Study On The Source,Transport And Radiative Effect Of Aerosols Over The Tibetan Plateau

The Tibetan Plateau?TP?,which is one of the highest and most complex plateaus in the world,can influence the atmospheric circulation and hydrologic cycle through dynamic and thermal forcing.Satellite observations show that there are substantial dust and anthropogenic aerosols over the TP.These aerosols have distinct effects on the energy balance,microphysical cloud properties and precipitation rates.Thus,thus figuring out the three-dimensional distribution,transport and radiation effects of aerosols over the TP is of great significance for revealing the role of TP playing in the regional and global climate change.Based on multiple satellite observations,reanalysis data and numerical models,the three-dimensional distribution,sources,transport process and radiation effects of aerosols over the TP are systematically studied in this paper.The main conclusions are as follows:?1?Three-dimensional distribution of aerosols over the TP and their mixture with cloudsAerosols over the TP are characterized by high in spring-summer and low in autumn-winter,and they present different seasonal variations in different regions.Aerosols over the northern slope of TP are mainly dust with an occurring frequency up to 60%,which can be extended to more than 6 km.The aerosol extinction coefficient is about 0.36 km^-1 decreasing with height.The aerosol types over the southern slope of the TP are complex,mainly including dust,polluted dust,soot and polluted continental aerosols.The aerosol extinction coefficient is about 0.30 km^-1.The aerosols over the main body of the TP are dominated by dust and polluted dust,and the aerosol extinction coefficient is only about 0.14 km^-1.Aerosols over the eastern slope of the TP?ESTP?are distributed below 5 km,and the aerosol extinction coefficient exceedes 0.56 km^-1.In summer,there are a large amount of aerosols over the south,north slopes and the main body of the TP with large aerosol extinction coefficient of⁰.56 km^-1,which can be rised even above 10 km.There are few aerosols mixed with clouds but the impact of aerosols on clouds cannot be ignored.Dusty clouds are mainly distributed over the southwestern and northerv slopes?¹3%?and the northeastern part?⁷%?of the TP.The polluted dust-clouds are mainly distributed over the southern,eastern and northeastern parts of the TP?⁸%?.?2?Source and transport of summer dust over the TPThe dust aerosols over the TP are mainly from the Taklimakan Desert and partially from the Gurbantunggut Desert and Great Indian Thar Desert.The northwesterly airflow carrying cold air is strengthened by the narrow terrain between the Tianshan and Altai mountains.The strengthened northwesterly airflow is divided into two branches due to the high terrain of the TP:one continues eastward and the other one turns to northeasterly blowing toward the TP.When a cold advection or front passes,dust particles are emitted into the atmosphere from the Taklimakan and Gurbantunggut deserts and then transported to the northern slope of the TP by the northeasterly.The correlation coefficient of the monthly frequencies of summer dust events over the TP and cold advections passing the Taklimakan and Gurbantunggut deserts are as high as 0.68 and 0.34,respectively.Differently,although the correlation is limited,the Tibetan dust is associated with the passing low-pressure system over the Great Indian Thar Desert.Dust aerosols emitted from the Great Indian Thar Desert are further transported to the southern slope of the TP by the prevailing westerly wind.The dust aerosols are generally polluted by anthropogenic aerosols during the transport.Because of the strong upward current,aerosols tansported to the TP can extend upward to approximately 7-8 km.?3?Distribution,source and aggregating mechanism of anthropogenic aerosols over the TPThe aerosol optical depth?AOD?over the ESTP indicates extremely large and even larger than that over some industrialized regions and deserts.The main aerosol type over the ESTP is sulfate,followed by dust and carbonaceous aerosols.It is similar for aerosols over the Ganges plain in the southern part of the plateau.Local emissions related to human activity directly contribute to the accumulation of sulfate and carbonaceous aerosols.In addition,in spring,abundant carbonaceous aerosols emitted from forest,grassland and savanna fires in Southeast Asia can be transported to the ESTP by the prevailing southwesterly wind.The dust AOD over the ESTP peaks in spring because of the transport from the Taklimakan and Gobi deserts.The transported dust aerosols are often polluted by the local anthropogenic aerosols.Additionally,the high aerosol loading over the ESTP is also directly related to the meteorological background.Due to the special topography,the terrain-driven circulation can trap aerosols in the Sichuan Basin.This leads to the high load of aerosols over the ESTP.Crop burning in the Ganges Plain leads to high loading of the local carbonaceous aerosols.The Hengduan Mountains are not conducive to the eastward transport of aerosols over the Ganges Plain.The aerosol loading is lowest in summer because of the larger precipitation and better diffusion due to larger vertical temperature gradients and ascending air movement enhanced by the plateau heat pump effect.In contrast,the aerosol loading is greatest in winter.?4?Radiation effects of dust,sulfate and carbonaceous aerosols over the TPAlthough the algorithm has been improved,there are still some missing values in the Version 4.10 CALIPSO L2 product due to the misjudgment of aerosol and cloud in daytime.The aerosol extinction coefficient calculated in this paper complements the missing data in the CALIPSO APro product,and shows the vertical distribution of aerosols over the TP reasonably.The energy budget estimation obtained by a radiative transfer model using the calculated aerosol extinction coefficient as an input shows better agreement with the satellite observations than that with the inputs from the Version 4.10 CALIPSO L2 product.Aerosols significantly affect the radiative energy budget and thermodynamic structure of the atmosphere over the TP,mainly by altering the shortwave radiation budget.Dust has the greatest contribution to the radiative forcing of aerosols over the TP,and the radiative forcings of sulfate and carbonaceous aerosols are relatively small.Dust aerosols heat the atmosphere by retaining the energy in the layer and the instantaneous heating rate can be as high as 5.5 K day^–1 depending on the density of the dust layers.The simulation results from 2015-2017 show that dust aerosols over the TP can significantly cool the surface(with net radiative forcing of-16 W m^-2over the south slope and-30 W m^-2 over the north slope)and the top of the atmosphere(with net radiative forcing of-8 W m^-2 over the south slope and-16 W m^-2 over the north slope south),which has a heating effect on the atmosphere(with net radiative forcing of⁸ W m^-2 over the south slope and¹6 W m^-2 over the north slope).Sulfate aerosols have a cooling effect on the top of the atmosphere and the surface(about-5 W m^-2),which has less impact on the atmosphere.Although the AOD is small,carbonaceous aerosols can significantly heat the atmosphere with net radiation forcing of⁶ W m^-2.