On Impact of Drop Size Distribution Models on Radar Measurement

The radar reflectivity factor as well as the rain attenuation are proportional to the rain rate, however, the rain rate is a function of the drop size distribution (DSD). The same rain rate can cause different attenuation effects by different DSDs which could be categorized into typical rain types (thunderstorm, shower, continuous rain and drizzle). Identical observation can be deduced for the radar echo. The mostly used exponential DSD model overestimates the number of small rain drops. This inaccuracy does not play an important role on low frequencies but it causes more problems on higher frequencies because of a more important role of small rain drops. The accuracy improvement is obvious when replacing the exponential DSD model by the Gamma one. This contribution compares also different methods finding parameters of DSD models (least squares, method of moments). Examples of these implications are added and discussed using the Mie scattering computations and the DSD measurement of the DLR in Germany.