Development of Low-Level Jets during TAMEX

Abstract

Detailed synoptic-scale map analysis and diagnosis for all six low-level jet (LLJ) cases that occurred during the Taiwan Area Mesoscale Experiment (TAMEX) were made to study the structure and the development of the LLJs. It was shown that the development of these LLJs was closely linked to the development of a lee trough to the cast of the Tibetan Plateau. Seasonal change had a profound influence on the secondary circulation across the jet-front system. Before the seasonal transition, the formation of a lee trough with closed isobars in low levels east of the Tibetan Plateau occurred as an upstream short-wave trough moved eastward with strengthening westerlies over the Tibetan Plateau in upper levels. After the development of the lee trough, the southwest flow appeared in the southeast quadrant of the Ice trough. As the upstream trough moved farther eastward, it merged with the lee trough, and the low-level southwesterly flow intensified. For most cases, a midlatitude deep trough was present to the north or northeast at this time; the northwesterlies behind the midlatitude trough brought in the cold air to the rear of the merged trough. The upper-level jet ahead of the merged trough strengthened. The secondary circulation across the jet-front system was characterized by a thermally direct circulation across the baroclinic zone, with warm air rising within the southwest monsoon flow, cold air sinking in the postfrontal northeasterlies, and a weak thermally indirect circulation to the south. After the seasonal transition, the major baroclinic zone shifted northward, and the upper-level south Asian high pressure cell moved over the Tibetan Plateau. The Tibetan high dominated the upper-level circulations with easterlies along the southern China coast, preventing baroclinic disturbances from moving into southern China. At 1200 UTC 22 June 1987, an orographically induced lee trough formed in the lee side of the Tibetan Plateau. An LLJ was observed during 0000 UTC 23 June-0000 UTC 25 June 1987 over the southern China coast as a southwest vortex developed within the lee trough. The formation of the vortex was driven by latent heat release in a barotropic environment. The secondary circulation associated with the LLJ was characterized by a rising motion northwest of the LLJ axis and a broad sinking motion to the south.