Retrieving Raindrop Size Distribution Parameters and Vertical Air Motion from Micro Rain Radar Observations

Micro Rain Radars (MRRs) are small 24-GHz frequency modulated continuously wave (FMCW) commercial meteorological vertically pointing radars manufactured by Metek Meteorologische Messtechnik GmbH in Germany. The MRR is a robust radar capable of operating during rain and snow and provides real-time products including vertical profiles of radar reflectivity factor and rain rate. The MRR real-time algorithms assume zero air motion in the precipitating column, which may be valid during stratiform rain, but is not valid during convective rain. To advance our understanding of microphysical rain and snow processes, this study developed a new retrieval algorithm using MRR observations that estimates vertical air motion and raindrop size distributions throughout the precipitating rain column.

The new retrieval algorithm inputs include profiles of recorded MRR Doppler velocity power spectra and the algorithm outputs are profiles of five parameters: two parameters for a Gaussian shaped air motion distribution (i.e., mean motion and turbulent spectrum broadening), and three parameters for a modified Gamma shaped RSD (i.e., number concentration, mean diameter, and shape factor). The new retrieval algorithm is based on a forward radar spectrum simulator developed from prior work retrieving vertical air motions and RSDs from vertically pointing radars (VPRs) operating at UHF, S-, Ka-, and W-band frequencies.

The 24-GHz retrieval algorithm was developed using MRR observations during the Midlatitude Continental Convective Clouds Experiment (MC3E) held in northern Oklahoma (USA) during the Spring of 2011. The recorded MRR spectra were calibrated using a collocated surface disdrometer and the retrieval algorithm incorporates path attenuation through the rain. The MRR retrieved air motion, RSD parameters, and integrated quantities (e.g., rain rate and attenuation corrected reflectivity factor) were compared with previously published MC3E results. Specifically, the air motions were compared with 449-MHz VPR Bragg scattering vertical motions, and the RSD and integrated quantities were compared with a dual-frequency S- and Ka-band retrieval method. Retrieved vertical air motions included a convective core updraft exceeding 8 m/s upward at 1 km above ground level during convective rain, which was confirmed using the 449-MHz and S-band air motion retrievals.

This presentation will describe the new 24-GHz retrieval algorithm and will compare MRR retrievals with previous published UHF, S-, and Ka-band air motion and RSD parameter retrievals.