Radar Measurement of Rainfall with and without Polarimetry

Abstract

Raindrop size distributions (DSDs) for tropical convective storms are used to examine the relationships between the parameters of a gamma DSD, with special emphasis on their variation with the stage of the storm. Such a distinction has rarely been made before. Several storms from a variety of tropical locations are divided into storm stages according to the temporal dependence of their reflectivity factor Z, rainfall rate R, and median volume diameter D0. In most cases it is found that the DSD parameter D0 is approximately constant in time during the convective, or C, stage, which leads to a Z?R relation of the form Z = AR, that is, a linear relationship between Z and R. This finding implies the existence of equilibrium DSDs during the C stage. The convective stage is sometimes marked by pulsations in draft strength so that D0, R, and Z and associated values of the shape parameter ? decrease in a quasi-transition stage before increasing once more. Theoretical relations between the differential reflectivity ZDR and the ratio Z/R as functions of the DSD parameter ? are derived by assuming a gamma DSD and an accurate raindrop fall speed law. It is found that data derived from disdrometer observations lie along a ? = 5 isopleth for tropical continental C stages (Puerto Rico and Brazil) and along a ? = 12 isopleth for tropical maritime C stages [Tropical Ocean and Global Atmosphere Coupled Ocean?Atmosphere Response Experiment (TOGA COARE)]. Small values of ? that occur in the weak updraft intervals do not impact the rainfall measurements because they correspond to relatively small R. The latter features imply that the measurement of rainfall for the convective stages can be performed with standard polarimetry involving only two measurables rather than three, provided knowledge of ? is available a priori. A new rain parameter diagram is presented in which isopleths of the generalized number concentration and D0 are superimposed on the Z?R plot. It is proposed that it is possible to estimate D0 from climatological and observable storm structural features, which, with Z, provide estimates of R. Such an approach is necessary for use with conventional radars until polarimetric radars are more widely available.