Composite structure of a low-level jet over southern China observed during the TAMEX period

Three cases of southwesterly low-level jet (LLJ) were selected during the 1987 TAMEX field phase (May-June) to reveal the structure and evolution of the LLJ. To suppress minor differences among different cases of LLJs, a composite technique was employed to obtain all the meteorological parameters with respect to the 700 hPa jet axis. Results showed that the LLJ formed and intensified in association with a non-quasigeostrophic trough intensification process. It had a highly supergeostrophic nature. Warm advection prevailed over the jet region and moisture flux convergence occurred over the region downstream from the jet core. The distribution of divergence and vertical velocity was mainly characterized by an antisymmetrical pattern upstream and downstream with upward motion caused by the along-jet ageostrophic wind speed convergence over the exit region. It also showed an antisymmetrical pattern to a lesser degree on both the left and right sides. The characteristics of secondary circulations across the jet axis were quite different from those of the upper-level jet. A thermally indirect circulation across the jet axis was observed in the entrance and the jet core regions and a thermally direct circulation existed over the exit region.