Role of Mesoscale Convective Systems Developed around the Eastern Tibetan Plateau in the Eastward Expansion of an Upper Tropospheric High during the Monsoon Season

In this study, we revealed and discussed the role of mesoscale convective systems (MCSs) in the eastward expansion of an upper tropospheric high (UH, that is South Asian High or Tibetan High) with a timescale of a few days over the Asian monsoon region in the summers of 1999-2008. The MCSs was extracted using hourly satellite images. The distribution of mature MCSs was inhomogeneous in the Asian summer monsoon region. At three specific regions, i.e., around the eastern part of Tibetan Plateau (TP), over the Bay of Bengal, and in northern Vietnam, the percentage of MCSs larger than 1.2 x 10(5) km(2) to all mature MCSs counted in each area was higher and exceeded 25%. Focusing on these regions, the UH extension was examined around the day of large MCSs (LMCSs) formation using objective analysis data sets. Only the LMCSs of eastern TP areas contributed to a significant increase of the geopotential height at 200 hPa with a temperature increase in the mid-upper troposphere. At the same time, the zone of active cloud convection (ZACC) was formed along 30-35 degrees N over central and east China. Cyclonic circulation was strengthened in the eastern part of TP by the LMCS formation and transported moist air north and northeastward toward the inland region, such as the northeastern part of the Sichuan Basin, in the lower troposphere. This moisture advection corresponded to the meandering of the ZACC. The ZACC was maintained until a few days after the LMCS formation, and was also a contributor of greater extension of UH to the east and eastward propagation of the positive anomaly area of soil moisture.