1Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China
2Faculty of Geo-Information Science and Earth Observation of the University of Twente, Enschede, The Netherlands
3Eindhoven University, Eindhoven, The Netherlands
Abstract. Due to its harsh natural conditions, there had not been any intensive radiosonde observations over the Tibetan Plateau (TP) until the year 2008, when a regional radiosonde observation network was implemented through a Sino–Japan joint cooperation project. This paper reports new findings on the structure of upper troposphere and lower stratosphere (UTLS) layer, and provides evidence for stratosphere and troposphere exchange (STE) over the TP.
Due to sparseness of high resolution sonde data, many previous studies assumed that there was only one thermal tropopause over the TP. Actually the radiosonde temperature profiles at pre-onset time of the Asian monsoon over the TP often exhibit a multiple tropopause (MT). The MT occurs in winter time with much higher frequency than any previous estimations over the Plateau. The MT during this time period is associated with tropopause folding near the subtropical westerly jet. The MT consistently varied with the movement of the jet. The MT becomes a single tropopause with the development of the monsoon. According to their height distribution, the MT can be divided into tropical and polar characterized tropopauses. Detailed analyses of MT characteristics are reported in this paper. Although some scientists have analyzed global MT events (with data including GPS radio occultation, ERA40 data and Integrated Global Radiosonde Archive database), the frequency of their MT occurrences in winter season over the plateau is largely under-estimated. This significant difference must be caused by the coarse vertical resolution of these data.
The stratospheric intruding episodes are generally associated with the presence of subtropical westerly jet stream over the Plateau. The subtropical jet causes dynamic tropopause foldings over the plateau, which have been observed by us as thermal MT events. Intrusions of high latitude stratospheric ozone rich air into the troposphere over the plateau give us a new explanation to why total column ozone in winter is higher than that in summer.