Unbalanced Low-Level Jet and Solenoidal Circulation Associated with

Abstract

A southerly low-level jet appeared over Kyushu on July 7, 1970 when a large amount of precipitation was observed to the west of jet axis. The mesoscale thermal structure around the jet core shows an indirect indication of adjustment with the wind field but it is found to be highly unbalanced.The warmer area appearing below the jet core to the east is characterized by dryness, which is possibly generated by an indirect solenoidal circulation.Similar relationship among wind, temperature and relative humidity field is found in the lower troposphere even for the monthly mean values of June 1970.

Composite Structure of a Low-Level Jet over Southern China Observed during the TAMEX Period

“…The secondary circulation across the jet axis would be expected in response to this unbalanced situation between the motion field and the mass field. A thermally indirect circulation would be expected below the jet level with upward motion on the cyclonic side of the jet axis and downward motion on the anticyclonic side under this condition, as discussion by Matsumoto (1972). This is in good agreement with the observed vertical motion field on the left and right sides as presented in Fig.…”

“…In other words, the 700 hPan LLJ was highly ageostrophic with the observed wind speed about 1.6-1.8 times that of the geostrophic wind. Over this area, the ageostrophic wind direction was mainly along the jet axis indicating the supergeostrophic nature of the LLJ similar to that observed over Japan area (e.g., Matsumoto et al, 1971;Matsumoto, 1972). The ratio of the ageostrophic wind to the geostrophic wind reached a maximum value at the LLJ level.…”

“…A close relationship between extremely heavy rainfall or a mesoscale convective system (MCS) and an LLJ in the 850-700hPa layer during the Mei-Yu season has been found in many observational case studies over various geographic locations (Matsumoto, 1972;Akiyama,1973;Ninomiya and Akiyama,1974;Chen and Chi, 1978;Chen, 1979;Chen, 1983;Tao and Chen, 1987;Tsay and Chain, 1987). Chen and Yu (1988) examined 35 heavy rainfall events over northern Taiwan in May and June during the years 1965-84.…”

“…In the latter case, a thermally direct circulation would occur in the entrance region below the jet level in the troposphere with a thermally indirect circulation in the exit region (e.g., Uccellini and Johnson, 1979). Finally, a thermally indirect circulation would be expected below the jet level over the jet core region, as the wind was highly supergeostrophic along the jet axis such that the observed vertical shear (-) was much greater than the geostrophic shear (J)z (e.g., Matsumoto, 1972).…”

“…The physical processes through which the existence of an LLJ leads to heavy rainfall have been proposed by various researchers. For example, Matsumoto (1972) suggested that the upward branch of the secondary circulation associated with the LLJ is responsible for the heavy rainfall. Chen's (1977a) study, on the other hand, suggested that the LLJ is the main mechanism for creating potential instability to the warm side of a Mei-Yu front and for generating the convective heavy rainfall by the associated convergence downstream.…”