Relationships between Radar Properties at High Elevations and Surface Rain Rate: Potential Use for Spaceborne Rainfall Measurements

Abstract

Ground-based radar data have been used to investigate the relationship between reflectivity at high elevations and surface rain rates. Such relations are useful for rainfall measurements by spaceborne radars at attenuating wavelength such as the 2.2-cm Precipitation Radar (PR) on board the Tropical Rainfall Measuring Mission (TRMM) satellite. In addition to attenuation, these relations are complicated by partial beamfilling, hail, and windshear sorting of particles, among others that affect the corresponding ground-based radar reflectivity?surface rain-rate relationships. TRMM-PR observations were simulated based on radar data from Darwin, Australia. The three-dimensional simulated data were classified by rain type according to several radar properties at high altitudes that are not seriously affected by attenuation. These properties are physical parameters relevant to the variations in the desired relationships. The resulting relationships are robust and permit the classification of near-surface rain and the estimation of its intensity. These findings demonstrate the feasibility of such a classification scheme to spaceborne estimates of rainfall by radar such as that on TRMM. A rain retrieval algorithm, which combines space- and ground-based radar data, is proposed. The basic statistical methodology in the algorithm is an adaptation of the Classified Window Probability Matching Method, developed for ground-based radars. In the proposed method, probabilities of the PR-observed reflectivities, taken from nonattenuated regions, are matched to probabilities of ground-based, radar-estimated rain intensities that have been classified into the same precipitation type.