Effects of biomass burning-derived aerosols on precipitation and clouds in the Amazon Basin: a satellite-based empirical study

Biomass burning in the Amazon provides strong inputs of aerosols into the atmosphere, with potential implications for precipitation, cloud properties, and atmospheric circulation. However, no study to-date has systematically examined these effects at the scale of the Amazon Basin, over an entire burning season, using available datasets. We examine empirically the relationship between aerosol optical depth (τ_a) versus rainfall and cloud properties measured by spaceborne sensors over the entire Brazilian Amazon during the dry, biomass burning seasons (Aug~Oct) of 2000 and 2003. τ_a and cloud properties derived from the Moderate Resolution Imaging Spectroradiometer (MODIS); rainfall was measured by the TMI and PR instruments onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. We attempted to account for synoptic scale forcing by stratifying the satellite data by the vertically-integrated moisture flux convergence (MFC) calculated from the NCEP Reanalysis meteorology. Elevated τ_a was associated with increased rainfall as measured by both the TMI and PR in both 2000 and 2003. Furthermore, with enhanced τ_a cloud cover increased significantly, and cloud top temperature decreased (suggesting higher clouds). While the empirical correlations by themselves do not unequivocally establish the causal link from aerosols, these results are consistent with previous observational [Andreae et al., 2004] and modeling [Khain et al., 2005] studies that pointed to dynamical effects from aerosols that invigorate convection, leading to higher clouds, enhanced cloud cover, and stronger rainfall. We speculate that the changes in precipitation and cloud properties associated with aerosol loading observed in this study could have important radiative, hydrological, and dynamical effects on the climate system.