Classification of Rain Regimes by the Three-Dimensional Properties of Reflectivity Fields

Abstract

An automated scheme to characterize precipitation echoes within small windows in the radar field is presented and applied to previously subjectively classified tropical rain cloud systirns near Darwin, Australia. The classification parameters are (a) Ee, effective efficiency, as determined by cloud-top and cloud-base water vaporsaturation mixing ratios; (b) BBF, brightband fraction, as determined by the fraction of the radar echo area in which the maximal reflectivity occurs within +1.5 km of the 0C isotherm level; and (c) ?rZ, radial reflectivity gradients (dB km-1). These classification criteria were applied to tropical rain cloud systems near Darwin, Australia, and to winter convective rain cloud systems in Israel. Both sets of measurements were made with nearly identical networks of C-band radars and rain gauge networks. The results of the application of these objective classification criteria to several independently predetermined rain regimes in Darwin have shown that better organized rain systems have smaller ?rZ and larger BBF. Similarly, smaller ?rZ and larger BBF were also observed from maritime rain cloud systems, as compared to continental rain cloud systems with the same degree of organization. Continental rain cloud systems, regardless of their degree of organization, have larger depths, as expressed by E1. The results suggest that ?rZ BBF, and E1 can be used in an automated way over small domains, of the order of 100 km2, for the classification of the rain types into physically meaningful regimes. The classification by E1 is particularly useful for rainfall estimation by area-time integral techniques; ?rZ is useful in assessing the bias errors of rainfall measurements by remote sensing, which are due to partial beam filling. The classification by BBF is useful for assessing the convective-stratiform partitioning of the rainfall volume. Our calculations suggest that previous estimates of the amount of stratiform rainfall in the Tropics may have been too large, by up to afactor of 2. The rainfall analyses presented in this study are based exclusively on rain gauge measurements, while radar information was used only to classify the individual gauge measurements.