Polarimetrically Tuned R(Z) Relations and Comparison of Radar Rainfall Methods

Abstract

The following rainfall measurements are compared: 1) the reflectivity factor?rain rate or R(Z) relation, whereby the rain is estimated point by point for mapping or area integration; 2) use of a specific differential phase KDP (between vertical and horizontal polarization) in a relation with rainfall rate for point-by-point mapping and subsequent integration over areas and time; 3) use of a R(KDP) relation together with a relation between KDP and Z to derive a polarimetrically tuned or matched R(Z) relation; and 4) use of empirical relations between the rainfall volume and the time integral of the storm area in which reflectivity is larger than a selected threshold. These methods are tested on five cases?two summer-type convections, one winter convective case, and two events of stratiform rain with embedded convection. Accumulations of rain in a dense gauge network in Oklahoma are used as a standard for comparison with radar measurements. In four of the five cases the rain totals obtained from the R(KDP) relation agree very well with actual gauge accumulations. This is significantly better than the Marshall?Palmer R(Z) relation, which agrees well with gauges for only one event. Matching Z to KDP brought the R(Z) derived rain total to better agreement with gauges in three more cases.