Direct and indirect effects of anthropogenic aerosols on regional precipitation over east Asia

A regional coupled climate-chemistry-aerosol model is developed. It is used to assess the direct and indirect effects of anthropogenic sulfate and carbonaceous aerosols on regional climate over east Asia with a focus on precipitation. The simulated direct and first indirect effects for the most part reduce the solar radiation and hence decrease the surface temperature, while the second indirect effect generates both negative solar forcing and a substantial positive long-wave forcing. It decreases the precipitation, but because of the cancelling effect, surface temperature does not change very much. With the interactively model-calculated current aerosol loading and the combined direct/semidirect/ first indirect effect, the simulated precipitation is reduced by about 10% in the fall and winter and by about 5% in the spring and summer. The second indirect effect has the largest impact, by itself decreasing the fall and winter precipitation from about 3% to 20%, depending on the autoconversion scheme assumed. The semidirect effect on precipitation is relatively small. An empirical orthogonal function analysis of climatological precipitation over east Asia since the last century shows a decreasing trend of the leading modes over most of China in the fall and winter, which is generally geographically consistent with the distribution of the model-simulated precipitation reduction from anthropogenic aerosols.