TRMM observations of polarization difference in 85 GHz: Information about hydrometeors and rain rate

Observations made by the Precipitation Radar (PR) and the Microwave Imager (TMI) radiometer on board the Tropical Rainfall Measuring Mission (TRMM) sa tellite help us to show the significance of the 85 GHz polarization differe nce, PD85, measured by TMI. Rain type, convective or stratiform, deduced fr om the PR allows us to infer that PD85 is generally positive in stratiform rain clouds, while PD85 can be markedly negative in deep convective rain cl ouds. Furthermore, PD85 increases in a gross manner as stratiform rain rate increases. On the contrary, in a crude fashion PD85 decreases as convectiv e rain rate increases. From the observations of TMI and PR, we find that PD 85 is a weak indicator of rain rate. Utilizing information from existing po larimetric radar studies, we infer that negative values of PD85 are likely associated with vertically-oriented small oblate or wet hail that are found in deep convective updrafts.