Scintillometry has been increasingly used over the last decade for the experimental determination of area-averaged turbulent fluxes at a horizontal scale of a few kilometres. Nevertheless, a number of assumptions in the scintillometer data processing and interpretation still call for a thorough evaluation, in particular over heterogeneous terrain. Moreover, a validation of the path-averaged structure parameters derived from scintillometer data (and forming the basis for the flux calculations) by independent measurements is still missing. To achieve this, the LITFASS-2009 field campaign has been performed around the Meteorological Observatory Lindenberg -Richard-Aßmann-Observatory of the towerbased in-situ turbulence measurements, field-scale laser scintillometers, long-range optical (large-aperture) and microwave scintillometers, and airborne turbulence measurements using an automatically operating unmanned aircraft. The paper describes the project design and strategy, and discusses first results. Daytime near-surface values of the temperature structure parameter, C 2 T , over different types of farmland differ by more than one order of magnitude in their dependence on the type and status of the vegetation. Considerable spatial variability in C 2 T was also found along the flight legs at heights between 50 and 100 m. However, it appeared difficult to separate the effects of heterogeneity from the temporal variability of the turbulence fields. Aircraft measurements and scintillometer data agreed in magnitude with respect to the temporal variation of the path-averaged C 2 T values during the diurnal cycle. The decrease of C 2 T with height found from the scintillometer measurements close to the surface and at 43 m under daytime convective conditions corresponds to free-convection scaling, whereas the aircraft measurements at 54 and 83 m suggest a different behaviour.

We derive the turbulent structure parameters of temperature C 2 T and humidity C 2 q from high-resolution large-eddy simulations (LES) of a homogeneously-heated convective boundary layer. Boundary conditions and model forcing were derived from measurements at Cabauw in The Netherlands. Three different methods to obtain the structure-parameters from LES are investigated. The shape of the vertical structure-parameter profiles from all three methods compare well with former experimental and LES results. Depending on the method, deviations in the magnitude up to a factor of two are found and traced back to the effects of discretization and numerical dissipation of the advection scheme. Furthermore, we validate the LES data with airborne and large-aperture scintillometer (LAS) measurements at Cabauw. Virtual path measurements are used to study the variability of C 2 T in the mixed layer and surface layer and its implications for airborne and LAS measurements. A high variability of C 2 T along a given horizontal path in the LES data is associated with plumes (high values) and downdrafts (low values). The path average of C 2 T varies rapidly in time due to the limited path length. The LES results suggest that measured path averages require sufficient temporal averaging and an adequate ratio of path length to height above the ground for the LAS in order to approach the domain average of C 2 T .

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