Land Interactions in an LES from the Perspective of Heterogeneity Propagation

  • 1. Institute of Geophysics and Meteorology, University of Cologne, 50969, Germany
  • 2. National Satellite Meteorological Center, China Meteorological Administration, Beijing 100081

Abstract: Atmosphere-land interactions simulated by an LES model are evaluated from the perspective of heterogeneity propagation by comparison with airborne measurements. It is found that the footprints of surface heterogeneity, though as 2D patterns can be dissipated quickly due to turbulent mixing, as1D projections can persist and propagate to the top of the atmospheric boundary layer. Direct comparison and length scale analysis show that the simulated heterogeneity patterns are comparable to the observation. The results highlight the model's capability in simulating the complex effects of surface heterogeneity on atmosphere-land interactions.

1. Introduction
  • The atmosphere interacts closely with the land surface through fluxes of energy, mass and momentum at the interface. The land surface in nature is heterogeneous in certain ways, and biases in flux estimates due to surface heterogeneity have long been recognized (e.g., Avissar and Pielke, 1989). The effects of surface heterogeneity that are closely related to the high nonlinearity of the atmosphere-land interaction are known as aggregation effects (Giorgi and Avissar, 1997). In practice, aggregation effects can be more or less represented by the effective parameter approach (e.g., Wood and Mason, 1991; Mahrt et al., 1994), the PDF method (e.g., Avissar, 1991, 1992), or the mosaic approach (e.g., Avissar and Pielke, 1989). The situation can be more complex when the heterogeneities of the surface are sufficiently strong to induce significant circulations. The effects of these circulations are known as dynamical heterogeneity effects (Giorgi and Avissar, 1997) and their impact usually extends beyond the atmospheric surface layer. To represent the complex dynamical effects in atmospheric models, general mechanisms need to be developed based on high-resolution 4D atmosphere-land data. This kind of data is extremely difficult, if not impossible, to obtain through field measurements. A more convenient way of obtaining such high-quality data is by LES modeling. The strength of LES lies in its explicit calculation of the energy-containing turbulent eddies with the unresolved small eddies parameterized.

    LES models have been under development since the 1960s (Smagorinsky, 1963; Deardorff, 1970, 1972; Moeng, 1984; Sullivan et al., 1994; Moeng et al., 2007). Models are developed for application and model application in turn relies much on the model development and evaluation. Earlier LES models were not coupled with land surface schemes, and most simulations were performed with fixed surface flux forcing (e.g., Hechtel et al., 1990; Avissar and Schmidt, 1998; Albertson and Parlange, 1999; Raasch and Harbusch, 2001; Letzel and Raasch, 2003; Huang et al., 2008). More recently, LES models have been coupled with land surface schemes for investigating atmosphere-land interactions over heterogeneous land surfaces (e.g., Patton et al., 2005; Huang et al., 2009; Huang and Margulis, 2010; Brunsell et al., 2011; Liu and Shao, 2013; Shao et al., 2013). Based on these LES studies, some interesting insights have been achieved with regard to the impact of surface heterogeneity on atmosphere-land interaction (e.g., Patton et al., 2005). However, the LES models applied were evaluated more from the perspective of bulk characteristics, such as profiles of state variables and higher-order moments. Whether the surface heterogeneity signals can be propagated properly in the atmosphere has been less well examined. If the propagation of surface heterogeneity cannot be simulated reasonably, model results in terms of the impact of surface heterogeneity tend to be less credible. In other words, before applying any LES model in studying the impact of surface heterogeneity, the model's ability to simulate the propagation of surface heterogeneity needs to be carefully examined.

    In this paper, atmosphere-land interactions simulated by an LES model are evaluated by comparison with airborne measurements in terms of heterogeneity propagation. The evaluation is based on a large-eddy atmosphere-land coupled model simulation over a natural heterogeneous land surface. The simulated heterogeneity propagation is investigated from the perspective of pattern persistency and is compared with the observed behavior. The consistency between observation and simulation adds credibility to the application of model results in studying the impact of surface heterogeneity.

2. Data and methodology
  • The airborne measurements used in this study consist of 13 flight legs flown over the German research collaborative SFB/TR 32 (TR32) (Vereecken et al., 2010) Selhausen-Merken field site——an area of 7.5× 6.0 km2, centered at (50°51'N, 6°25'E) and 120-200 m above ground——between 1300 and 1400 UTC5 August 2009. The weather conditions during this period were fair, with mainly easterly wind of about 3 m s-1 in the entire boundary layer and no cloud cover. The measurements were carried out using a Swiss Met Air atmospheric research aircraft (Dimona) equipped with sensors for numerous atmospheric quantities. The data used in this study include 3D wind speed, temperature, humidity, pressure, and above ground altitude. All data were recorded at a frequency of 10 Hz, which corresponds to a spatial resolution of 5 m considering an average flight speed of 50 m s-1.

    Figure 1.  Land cover map of the simulation area. A selected subset of the aircraft flight legs (white lines) is projected onto the map. All flights were between 120 and 200 m above ground level. The numbers stand for the start and end time (UTC) of the selected flights.

