Simultaneous flight measurements with the research aircraft Do 128 and the helicopter-borne turbulence probe Helipod were performed during the LITFASS-98 field experiment in Germany. The area-averaged turbulent vertical fluxes of momentum, sensible and latent heat were determined on a 15 km x 15 km and a 10 km x 10 km flight pattern, respectively. The flights were carried out over heterogeneous terrain at different altitudes within a moderately convective boundary layer with Cumulus clouds.

Co-spectra-analysis demonstrated that the small scale turbulent transport was completely sampled, while the comparatively small flight patterns were possibly of critical size regarding the large-scale turbulence. The phygoide of the airplane was identified as a significant peak in some co-spectra - as far as the authors know - for the first time in meteorological flight measurements. No further systematic errors were identified in the airborne measurements. The turbulent fluxes of momentum and sensible heat at 80 m above the ground showed systematic dependence on the location of the flight legs above the heterogeneous terrain. This was not observed for the latent heat flux, due to the vertical distribution of humidity in the boundary layer.

Statistical error analysis of the fluxes showed that the systematic statistical error was one order of magnitude smaller than the standard deviation. The area-averaged fluxes derived from simultaneous Helipod and Do 128 measurements were in remarkable agreement. Their difference was much smaller than their standard deviations, indicating that the systematic statistical error was possibly over-estimated by the usual method.

In the upper half of the boundary layer the airborne-measured sensible heat flux agreed well with windprofiler/RASS data. A linear fit was the best approximation for the height dependence of all three fluxes. The linear extrapolations of the latent and sensible heat fluxes to the ground were in good agreement with tower, scintillometer, and averaged ground-station measurements on various surface types. Systematic discrepancies between airborne and ground-based measurements - as reported from other field experiments - were not found. The results show that the Helipod - designed as a small-scale turbulence probe - and the presented flight strategy with a comparatively small flight pattern are well suited for the moderately convective ABL.