Comparative analysis of dry intrusion in the different position of pre-TC squall line on typhoon Lekima (1909) and Matsa (0509)

Using conventional observation data, FY2 satellite data, typhoon data and NCEP reanalysis data, the different position of Pre-TC squall line accompanying the similar tracks with “Lekima” (No. 1909) and “Matsa” (No. 0509) is studied. The article analyzes the circulation background and trigger mechanism generated by the two pre-TC squall lines, and uses various physical quantities such as low-level vertical wind shear, humidity, wind field, wet potential vorticity, and vertical vorticity. Also, try to study the dynamic mechanism of dry intrusion during the process using the Slantwise Vorticity Development (SVD) theory. The results show that: the high-altitude system and the ground trigger mechanism produced are significantly different. On the high-altitude system, the “Lekima” squall line is generated between the continental high pressure and the typhoon with the northeast jet. The “Matsa” squall line occurred between the typhoon and the subtropical high, and near the typhoon inverted trough, which was generated with the strong easterlies. On the ground trigger mechanism, the “Lekima” squall line is generated at the convergence line under the intersection of northeast and northwest air currents. The “Matsa” squall line is generated in the ground inverted trough moves westward and meridional degree increases. During the two pre-TC squall lines, low-level vertical wind shear values are above 16 m·s⁻¹.The dry air intrusion in the lower layer came from the upper and middle troposphere in the vertical direction, and in the east from the west along the latitude in the horizontal direction. Theoretical analysis base on the development of wet potential vorticity, vertical vorticity, and inclined vorticity indicate that the downward transmission of the high potential vorticity band on the upper troposphere, the level of dry invasion, and the air mixing of equivalent potential temperature at high and low levels play an important role in the generation and development of the pre-TC squall line. The slantwise vorticity has a good corresponding relationship with the vertical vorticity over the pre-TC squall line. The two development processes of the pre-TC squall line can be better explained by using the SVD theory.