Large-scale advection of continental aerosols during INDOEX

In this paper, we present passive and active remote sensing measurements of atmospheric aerosols over the North Indian Ocean (NIO) during the Intensiv e Field Phase (IFP, January to March 1999) of the Indian Ocean Experiment. The variability of the aerosol load over NIO is discussed based on three-di mentional numerical simulations made at a local scale by use of Regional At mospheric Modeling System (RAMS) and at a regional scale using the zoomed L aboratoire de Meteorologie Dynamique global circulation model (LMD-Z versio n 3.3). Ground-based measurements of the columnar aerosol optical thickness (AOT) and of surface black carbon (BC) concentration were carried out at t wo different sites in India: Goa University on the NIO coast and Dharwar 15 0 kin inland. Local-scale investigations point out that the trend in BC con centration at the ground is not correlated with AOT. Lidar profiles obtaine d both from the surface at Goa and in the NIO from the Mystere-20 research aircraft indicate that a significant contribution to the total AOT (more th an 50%) is due to a turbid monsoon layer located between I and 3 kin height . RAMS simulation shows that the advection of aerosols in the monsoon layer is due to the conjunction of land-sea breeze and topography. We present th e regional-scale extent of the aerosol plume in terms of AOT derived from t he visible channel of Meteosat-5. During March, most of the Bay of Bengal i s overcast by a haze with a monthly average AOT of 0.61 +/-0.18, and a spat ially well-defined aerosol plume is spreading from the Indian west coast to the Intertropical Convergence Zone with an average AOT of 0.49 +/-0.08. Th ose values axe bigger than in February with AOT at 0.35 +/-0.18 and 0.37 +/ -0.09 for the Bay of Bengal and the Arabian Sea, respectively. One of the p rincipal findings of this paper is that a significant contribution to the a erosol load over the NIO is due to the advection of continental aerosols fr om India in a well-identified monsoon layer above the marine boundary layer . Moreover, it is suggested that the increase in biomass burning plays a si gnificant role in the increasing trend in AOT during the winter dry monsoon season.