Relationships between cloud droplet effective radius, liquid water content, and droplet concentration for warm clouds in Brazil embedded in biomass smoke

During the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) project, the microphysical properties of over 1000 warm, nonprecipitating, clouds were measured from the University of Washington research aircraft. The clouds were partially embedded in the continental‐scale, smoky haze that envelops much of Brazil during the biomass‐burning season. For the entire data set, the most universal parameterization for the effective cloud droplet radius ( r eff ) is as a function of the ratio of cloud liquid water content (LWC) to droplet concentration (essentially the volume mean radius, r v ); this agrees with previous studies under less polluted conditions. Comparisons of SCAR‐B data with data from the east coast of the United States and clean oceanic areas show that the r eff ‐ r v relationship is similar in all three cases, suggesting that even the extreme case of clouds impacted by large biomass fires can be treated similarly to more typical clouds. Beyond a certain ambient concentration of accumulation‐mode particles (∼3000–4000 cm −3 ), cloud drop number concentrations for cumulus clouds in Brazil were almost constant, so that further increases in the ambient particle concentration did not change r eff and r eff correlates well with LWC alone. For example, a cumulus cloud, which capped a particularly large smoke plume with total particle concentrations >150,000 cm −3 , had the same r eff ‐LWC relationship as other clouds in the region where the ambient particle concentrations were ∼3000 cm −3 . In this study the values of r eff for cumulus clouds in Brazil affected by smoke were between 3 and 8 μm, compared to 9 to 14 μm inferred from satellite measurements of cloud reflectivity at 3.7 μm by Kaufman and Fraser [1997].