Comparative study on the characteristics of shear lines in the subtropical plain and plateau areas of East Asia in summer

The Yangtze-Huaihe shear lines (YHSLs) generated in the eastern plain and the Tibetan Plateau shear lines (TPSLs) generated in the western Tibetan Plateau are located in the same latitude zone of subtropical East Asia. In order to deepen the understanding of the YHSLs and the TPSLs located at different terrain heights, based on the ERA-interim reanalysis data and composite analysis, a comparative study is carried out on the relationship between shear lines and rainstorms, three-dimensional structural characteristics of shear lines, characteristics of wind field and circulation near shear lines, and the thermal mechanism in the structural evolution of shear lines. The results show that: (1) The YHSLs can be divided into four types, namely warm type, cold type, quasi-stationary type and vortex type. The TPSLs are classified into horizontal TPSLs and vertical TPSLs. Both of them are closely related to rainstorms. In summer, nearly 70% of the YHSLs can produce rainstorms. Warm YHSL-induced rainstorms have the largest contribution to total rainfall of YHSL-induced rainstorms, while the vortex YHSL-induced rainstorms have the largest rainfall intensity but low occurrence frequency. Nearly 60% of the horizontal TPSLs bring rainstorms to the main area of the Tibetan Plateau, and more than 55% of the vertical TPSLs cause rainstorms to the east side of the Tibetan Plateau and its adjacent areas. (2) Both the YHSLs and the TPSLs are boundary-layer systems, and the characteristic levels are located at 850 hPa and 500 hPa, respectively. On the temporal and spatial scales, the horizontal dimensions of cold YHSLs and horizontal TPSLs can reach 1000 km and 2000 km, their vertical extension thicknesses can reach 5 km and 2 km, and their life spans can be up to 48 h and 96 h, respectively. Both of the YHSLs and TPSLs are inclined northward from lower to upper levels. (3) There are differences in wind field and circulation characteristics between cold YHSLs and horizontal TPSLs. The north side of a cold YHSL is northeasterly wind, and the south side is southwesterly wind; wind fields in the east and west of a horizontal TPSL are different as the west section is similar to cold YHSL and the east section varies significantly in different development stages. (4) There are differences in dynamic and thermal structures between cold YHSLs and horizontal TPSLs. In terms of dynamic structure, the YHSLs and horizontal TPSLs are all located in the positive vorticity zone, and the strength of the positive vorticity center reaches the maximum at the strong stage. In terms of thermal structure, a cold YHSL is near the low-level frontal zone, its western section is located in the warm and humid zone and its eastern sections is located in the dry and cold zone. The south side of a horizontal TPSL is highly warm and humid, and there is a frontal zone structure on the north side of the TPSL. (5) The diabatic heating near the shear line is closely related to the evolution of the cold YHSL and the horizontal TPSL. The vertical diabatic heating is the most important factor that causes the development and enhancement of the cold YHSL and the horizontal TPSL. There are differences in thermal structure and weakening mechanism between cold YHSLs and horizontal TPSLs. The invasion of dry and cold air will lead to the weakening or even extinction of a horizontal TPSL, while the weakening of the intensity of a cold YHSL is related to the northward invasion of warm and humid air from the south.