Raindrop size distribution parameters estimated from polarimetric radar variables in convective cells around Okinawa Island during the Baiu period

Abstract

Convective cells with low echo-top heights are commonly observed in the humid environments typical of the Baiu season around Okinawa Island. Representative convective cells with low echotop heights include convective cells in the stratiform and convective rain zones accompanying a precipitation system, as well as isolated convective cells. Raindrop size distribution (DSD) parameters (the median volume diameter D0 and the normalized intercept parameter Nw) for the three types of convective cells were clarified by estimation from polarimetric radar measurements. The estimated D0 values in the mature stages of a convective cell in a stratiform rain zone were approximately 1.2 mm, with radar reflectivity (Zh) of 40–45 dBZ, and the estimated Nw ranged from 32000–100000 mm−1 m−3. In contrast, for a convective cell in a convective rain zone, and also for an isolated convective cell, the estimated D0 was approximately 2 mm with Zh of 40–45 dBZ, and the estimated Nw values were in the range 1000–10000 mm−1 m−3. The variations in the estimated D0 and Nw within the lifetimes of the convective cells suggest the following microphysics: for the convective cell in the stratiform rain zone, a high number-concentration of small raindrops contributed to the increase in Zh, whereas for the convective cell in the convective rain zone and the isolated convective cell, large raindrops contributed to the increase in Zh, and raindrop coalescence processes were predominant at altitudes below 2 km ASL. This study characterized precipitation particle distributions for three convective cells, with low echo-top heights, representing the humid environment of Okinawa Island during the Baiu period. In particular, the precipitation particle distribution for the convective cell in the stratiform rain zone, which differs markedly from those of the convective cell in the convective rain zone and the isolated convective cell, was characterized.