CALIPSO observations of transatlantic dust: vertical stratification and effect of clouds

摘要

We use CALIOP nighttime measurements of lidar backscatter, color anddepolarization ratios, as well as particulate retrievals during thesummer of 2007 to study transatlantic dust properties downwind of Saharansources, and to examine the influence of nearby clouds on dust. Our analysissuggests that (1) under clear skies, while lidar backscatter and color ratiodo not change much with altitude and longitude in the Saharan Air Layer(SAL), depolarization ratio increases with altitude and decreases westwardin the SAL; (2) the vertical lapse rate of dust depolarization ratio,introduced here, increases within SAL as plumes move westward; (3) nearbyclouds barely affect the backscatter and color ratio of dust volumes withinSAL but not so below SAL. Moreover, the presence of nearby clouds tends todecrease the depolarization of dust volumes within SAL. Finally, (4) theodds of CALIOP finding dust below SAL next to clouds are about ⅔ of thosefar away from clouds. This feature, together with an apparent increase indepolarization ratio near clouds, indicates that particles in some dustvolumes loose asphericity in the humid air near clouds, and cannot beidentified by CALIPSO as dust.