| Water vapor,as a greenhouse gas,exerts significant influence on weather and climate variations by modulating terrestrial longwave radiation.Water vapor is a primary source of hydroxyl radicals,which regulates the depletion rate of stratospheric ozone and oxidation of stratospheric methane,thus critically influence Earthatmosphere system’s energy equilibrium.About 75% of atmospheric water vapor is concentrated in the lower troposphere.Although the water vapor content in the upper troposphere-lower stratosphere(UTLS)is relatively low,it plays an important role in global climate change.Water vapor in the UTLS region facilitates the maintenance of relatively low temperatures(strong temperature gradients)at the tropopause,which influences the distribution and thermodynamic properties of atmospheric constituents and aerosols.As an intricate and essential component of the UTLS region,water vapor has important impacts on cloud and precipitation formation,Earth-atmosphere radiative balance,climate change,and pollutant transport processes.As its high and broad terrain,the Tibetan Plateau(TP)plays a significant role in regional and global atmospheric dynamics,thermodynamic structures and processes,particularly in the formation of the Asian monsoon.The TP and its surrounding areas constitute a key region for stratosphere-troposphere exchange and are global hotspots.Moreover,it is a significant extratropical source of stratospheric water vapor and an important material transport pathway,which profoundly impacts the global hydrological cycle.Frequent tropopause folds in this region contribute to the redistribution of water vapor between the stratosphere and troposphere.Therefore,accurately quantifying the influence of tropopause folds over the TP on water vapor transport has become a central issue for a comprehensive understanding of troposphere-stratosphere coupling processes and variations in atmospheric constituents in the UTLS.Building upon the 3D labeling algorithm of identification tropopause folds proposed by ?kerlak et al.(2015),the algorithm and filters have been improved.The parallel acceleration has also been achieved in the implementation.The improved identification performance is validated with ERA5 reanalysis and observation data.Results demonstrate that the improved 3D labeling algorithm identifies the spatial continuity of folds,with fold locations and depths showing good consistency with radiosonde profile observations.The spatial distribution of tropopause folds over the TP from 1979 to 2019 using ERA5 reanalysis data,seasonal variations in fold frequencies and fold height are investigated in the thesis.It is revealed that the spatial distribution of tropopause folds corresponds well with the location of the subtropical jet stream moving with the latitudinal shifts of the mid-latitude westerlies.Fold occurrences are predominantly near the southern TP in winter and spring,more frequent in the northwest of the TP in summer,but relatively small in autumn.Moreover,the frequency of medium folds in summer is roughly equivalent to that of shallow folds,which is contrast with previous research findings.This is due to the previous studies’ use of 3D labeling algorithms that did not consider the continuity of folded distributions.However,this paper has made improvements addressing this issue.The results also indicate an overall increasing trend in the frequency of tropopause folds over the TP during the past 41 years.Furthermore,fold occurrence heights decrease with increasing latitude,which is enhanced at north of 36°N.In this thesis,the impact of tropopause folds with various depths and regions on water vapor flux in the UTLS over the TP is investigated.The influence mechanism of fold impacts on water vapor in the UTLS is presented by analyzing their effects on the secondary circulation and sub-grid water vapor flux.It is found that tropopause folds occur with the stratospheric air intruding to the troposphere,which leads to the enhancement of horizontal divergence in the UTLS over the TP and increase of the horizontal transport of water vapor away from the TP to outside of it.However,the vertical water vapor flux in the UTLS region increases significantly due to tropopause folds,which not only compensates for the water vapor reduction caused by the horizontal divergence but also leads to the overall increase of water vapor mixing ratio in the UTLS.The increase in water vapor is more pronounced with the occurrence of medium folds.The water vapor in the UTLS increases with deep folds in the northern and central TP,while it decreases with deep folds in the south.Moreover,the increase in water vapor is gradually intensified as medium and shallow folds shift from the southern to the northern TP.Further analysis reveals that the presence of secondary circulation anomalies and cyclonic anomalies accompanying folds enhance vertical transport of water vapor.In addition,the sub-grid perturbation increases the upward transport of water vapor and the downward transport of dry air with the occurrence of shallow and medium folds,especially significant in medium folds.Furthermore,two cases(in winter and summer,respectively)are simulated by the WRF model to investigate the structure and evolution of tropopause folds,and their impacts on water vapor flux and upper-level stability.It is found that the occurrence and development of folds are influenced by weather systems and the upper-level westerly intensity.Simulation results indicate that there is downward water vapor flux in the western region of the folds,while upward water vapor flux occurs in the eastern region,which contributes the transport of moist tropospheric air to the UTLS.The results correspond well to that of synthetic analysis of tropopause folds climatology.It is also presented that the separated unstable air from folds area enhances mixing int the troposphere.The depth of stratospheric intrusion is more deeply in winter due to the stronger jet stream,which results in a broader impact on vertical water vapor flux and water vapor mixing ratio.Moreover,folds can trigger turbulence in the UTLS region,which probably enhances turbulence mixing in the troposphere.The effects of folds on the stability even can extend down to the lower troposphere.The distribution of unstable regions is more wider in the winter case than that in summer,because the relative shallower folds occur in summer.The results of spatiotemporal distribution characteristics of tropopause folds and their impacts on UTLS water vapor in the UTLS over the TP investigated in this thesis,will provide a scientific basis for a better understanding of the stratosphere-troposphere exchanges,as well as material transport processes. |
