Abstract
The time evolution of the large-scale precipitation, low-level (850 mb) wind, moisture and vertical motion fields over the Tibetan Plateau and surrounding areas during a 40-day period from late May to early July 1979 is studied based on the objectively analyzed FGGE Level II-b data set. During this period the general circulation over East Asia undergoes a distinct change characterizing the onset of the summer monsoon circulation.
The Tibetan Plateau exerts profound orographic and thermal influences upon the low-level wind field. The inflow towards the eastern part of the Plateau with a marked diurnal change in its intensity is the most prominent feature of the low-level wind field. The areas of organized precipitation are well related to synoptic systems seen in the 850 mb flow: the quasi-stationary Burma-India trough, the disturbances forming on the trough, the “Mei-yü (Baiu)” front, and the “transverse trough” extending eastward from the Plateau. There is a sharp contrast between the western and eastern Plateau in terms of precipitation and moisture distributions. The eastern Plateau acts as a huge chimney funneling water vapor from the lower to the upper troposphere. Maxima of 40-day mean upward velocities are located above the eastern Plateau, above the Assam-Bengal region and along the Mei-yü frontal zone. The vertical motions above the Plateau are more upward in the evening than in the morning. The reverse is true in the surrounding areas. Detailed examinations of daily values of the areal mean vertical p-velocity, mixing ratio and rainfall are made for four heat source regions (the western Plateau and adjacent areas, the eastern Plateau, the Yangzi River, the Assam-Bengal region) as a preliminary to the discussion of heating mechanisms operating in these regions.
