Analysis of smoke impact on clouds in Brazilian biomass burning regions: An extension of Twomey's approach

Satellite remote sensing of smoke aerosol-cloud interaction during the rece nt Smoke, Clouds, and Radiation-Brazil (SCAR-B) experiment is analyzed to e xplore the factors that determine the magnitude of the cloud response to sm oke aerosol. Analysis of 2 years worth of data revealed that the response i s greatest in the north of Brazil where aerosol optical depth is smallest, and tends to decrease as one moves southward, and as aerosol optical depth increases. Saturation in this response occurs at an aerosol optical depth o f 0.8 in 1987 and 0.4 in 1995. To explore the reasons for this, a framework is developed in which the satellite-measured response can be compared to s imple analytical models of this response and to numerical models of smoke a erosol-cloud interaction. Three types of response are identified: (1) cloud droplet concentrations increase with increasing aerosol loading, followed by saturation in the response at high concentrations; (2) as in type 1, fol lowed by increasing droplet concentrations with further increases in aeroso l loading. This increase in droplet concentration is due to the suppression of supersaturation by abundant large particles, which prevents the activat ion of smaller particles. This enables renewed activation of larger particl es when smoke loadings exceed some threshold; (3) as in type 1, followed by a decrease in droplet number concentrations with increasing aerosol loadin g as intense competition for vapor evaporates the smaller droplets. The lat ter implies an unexpected increase in drop size with increasing smoke loadi ng. The conditions under which each of these responses are expected to occu r are discussed. It is shown that although to first-order smoke optical dep th is a good proxy for aerosol indirect forcing, under some conditions the size distribution and hygroscopicity can be important factors. We find no e vidence that indirect forcing depends on precipitable water vapor.