Multi-instrumental observation of an exceptionally strong Saharan dust outbreak over Portugal

Abstract

An exceptionally strong outbreak of Saharan dust was observed in southern Portugal from 4 to 9 April 2011. The event was monitored with a multiwavelength Raman lidar, a CIMEL Sun photometer, the Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) and two Moderate Resolution Imaging Spectroradiometer (MODIS). Those instruments provided vertically resolved and columnar information about the optical properties of the mineral dust plume. Maximum aerosol optical depths in the free troposphere of 1.8 and 1.9 were observed with Raman lidar at 355 and 532nm, respectively. The mean extinction-related Ångström exponents (355, 532nm) from Raman lidar averaged over the whole period were 0.0 ± 0.2. Backscatter-related Ångström exponents (355, 532nm and 532, 1064nm) were 0.4 on average. Mean lidar ratios calculated from Raman lidar measurements were 45 ± 8sr at 355nm and 53 ± 7sr at 532nm. The mean linear particle depolarization ratio at 532nm was 0.28 ± 0.04. Furthermore, intrusion of Saharan dust into the planetary boundary layer could be observed with different ground-based in-situ instruments. Maximum particle mass concentration values (PM10) of 162 μg m-3 were detected at ground. The daily threshold of 50 μg m-3 for PM 10 was exceeded on three subsequent days during the considered period. The ratio of coarse to fine mode particle number concentration reached maximum values larger than 1, coinciding with the maximum in PM10 mass concentration. A comparison of ground-based Raman lidar measurements and CALIOP Level 2 data was done for one CALIPSO overpass. A good agreement was found for backscatter coefficients. Copyright 2011 by the American Geophysical Union.