A Study On The Changes Of Tropopause Folds And Its Impact On The Ozone Changes Over The Tibetan Plateau

Using NCEP/NCAR FNL data. European Centre for Medium-Range Weather Forecasts (ECMWF) Interim data and satellite observation data, together with a regional atmospheric chemical model (WRF-Chem) and a trajectory model (HYSPLIT), a tropopause folding event caused by a cold trough during 19-21 March 2012 was simulated to analyse the details and features of the transport of the stratospheric ozone to the troposphere caused by tropopause folds over the Tibetan Plateau (TP). The influence of the high orography on the intrusion of stratospheric ozone is also discussed. In addition, the spatial and temporal characteristics and long-term trends of the tropopause folds frequencies over the TP during 1979-2013 are analysed quantitatively. And the effects of the tropopause folds frequencies on the long-term trends of the tropospheric ozone over the TP are discussed. The main conclusions are summarized as follows:(1) Using the mesoscale model WRF-Chem, the details and characteristics of the transport of the stratospheric ozone to the troposphere over the northwest side of the TP in spring are analyzed. The results show that tropopause folding event occurred in the northwest side of the TP locates in the transition zone between tropical tropopause and mid-latitude tropopause. Due to steep north-south gradient of the tropopause, stratosphere and troposphere exchange (STE) in this region is more persistent and stronger than that caused by the east-west tropopause folding. The main form of STE caused by the east-west tropopause folding is breaking up of the stratospheric intrusion tongue. But mass exchange across the isentropic surface and turbulent mixing has significant contributions to STE in the region of steep north-south gradient of the tropopause. High orography has a significant influence on the STE processes with its effect having an evident diurnal variation. Comparative analysis shows that height of the orography that turbulent mixing acts highlights at 2.5 km. Above this height, contribution of turbulent mixing to the ozone change increases about 1% when the average height of the orography rises 100 m.(2) The spatial and temporal characteristics of the tropopause folds frequencies caused by different folding intensities over the TP and their impact factors during 1979-2013 are analysed by using the high spatial and temporal resolution ECMWF ERA-Interim data. The results imply that annual variations of all the folds frequencies have an apparent bimodal structural feature except for the deep folds. The annual variation of the deep fold frequency has an unimodal structural feature. Total folds frequencies over the TP are mainly determined by the shallow folds frequencies. Tropopause folds frequencies of different folding intensities have shown increasing trends in most of the months during the 35 years and the increasing amplitude of the deep fold frequency for every month is small. From summer to winter, total and shallow folds over the TP will occur with a northward shift due to the movement of the upper level jet. And the frequencies of total and shallow folds are jointly determined by the strength of the upper level jet and the intensity of the horizonal gradient of the upper level isentropic surface. The occurrences of medium and deep folds are not only affected by the upper level jet and horizonal gradient of the upper level isentropic surface, but also influenced by the high orography of TP and Asian summer monsoon.(3) The long-term trends of the tropopause folds frequencies and effects of the trends on the changes of tropospheric ozone over the TP during 1979-2013 are studied with ozone mixing ratio and total column ozone from ECMWF ERA-Interim reanalysis data and MOPITT nearly 14 years observation data of tropospheric carbon monoxide. The results indicate that the significantly increased regions, especially regions with the highest amplitudes of total and shallow folds frequencies are corresponding to the regions where the horizonal gradient of the upper level potential temperature steepens notably at the entrance of upper level jet over the TP and its vicinity. From winter to summer, regions of the increased tropospheric column ozone, especially for the upper tropospheric column ozone shift from southeast to northwest due to the movement of the regions where the tropopause folds frequencies increase over the TP. The dynamic transport process of tropopause folds makes a greater contribution to the tropospheric column ozone increase, especially to the the upper tropospheric column ozone increase over the TP during the winter half year. But parts of the increased upper tropospheric column ozone are caused by the photochemical reactions of the ozone precursors in May. Because the solar radiation is significantly enhanced and large quantities of pollutants which come from the lower level troposphere can be brought into the UTLS by the TP as a pathway for STE due to the effect of the Asian summer monsoon onset during this time.