    A detailed description of the aircraft equipment can be found in (Neininger et al., 2001). The flight pattern over the investigation area, as well as the land cover map used for modeling, are given in Fig. 1.

    All signals were low-pass filtered at 1 Hz to remove noise, but with energy-containing eddies retained and high-pass filtered at 0.007 Hz to minimize the variability from leg to leg (Zacharias et al., 2012). For each flight leg, the sensible heat flux (H) and latent heat flux (LE) for a given location were computed as \begin{equation} H=\rho c_pw'\theta' (1)\end{equation} and \begin{equation} \label{eq1} {LE}=\rho Lw'q' , (2)\end{equation} where ρ is air density, cp is the specific heat of air at constant pressure, and L is the latent heat coefficient of vaporization. The perturbations of vertical velocity (w'), potential temperature (θ'), and water vapor mixing ratio (q') are defined as \begin{equation} \phi'=\phi-\langle\phi\rangle , (3)\end{equation} where φ represents one of the variables and \(\langle\cdot\rangle\) denotes the leg average. All leg fluxes were then remapped to the 2D study domain with a spatial resolution of 60 m——the same as the model grid spacing. For a given 60× 60 m2 grid cell, the arithmetic mean of the projected flux within the cell was taken as its 1-h (1300-1400 UTC) average flux. By doing so, the 2D airborne flux patterns were obtained (Fig. 2).

    Figure 2.  The remapped 2D maps of (a) $H$ (W m$^-2$), (b) LE (W m$^-2$), (c) temperature ($^\circ$C) and (d) specific humidity (g kg$^{-1}$) based on the 1-h (1300-1400 UTC) time averages of the airborne measurements.

  • The model used in this study was a large-eddy atmosphere and land surface coupled model (LES-ALM, Shao et al., 2013),which couples the WRF large-eddy flow model (Skamarock et al., 2008) and the Noah land surface model (LSM) (Chen and Dudhia, 2001). Substantial improvements have been made to the Noah LSM to ensure its adequacy for LES of the atmosphere and land surface processes (Liu and Shao, 2013; Shao et al., 2013). These include: (1) a multi-layer canopy scheme; (2) a method for surface flux estimates consistent with the large-eddy sub-grid closure; and (3) an appropriate soil-layer configuration.

    For the study domain described in section 2.1, a 12-h simulation has already been carried out using LES-ALM by (Shao et al., 2013). For simplicity, the original nine land-use types were regrouped into five types. The land cover pattern used in the present study is as shown in Fig. 1. The model state at 1200 UTC in (Shao et al., 2013) was taken as the initial condition for the simulation in this study. A summary of the model settings is given in Table 1.

  • The technique used for scale analysis was the newly developed orthogonal PDF decomposition (OPD) (Liu et al., 2015). The OPD decomposes a signal based on the reconstructed fields via an orthogonal transform in the PDF domain by adopting the idea of "patches". It recognizes patches over patches of arbitrary shapes, wherein the larger patches are divided into smaller ones and smaller ones are superimposed on larger ones. The energy contained at scale Lm is given by \begin{eqnarray} E_m=\int_{-\infty}^\infty|D_m(x)|^2{d}x , (4)\end{eqnarray}

    where Dm(x) is the decomposed detail at scale Lm for location x. The energy spectrum can identify how much energy of a signal is associated with a particular scale. The OPD length scale is defined based on the patches in the signals and it is about the spatial extent. For a 1D signal, for example, such length scales are simply the lengths of the 1D patches recognized in the signal. Compared to the wave-based wavelet transforms, the OPD can reconstruct the original signal more effectively and its energy spectrum can represent the multiscale variation more reasonably. For details, please refer to (Liu et al., 2015).

3. Results
  • The signals of surface heterogeneity are transmitted in the atmospheric boundary layer via turbulent eddies. Turbulence and fluidity make it easy for the air to deform. Therefore, the patterns of atmospheric quantities evolve differently from that of the solid land surface. Close to the surface (several meters above ground), the patterns of atmospheric quantities may resemble the land surface patterns (Fig. 3c). However, as 2D patterns, the footprints of surface heterogeneity can be strongly blurred by turbulent mixing and quickly become unrecognizable in the atmosphere (Figs. 3a and b). That 2D footprints become unrecognizable does not mean the impact of surface heterogeneity vanishes. It is widely recognized that the structures of the boundary layer are mainly determined by the macroscopic contrast between the surface and top-boundary conditions. Therefore, as non-dimensional quantities, the bulk characteristics of land surface properties do affect the whole boundary layer. Note that, as 2D patterns, the footprints of surface heterogeneity diminish quickly due to turbulent mixing. Consequently, the question arises: as 1D patterns, how do the footprints of surface heterogeneity evolve?

    Figure 3.  Model-simulated instantaneous patterns of potential temperature (deviation from the horizontal mean normalized by the spatial standard deviation) for the levels of (a) 32 m, (b) 8 m, and (c) 2 m.

