Lidar ratio of Saharan dust over Cape Verde Islands: Assessment and error calculation

Abstract

[1] Lidar ratios Sp of Saharan dust were determined at the Cape Verde Islands in the framework of the second phase of the Saharan Mineral Dust Experiment to investigate possible changes during the first few days of transport from the source regions. Lidar ratios were retrieved from vertical profiles of particle extinction coefficients and backscatter coefficients measured simultaneously at 355 and 532 nm with the two Raman lidar systems MULIS and POLIS of the Meteorological Institute of the Ludwig-Maximilians-Universitat. As previous studies are lacking, one of the main foci of this paper is the elaboration of an extensive error analysis of the Sp retrieval. Only the determination of systematic errors, resulting from the uncertainties of the input parameters of the retrieval, and statistical errors of Sp due to signal noise allows one to compare measurements at different sites and to gain insight into the changes of the dust lidar ratio with time. Suitable conditions for the assessment of Sp and the error analysis were met during a strong Saharan dust outbreak from 28 to 30 January 2008, where particle extinction coefficients of the dust layer as high as 0.2–0.4 km−1 in the UV and visible spectral region were observed. We found an average lidar ratio of dust over Cape Verde Islands of 63 ± 6 sr at 355 and 532 nm.