| As the highest plateau in the world,the Tibet Plateau(TP)is known as the "Water Tower of Asia",tall terrain influences the local hydrological cycle and climate change through dynamic and thermal forcing.At the same time,the Tibet Plateau is also one of the most sensitive regions to global climate change.Therefore,the Aerosol-Cloud Interaction(ACI)on the Tibet Plateau is an important physical process.This paper focuses on the temporal and spatial variations of aerosols,clouds and precipitation over the Tibet Plateau and their interactions.The Principal Component Analysis was used to separate the influence of aerosols on the Cloud-Precipitation-Radiation system from the influence of the meteorological field,and to quantify the influence of aerosols on cloud characteristics and precipitation on the Tibet Plateau.This provides a reference for Aerosol-Cloud Interaction parameterization schemes in numerical models.The main conclusions are as follows:(1)The MERRA2(The Modern-Era Retrospective analysis for Research and Applications,Versions 2)data can distinguish aerosol types more clearly and specifically,and better retrieval the Aerosol Optical Depth(AOD)on the Tibet Plateau.Outside the TP,the north and southwest sides are mainly dust,and the east and south sides are sulfate aerosols and organic carbon.The northern part of the plateau is dust,and there is a small amount of sulfate aerosol on the eastern edge.Since the aerosol is very little on the Tibet Plateau,its diurnal variation is also very small.One of the two peaks occurred at 11:00 in the middle of the night and the other one at 2:00 in the morning.Seasonal variation of aerosol optical depth from large to small is:spring>summer>autumn>winter,which contributed 35.7%,30.6%,18.5% and 15.2%of the whole year,respectively.The aerosol optical depth peaked throughout the year in May.From the perspective of interannual variation: the total aerosol optical thickness on the plateau decreases gradually over time.(2)The average of total cloud fraction on the Tibet Plateau is about 50%,mainly distributed in the altitude of 6-9 km,and the cloud fraction varies significantly inside and outside the Tibet Plateau,and the plateau is greater than the area around the plateau.The main cloud types on the plateau are altostratus,dominated by ice clouds.There are two kinds of ice clouds with different properties,namely,anvil-like cirrus clouds formed after the cloud anvil generated by fall off from convective clouds and thin cirrus clouds formed naturally.The precipitation in the plateau shows a decreasing distribution pattern from southeast to northwest,more in the south and less in the north.In the area near the outer edge of the plateau,the maximum precipitation basically occurs at night in local time,while in the plateau,except for the Qaidam Basin,the maximum precipitation occurs almost during the day.The diurnal variation of precipitation exhibits a unimodal form,with the precipitation peak between 15:00-20:00 local time.The most precipitation season is summer,with an average precipitation of 425.2 mm,contributing 60% of the annual precipitation.In the past 14 years,the annual precipitation in the Tibet Plateau has shown an increasing trend,and it has become wetter at an average rate of 5.9 mm/year.(3)After using the Principal Component Analysis(PCA)to extract the temperature,relative humidity and convection stability of 250 h Pa,as well as the U wind speed and vertical speed of 300 h Pa over the Plateau.It was found that the smaller the first principal component(PC1),corresponding to the lower the temperature and the lower the relative humidity,the convection is stable,the higher the wind speed of the U wind,and the prevailing sinking movement,which is not conducive to the convection occurs,so it is not conducive to the formation of upper high clouds.But conversely,the larger the value of the first principal component,corresponding to the higher the temperature in the lower layer,the higher the relative humidity,the smaller the U wind speed,the unstable convection,the large vertical speed,the violent upward movement,the strong convection development in the upper atmosphere.It is very beneficial to the occurrence and development of high clouds.At the same time,we use partial correlation analysis to show that the meteorological field plays a leading role in the change of cloud macroscopic shape.For cloud microphysical properties and cloud radiation characteristics,the aerosol optical depth seems to have a greater impact on them than meteorological fields.Under the extremely unfavorable meteorological conditions,aerosols can promote the Cloud-Precipitation-Radiation system at the edge of the Tibet Plateau.Under extremely favorable conditions,the increase of aerosols will produce a large amount of precipitation,and the precipitation plays a wet-scavenging effect on aerosols,which in turn inhibits the Cloud-Precipitation-Radiation system,mainly in the central and western regions of the plateau.With very small Ice Water Paths(IWP),aerosols contribute to the Cloud-Precipitation-Radiation system in parts of the northern and eastern margins of the Tibet Plateau.When the Ice Water Path reaches a certain level,the precipitation also induced by aerosols will remove a large amount of aerosols,thus inhibiting the development of the Cloud-Precipitation-Radiation system,and when the Ice Water Path is increased,the meteorological field plays a leading role in the entire system,so the response of each parameter to the aerosol is very small.At the same time,we also quantified the radiative forcing effects of aerosols.The changes in four parts of cloud optical depth(COD),infrared emissivity(IRE),cloud fraction(HCF)and icewater path(IWP)explained the aerosols 79% of the effects of longwave radiative forcing(CRF)at the top of the atmosphere.Cloud top longwave emissivity effect and cloud optical depth effect are negative contributions to longwave radiative forcing,while cloud fraction effect and ice water path effect are positive contributions to longwave radiative forcing. |
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