MODEL-ESTIMATED MICROWAVE EMISSIONS FROM RAIN SYSTEMS FOR REMOTE-SENSING APPLICATIONS

A simple model for estimating the upward and downward microwave emissi on from rain layer types above ground is presented. The emission prope rties of the rain layers are estimated from physical quantities such a s the optical depth, the single-scattering albedo, the physical temper ature, and a given drop size distribution for Mie scattering calculati ons. The underlying surface is characterized by the emissivity and the physical temperature. The transparency coefficient q and the reflecti on coefficient r of the rain layer are expressed by these physical qua ntities. The brightness temperature then is given by the physical temp erature T, q, and v. The radiation transfer is estimated by the method of layer addition, described by Sobolev [1956], which avoids the nece ssity of solving the equation of radiation transfer. The accuracy of t his simple model was estimated by comparisons with three-dimensional M onte Carlo calculations. The error is estimated to be less than 3 K fo r common situations and less than 8 K for unrealistic high optical dep ths. It is shown that any one of the quantities rain rate, rain layer depth, and physical temperature can be estimated with sufficient accur acy if the others are known. The basic model has been extended for app lication to inhomogeneous cloud layers and to include differences in b rightness temperatures for horizontal and vertical polarizations for o blate raindrops. The main intended application of this model is rain r ate estimation from space with low data processing efforts, especially for the Priroda mission. The model was tested for the downwelling emi ssion during the field experiment CLEOPATRA by measurements with a pol arimetric weather radar and rain gauges. The results verify the princi ples, and promising agreement was found at least for stratiform rain. The polarimetric extension of the model too showed promising results u nder quite different measurement conditions in Russia and southern Ger many.