Ground-based validation of CALIPSO observations of dust and smoke in the Cape Verde region

Abstract

Ground-based Raman lidar measurements during the second Saharan Mineral Dust Experiment (SAMUM-2) in 2008 were used for validation of measurements of the lidar aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite within the dusty environment of the Cape Verde region. SAMUM-2 featured two one-month campaigns in January/February and May/June 2008 to cover different modes of aerosol transport to the tropical Atlantic: dust from northern Africa and biomass-burning smoke from western Africa during winter, and pure Saharan dust during summer. During the investigated time period, 33 CALIPSO overflights occurred at a distance of less than 500 km from the location of the ground-based lidar. Fifteen out of these 33 cases were found suitable for comparing the findings of the two instruments. The parameters for this comparison are the particle backscatter coefficient at 532 and 1064 nm, the extinction coefficient, the lidar ratio (aerosol type), and the particle depolarization ratio at 532 nm, as well as the backscatter-related angstrom ngstrom exponent for the wavelength pair 532/1064 nm. Best agreement was found for the 532 nm backscatter coefficient, while the 532 nm extinction coefficient is underestimated by up to 30%. The latter is due to the use of an effective dust lidar ratio that gives reliable backscatter coefficients but is not suitable to transform these to extinction coefficients. CALIPSO particle depolarization ratios provided in the current (version 3.01) aerosol profile product were found to be affected by a computing error and should be calculated from the perpendicular and total particle backscatter coefficients provided in the same data file. CALIPSO aerosol classification was found to be mostly correct but a demand for homogeneous aerosol layers could improve the retrieval. Suggestions for the improvement of the CALIPSO retrieval by introducing iterative procedures are provided.