Edge Effects and Energy Balance Closure from High-Resolution Large-Eddy Simulation for the LITFASS-2003 Campaign

The LITFASS-2003 campaign in northeastern Germany was designed to test and to assess different strategies for the area-averaging of surface fluxes over a heterogeneous land surface. The measurement program included measurements of local fluxes over different types of land surface. These micrometeorological measurements provide an ideal dataset to study the energy balance closure problem for a complex land surface where secondary circulations from the surface heterogeneity are expected to influence the imbalance in the surface energy budget. In our study, we perform large-eddy simulations for a subregion of the LITFASS area (horizontal extent of 5.4 x 5.4 km² with a flat topography and mainly agricultural land) initialized and driven by experimental data collected on 30 May 2003. The composite fluxes calculated from multiple towers in the experiment are used to prescribe the surface fluxes of latent and sensible heat flux in the simulated domain, such that the diurnal and spatial variation of the fluxes are captured by the model. A newly developed vertically nested LES-within-LES model is used to achieve a grid resolution of 1 m and 2 m in the vertical and horizontal directions, respectively, up to 115 m from the ground. An ensemble of simulations is performed to separate the heterogeneity effect from the turbulence fluctuations and to achieve more representative statistics of the virtual measurements. To come to a better process understanding, we study the height dependence of mean imbalance and the energy partitioning, by separating the residual into its constituent terms: advection by the mean flow, horizontal flux-divergence and storage. We investigate the spatial variability of the energy balance ratio, where we observe pronounced edge effects. In the lee of the intersection of different land types the ratio can be either positive or negative. We investigate the spatial structure of the edge effects and their height-dependence. Furthermore, the role of secondary circulations is studied by comparing the ensemble results with the experimental data from multiple towers.