| The Tibetan Plateau(TP),which is known as the"Asian Water Tower",can significantly affect the hydrologic cycle and climate at regional and global scales.Recently,satellites observed frequently the aerosol pollution events over the TP.The aerosols mainly come from local emissions over the TP and long-range transport from surroundings.With the intensification of human activities,local emissions of atmospheric aerosols increased gradually over the TP.Driven by the thermal effects of the TP,aerosols are transported from several natural deserts and anthropogenic pollutants sources in the vicinity of TP to the slopes of the TP,and further elevated to the atmosphere above the TP.Meanwhile,in summer,the water vapor around the TP can be converge around the plateau due to the influence of the monsoon and then further ascend to accumulate above the TP,forming an aerial wet island over the TP.Abundant water vapor causes the formation and development of strong convective clouds over the plateau,in which some clouds are polluted by the aerosols uplifted to the plateau.The physical properties of the clouds could be altered by the aerosols mixed with clouds.Therefore,the properties and interaction of aerosol and cloud over the TP are not only the important international frontier scientific issue,but also the new challenges in this research of TP’s role in regional and global climate change.Due to the special terrain and geographical location of the TP,lack of observation over the TP has significantly limited the progress on the issue relating aerosol and cloud over the TP.Therefore,there is still a lack of comprehensive understanding on the aerosol-cloud interaction over the TP,especially the ice clouds.In this study,combining multi-sourced satellite data,reanalysis data,Nonhydrostatic Icosahedral Atmospheric Model coupled with Spectral Radiation-Transport Model for Aerosol Species(NICAM-SPRINTARS)and Coupled Model Intercomparison Project Phase 5(CMIP5),the properties of aerosol and cloud and aerosol effects on the water and ice clouds over the TP are investigated systematically.This study can provide a scientific basis to understand the aerosol-cloud interaction and improve the numerical model over the TP.Main conclusions are summarized as follows:(1)Properties of aerosol and cloud distributions over the TP from 2000 to2015 were analyzed.Basing on the analysis,it is found that the aerosols are mainly distributed over the north slope of the TP.The aerosol optical thickness(AOD)increases gradually from the south to the north of the TP while the Angstrom index(AE)decreases gradually from the south to the north of the TP.The aerosol index(AIn)was the smallest(0.01)over the southern margin of the TP and the largest(0.62)over the northern margin of the TP.On the other hand,although the high-value areas of the ice cloud and water cloud fraction are in the southeast of the TP,both of them are mainly over the margins of the TP.Compared with water clouds,ice clouds appear more frequently over the TP,especially over the northern and western slopes.Meanwhile,combining CALIPSO(Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations)and CloudSat data,the frequency of aerosol and cloud mixture is higher over the margin areas of the plateau than over the central TP.The properties of aerosols and clouds distribution suggesting that a potential relationship may exist between the aerosols and clouds over the TP.(2)Based on the properties of aerosol and cloud distributions,effects of aerosol on the properties of ice and water clouds over the TP were investigated comprehensively.It was found that,for the macro-physical characteristics of cloud,the correlation of aerosol with the ice cloud indicates greater significance than that with the water cloud.Due to the addition of more aerosols,the water clouds become higher and thicker,and the ice clouds become higher but thinner.For the micro-physical characteristics of cloud,the effects of aerosol on ice cloud are more significant than those on water clouds.When the AIn increases,the ice cloud droplet radius(ICDR)during the daytime decreases,while the ICDR during the nighttime remains almost constant.The ice water path(IWP)during the daytime decreases slightly due to the saturation effect,while the nocturnal IWP increases significantly.The changes in ice cloud optical depth(ICOD)during daytime and nighttime show significant and completely opposite trends,similarly with the change of water cloud optical depth.The removal of the influence of meteorological factors shows that aerosols have a more dominant influence than meteorological conditions on cloud properties.The simulation results of NICAM-SPRINDARS model are consistent with the observation results.Overall,the effects of aerosols on the physical properties of ice clouds over the TP are more significant than those on water clouds.(3)Combining satellite observations with CMIP5 model simulations,the indirect effects of aerosol on the water and ice clouds over the TP were further compared and quantified.Satellite observations show that the aerosol indirect effects on the total radiative forcing of water cloud and ice cloud over the TP are-0.31±0.02W/m~2 and-0.33±0.09W/m~2,respectively,in which the shortwave effect on the ice cloud is dominated.Simulation results by the CMIP5 models show that,due to the variation of aerosols,changes in the ice cloud radiative forcing(-0.73±0.03W/m~2)cover most of the TP,while changes in the water cloud radiative forcing(-0.34±0.03W/m~2)mainly appear over the southern edge of the TP.Overall,both the model simulations and satellite observations show that the effect of aerosols on the ice cloud radiative properties is more pronounced than that on water cloud.(4)Basing on the changes in cloud radiative properties,contribution of cloud on the accelerating warming over the TP were quantified.With the global warming slowdown,an accelerating warming has been identified over the TP.Compared with the+0.04°C/decade warming from 1961 to 1999,the warming greatly accelerated for the period 2000–2015 at a rate of+0.30°C/decade over the TP.During the same period,warming in the cold season(November to March)was more pronounced than in the warm season(May to September)over the TP.In addition,the middle-level cloud decreased,while the high-level cloud increased,and net cloud radiative forcing is positive over the TP during the cold season,that is,a heating effect of clouds.Observation result suggesting that the decreased albedo effect of middle cloud and the increased longwave greenhouse effect of high cloud may have partially contributed to the sustained warming,especially in the cold season.The analysis based on a model of CMIP5 shows that the net contribution of cloud change is+0.88°C,in which the increase of shortwave effect caused by middle cloud decrease is the dominant. |
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