Estimation of vertical profiles of raindrop size distribution and cloud microphysical processes in stratiform rainfall using vertical-pointing X- and VHF-band radars

Abstract. Simultaneous vertical pointing observations by X- and VHF-band radars were conducted in Japan, and these data were used to estimate vertical profiles of drop size distribution (DSD) parameters of raindrops, assuming a gamma distribution, for a stratiform rainfall event. We used X-band reflectivity, vertical Doppler velocity, and spectral width, combined with VHF-band vertical air motion data. The estimation considers non-Rayleigh scattering and the influence of vertical air motion, and accounts for the contamination of spectral broadening using a forward convolution technique. We demonstrated that for stratiform rainfall, broadening by wind shear may be neglected, even with a relatively coarse radar range resolution of 150 metres. Cloud physical quantities (median volume diameter, liquid water content, normalised intercept parameter) retrieved from the estimated DSD parameters were compared with operational X-band polarimetric radar data and found to be highly accurate. We also point out the potential applicability of this method to satellite-borne radars. Among the estimated parameters, the shape and slope parameters generally increased with decreasing altitude. These changes are attributed to collision-coalescence and breakup based on variations in the cloud physical quantities, likely due to the humid environment. This study suggests that retrieving cloud physical quantities from DSD parameters estimated from vertical observations enables robust discussions on cloud physical processes.