The Significance of Giant Aerosol in Continental Cloud

Continental clouds are generally thought to produce drizzle less easily than maritime clouds, because of their high droplet number concentrations and the resulting small droplet sizes. Consistent with this idea are the results of previous studies of wintertime stratiform clouds in the Northeastern U.S. and Canada, that have suggested that supercooled large drops, a substantial hazard to aviation, are more prevalent in clouds with lower drop concentrations, presumably having formed in “cleaner” air masses. Recent studies using data from the Alliance Icing Research Study II (AIRS II) have suggested, however, that giant aerosol particles may be responsible for the formation of supercooled large drops in some of these wintertime stratiform clouds. These giant aerosol particles appear most prevalent in air masses having passed over major urban and industrial regions, which would be expected to contain more cloud condensation nuclei and thus initiate clouds with higher droplet concentrations. In order to understand this discrepancy in these studies of supercooled droplet formation, new observations of cloud condensation nuclei and giant aerosol particles collected during AIRS II are quantified and compared. Simple adiabatic model calculations are presented to understand the importance of GA in the formation of supercooled drizzle drops, and their possible effects in limiting droplet number concentration. Implications of the findings for the forecasting of supercooled liquid drops will be discussed.