Research On Aerosol Effects On Precipitation In Mountainous Areas

Orographic precipitation is highly impacted by air coming from the upwind area, which is usually a low elevation plain with severe air pollution in many locations of China. There are many cities located at the upwind of mountain in China, which provide sources of the anthropogenic pollution. More and more researchers focus on how the anthropogenic aerosols from cities affect the downwind mountain precipitation. To examine the mechanism of how aerosols affect orographic precipitation, we use WRF-Chem to conduct research for precipitation at two mountain locations – a flood event at the northwest of Sichuan Basin and summer precipitation at Mt. Hua. We simulate a flood event occurring at the mountainous area of Sichuan Basin on 2013 July 8 to 9, and explore how the anthropogenic aerosols from Sichuan Basin contribute to the flood. For Mt. Hua, we conduct a monthly-long simulation over the Guanzhong plain and mountain Hua(Mt.Hua) area, to study how the aerosols from Guanzhong plain affect precipitation at Mt.Hua. Four sets of simulation experiments are conducted for both cases: P_ALL(the polluted condition with both aerosol-radiation interactions and aerosol-cloud interactions), C_ALL(the clean condition with both aerosol-radiation and aerosol-cloud interactions), P_NORAD(the polluted condition without aerosol-radiation interaction) and C_NORAD(the clean condition without aerosol-radiation interaction). The study found that the absorbing aerosols mainly black carbon strongly impact orographic precipitation through aerosol-radiation interaction, but the impacts can be an increase or decrease, depending on different topographic features and meteorological conditionsFor the case of a heavy rainfall occurring at the mountainous area of Northwest Sichuan on 2013 July 8 to 9, the main conclusions are:1. The precipitation event is mainly controlled by a weather system. All cases predict obvious rainfall at the northwest mountain of Sichuan basin. But only the P_ALL case reproduce the observed precipitation intensity. It estimates more than 125 mm rainfall at the strong precipitation center. Other cases have less than 100 mm precipitation, The average reduction is about 40% related to P_ALL.2. The absorbing aerosols reduce the solar radiation reaching the surface of Sichuan Basin and decrease surface temperature, while heats the atmosphere, which stabilizes atmosphere at the basin. Therefore, convection at the basin is suppressed during the daytime. From 2 pm to 8 pm, precipitation in P_ALL at basin is decreased about 60% relative to C_ALL.3. Because the suppression of convection at basin during 2pm to 8pm. The moisture and energy arriving at Sichuan Basin area are not released in daytime over the Basin. The moisture and energy are transported to the mountainous area downwind by the prevailing wind, and orographically triggering produces much stronger convection and heavier precipitation compared with the clean case. precipitation in P_ALL at the mountainous area increased 20%-60% relative to C_ALL.For the summer precipitation at Mt.Hua, we find aerosols from Guangzhong plain contribute to the suppression of precipitation over the Mt. Hua, through the same aerosol-radiation interactions(ARI):1. The precipitation of P_ALL at Mt.Hua is decreased by 40% compared with C_ALL. And the decrease is mainly caused by ARI which occurs during daytime.2. The cloud systems are mainly controlled by valley-mountain circulation during the one-month simulation period. We find that the weakened the valley-mountain circulation induced by ARI is the main reason for the suppressed precipitation. That is, the strong absorbing properties of aerosols in the plain reduce the temperature differences between the valley and mountain at the elevated altitudes and cool the plain surface evidently, both of which lead to a weakened valley breeze at the daytime. The valley-mountain breeze is reduced by 0.3 m/s on average.3. The weakened valley breeze circulation along with reduced water vapor in the valley due to reduced evapotranspiration as a result of surface cooling dramatically reduce the transport of water vapor from the valley to mountain as well as the RH over the mountain, resulting in much weakened convection and reduced precipitation by suppressing convection intensity and therefore the ice-precipitating hydrometeor production. The relative humidity is reduced more than 3% over the Mt. Hua.Aerosols modify orographic precipitation in an opposite way but through the same aerosol-radiation interactions for these two cases. The reasons for the differences are likely because of different cloud systems with very different dynamic and thermodynamic conditions. For the Sichuan flood case, the low-level monsoonal flow provides ample water vapor and strong transport of water vapor and heat from the Basin to the Mountain. However, for the Mt.Hua case, the local circulation(i.e., valley mountain circulation) dominates and the weakened valley breeze leads to a drier condition over the mountain, therefore reduces precipitation.Aerosol indirect effects by acting as CCN could suppress warm rain due to less efficient collision-coalescence process. It could also invigorate ice phase clouds and precipitation. However, in the cases of the studies here, due to strong absorbing properties of aerosols, aerosol-radiation interactions dominate so the contribution of aerosol-cloud interactions to precipitation are very small. In addition, since two-moment bulk schemes have significant limitations in representing aerosol indirect effects, Aerosol indirect effect might have large uncertainty in this research.