Helipod measurements in the LITFASS-2003 field experiment: Comparison with DIAL, LAS, tower, ground-station, and LES

The measurements in the EVA-GRIPS (Regional Evaporation at Grid/Pixel Scale over Heterogeneous Land Surfaces) project were a contribution to the German climate research programme DEKLIM. The main joint field campaign was carried out in the framework of the LITFASS (Lindenberg Inhomogeneous Terrain - Fluxes between Atmosphere and Surface: a Long-Term Study) experiment. The measurement area near the meteorological observatory Lindenberg (about 60 km south-east of Berlin) is a mix of forest, grassland, agriculture, and lakes that is typical for mid-Europe.

The turbulent structure of the atmospheric boundary layer (ABL) was examined on several flight patterns (horizontal and vertical grids, vertical profiles) in a well-mixed layer using the turbulence measurement system Helipod. The Helipod is an autonomously operating sensor system, attached to a 15 m rope and operates at 40 m/s air speed. During the experiment LITFASS-2003 (19.05. - 17.06.2003) the turbulence probe Helipod performed 27 measurement flights on 16 days. During 65 flight hours the surface and air temperature, humidity, and wind vector were measured at 100 Hz sampling rate.

The Helipod measurements link observations performed by ground-based micro-meteorological stations, large aperture scintillometers (LAS), SODAR, RASS, differential absorption LIDAR (DIAL), a 99 m tower, satellites, and other aircraft (Tornado), as well as numerical models like large eddy simulation (LES), performed by other groups. The comparison of all systems with the Helipod yield in general good or even very good agreement. In the following some case studies are introduced.

The Helipod time series of the humidity and the potential temperature were compared (e.g., power and co-spectra, statistical distributions) with the DIAL observations. The agreement of observed convective structures and convective length scales in the spectra is remarkable. Discrepancies in turbulent flux measurements at several heights were finally explained with the influence of the heterogeneous terrain: While the Helipod measurements were averaged along a 10 km flight path, the DIAL time series were averaged over the fetch due to the mean wind direction and speed. Since the latter was usually low, the DIAL measurements were mainly influenced by the closer vicinity of the DIAL position. These findings and considerations were in very good agreement with LES runs that used realistic surface parameters and were initialized with data from the micro-meteorological stations.

During the LITFASS-2003 experiment the area-averaged turbulent sensible and latent heat fluxes were calculated at several altitudes using Helipod measurements. The area-averaged surface fluxes were then calculated using an inverse model. A comparison of these surface fluxes with the observations (tower) of the global radiation during the field experiment showed a clear dependence. The surface fluxes were used for the initialization of numerical models that simulated several days within the experimental period. Additionally, the area-averaged surface fluxes are now used to development an averaging strategy for ground-based measurements above various surface types within a very heterogeneous terrain.