A DIAGNOSTIC STUDY OF THE LOW-LEVEL JET DURING TAMEX IOP-5

During 31 May-2 June 1987 a low pressure center developed around 28-de grees-N, 102-degrees-E in the lee of the Tibetan Plateau during the pa ssage of a midlatitude trough. It moved northeastward and intensified over the Yangtze River valley and produced widespread rainfall over so utheastern China. A strong southwest flow developed in the lower tropo sphere over southern China as the cyclone deepened. At 0000 UTC 1 June , a well-defined low-level jet (LLJ) was found between the 850- and 70 0-hPa levels with wind speed exceeding 15 m s-1 at the 850-hPa level a nd 20 m s-1 at the 700-hPa level. The LLJ transported warm, moist air from the subtropical ocean, increased the moist static energy, and pro vided a favorable environment for the development of widespread precip itation. It is shown that the development of the LU is closely related to the developing lee cyclone to the east of the Tibetan Plateau. Our analysis reveals that the diagnosed secondary circulation associated with the jet-front system differs from those predicted by previous the oretical studies. The transverse secondary circulation across the fron t is characterized by a thermally direct circulation across the barocl inic zone with warm air rising within the southwest monsoon flow and c old air sinking in the postfrontal northeasterlies. A very weak therma lly indirect circulation was observed to the south.