Using perturbed physics to study sulfate aerosol effects on the South Asian monsoon
ClimatePrediction.net is a project to investigate the influence of approximations that must be made in global climate models, and sample uncertainty in model forecasts. By running the Hadley Center Coupled Model (HadCM3) within a distributed computing framework, it has yielded several thousand simulations in each of different model experiments; where each simulation corresponds to a unique set of HadCM3 parameter values. We analyze results from one such experiment, and our goal is to examine influences of increasing sulfur dioxide (SO2) emissions on South Asian monsoon precipitation. The resulting increase in sulfate aerosols can affect monsoon precipitation by various pathways.
We identify the main effects of increasing SO2 that are robust to parameter uncertainties in HadCM3, and introduce a simplified box model to explain these effects. The box model describes growth and depletion of cloud cover. Using the box model we learn that the effects of increasing SO2 can be explained by a longer timescale for clouds to precipitate or a shorter timescale for them to mix with air from the environment. This indeterminacy is resolved by exploiting variation in HadCM3 parameter values, suggesting that effects of increasing SO2 can be understood via longer precipitation timescales. In the presentation we also describe main effects of HadCM3 parameters on precipitation, and suggest explanations for the effects.