The structure of the haze plume over the Indian Ocean during INDOEX: tracer simulations and LIDAR observations

Three-dimensional nested tracer simulations of a pollution plume originating from the Indian sub-continent over the Indian Ocean in the framework of the Indian Ocean Experiment (INDOEX) between 5 and 9 March 1999 were performed with the Regional Atmospheric Modeling System (RAMS) to provide insight into the transport patterns of the pollutants as well as investigate the dynamical mechanisms controlling the vertical structure of the plume and its evolution in the vicinity of the Maldives Islands. Airborne and ground-based LIDAR observations of the structure of the haze plume made on 7 March 1999 were used to assess the quality of the simulations as well as the impact of grid resolution on the vertical structure of the simulated plume. It is shown that, over the Arabian Sea, in the vicinity of the Maldives Islands, the pollutants composing the plume observed by the airborne LIDAR essentially originated from the city of Madras and that the vertical structure of the plume was controlled by the diurnal cycle of the continental boundary layer depth. A combination of tracer simulations and remote sensing observations (airborne LIDAR, ship-borne photometer, ground-based LIDAR in Goa) was used to analyse the diurnal evolution of the haze plume over the sea. We find evidence that the sea breeze circulation and orographic lifting taking place in the southern part of the Indian sub-continent during the day time play a crucial role in the modulation of the continental boundary layer depth, and in turn, the haze plume depth. The dynamical processes, as well as entrainment at the top of the marine atmospheric boundary layer, also play a crucial role in re-circulating a fraction of the tracers transported over ocean by the monsoon flow above the marine atmospheric boundary layer in the landward (southwesterly) branch of breeze circulation during the next day. This in turn leads to pollutant accumulation in the vicinity of the Indian coastline. Nevertheless, this contribution to the total aerosol load observed during the INDOEX intensive field phase is shown to small compared to that related to massive advection of aerosol from the continent. The nesting of a high horizontal resolution domain (5 km, with 39 vertical levels below 4000 m above mean seal level), allows a better representation of local dynamics, sea and mountains breezes circulations, and therefore a noticeable improvement in the representation of the pollutants plume in the simulation.