A Study On The Characteristics Of Water Vapor Content And Water Vapor Transport Of The Tibetan Plateau

Water vapor is an important part of the atmosphere and the basis of precipitation,which not only can affect local weather and climate conditions,but also can influence global water cycle and energy balance through the atmospheric circulation and phase change processes.In addition,water vapor is the most abundant greenhouse gas in the atmosphere,which has an important contribution to the greenhouse effect.The Tibetan Plateau(TP)is also called 'The Roof of the World' and 'The Third Pole' due to its unique topography.Its high elevation makes it directly heating atmosphere of middle and troposphere and transport water vapor to the upper troposphere.The TP is located in the north of the South Asian monsoon region and the west of the East Asia monsoon region.It can be affected by the large-scale circulation of the South Asia monsoon,East Asia monsoon and the mid-latitude westerlies at the same time,which determines TP's complex water vapor transport.The TP is also the origin of important rivers in Asia,so its water vapor budget has an important impact on the drought and flood in its surrounding areas and even the whole Asia.Therefore,studying the characteristics of water vapor content and water vapor transport over the TP is very important for understanding the climate change in the TP and surrounding regions.However,there is a lack of long-term trends study on atmospheric water vapor content and the understanding of the characteristics of horizontal and vertical water vapor transport is limited.Therefore,based on ERA5 reanalysis data,JRA55 reanalysis data and GPS observation data,this thesis will focus on the long-term changing characteristics of atmospheric water vapor content in the TP as well as its relationship with precipitation and radiation,and finally reveal the three-dimensional transport characteristics of water vapor quantitatively.Firstly,the atmospheric water vapor content calculated from ERA5 reanalysis data was verified based on JICA sounding data.The results show that the correlation coefficient and the root mean square error of atmospheric water vapor content of four stations are 0.93 and 3.78 mm respectively.Their seasonal changes are also consistent with each other.In general,the water vapor content in the TP is significantly lower than that in the surrounding regions,with clear seasonal variation characteristics.The results of SVD analysis show that there is a significant positive correlation between water vapor content and precipitation.When the water vapor content increases(decreases),the precipitation also increases(decreases).The correlation between the atmospheric water vapor content and the surface downward shortwave radiation(DS WR)is negative while the correlation between the atmospheric water vapor content and the surface downward longwave radiation(DLWR)is positive.The results of the SBDART model show that when the water vapor increases by 5 times,the DSWR of 0.8 ?m,1 ?m,and 2 ?m decreases by 5.39%,1.58%,and 46.81%of the original value,while the 10 ?m,12 ?m,and 14 ?m DLWR increased by 313.23%,454.29%and 0.94%of the original value.Secondly,the ERA5 and JRA55 reanalysis data were used to analyze the characteristics of water vapor horizontal transport over the TP in 1979-2013.The results show that the spatial patterns,magnitude and spatial-temporal characteristics of above two datasets are in good agreement with each other.Water vapor flows into the TP from the western and southern boundary and flows out from the eastern boundary of the TP.The whole plateau is a water vapor convergence area,and the net water vapor income is 11.85 × 106 kg·s-1.Under the influence of Asian summer monsoon,the water vapor input from the southern boundary in summer increased significantly,and its net water vapor income was three times of the annual average,which is about 29.60 × 106 kg·s-1.Over the past 40 years,the TP water vapor income has increased significantly,mainly due to the increasing input from the southern boundary and the decreasing output from the eastern boundary.The change of net water vapor budget is more significant in summer.The results of simulation with the Lagrangian backward trajectory model show that there are two main water vapor channels in the TP.They are the west channel associated with the mid-latitude westerlies and the southern channel associated with the South Asian monsoon.In the past 40 years,the contribution of the mid-latitude westerlies to the plateau water vapor has increased,while that of the South Asian monsoon has decreased.Finally,the characteristics of vertical water vapor transport in the TP were discussed,and the water vapor flux exchange between the troposphere and stratosphere was quantitatively revealed.In summer,the TP is a water vapor convergence region as a whole,and the middle part of the TP is a region with strong atmospheric upward motion.Water vapor can be transported to a higher level of more than 200 hPa.While in winter,the TP is a water vapor divergence region as a whole.From 1979 to 2018,the downward water vapor transport in the middle and lower layers was weakened,while the upward water vapor transport in the upper layers was enhanced.In summer,the water vapor over the plateau is transported from the troposphere to the stratosphere.While in spring,autumn and winter,it is transported from the stratosphere to the troposphere.In summer,the water vapor flux from the troposphere to the stratosphere is 2.51 × 10-9 kg·m-2·s-1.In winter,the water vapor flux from the stratosphere to the troposphere is 10.01 × 10-9 kg·m-2·s-1.Under the background of global warming,the water vapor vertical exchange was strengthening.