Impacts of Anthropogenic Air Pollution on Precipitation in Downwind Mountains

Precipitation over mountain ranges is especially important since it provides a major source of water supply for agriculture and domestic and industrial use. Orographic precipitation is highly impacted by air coming from the upwind area, which is usually a lower elevation plain with severe air pollution in many locations of China. How aerosols in the upwind plain area change precipitation over the mountainous region downwind could be dependent on many factors such as geographic locations, topographic features, aerosol properties, and meteorological conditions. Here we show that, at the two locations of China, aerosols modify orographic precipitation in an opposite way but through the same aerosol-radiation interactions. At the Longmen mountain that rises up to the Tibetan plateau and downwind of Sichuan Basin, we find aerosols to significantly enhance rainfall intensity in the mountain during a flood event through “aerosol-enhanced conditional instability” - aerosols suppress convection by absorbing solar radiation and increasing atmospheric stability in the basin during daytime, which allows excess moist air to be transported to the mountainous areas in the evening and orographically lifted to produce much stronger convection and heavier precipitation. While flood is enhanced when a polluted air mass was synoptically forced uphill during the Sichuan flood, rain suppression over the hills occurs without such a forcing mechanism. This is manifested in the Mt. Hua region downwind of Guanzhong Plain (Xi'an is located in that plain), where aerosols significantly suppress precipitation over Mt. Hua on a climatological basis. We find that one major mechanism contributing to the reduction is the weakened valley-mountain circulation, which reduces the transport of water vapor from the valley to mountain. Then the reduced relative humidity in the mountain weakens convection and precipitation. Modeling combined with analysis of detailed physical and dynamical mechanisms are key to understanding the impacts of severe aerosol pollution in China on weather and precipitation.