EVALUATION OF PASSIVE MICROWAVE PRECIPITATION ALGORITHMS IN WINTERTIME MIDLATITUDE SITUATIONS

The second intercomparison project of the Global Precipitation Climato logy Project examined the estimation of midlatitude, cool-season preci pitation. As part of that effort, the authors report here on the resul ts of two microwave techniques, the Goddard scattering algorithm and t he physical retrieval algorithm of Kummerow. Results from the estimati on of instantaneous rain rate for five overpasses of the Special Senso r Microwave/Imager (SSM/I) are presented in a case study mode to illus trate both the strong and weak points of each technique. These five ca ses represent a sampling of the various types of precipitating systems observed. Results for the complete set of 20 swaths chosen by the Uni ted Kingdom Meteorological Office are then categorized by scatterplots and statistics of instantaneous radar versus microwave-estimated rain rate, rain/no-rain contingency tables, and scatterplots of areal cove rage of rainfall. Neither algorithm produced a good statistical correl ation with the radar data, yet in general, both did well at determinin g rainy areas. Two reasons are suggested for the low correlation coeff icients between both algorithms and the radar data. Time differences b etween the SSM/I overpass and the radar observations can occasionally account for some of the differences. The primary reason for the low co rrelations, however, appears to be the predominance of very light rain in the area of interest during the winter. Both algorithms are in goo d spatial agreement with the radar when the radar data are restricted to rates above 1 mm h-1. When all radar rain rates are included, the r adar areal coverage increases by as much as a factor of 10 in some cas es. Because the Kummerow algorithm does not handle such low rain rates over land very well, and because the Goddard scattering algorithm use s 1 mm h-1 as the minimum reliably detectable rain rate, regimes that contain large areas of very light rain present inherent difficulties f or these retrieval methods. Therefore, the proliferation of low rain r ates observed during the experiment is the main contributor to low cor relation coefficients and high root-mean-square differences. Misidenti fication of cold surface (e.g., snow cover) as precipitation was also a problem in several instances.