Comparison of methods for area-averaging surface energy fluxes over heterogeneous land surfaces using high-resolution non-hydrostatic simulations

Abstract

The quantification of subgrid land surface heterogeneity effects on the scale of climate and numerical weather prediction models is of vital interest for the energy budget of the atmospheric boundary layer and for the atmospheric branch of the hydrological cycle. This paper focuses on heterogeneity effects for the exchange processes between land surfaces and the atmosphere. The results are based on high-resolution non-hydrostatic model simulations for the LITFASS area near Berlin. This area represents a highly heterogeneous landscape of 20 × 20 km 2 around the Meteorological Observatory Lindenberg of the German Weather Service (DWD). Model simulations were carried out using the non-hydrostatic model FOOT3DK of the University of K¨ oln with resolutions of 1 km and 250 m. The performance of different area-averaging methods for the turbulent surface fluxes was tested for the LITFASS area, namely the aggregation, mosaic and tile methods. For one tile method (station-tile), the experimental setup of the surface energy balance stations of the LITFASS98 experiment was investigated. Two different simulation types are considered: (1) realistic topography and idealized synoptic forcing; (2) realistic topography and realistic synoptic forcing for LITFASS98 cases. A double one-way nesting procedure is used for nesting FOOT3DK in ‘Lokalmodell’ of the DWD. The mosaic method shows good results, if the wind speed is sufficiently high. During weak-wind convective conditions, errors are particularly large for the latent heat flux on the 20 × 20 km 2 scale. The aggregation method yields generally higher errors than the mosaic method, which even increase for higher wind speeds. The main reason is the strong surface heterogeneity associated with the lakes and forests in the LITFASS area. The main uncertainty of the station-tile method is the knowledge of the area coverage in combination with the representativity of the stations for the land-use type and surface conditions. The results of this study lead to the recommendation to use a mosaic approach or at least a tile approach for downscaling fluxes over heterogeneous surfaces in mesoscale and regional climate models. Copyright  2005 Royal Meteorological Society.