The Impact of Amazonian Deforestation on Dry Season Rainfall

Abstract

Many modeling studies have concluded that widespread deforestation of Amazonia would lead to decreased rainfall. Geosynchronous visible and infrared satellite data over southwest Brazil are analyzed with respect to percent cloudiness, and rain estimates are analyzed from both the Tropical Rainfall Measuring Mission and Special Sensor Microwave Imager. The studies conclude that in the dry season, when the effects of the surface are not overwhelmed by synoptic-scale weather disturbances, shallow cumulus cloudiness, deep convective cloudiness, and rainfall occurrence all are larger over the deforested and nonforested (savanna) regions than over areas of dense forest. This paper speculates that this difference is in response to a local circulation initiated by the differential heating of the region's varying forestation. Analysis of the diurnal cycle of cloudiness reveals a shift in the onset of convection toward afternoon hours in the deforested and toward the morning hours in the savanna regions when compared to the neighboring forested regions. Analysis of 14 years of monthly estimates from the Special Sensor Microwave Imager data revealed that in August there was a pattern of higher monthly rainfall amounts over the deforested region. Analysis of available rain gauge data showed an increase in regional rainfall since deforestation began around 1978.