Satellite-Based Studies of Smoke-Cloud Interactions in the Atlantic Ocean off the coast of South America

The influence of aerosols on cloud properties is important because aerosols can change cloud radiative properties, cloud lifetime and rain patterns. However, observational studies linking aerosol effects to clouds properties have been sparse; in particular there are no studies focusing on the effects of aerosols downwind of the biomass burning regions of South America. We analyzed collocated aerosol and cloud parameters derived by the MODIS instrument on the Terra and Aqua satellites and precipitation parameters from the instruments onboard the TRMM satellite. The MODIS parameters used (Level 3 daily) in this study were aerosol optical depth (AOD), cloud optical thickness (COT), cloud effective radius (CER), cloud top pressure (CTP) and cloud fraction (CF). The cumulative rainfall (CR, level 3B42RT) parameter was obtained from TRMM. Data were obtained from the GES-DISC Interactive Online Visualization ANd aNalysis Infrastructure (Giovanni) interface developed by NASA (http://giovanni.gsfc.nasa.gov/). The data were selected in the South Atlantic Ocean downwind of Southern Brazil for the most active biomass burning months along a perpendicular axis to the coast and at the center of the smoke plume as determined in monthly maps of AODs. We analyzed the variation of COT and CER for low and high clouds (defined by a threshold set at 650 mb) as well as CF and CR along the axis as a function of AOD in monthly and 4-day averages for the years 2001-2005. We found evidence suggesting a possible influence of smoke on cloud properties. Elevated aerosol concentrations correlated with smaller CER and higher COT. Furthermore, during the most active burning seasons, we observed lower rainfall in low clouds, but higher average rainfall in high clouds. This suggests that high aerosol concentrations may lead to an initial lag in rainfall but subsequently invigorating convecting clouds and producing more active, higher-precipitation thunderstorms.