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Abstract

A 6.4 kHz Doppler sodar has been used in a feasibility study to retrieve rain rate and drop
size distribution from sodar Doppler spectra. For rain rates above about 0.5 mm/h raindrop
and turbulence spectra can be separated, thus allowing a drop size distribution measurement in
the Prandtl layer without assumptions on the size distribution type for drop diameters above 1
mm. The noise of the sodar used mostly forced us to 10 min averages for the rather light rain
episodes (< 3 mm/h rain rate) we have encountered in January 1993 at our experimental site in
Northern Germany. In order to arrive at consistent rain parameter retrievals we have corrected
both the mean vertical wind influence and the drop size dependent reaction to turbulent motion.
The comparison to a drop counter at the site gives good results only for the periods with rain
rates above 1 mm/ 11 at the absence of drizzle. Assuming a height independent rain rate in the
Prandtl layer even gives the possibility for a near surface temperature profile retrieval. For
higher sodar frequencies above 15 kHz a clearer separation of turbulence and rain spectra would
lower the threshold drop size and thus improve rain parameter retrievals but be more strongly
dependent on the near surface temperature and humidity profiles.