Drizzle suppression in ship tracks

Although drizzle was a relatively infrequent occurrence during the Monterey Area Ship Track study, diverse measurements from several sources produced data signals consistent with a reduction in drizzle drops in stratus clouds affected by ship effluents. Concurrent increases in liquid water in the cl oud droplet size range, due to redistribution from the drizzle mode, were n ot always observed, possibly because of the relatively small and often negl igible amounts of water in the drizzle mode. Significant changes in cloud d roplet size distribution, as well as reductions in drizzle flux and concent rations of drops >50-mu m radius, were observed in ship tracks when drizzle was more uniformly present in the ambient cloud. Radiometric measurements showed that increased droplet concentrations in sh ip tracks, which resulted in reduced droplet sizes, can significantly alter the liquid water path. Radar observations indicated that the reduced refle ctivities of ship tracks compared with ambient clouds may be due to reducti ons in the concentrations of larger drops and/or reductions in the sizes of these drops. Two independent modeling studies showed decreases in drizzle in ship tracks due to the presence of smaller cloud droplets that reduced t he efficiency of drop growth by collisions.