| The Tibeten Plateau(TP)has a significant impact on regional and global weather and climate.In summer,the“heat pump”effect caused by the heat source over the TP drives the water vapor converge over the TP,leading to the water resources abundant on the TP.The complex topography and abundant water vapor in the atmosphere induce strong convective activity over the TP.Meanwhile,the TP is located at the intersection of several deserts and areas of intensive human activities,and aerosols from surrounding areas can be transported to the TP.From the perspective of cloud microphysical processes,both aerosols and water vapor are essential and important conditions in the cloud formation process.Water vapor and convective clouds over the TP not only can contribute to local precipitation,but also can move further out of the TP under the influence of the westerly wind belt,becoming an important factor affecting the weather and climate in the downstream regions of the TP.In these processes,it is also an important frontier scientific issue to consider the effects of aerosols on cloud microphysical processes and on local and downstream precipitation of the TP.To address the above issues,this study combines multi-sourced satellite observations,reanalysis data,Weather Research and Forecasting(WRF)model,and Nonhydrostatic Icosahedral Atmospheric Model coupled with Spectral Radiation-Transport Model for Aerosol Species(NICAM-SPRINTARS)to systematically investigate the effects of water vapor and cloud water resources transport from the TP on precipitation over downstream regions.Besides,the aerosol-cloud interactions over the TP were also considered in the study,providing a strong scientific basis for revealing the impact of water resources transport from the TP on downstream weather and climate.The main conclusions are summarized as follows:(1)The distributions and variation characteristics of the water vapor and cloud properties over the TP are revealed.The whole layer water vapor content is mainly distributed in the southeast TP and gradually decreases from the southeast to the northwest TP,and the annual averaged water vapor content shows a significant increasing trend.The water vapor content over the TP shows obvious seasonal difference,with the highest values in summer but the lowest in winter.The water vapor over the TP is mainly transported from west to east,and the annual averaged water vapor flux shows a significant increasing trend.The annual mean cloud fraction over the TP shows higher over the east and west of the TP,and lower over the middle of the TP,indicating an insignificant decreasing trend.The annual mean liquid water path shows higher over the western TP and lower over the eastern TP,and the tendency is not significant.The ice water path inside the main body of the TP is higher in the central region and lower in the edge region,showing a significant decreasing trend.In addition,the high values of ice water path apper over the edge of the southern slope of the TP.(2)The impact of the Asian subtropical westerly jet(SWJ)on water vapor transport and downstream precipitation of the Tibetan Plateau is clarified.Over the past 40 years(1981-2019),the significant northward shift of the center of the Asian SWJ caused a weakening of the latitudinal winds in the southern Asian subtropical region,resulting in a decrease in water vapor outflowing from the Tibetan Plateau to the downstream regions,which is positive to the precipitation over North China.The effect of the change in water vapor transport from the TP on the precipitation over North China was further quantified by the WRF model,and it was found that if the water vapor outflowing from the eastern border of the TP decreases by 52.74%and 35.12%,the precipitation in North China would be decreased by 12.69%and 7.98%,respectively.On the contrary,if the water vapor outflowing from the eastern border of the TP increases by 35.12%and 70.23%,the precipitation in North China would be increased by 1.79%and 6.31%,respectively.The mechanism is that the northward shift of the Asian SWJ center weakens the latitudinal winds over the southern Asian subtropics,leading to the reduction of water vapor transport from the TP to North China,which in turn affects the formation of clouds over North China and finally weakens the precipitation.(3)The effects of the Taklamakan Desert dust aerosols on the cloud properties over the Tibetan Plateau are investigated.The Taklamakan Desert dusts can be lifted up to the northern slope of the TP and mix with convective clouds over the TP,and the addition of dust aerosols in the clouds can decrease the cloud drop radius and prolong the cloud lifetime,causing the convective clouds develop vertically vigorously,consequently,the cloud top height and cloud fraction are increased.Meanwhile,the numerical simulations also confirm that the cloud drop radius decreases and the cloud optical thickness and cloud water content increase as the dust aerosols over the TP increases.In addition,when the dust aerosols over the TP begin to decrease,the cloud drop radius gradually increases and the cloud optical thickness and cloud water content begin to decrease.The maximum responses of cloud particle radius,water and ice cloud optical thickness,liquid water and ice water paths to dust aerosol were-1.06?m,1.00,1.91,16.99 g m-2,and 42.74 g m-2,respectively.(4)The impact of convective clouds outflowing eastward from the TP on downstream precipitation is quantified.Convective clouds outflowing out from the TP can cause low-level convergence rise and high-altitude dispersion motion over the Yangtze River basin,which is conducive to the development of low-level vortices and the intensification of upward motion,affecting the precipitation over the downstream regions.The numerical simulations show that when the water vapor over the TP is halved,the local convective activity over the plateau is significantly weakened,causing a decrease in precipitation in the middle and lower reaches of the Yangtze River,which can reduce precipitation in the Yangtze River basin by up to 74.4%.It is also found that the aerosol-cloud interactions over the TP further develop convective clouds over the plateau,and the convective clouds influenced by dusts moving eastward out of the TP can delay the heavy rainfall over North China for about 12 hours,but enhance the intensity of precipitation up to 53.3%,which in turn intensifies the heavy precipitation process over North China. |
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