Abstract
Abstract. We use CALIOP nighttime measurements of lidar backscatter, color and depolarization ratios, as well as particulate retrievals during the summer of 2007 to study transatlantic dust properties downwind of Saharan sources, and to examine the influence of nearby clouds on dust. Our analysis suggests that (1) under clear skies, while lidar backscatter and color ratio do not change much with altitude and longitude in the Saharan Air Layer (SAL), depolarization ratio increases with altitude and decreases westward in the SAL; (2) the vertical lapse rate of dust depolarization ratio, introduced here, increases within SAL as plumes move westward; (3) 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, (4) the odds of CALIOP finding dust below SAL next to clouds are about ⅔ of those far away from clouds. This feature, together with an apparent increase in depolarization ratio near clouds, indicates that particles in some dust volumes loose asphericity in the humid air near clouds, and cannot be identified by CALIPSO as dust.
Highlights
Atmospheric mineral dust particles have significant effects on the climate and the environment
The recent study of co-located MODIS and CALIPSO observations demonstrates that off-track clouds do not qualitatively change the aerosol property trends obtained from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data alone, and that the contributions from off-track clouds can be taken into account by using a scaling factor (Varnai and Marshak, 2012.)
In order to reduce the impact of misclassifications between clouds and aerosols, this paper examines aerosol layers only if the Cloud-Aerosol Discrimination (CAD) product (Liu et al, 2004, 2009) – based on probability distribution functions obtained from expert
Summary
Atmospheric mineral dust particles have significant effects on the climate and the environment. This question is pertinent as recent studies have shown that optical properties of clear sky aerosols are different in the vicinity of clouds from those far away from clouds (e.g., Clarke et al 2002; Twohy, et al, 2002, 2009b; Koren et al, 2007; Su et al, 2008; Redemann et al, 2009; Tackett and Girolamo, 2009; Varnai and Marshak, 2011). The answer to this question is likely to improve our understanding of dust-cloud interactions and yield better estimates of direct radiative forcing. The recent study of co-located MODIS and CALIPSO observations demonstrates that off-track clouds do not qualitatively change the aerosol property trends obtained from CALIOP data alone, and that the contributions from off-track clouds can be taken into account by using a scaling factor (Varnai and Marshak, 2012.)
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