The impact of observed precipitation upon the transport of aerosols from South Asia

Abstract

Deposition of atmospheric aerosols by precipitation is investigated by assimilating near-global 3-hourly precipitation estimates from satellites at 0.25¼ resolution into a global chemical transport model. Simulations of 21 d duration during February 2002 are performed where satellite observations of surface rain rate are incorporated into the computation of the aerosol removal rate. A steady source of a tracer simulating a soluble aerosol species is imposed on the bottom layer of the model over the Indian Subcontinent at a rate of 0.08 μg m−2 s−1. The impacts of wet deposition by observed precipitation patterns upon regional-scale and global-scale transport of aerosols are examined. The results are compared with a model simulation where wet deposition has been turned off and a simulation which uses model-simulated precipitation to compute wet deposition. Observed precipitation patterns are responsible for reducing aerosol amounts over the Indian Ocean basin by about 25% compared with a case with no wet deposition. Over the remainder of the Northern Hemisphere the aerosol amounts are reduced by nearly 80%. Precipitation in the region of the intertropical convergence zone (ITCZ) acts as an effective barrier to the transport of aerosol to the Southern Hemisphere. Virtually all of the wet deposition over the Indian Ocean occurs in grid cells experiencing rain rates of less than 1 mm h−1, even though these events account for only 20% of the total precipitation observed in the region. Light-rain events to the north of the ITCZ occur frequently enough, and scavenge aerosol efficiently enough, to remove much of the aerosol before southward-moving air masses reach the ITCZ. Model-simulated precipitation over the Indian Ocean occurs more frequently than observed by satellite, by as much as 50%, and is characterized by more drizzle events and fewer heavy-rain events than observed. The differences in simulated and observed rain rate distributions result in only a 6% difference in aerosol amount over the Indian Ocean, and an 8% difference over the remainder of the Northern Hemisphere. Satellite observations also reveal that rain events covering more than 80% of the horizontal area of a model grid cell occur up to an order of magnitude more frequently than predicted by a common parametrization of precipitation scavenging. A test of wet deposition based on the observed spatial coverage of surface precipitation suggests that incorrect assumptions about the physical sizes of rain events may result in as much as a factor of 2 over-estimate of the amount of aerosol transported out of the South Asia region to the remainder of the Northern Hemisphere in global atmospheric transport models.