Characterization of aerosol spatial distribution and optical properties over the Indian Ocean from airborne LIDAR and radiometry during INDOEX'99

Abstract

The three-dimensional structure and the optical properties of the Indian pollution plume has been investigated from airborne LIDAR and radiometric measurements over the Indian Ocean during three consecutive days (7, 8, and 9 March 1999) of the INDOEX'99 intensive field phase. The vertical structure of the plume consisted of two layers: the marine atmospheric boundary layer (MABL) and the so-called land plume aloft. The depth of the land plume was observed to depend on the history of the air masses; shallow plumes were associated with air masses coming from the Gulf of Bengal, while deeper plumes were associated with air masses coming from the Indian subcontinent. The larger aerosol optical depths (AODs) observed with Meteosat-5 over the Arabian Sea were associated with the deeper land plume. A combination of airborne light detection and ranging (LIDAR) measurements and Sun photometer measurements at Kaashidhoo Observatory and in Male were used to determine the column-equivalent backscatter-to-extinction ratio needed to retrieve aerosol extinction coefficient profiles and AOD from LIDAR measurements. Direct aerosol forcing was analyzed using a simple radiative model as well as LIDAR-derived AOD and visible flux measurements. Average values of the vertically integrated single scattering albedo of about 0.85 ± 0.05 and 0.8 ± 0.1 were found to be associated with the shallower and the deeper part of the land plume, respectively.