EVA_GRIPS and LITFASS-2003: What have we learned ?

Land surface - atmosphere interaction processes play an important role in the energy and water cycles over a wide magnitude of scales. An adequate description of these processes in numerical weather prediction and climate models is fundamental for a reliable simulation of near surface weather and climate conditions. However, considerable deficits still exist concerning our understanding and ability to properly describe these processes consistently over a variety of scales ranging from the local patch to the regional landscape scale.

The EVA_GRIPS (EVAporation at GRId and Pixel Scale) project was realized as a part of the German Climate Research Program in order to study surface – atmosphere interaction processes over a heterogeneous land surface. EVA_GRIPS focused on the determination of the area-averaged evaporation at the scale of a grid box of a regional numerical weather prediction or climate model and / or at the scale of a pixel of a satellite image. The project combined surface based and airborne measurements, satellite data analysis, and numerical modeling activities. A meso-scale field experiment, LITFASS-2003, was carried out in the heterogeneous landscape around the Meteorological Observatory Lindenberg (MOL) of the German Meteorological Service during the main growing season in May and June, 2003. The experiment was embedded in the comprehensive, operational measurement program of the MOL. Experimental determination of surface fluxes on a variety of spatial scales was achieved by employing micrometeorological flux stations, scintillometers, a combination of ground-based remote sensing instruments, and the Helipod, a turbulence probe carried by a helicopter. Surface energy fluxes were also derived from satellite data. Modeling work included the use of different SVAT schemes, a large-eddy simulation model and three meso-scale atmospheric models.

The paper will give an overview on the major results of the EVA_GRIPS project both from the experimental and modeling work which can be summarized in the following theses: - The local energy and water vapor fluxes show considerable variability across the heterogeneous landscape. - Area-averaged fluxes determined from an aggregation of local eddy-covariance measurements and from spatially averaging measurement systems (scintillometers, airborne turbulence probe, flux profiles from ground-based remote sensing) are widely consistent. - The water vapor flux profile in the convective boundary layer is dominated by larger scales than the sensible heat flux profile, and it is strongly affected by the surface heterogeneity and by the entrainment of dry air. - Offline data assimilation of precipitation data and different treatments of subgrid-scale processes lead to an improved representation of soil moisture and surface-atmosphere exchange processes in meso-scale models. - Meso-scale circulations can provide a significant contribution to the area-averaged water vapor fluxes under weak wind conditions.