A Polarimetric Radar Analysis of Ice Microphysical Processes in Melting Layers of Winter Storms Using S-Band Quasi-Vertical Profiles

摘要

Quasi-vertical profiles (QVPs) obtained from a database of U.S. WSR-88D data are used to document polarimetric characteristics of the melting layer (ML) in cold-season storms with high vertical resolution and accuracy. A polarimetric technique to define the top and bottom of the ML is first introduced. Using the QVPs, statistical relationships are developed to gain insight into the evolution of microphysical processes above, within, and below the ML, leading to a statistical polarimetric model of the ML that reveals characteristics that reflectivity data alone are not able to provide, particularly in regions of weak reflectivity factor at horizontal polarization Z(H). QVP ML statistics are examined for two regimes in the ML data: Z(H) >= 20 dBZ and Z(H) < 20 dBZ. Regions of Z(H) >= 20 dBZ indicate locations of MLs collocated with enhanced differential reflectivity Z(DR) and reduced copolar correlation coefficient rho(hv), while for Z(H) < 20 dBZ a well-defined ML is difficult to discern using Z(H) alone. Evidence of large Z(DR) up to 4 dB, backscatter differential phase delta up to 8 degrees, and low rho(hv) down to 0.80 associated with lower Z(H) (from -10 to 20 dBZ) in the ML is observed when pristine, nonaggregated ice falls through it. Positive correlation is documented between maximum specific differential phase K-DP and maximum Z(H) in the ML; these are the first QVP observations of K-DP in MLs documented at S band. Negative correlation occurs between minimum rho(hv) in the ML and ML depth and between minimum rho(hv) in the ML and the corresponding enhancement of Z(H) (Delta Z(H) = Z(Hmax) - Z(Hrain)).