Direct and Indirect Effects of Anthropogenic Aerosols on Regional Precipitation Over East Asia

A regional coupled climate-chemistry-aerosol model is developed and used to assess the various effects of anthropogenic sulfate and carbonaceous aerosols on regional climate over East Asia, with a focus on precipitation. The model includes three major types of anthropogenic aerosols: sulfate, black carbon and organic carbon; and four types of aerosol radiative/cloud effects: the direct effect, semi-direct effect and 1st and 2nd indirect effects. Our simulations suggest that the direct effect and 1st indirect effect mainly reduce the solar radiation and hence decrease the surface temperature, while the 2nd indirect effect generates a substantial positive long-wave forcing compared to the negative solar forcing, and decreases the precipitation, but less effect on surface temperature. The simulated precipitation reduction by the combined direct/semi-direct/1st indirect effect is about -10% in the fall and winter, and about -5% in the spring and summer. The 2nd indirect effect by itself decreases the fall and winter precipitation by about -20 ~ -30% using BH94 scheme, and about -3% using TC80 scheme, thus it is strongly dependent on the autoconversion scheme. The semi-direct effect redistributes the vertical temperature structure of atmosphere, and possibly affects the precipitation pattern over China, but its overall effect on precipitation is negligible relative to the other aerosol effects. By allowing the feedbacks between aerosol perturbation and climate change, the coupled model generally decreases the discrepancies between the model-simulated and observed precipitation and aerosols over the region. The EOF analysis of the climatological precipitation over East Asia from last century shows a decreasing trend of the 1st or 2nd leading modes over most area of China in the fall and winter, which is generally geographically consistent with the distribution of the model-simulated precipitation reduction from anthropogenic aerosols. However, the model predicted precipitation reduction trend is larger than that in the observation, even without the inclusion of the 2nd indirect effect.