Removal of aerosols in the Indian Ocean ITCZ

Measurements taken during the Indian Ocean Experiment (INDOEX) showed significant aerosol concentrations of continental origin (mineral dust, carbonaceous and sulfate particles) over large areas of the Northern Indian Ocean. This aerosol accumulation was favored by the dry regime of the winter monsoonal circulation that allows atmospheric particles to be transported during 6-7 days toward the tropical Indian Ocean. In contrast, measurements across and South of the ITCZ showed a dramatic decrease in aerosol concentrations, caused mainly by precipitation removal. Since the aerosols impact large regions of the Indian Ocean and have consequences for regional climate changes, there is a great interest in understanding their natural removal by clouds and precipitation.

We use the Geophysical Fluid Dynamics Laboratory cloud resolving model with aerosol- cloud interactions capabilities to investigate the removal of aerosols by clouds and precipitation in the Indian Ocean ITCZ. The model is driven by NCEP meteorological data and by aerosol measurements taken during various phases of INDOEX. Our findings suggest that the removal of aerosols is highly dependent of the intermittent nature of deep convection and precipitation. Convective systems are pumping aerosols from the lower part of the atmosphere at higher levels during intense updrafts. By nucleation scavenging aerosol particles are included in cloud droplets. Rain drops are effective collectors of aerosols both in cloud and sub-cloud during precipitation. While, the area coverage of convective precipitation is small at a given time (about 5-10 %), repetitive precipitation events are effective in removal of most of the aerosol in a matter of days. Areas of the ITCZ with weak precipitation events can allow some aerosol to escape into the air masses of Southern Hemisphere.