Assessments of the Direct and Indirect Effects of Anthropogenic Aerosols on Regional Precipitation over East Asia Using a Coupled Regional Climate-Chemistry-Aerosol Model
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An aerosol module is developed and coupled to a regional climate model to investigate the direct and indirect effect of anthropogenic aerosols (sulfate and carbonaceous aerosols) on climate with a focus on precipitation over East Asia. This fully coupled regional climate-chemistry-aerosol model is capable of understanding the interactions between the aerosol perturbation and climate change. The simulated aerosol spatial and seasonal distributions are generally consistent with the observations. The magnitude of the simulated total aerosol concentration and optical depth is about 2/3 of the observed value, suggesting the estimated climatic effects in this work are reasonable and conservative. With the implementation of various aerosol effect, i.e., direct, semi-direct, 1st and 2nd indirect effect, the aerosols?impacts on climate are assessed over the region. The direct, semi-direct and 1st indirect effects generate a negative surface solar forcing, leading to a surface cooling, and the semi-direct effect also heats the atmosphere by BC absorption. This, in turn, increases the atmospheric stability and tends to inhibit the precipitation. The precipitation reduction is largest in the fall and winter, up to -10% with the inclusion of both direct and 1st indirect effects. The 2nd indirect effect using BH94 scheme produces a comparable magnitude in long-wave heating as the solar cooling, leading to the nighttime temperature warming of 0.5K, and a reduction in the diurnal temperature range. The precipitation reduction from the 2nd indirect effect strongly depends on the auto-conversion scheme, with about -30% in the fall and winter, and -15% in the spring and summer using BH94 scheme, while less than -5% using TC80 scheme. By allowing the feedbacks between aerosols and climate, 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 from last century over East Asia shows a decreasing mode in the EOF leading modes in the fall and winter, and is generally geographically consistent with the distribution of the model simulated precipitation reduction from anthropogenic aerosols.
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