Aerosol direct forcing of the summer Indian monsoon as simulated by the NCAR CAM3

In this study, the effects of aerosols on the simulation of the Indian monsoon by the NCAR Community Atmosphere Model CAM3 are measured and investigated. Monthly mean 3D mass concentrations of soil dust, black and organic carbons, sulfate, and sea salt, as output from the GOCART model, are interpolated to mid-month values and to the horizontal and vertical grids of CAM3. With these mid-month aerosol concentrations, CAM3 is run for a period of approximately 16 months, allowing for one complete episode of the Indian monsoon. Responses to the aerosols are measured by comparing the mean of an ensemble of aerosol-induced monsoon simulations to the mean of an ensemble of CAM3 simulations in which aerosols are omitted, following the method of Lau et al. (2006) in their experiment with the NASA finite volume general circulation model. Additionally, an ensemble of simulations of CAM3 using climatological mid-month aerosol concentrations from the MATCH model is composed for comparison. Results of this experiment indicate that the inclusion of aerosols results in drops in surface temperature and increases in precipitation over central India during the pre-monsoon months of March, April, and May. The presence of aerosols induces tropospheric shortwave heating over central India, which destabilizes the atmosphere for enhanced convection and precipitation. Reduced shortwave heating and enhanced evaporation at the surface during April and May results in reduced terrestrial emission to cool the lower troposphere, relative to simulations with no aerosols. This effect weakens the near-surface cyclonic circulation and, consequently, has a negative feedback on precipitation during the active monsoon months of June and July.