Saturday, 13 January 2007
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 lifetimes, and rain patterns. However, observational studies linking aerosol effects to cloud properties have been sparse; in particular there are very few studies focusing on the effects of aerosols downwind of the biomass burning regions of South America. We analyzed collocated aerosol and cloud parameters derived from the MODIS instrument on the Terra and Aqua satellites and precipitation parameters from the instruments onboard the TRMM satellite. The MODIS parameters used in this study (Level 3 daily) 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 an axis perpendicular to the coast and at the center of the mean location of the smoke plume—as determined by 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 CF and 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 initially suppress rainfall but subsequently invigorate convective clouds and produce more active, higher-precipitation thunderstorms.