MICROPHYSICAL RETRIEVALS OVER STRATIFORM RAIN USING MEASUREMENTS FROMAN AIRBORNE DUAL-WAVELENGTH RADAR-RADIOMETER

The need to understand the complementarity of the radar and radiometer is important not only to the Tropical Rain Measuring Mission (TRMM) p rogram but to a growing number of multi-instrumented airborne experime nt that combine single or dual-frequency radars with multichannel radi ometers, The method of analysis used in this study begins with the der ivation of dual-wavelength radar equations for the estimation of a two -parameter drop size distribution (DSD), Defining a ''storm model'' as the set of parameters that characterize snow density, cloud water, wa ter vapor, and features of the melting layer, then to each storm model there will usually correspond a set of range-profiled drop size distr ibutions that are approximate solutions of the radar equations, To tes t these solutions, a radiative transfer model is used to compute the b rightness temperatures for the radiometric frequencies of interest, A storm model or class of storm models is considered optimum if it provi des the best reproduction of the radar and radiometer measurements, Te sts of the method are made for stratiform rain using simulated storm m odels as well as measured airborne data, Preliminary results show that the best correspondence between the measured and estimated radar prof iles usually can be obtained by using a moderate snow density (0.1-0.2 g/cm(-3)), the Maxwell-Garnett mixing formula for partially melted hy drometeors (water matrix with snow inclusions), and low to moderate va lues of the integrated cloud liquid water (less than 1 kg/m(-2)), The storm-model parameters that yield the best reproductions of the measur ed radar reflectivity factors also provide brightness temperatures at 10 GHz that agree well with the measurements, On the other hand, the c orrespondence between the measured and modeled values usually worsens in going to the higher frequency channels at 19 and 34 GHz. In searchi ng for possible reasons for the discrepancies, it is found that change s in the DSD parameter mu, the radar constants, or the path-integrated attenuation can affect the high frequency channels significantly, In particular, parameters that cause only modest increases in the median mass diameter of the snow, and which have a minor effect on the radar returns or the low frequency brightness temperature, can produce a str ong cooling of the 34 GHz brightness temperature.