REGIONAL RAINFALL CLIMATOLOGIES DERIVED FROM SPECIAL SENSOR MICROWAVEIMAGER (SSM I) DATA/

Climatologies of convective precipitation were derived from passive mi crowave observations from the Special Sensor Microwave Imager using a scattering-based algorithm of Adler et al. Data were aggregated over p eriods of 3-5 months using data from 4 to 5 years. Data were also stra tified by satellite overpass times (primarily 0600 and 1800 local time ). Four regions [Mexico, Amazonia, western Africa, and the western equ atorial Pacific Ocean (TOGA COARE area)] were chosen for their meteoro logical interest and relative paucity of conventional observations. Th e strong diurnal variation over Mexico and the southern United States was the most striking aspect of the climatologies. Pronounced morning maxima occurred offshore, often in concavities in the coastline, the r esult of the increased convergence caused by the coastline shape. The major feature of the evening rain field was a linear-shaped maximum al ong the western slope of the Sierra Madre Occidental. Topography exert ed a strong control on the rainfall in other areas, particularly near the Nicaragua/Honduras border and in Guatemala, where maxima in excess of 700 mm month-1 were located adjacent to local maxima in terrain. T he correlation between the estimates and monthly gage data over the so uthern United States was low (0.45), due mainly to poor temporal sampl ing in any month and an inadequate sampling of the diurnal cycle. Over the Amazon Basin the differences in morning versus evening rainfall w ere complex, with an alternating series of morning/evening maxima alig ned southwest to northeast from the Andes to the northeast Brazilian c oast. Areal extent of rainfall in Amazonia was slightly higher in the evening, but a maximum in morning precipitation was found on the Amazo n River just east of Manaus. Precipitation over the water in the ITCZ north of Brazil was more pronounced in the morning, and a pronounced l and- /sea-breeze circulation was found along the northeast coast of Br azil. Inter-comparison of four years revealed 1992 to be the driest ov er Amazonia, with about a 23% decrease in mean rain rate compared to t he 4-year mean estimated rain rate. The major rain feature of tropical western Africa was found on the west coast-a pronounced overland even ing maximum directly between the coast and a high mountain peak, and a morning maximum directly off shore. An intense, localized morning max imum of over 1 000 mm month-1 was found at a concavity in the coast at the Bight of Bonny. In the region of the TOGA COARE experiment, preci pitation in the ITCZ was greater in November 1989 February 1990, compa red to the same period in 1988-1989, notably in the region five degree s either side of the equator from 160-degrees-E to the dateline. There was a clear preference in both seasons for morning precipitation over the water. Interesting diurnal effects were found over and offshore o f New Guinea and the Solomon Islands. For the eight months studied, av eraging both the gauges and Goddard Scattering Algorithm estimates to 2.5-degrees grid boxes yielded a correlation of 0.73, bias of -59.5 mm , and a root-mean-square difference of 131.8 mm-29% and 64%, respectiv ely, of the mean monthly observed rainfall.