Polarimetric Rainfall Measurements in Localized Strong Convection

Validation of rainfall estimation performed with the polarimetric WSR-88D radar during the Joint Polarization Experiment (JPOLE) clearly demonstrated the advantages of polarimetric measurements of rain. Particular success was achieved if the combination of radar reflectivity Z, differential reflectivity ZDR, and specific differential phase KDP were used according to a “synthetic” algorithm. However, difficulties remain for situations of localized intense convection due to the relatively poor radial resolution of KDP, possible contamination of Z and ZDR with hail, and nonuniform beam filling. The latter causes spurious oscillations of KDP that progress with increasing distance from the radar. As a result, occasional outliers in the synthetic algorithm estimates occur for isolated convective cells.

These problems are illustrated for a number of rain events observed with the KOUN WSR-88D radar in Oklahoma. A new procedure for KDP estimation that mitigates the effects of inadequate resolution and nonuniform beam filling is evaluated. Its impact on the accuracy of polarimetric rain retrieval is examined using in-situ rain measurements by gages at close and distant ranges from the radar. The origin and nature of outliers in polarimetric rain estimates is discussed and possible ways to alleviate them are suggested.