The Dependence of Energy Budget Components on the surface Characteristics of a shallow pre-Alpine Valley

The imbalance of the surface energy budget in eddy-covariance measurements is still a pending problem. A possible cause is the presence of land surface heterogeneity. The influence of surface heterogeneity on the atmospheric boundary layer was intensively investigated about one to two decades ago. It was found that structures with length scales of the order of the boundary layer height or larger are most effective in influencing the boundary layer turbulence. Subsequent large-eddy simulations showed that also the near-surface turbulent fluxes are influenced by organized structures in the boundary layer. However, the precise influence of the surface characteristics on the energy imbalance and its partitioning is still unknown. In previous work, we have investigated the influence of idealized surface characteristics on energy budget components by means of a systematic parameter study. We now extend that study to an existing land surface including topography, in case the TERENO field site of Fendt, a shallow valley in the German pre-Alps. Besides the presence of a mild topography the region around the field site also contains a variation in landuse with length scales of O(100m - 10 km). We employ the Fendt landscape as a backbone to inspect the energy budget response to varying meteorological conditions, by positioning several virtual towers and virtual control volumes in the domain at functionally different positions (e.g. the center of convergence and divergence zones and the borders between them). The virtual control volume approach allows us to also characterize advection, flux-divergence and storage terms of the energy budget at the virtual measurement site, in addition to the standard turbulent flux. As in the idealized study with flat topography, we seek atmospheric correlators for the energy balance ratio and the residual, e.g. by investigating their correlation with measurable atmospheric quantities as u*, temperature and moisture gradients and boundary layer depth. In addition, our work connects to the ScaleX field campaign of 2015, which allows comparison between our idealized meteorological conditions and real events, including the validation of the simulated energy fluxes with the standard on-site eddy-covariance measurements and additional LIDAR measurements during the field campaign.