A REGIONAL RAINFALL CLIMATOLOGY OVER MEXICO AND THE SOUTHWEST UNITED-STATES DERIVED FROM PASSIVE MICROWAVE AND GEOSYNCHRONOUS INFRARED DATA

A three-year climatology of satellite-estimated rainfall for the warm season for the southwest United States and Mexico has been derived fro m data from the Special Sensor Microwave Imager (SSM/I). The microwave data have been stratified by month (June, July, August), year ( 1988, 1989, 1990), and time of day (morning and evening orbits). A rain alg orithm was employed that relates 86-GHz brightness temperatures to rai n rate using a coupled cloud-radiative transfer model. Results identif y an early evening maximum in rainfall along the western slope of the Sierra Madre Occidental during all three months. A prominent morning r ainfall maximum was found off the western Mexican coast near Mazatlan in July and August. Substantial differences between morning and evenin g estimates were noted. To the extent that three years constitute a cl imatology, results of interannual variability are presented. Results a re compared and contrasted to high-resolution (8 km, hourly) infrared cloud climatologies, which consist of the frequency of occurrence of c loud colder than -38-degrees-C and -58-degrees-C. This comparison has broad implications for the estimation of rainfall by simple (cloud thr eshold) techniques. By sampling the infrared data to approximate the t ime and space resolution of the microwave, we produce ratios (or adjus tment factors) by which we can adjust the infrared rain estimation sch emes. This produces a combined microwave/infrared rain algorithm for m onthly rainfall. Using a limited set of raingage data as ground truth, an improvement (lower bias and root-mean-square error) was demonstrat ed by this combined technique when compared to either method alone. Th e diurnal variability of convection during July 1990 was examined usin g hourly rain estimates from the GOES precipitation index and the conv ective stratiform technique, revealing a maximum in estimated rainfall from 1800 to 2100 local time. It is in this time period when the SSM/ I evening orbit occurs. A high-resolution topographic database was ava ilable to aid in interpreting the influence of topography on the rainf all patterns.