Effects of Variations in Precipitation Size Distribution and Fallspeed Law Parameters on Relations between Mean Doppler Fallspeed and Reflectivity Factor

Abstract

A description is given of the influence of variations in the form of the hydrometeor particle size distribution and of variations in the form of the particle fallspeed law on relations between mean Doppler fallspeed vD and reflectivity factor Z. It is shown that for rain, variations in distribution shape can produce errors in vD of 20%?25%. The resultant errors in raindrop distribution parameters deduced from vD and Z are calculated. Uncertainties regarding hydrometeor phase (and thus in the particle fallspeed law) are shown to produce much larger errors in the value of vD deduced from Z of up to a factor of 4. These results are deduced theoretically, tested with experimental raindrop size spectra, and demonstrated with experimental Doppler radar data for a tropical thunderstorm. It is concluded that the use of theoretical or empirical vD-Z relations to deduce particle size distributions will produce large errors in most meteorological situations. However, in those cases where a reasonable assumption can be made about particle phase, the use of such relations may produce estimates of vertical winds with acceptable accuracy.