Regional Rainfall Climatologies Derived from Special Sensor Microwave Imager (SSM/I) Data

Abstract

Climatologies of convective precipitation were derived from passive microwave observations from the Special Sensor Microwave Imager using a scattering-based algorithm of Adler et al. Data were aggregated over periods of 3?5 months using data from 4 to 5 years. Data were also stratified by satellite overpass times(primarily 0600 and l800 local time). Four regions [Mexico, Amazonia, western Africa, and the western equatorial Pacific Ocean (TOGA COARE area)] were chosen for their meteorological interest and relative paucity of conventional observations. The 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 result of the increased convergence caused by the coastline shape. The major feature of the evening rain field was a linear-shaped maximum along the western slope of the Sierra Madre Occidental. Topography exerted 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. The correlation between the estimates and monthly gage data over the southern United States was low (0.45), due mainly to poor temporal sampling in any month and an inadequate sampling of the diurnal cycle. Over the Amazon Basin the differences in morning versus evening rainfall were complex, with an alternating series of morning/evening maxima aligned southwest to northeast from the Andes to the northeast Brazilian coast. Areal extent of rainfall in Amazonia was slightly higher in the evening, but a maximum in morning precipitation was found on the Amazon River just east of Manaus. Precipitation over the water in the ITCZ north of Brazil was more pronounced in the morning, and a pronounced land-/sea-breeze circulation was found along the northeast coast of Brazil. Inter-comparison of four years revealed 1992 to be the driest over Amazonia, with about a 23% decrease in mean rain rate compared to the 4-year mean estimated rain rate. The major rain feature of tropical western Africa was found on the west coast?a pronounced overland evening maximum directly between the coast and a high mountain peak, and a morning maximum directly offshore. An intense, localized morning maximum of over 1000 mm month?1 was found at a concavity in the coast at the Bight of Bonny. In the region of the TOGA COARE experiment, precipitation in the ITCZ was greater in November 1989-February 1990,compared to the same period in 1988?1989, notably in the region five degrees either side of the equator from 160°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 of New Guinea and the Solomon Islands. For the eight months studied, averaging both the gauges and Goddard Scattering Algorithm estimates to 2.5° 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%, respectively, of the mean monthly observed rainfall.