Study of a Horizontal Shear Line over the Qinghai-Tibetan Plateau and the Impact of Diabatic Heating on Its Evolution

Based on the 4 times daily 0.75A degrees x 0.75A degrees ERA-Interim data, the structural evolution of a Qinghai-Tibetan Plateau horizontal (east-west-oriented) shear line (TSL) during 15-19 August 2015 and the effect of diabatic heating on its evolution were analyzed. The results show that the TSL possessed a vertical thickness of up to 1.5 km (approximately 600-450 hPa), and was baroclinic in nature. Weak ascending motions occurred near the TSL, accompanied with more significant gradients in dew point temperature than in temperature. The TSL was characterized by diurnal variations in its appearance and structure. It was relatively full in shape (broken) and was the lowest (highest) in vertical extent at 0000 (1800) UTC, and veered clockwise (anticlockwise) during 0000-0600 (1200-1800) UTC. When the north-south span of the TSL increased, it was prone to fracturing; and it disappeared when the dew point temperature gradients to its either side decreased. When the TSL moved northward (southward), its western (eastern) section broke up, while the eastern (western) section inclined to regenerate or merge. The TSL tended to move towards the positive vorticity areas with significant increases in vorticity. When the positive vorticity center moved down, the height of TSL decreased. Further analysis shows that the plateau surface heating dominated the vorticity attribute of the TSL and its movement, with different contributions from local variation, horizontal advection, and vertical advection of the diabatic heating to the TSL at different heights.