An Attempt to Validate Scintillometer Measurements: The LITFASS-2009 Experiment

The turbulent exchange of heat and water vapour are essential land surface – atmosphere interaction processes in the local, regional and global energy and water cycles. Scintillometry can be considered as the only technique presently available for the quasi-operational experimental determination of area-averaged turbulent fluxes at a horizontal scale of a few kilometres needed to validate the fluxes simulated by regional atmospheric models or derived from satellite images. It has thus been increasingly used over the last decade. The scintillometer principle is based on the quantitative evaluation of intensity fluctuations of electromagnetic radiation propagating across the turbulent atmosphere over distances up to several kilometres.

Although the successful application seems to justify the broad use of this technique, a number of assumptions in the scintillometer data processing and interpretation still call for a thorough evaluation of the scintillometer principle, in particular over heterogeneous terrain. These open issues include the spatial variability and corresponding aggregation rules for structure parameters, the structure parameter vertical profiles, and the temperature-humidity correlation. Moreover, a validation of the path-averaged structure parameters of temperature and humidity derived from scintillometer data by independent measurements is still missing. To answer these questions, the LITFASS-2009 field experiment has been performed in July 2009 around the Meteorological Observatory Lindenberg – Richard-Aßmann-Observatory of the German Meteorological Service (DWD). It combined tower-based in-situ turbulence measurements, field-scale laser scintillometers, long-range optical large-aperture scintillometers, and airborne turbulence measurements using an autonomous unmanned aircraft.

Questions studied included the effect of tower vibrations on scintillometer measurements, the determination of the spatial (horizontal and vertical) and temporal variability of structure parameters (underlying the scintillometer principle), and the validation of scintillometer measurements over moderately heterogeneous terrain by airborne measurements. The paper will present exemplary results from LITFASS-2009. Suggestions will be derived for a continuation of this type of studies.