Systematic Variation of Observed Radar Reflectivity–Rainfall Rate Relations in the Tropics

Abstract

The Tropical Rainfall Measuring Mission Global Validation Program provides a unique opportunity to compare radar datasets from different sites, because they are analyzed in a relatively uniform procedure. Monthly observed radar reflectivity?rainfall rate (Ze?R) relations for four different sites that are surrounded by tipping bucket gauge networks (Melbourne, Florida; Houston, Texas; Darwin, Australia; and Kwajalein Atoll, Republic of Marshall Islands) were derived. The radar and gauge data from all sites are controlled for quality using the same algorithms, which also include an automated procedure to filter unreliable rain gauge data upon comparison with radar data. The relations are generated by two different methods. The first method is based on using a power law Ze?R with a fixed exponent of 1.4, and the second is based on matching unconditional probabilities of rain rates as measured by the gauge to radar-observed reflectivities and is known as the window probability matching method (WPMM). Both methods tune the radar observations to a network of quality-controlled gauges to adjust the total monthly rainfall to match the gauges. Separate relations are generated for convective and stratiform rain, as classified by the horizontal reflectivity structure. In the WPMM-based Ze?R relations, a given Ze was matched to a much lower R in convective rainfall than in stratiform rainfall. These relations were found to be curved lines in log?log space rather than a straight-line power law. The WPMM-based Ze?R curves demonstrated systematic variation between the convective and stratiform rain, but the power law?based Ze?R curves showed no systematic trend. The systematic variation in the relations shown here contradicts previous findings in which the classification is based only on the existence or nonexistence of brightband signature. The latter indicates a higher reflectivity in stratiform rain as compared with that in convective rain, for a given rain rate. Recent studies, based on disdrometer data, suggest that during a typical event there are three principal types of rain (convective, transition, and stratiform), each characterized by a different type of Z?R relation. The current study suggests that to distinguish each type, both the horizontal and the vertical reflectivity field structure should be analyzed.