On rainfall retrieval using polarization-corrected temperatures

This paper describes a technique for the delineation and retrieval of rainfall from passive microwave data using the polarization-corrected temperature (PCT) algorithm. The main advantage of the technique is its ability to reduce the effect of background surface emissivities making it possible to delineate areas of rainfall over varying surface types. This ability is most important when attempting to retrieve rainfall over coastal areas with emissivity signatures from sea, land and combinations of these two. This paper outlines the formulation of polarizationcorrected temperatures, and its use with data from the Special Sensor Microwave/ Imager. Central to the success of the technique is the appropriate assignment of values to two key variables: One effecting the calculation of the polarizationcorrected temperature, the other the threshold of the rain/no-rain boundary. A method is examined whereby both variables are generated from the analysis of the data. The technique is applied to one year of data over the United Kingdom to examine the dependence of these variables on the seasonal cycle. Initial results indicate that the technique is good at delineating rainfall and adjusting for seasonal fluctuations in the background surface temperature. However, retrieval of rainfall rates from the PCT using a constant relationship is less promising due to differences in cloud structure affecting the PCT to rain-rate conversion.