A 16-year climatology of global rainfall from SSM/I highlighting morning versus evening differences

This study examines 16 years of passive microwave observations from the Special Sensor Microwave Imager (SSM/I) processed through the Goddard Profiling (GPROF) rain estimation algorithm version 6. Orbit data (for 6 am and 6 pm satellites only) were aggregated in monthly intervals for the period July 1987-December 2003, and were stratified by satellite overpass times (0600 and 1800 local time). Space-time sampling (monthly at 0.5?) results in about 300 samples per grid box per month near the equator. Several regions were chosen for their meteorological interest and relative paucity of conventional observations to contrast the morning (AM) and evening (PM) estimates.

Strong diurnal variations near coastlines were the most striking aspect of the climatologies. Pronounced AM maxima occurred offshore, often in concavities in the coastline, the result of the increased convergence caused by the coastline shape. Examples of this phenomenon included the Gulf of Panama, the Bay of Bengal, parts of both coasts along the isthmus of Mexico, and the Gulf of Guinea (central Africa). A prominent AM maximum was found at the mouth of the Amazon R. at Belem, Brazil, extending eastward along the northern Brazilian coastline. Another striking aspect of the climatology was the offshore, AM maxima surrounding virtually all of the islands of the Maritime Continent, notably New Guinea. A preference for AM rainfall was noted in the western Pacific region of the ITCZ, as well as over the SPCZ. A notable AM maxima was found off the east coast of the U.S. coincident with the position of the Gulf Stream.

Mountain valley circulations were also apparent in the AM/PM rain difference fields. A linear-shaped AM maximum was noted along the west coast of Africa from Senegal to Liberia, in close proximity to a PM maximum over land associated with the Fouta Djallo, a local maximum in topography. The major feature of the North American PM 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.

Major rivers can also exert a strong control on the rainfall. A distinct minimum in the PM rainfall was found along the Amazon R. from Belem to Manaus, continuing westward where the Amazon forks into the Negro and Solimoes rivers. A maximum in morning precipitation was found on the Amazon River just east of Manaus.

It is hoped that by stratifying the rainfall by a climate index, such as Nino 3.4, patterns in the amplitude of the diurnal cycle related to interannual variability will be revealed TRMM data, both from the TMI and PR, with its better coverage of the diurnal cycle compared to the sun-synchronous SSMI’s, will also be examined. We also hope to examine the data set with respect to the effects of urban areas on the rainfall.