    To this end, for each level z we define a heterogeneity index (I h) along the x direction (aligned with the background wind) as: \begin{equation} \label{eq2} I_{h}(x,z)=\dfrac{\langle f\rangle_y-\langle f\rangle_{xy}}{\sigma_f} , (5)\end{equation} where \(\langle f\rangle_y\) and \(\langle f\rangle_xy\) represent the averages of f(x,y,z) in the y direction and over the whole horizontal domain, respectively; and σf is the horizontal standard deviation of f.

    The heterogeneity indices for the model-simulated surface temperature and moisture and sensible and latent heat fluxes were computed (Fig. 4). It can be seen that the variations of surface temperature correspond well with the patterns of sensible heat flux (H) in the entire boundary layer (Fig. 4a). In general, a warmer surface has higher surface H, and a cooler surface lower H. The patterns of H are maintained well near the surface and propagate upwards in the atmosphere. Owing to the predominantly easterly background wind, the patterns move steadily toward the west with height, but their main structures are sustained. As for the latent heat flux (LE; Fig. 4b), it responds to the surface moisture in a similar manner to the response of H to surface temperature. In general, a moister surface has a higher surface LE, and vice versa. Compared to H, the LE patterns at high levels are not so consistent to those near the surface. This is because, at high levels, downdrafts from the inversion layer may entrain dry air downwards and produce positive latent heat fluxes. Therefore, the LE patterns there are more affected by the dry downdrafts from the top than by the moist updrafts originating from the surface.

    Figure 4.  Model-simulated heterogeneity indices for (a) sensible and (b) latent heat flux (color-shaded) at different heights and corresponding surface (a) temperature and (b) moisture (curves). Data are time averaged from 1300 to 1400 UTC.

  • Comparison of the absolute values along the x direction shows that there are significant differences between the model simulation and airborne measurements, not only in fluxes, but also in temperature, specific humidity, and vertical velocity (Figs. 5a-e). However, the heterogeneity indices, as defined in Eq. (3), show that the model simulation and airborne measurements agree well with each other, except for the specific humidity (Figs. 4f-j). This suggests that, from the 1D patterns perspective, the model simulation can reproduce the observed heterogeneities well, even though the modeled phases shift behind their observed counterparts somewhat (Figs. 5f-h). This phase mismatch can be attributed to the lower easterly wind speed in the model simulation, which is due to the continuous momentum loss at the land surface. More importantly, the multi-scale variations exhibited in the observation seem to be able to be captured by the model simulation (Figs. 5f-h).

    For a better quantitative comparison, we applied the OPD multi-scale decomposition method to the 1D patterns shown in Fig. 5, to quantify their multi-scale variations. The OPD energy spectrum measures the energy associated with particular scales and can identify how the variation is distributed across those scales. The OPD energy spectra show different behavior for different quantities (Fig. 6). The turbulent velocity and the fluxes of sensible heat and latent heat vary more on smaller scales, while the scalars vary on scales covering a much wider range. The former three quantities exhibit three or four distinct energy peaks from small to large scales. For the two scalars, especially temperature, the energy spectra generally increase with scale. By comparison, the energy spectra of the model simulation agree quite well with that of the observation (Fig. 6). This indicates that the distribution of heterogeneity scales simulated by the model is consistent with that observed via the airplane flights. It suggests that the model can simulate the multi-scale processes quantitatively.

    Figure 5.  Comparison of the simulated and observed (a) sensible heat flux, (b) latent heat flux, (c) temperature, (d) specific humidity, and (e) vertical velocity at the 160-m level, averaged over the $y$ axis (north-south). Panels (f-j) are the same as (a-e) but for the heterogeneity indices defined in Eq. (3).

    Figure 6.  Comparison of the OPD energy spectra of (a) sensible heat flux, (b) latent heat flux, (c) temperature, (d) specific humidity, and (e) vertical velocity at the 160-m level between the model simulation and airborne observations.

4. Conclusions
  • In this paper, atmosphere-land interactions simulated by an LES model have been evaluated by comparison with airborne measurements in terms of heterogeneity propagation. The evaluation is based on a large-eddy atmosphere-land coupled model simulation over a natural heterogeneous land surface. The heterogeneity propagation is investigated from the perspective of pattern persistency. It is found that the footprints of surface heterogeneity, though as 2D patterns can be mixed quickly by turbulence, as 1D projections persist and propagate up to the top of the boundary layer. Direct comparison shows that the simulated heterogeneity patterns are comparable to the observation.

    Heterogeneous land-atmosphere systems involve multi-scale processes. Spatially, the process scale can be characterized in terms of spatial extent, period, or correlation length. For better comparison, we adopted the extent-based scale and applied the OPD multi-scale decomposition method to the modeled and observed patterns. The turbulent velocity and the fluxes show greater variation on smaller scales, while the temperature and specific humidity vary across a much broader range of scales. The former quantities exhibit three or four distinct energy peaks from small to large scales, while the latter generally exhibit higher energy on larger scales. In general, the energy spectra of the model simulation agree quite well with that of the observation. It suggests that the model can simulate the observed multi-scale processes quantitatively. The consistency regarding the multi-scale variation between the model simulation and observation adds credibility to the application of models in studying the impact of surface heterogeneity.

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