CALIPSO observations of transatlantic dust: vertical stratification and effect of clouds
1Goddard Earth Sciences Technology and Research, Universities Space Research Association, Columbia, MD 21044 USA
2NASA Goddard Space Flight Center, Greenbelt, MD 20771 USA
3Joint Center for Earth System Technology, University of Maryland at Baltimore County, Baltimore, MD 21228 USA
4Jet Propulsion Laboratory, 4800 Oak Grove Drive, Ms 180-401, Pasadena, CA 91109 USA
5Department of Physics, Michigan Technological University, Houghton, MI 49931 USA
Abstract. We use CALIOP nighttime measurements of lidar backscatter, color and depolarization ratios during the summer of 2007 to study transatlantic dust properties downwind of Saharan sources, and to examine the interaction of clouds and dust. Our analysis suggests that (1) while lidar backscatter doesn't change much with altitude in the Saharan Air Layer (SAL), depolarization and color ratios both increase with altitude in the SAL; (2) lidar backscatter and color ratio increase as dust is transported westward in the SAL; (3) the vertical lapse rate of dust depolarization ratio, introduced here, increases within SAL as plumes move westward; (4) nearby clouds barely affect the backscatter and color ratio of dust volumes within SAL but not so below SAL. Moreover, the presence of nearby clouds tends to decrease the depolarization of dust volumes within SAL. Finally, (5) the odds of CALIOP finding dust below SAL next to clouds are about 2/3 of those far away from clouds. This feature, together with an apparent increase in depolarization ratio near clouds, indicates that particles in some dusty volumes lose asphericity in the humid air near clouds, and cannot be identified by CALIPSO as dust.