A Study Of The Stratosphere-Troposphere Exchange Over Tibetan Plateau And Its Surroundings

The atmospheric constituents and dynamical process of the Tibetan Plateau (TP) and its surroundings make the stratosphere-troposphere exchange (STE) over the TP and its surroundings become a crucial and special research topic. Detailed investigations on the STE over the TP and its surroundings are not only crucial for understanding the characteristics and global budget of the STE, but also important for providing us with more information and theoretical basis to understand the impact of anthropogenic emissions on regional and global climate change. Using the MLS (Microwave Limb Sounder), AIRS (Atmosphere Infrared Sounder), CALIPSO (Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations) and TRMM (Tropical Rainfall Measuring Mission) observations together with the ERA-Interim of ECMWF (European Center for Medium Range Weather Forecasting) reanalysis data and NCEP (National Centers for Environmental Prediction) reanalysis data, a trajectory model (NOAA HYSPLIT) and a climate-chemistry model, characteristics and transport prepoperties of water vapor in the upper troposphere and lower stratosphere (UTLS) over the TP and its surroundings are investigated, and the effect of vertical transport by deep convective activities on the chemical constituents in UTLS over the TP and its surroundings are discussed in this thesis. In addition, the long-term trends of the STE as well as the main mechanisms responsible for STE trends over the TP and its surroundings are also analyzed. The main conclusions are summarized as follows:(1) The distributions and sources of atmospheric water vapor near tropopause region over the TP and the associated STE over the TP are investigated using satellite observations, NCEP/NCAR reanalysis data and a trajectory model. The results show that the distributions of water vapor over the TP are characterized by a minimum over the southern TP during March and April, and a maximum over the southern TP from July to October near tropopause region at100hPa. The water vapor mixing ratios have large values in the upper troposphere over the south slope of TP between March and April. The results suggest that transport of air masses from the troposphere to stratosphere occurs over south slope of TP because of the orographic lifting of TP and the westerly circulation. The low water vapor at215hPa over the center of the TP (80°E-90°E) is related to the sinking of dry air from the UTLS region. The water vapor over the TP is the highest in the exuberant monsoon season between July and August and is related to the Indian summer monsoon the associated anticyclonic circulation, which transports water vapor to the TP lower stratosphere by2-4days. The seasonal variation of water vapor mixing ratios near tropopause region (i.e., at100hPa) over the TP, the east and west of TP are consistent with each other, the minimum value of water vapor occurs in March.(2) The results show prominent difference and seasonal variation exisit in the STE patters over different regions of the TP. The water vapor in the UTLS adjacent to the northern TP (40°N-45°N) is found to be relatively higher than that in the surrounding regions of the same latitude in March and April. This relatively higher water vapor in the northern TP UTLS is proposed to be associated with approaching cold surges from the north and forced lifting of air by high orography. Another interesting feature detected in this study is the region of low water vapor values on the order of5-7ppmv, which is more pronounced from May to September at around200hPa and located at30°N-40°N western TP. This low water vapor region is found to be related to an anticyclone developed at the western TP which causes sinking of dry air from the stratosphere resulting lower water vapor values in the upper troposphere. In addition, the low water vapor and weak convective activity over the desert at the western TP lead to the lower water vapor in the upper troposphere at the western TP.(3) Using13years of TRMM data and NCEP reanalysis fields, the characteristics of deep convection distributions and variations over the TP and its surroundings are investigated. The results show that deep convective activities of radar echo≥20dBZ extending≥14km in height and radar echo≥40dBZ extending≥10km in height form preferentially over the Ganges Delta, the number of deep convection is low over the TP in the premonsoon season. During the monsoon season, the deep convective activities of radar echo≥20dBZ extending≥14km in height are primarily found over the south slope of the TP, along the Himalayan foothills, the southern and southeastern TP. While the deep convection with radar echo≥20dBZ extending≥17km in height occurred over the south slope of the TP is rare. The location of maximum occurrence of the deep convection (radar echo≥40dBZ extending≥10km in height) is the western Himalayan, which is consistent with the surface precipitation. The deep convection ove the TP (radar echo≥40dBZ extending≥10km in height) exhibits a weaker decreasing trend in the premonsoon and the deep convection has an increasing trend during1998-2010. Furthermore, the vertical transport of mass by deep convective activities occurred over the eastern TP shows that the air with high water vapor, low ozone and potential vorticity (PV) in the troposphere is transported to tropopause region by strong vertical motions over the eastern TP, but the higher and colder tropopause over the convective region leads to the stronger dehydration, which has an important impact on the chemical composition in the UTLS region over the TP.(4) The cross-tropopause mass flux (CTMF) and long-term trends of the STE over the TP and its surroundings are analyzed, and the main factors responsible for STE trends near the TP are investigated using ECMWF reanalysis data (ERA-Interim) and a general circulation model. The net CTMF shows a strong spatial dipole structure near the TP, which is mainly related to the horizontal exchange of mass along the tropopause associated with the sharp tropopause pressure meridional gradient or discontinuity of the thermal tropopause in winter. In summer, the net CTMF over the TP becomes smaller but overall upward. The prevailing monsoon circulation over the southeastern TP gives rise to significant upward vertical motions and upward cross tropopause transport of air in this region. The STE over the TP shows that the largest downward mass flux occurs in winter, and the net upward mass flux reaches maximum in summer. The net CTMF over the TP accounts for2.96%of the global net. The net CTMF over the TP exhibits a strong decreasing trend in winter during the period1979-2009, suggesting that the upward mass flux decreases and the downward mass flux increases over the TP in the past several decades. The strong decreasing trend of net CTMF in winter over the TP is resulted from the combined effects of the rising tropopause height and weakening winter monsoon. The summer time net CTMF exhibits a decreasing trend over the western TP and an increasing trend over the eastern TP. The sensitivity simulations with a climate model reveal that changes in land-sea surface temperatures over the South Asia can significantly affect the tropopause and the CTMF over the TP via changing the intensity of Asia monsoon.