Drizzle?

Dr. Bjorn Stevens, University of California at Los Angeles

Can one of nature's seemingly more mundane processes regulate the Earth's climate? A growing literature suggests that the aerosol-induced suppression of drizzle may increase stratiform cloudiness and help offset surface warming generally attributed to greenhouse gases. In reviewing this literature we show how its conclusions are predicated on the assumption that stratiform cloudiness during the pre-industrial age was limited in important respects by drizzle. At the same time past studies have been unable to quantify the role drizzle plays in the water budget of stratiform clouds. In particular it is generally unknown whether entrainment drying or drizzle is most responsible for offsetting the moistening tendency of surface fluxes in the marine boundary layer. Recent data collected during DYCOMS-II is used to shed light on these issues. These data provide definitive estimates of both entrainment and drizzle in stratocumulus cloud decks in the north-east Pacific. The data show that at times drizzle plays an important role in the dynamics of the marine boundary layer, perhaps even inducing actinoform clearings of clouds.