A Study Of The Mesoscade Stratosphere-Troposphere Echange Over Tibetan Plateau

The thermal and dynamical effects of the Tibetan Plateau (TP) have an important impact not only on regional and global weather and climate but also on the global Stratosphere-Troposphere Exchange (STE). Detailed investigations of the STE over the TP is important for understanding the impact of anthropogenic and natural emissions on the regional and global climate. Using a meso scale model (WRF) togather the ERA-Interim of ECMWF (European Center for Medium Range Weather Forecasting) reanalysis data and FNL (Final Operational Global Analysis) of NCEP (National Centers for Environmental Prediction) reanalysis data, satellite observations, various ground based measurements, and surface observations at routine meterologcal stations of China, the detailes of a few tropopause events and climatological characteristics of tropopause fold as well as the associated STE over the TP and its surrounding areas are analyzed in this thesis. In addition, the factors affecting the tropopause fold over the TP are also investigated. The main conclusions are summarized as bellow:(1) To verify the performance of WRF model in simulating tropopause fold events over the TP and its surrounding areas (25-45(?),70(?)-110(?)), the two tropopause events occurred in the middle of April and May2007were simulated. The result shows that the high-resolution meso scale model WRF can well capture the properties of the tropopause fold events and is able to provide more detail of the tropopause fold structures than FNL data, especially over regions with complex terrain. It is also found that the cumulus parameterization schemes have little impact on the tropopause fold simulation over TP. Both the modeled water vapor field and potential vorticity field can capture the morphology and structure of tropopause fold. The tropopause structure is more clearly defined in the modeled water vapor field while the tropopause distortion is more obvious in the modeled potential vorticity field. A comparison of these two tropopause fold events reveals that the fold duration will be longer when a fold is influenced directly by the TP-induced circulation systems and indirectly by the upper level synoptic systems, and the STE induced by fold will be more significant accordingly. The analysis also shows that orography has a significant impact on the cross-tropopause mass exchange. The leeside jet stream and a layer of wet air in the middle troposphere tend to develop when folds passed an elevated surface. The low-level jet was mainly due to downward transportation of momentum from the higher levels associated with the fold and mesoscale descent of air on the lee of the orography. The layer of wet air in the middle troposphere was lifted from the lower troposphere by the descent of dry air from the stratosphere. The leeside jet and the wet layer with high potential instability (PI) can give rise to deep upward motion on the leeside and inject tropospheric air into the lower stratosphere. On the other hand, when the flow encounters an elevated surface, forced lifting together with mid level wet layer can trigger deep convective motion on the windward slope. The troposphere-to-stratosphere transport was found to be persistent and almost stationary over a windward slope of high orography during the evolution of the fold. In particlular, if a fold is affected by the upper level front, the persistent cross-tropopause mass exchange will occur along the frontal surface, and the cross tropopause mass exchange is stronger than that caused by the forced of lifting air over the windward slope.(2) The climatlogical charateristics and factors of impacting on the long-term trend of tropopause fold over the TP during the time period from1989-2009are further analysed. A pattern maching method through seeking for collocated maxima of potential vorticity and Q-vector divergence is used for searching tropopause folds from the reanalysis data. A large number of folds selected by this method are compared and verified by the corresponding folds determined by the other means. The comparison shows that this pattern maching method is robust and scientificaly effective, and the rate of overlaped selections for the same fold event is below5%. The statistical analysis shows that the fold intensity is closely related to the Q-vector divergence on400hPa level with a larger Q-vector divergence corresponding to a stronger tropopause fold. The frequency of both strong and weak folds over the TP shows a increasing trend from1989to2009. This trend is possibly caused by the increased intensity of the subtropical westerly jet, enhanced frequency of cold air intrusion and decreased atmosphere stability in the upper troposphere over the TP. The annual variation of strong fold events have two obvious peaks with one in the winter monsoon season (Dec-Mar) and the other in the summer monsoon season (Jul-Aug). There are more the strong folds in winter than in summer while the weak folds mostly occur in summer monsoon season. The number of weak folds is larger than the number of strong folds over the TP. In addition, the seasonal distribution of tropopause fold events shows that the number of folds in summer over TP is the largest and there are no significant differences in fold numbers in the other seasons. A composite analysis of the wind and temperature fields with and without fold and corresponding fields associated with strong and weak folds is performed. The analysis indicates that the upper level westerly jet is the mainly factor influencing the frequency and intensity of fold during winter monsoon season while in summer monsoon season, the effect of the upper level westerly jet on folds over the TP is not significant.(3) The effects of the surface temperature and topography height on folds over the TP are investigated through a series of WRF sensitivity simulations. The results demonstrate that in a short integration time of6hrs there are little changes in potential temperature field and the upper level zonal wind field, as well as in the tropopause fold when the model surface temperature is arbitrarily increased by3K and6K. Nevertheless, changes in the topography height in the model can significantly influence the propagation speed of folds via changing intensity of upper level jet. Meanwhlie, the lower terrain results in weaker gravity waves, and this also has a possible impact on the STE over the TP and its surronding areas.(4) Finally, the number of tropopause events during the time period from1989to2009over the TP (70°E-110°E,20°N-45°N) is compared to that over the Western Pacific (120°E-160°E,20°N-45°N). The result indicates that the folds over both regions are mainly affected by the subtropical westerly jet in winter. But the number of folds over the Western Pacific is larger than that over the TP. Folds over the Weatern Pacific are mainly strong folds and the number of the strong folds is three time larger than the number of weak one. The possible reason and mechanisms of these differences between the two areas need further study.