Doppler Radar Observations of Drop-Size Distributions in a Thunderstorm

Abstract

Data obtained in a thunderstorm with a vertically pointing Doppler radar are analyzed to find the size distribution of raindrops at heights below the 0C level. Drop-size distributions were computed using the updraft UR deduced by Rogers' method and two other updrafts, namely, UR ? 1 and UR + 1 m sec?1. It is found that the drop-size data at all the heights may be well represented by the exponential equation, N (D) = N0 exp(? ?D), in which N (D)?D is the concentration of drops in the diameter range D to D + ?D, N0 = 0.07 R0.37 [cm?4], and ? = 38R?0.14 [cm?1], R being the rainfall rate (mm hr?1). For R ? 3 mm hr?1, the distribution is steeper and N0 is greater as compared to the Marshall-Palmer distribution. For radar reflectivity factors Z in the range 1?105 mm6 m?3, the relationship between the mean Doppler velocity and Z for the distribution agrees with that given by Rogers to within 1 m sec?1. The following equations have been found between the water content M, median volume diameter D0, radar reflectivity factor Z, and the rainfall rate R. M = 0.052 R0.94 [gm m?3], D0 = 0.13 R0.14 [mm] Z = 300R1.35 [mm6 m?3].