Effect of biomass burning on marine stratocumulus clouds off the California coast

Aerosol-cloud interactions are considered to be one of the most important and least known forcings in the climate system. Biomass burning aerosols are of special interest due to their radiative impact (direct and indirect effect) and their potential to increase in the future due to climate change. Combining data from GOES and MODIS with passive tracers from the FLEXPART Lagrangian Particle Dispersion Model, the impact of biomass burning on marine stratocumulus clouds has been examined in June and July of 2006-2008 off the California coast. Using a continental tracer, the indirect effect of biomass burning aerosols has been isolated by comparing the average cloud fraction and cloud albedo for different meteorological situations, and for clean versus polluted (in terms of biomass burning) continental air masses. Within a 500 km-wide band along the coast of California, biomass burning aerosols, which tend to reside above the marine boundary layer, increased the cloud fraction by 0.143, and the cloud albedo by 0.038. The combined effect is an indirect radiative forcing of -7.45% (cooling effect) on average, with a bias due to meteorology of +0.89%. Further away from the coast, the biomass burning aerosols, which are located within the boundary layer, reduce the cloud fraction by 0.023 and the cloud albedo by 0.006, resulting in an indirect radiative forcing of +1.33% (warming effect) with a bias of +0.49%. These results underscore the dual role that absorbing aerosols play in cloud radiative forcing.