Characteristics Of Atmospheric Water Vapor Budget Over Summer Tibetan Plateau Based On Multi-Source Datasets

As the most active atmospheric component,water vapor is one of the greenhouse gases contributing to global warming.It plays an important role in the radiative forcing process between land and atmosphere,and directly participates in the formation of clouds and precipitation.The clouds and precipitation will in turn aflfect the spatial and temporal distribution of water vapor.The plateau is known as the "water tower of Asia"and plays an important role in the regional and even global atmosphere-land-ocean water cycle,so the spatial and temporal variation of water vapor over the Tibetan Plateau(hereafter referred to as "plateau")has been the focus in atmospheric science.The changes in water vapor distribution and transport between the plateau and surrounding regions will greatly affect the weather and climate on the plateau and eastern China.Therefore,it is of great scientific and practical importance to investigate the atmospheric water vapor transport between the plateau and its surrounding regions.In this paper,we mainly use the observations from the Tropical Rainfall Measuring Mission(TRMM)with precipitation radar(PR)and visible/infrared scanner(VIRS),and the Global Precipitation Measurement(GPM)with dual-frequency precipitation radar(DPR),together with the ECMWF reanalysis data ERAS.First,the spatial distribution of wind field,water vapor flux and different intensity of water vapor transport between the plateau and the surrounding regions are analyzed.Meanwhile,the climatic distribution and multi-year trends of regional water vapor budget,precipitable water and precipitation rate are then investigated.Second,the climatic trends of the regional water vapor budget,precipitable water and precipitation rate are explored among the diverse regions over the plateau.At the same time,the characteristics of water vapor transport in the steep terrain of the southern slopes of the Himalayas and the influence of large-scale circulation systems(westerlies and South Asian monsoon)on water vapor transport are carefully studied.The main results and innovations in this thesis are as follows.1.The spatial distribution and characteristics of atmospheric water vapor transport between the summer plateau and surrounding regionsThe results show that the water vapor transporting to the plateau is most active on the southern part of the western boundary,the Sichuan Basin,the southeastern Grand Canyon of the plateau and the eastern part of the southern boundary in the lower layer.The transport of water vapor from the plateau to the surrounding regions mainly occurs in the middle layer on the eastern boundary,the middle and lower layers on the eastcentral part of the southern and northern boundaries.In summer,the plateau is a "water vapor sink" with a net water vapor input of 153.28 × 106 kg s-1,with the largest contribution from the southern boundary.The regional water vapor budget increases slowly.The precipitable water and precipitation rate over the plateau are the lowest among the surrounding regions.The precipitable water shows an increasing trend(passing the 95%significance test)and the precipitation rate shows a decreasing trend over the recent years.2.The characteristics of atmospheric water vapor transport and water vapor budget in different regions within the summer plateauThe results show that the inner regions of the plateau basically gain water vapor,but the southeastern slope of the plateau appears the loss of water vapor.The western,central and eastern plateau are dominated by zonal water vapor transport.The Yarlung Zangbo River region is influenced by southerly water vapor transport,resulting in more significant meridional water vapor transport.The southeastern slope of the plateau is influenced by the water vapor transporting from the Bay of Bengal to the southeastern Grand Canyon of the plateau,resulting in strong zonal water vapor transport.Over the years,the regional water vapor budget within the plateau has shown an increasing trend,with the slowest increasing trend in the Yarlung Zangbo River region.The precipitable water shows an increasing trend,with the southeastern slope of the plateau showing the flattest increasing trend.Precipitation rates show increasing trends in the western,central and eastern plateau,but decreasing trends in the Yarlung Zangbo region and the southeastern slopes of the plateau.3.The effects of steep topography on atmospheric water vapor transport on the southern slope of the HimalayasThe analysis show that the southern slope of the mountain range presents a very clear distribution of steep topography.The water vapor is continuously depleted during the transport to the southern slope of the plateau.The net water vapor budget differences in the middle part of the southern slope can explain the stronger precipitation intensity in the foothills and the higher frequency distribution of precipitation on the slope.In the water vapor transport process,the middle southern slope and the eastern side of western sections of the southern slope become the most important channel for water vapor transporting towards the plateau,but the obvious water vapor flux appears in the lower layer of the eastern section of the southern slope.In the area with a more uniform distribution of topographic steepness,topographic vertical velocity occupies the major part of regional vertical velocity.The vertical transport of water vapor is strongest in the western part of the eastern section of the southern slope.The topographic,regional vertical water vapor transport are similar in intensity in the middle and eastern sections of the southern slope.The vertical water vapor flux in the western section of the southern slope is generally smaller than that in the middle and eastern sections.4.The impacts of large-scale circulation system on atmospheric water vapor transportThe results show that the influence of strong(weak)monsoon on water vapor transport are as follows:the distribution of cyclonic water vapor flux on the south of the southern boundary is stronger(weaker);the southerly water vapor transporting from the Bay of Bengal to the southeastern Grand Canyon of the plateau is weaker(stronger)compared with the zonal water vapor transport;the influence region of water vapor transporting from the monsoon region to eastern China is northward(southward).The influence of strong(weak)westerlies on water vapor transport are mainly reflected in the stronger(weaker)water vapor flux and larger(smaller)influence region of water vapor transport on the northern plateau.Therefore,the water vapor transport is strongest on the northern plateau in strong westerlies years.The southerly water vapor transport on the south of the southern boundary is stronger in weak westerlies years than that in strong westerlies years.In the strong monsoon years,the zonal water vapor transport is strong along the southern boundary towards the west and the water vapor transporting to the eastern China is northward.The meridional water vapor transport on the south of the southern boundary is obvious in the weak monsoon years.The calculation of water vapor budget over the plateau shows that the net regional water vapor input is small in the strong westerlies years due to the weaker southerly water vapor transport on the south of the southern boundary.In the weak westerlies years,the net water vapor output increases at the eastern boundary and the net water vapor budget input increases at the northern boundary,resulting in a slightly larger net water vapor input than that in the strong westerlies year.The regional net water vapor input is largest in the strong monsoon years and smallest in the weak monsoon years.