Analysis of evolution and abnormal characteristics on the pre-TC squall line of typhoon Lekima (2019)

Based on the Doppler radar, wind profile radar, FY-4A black body temperature (TBB), ground-based microwave radiomter, automatic ground stations and ERA5 reanalysis data, the process evolution and abnormal characteristics of the location, direction of movement and long life of the pre-TC squall line caused by typhoon Lekima (2019) are analysed. Results are as follows. (1) The evolution of the pre-TC squall line experiences three stages: development and formation, strengthening and maturation, and weakening and extinction. In the second stage, the influence area is the most extensive and the intensity is the strongest, and the changes of meteorological parameters caused by the pre-TC squall line crossing have extreme characteristics. The characteristics of mesoscale thunderstorm high pressure, pre-squall hot depression, cold pool and warm center are obvious. (2) The pre-TC squall line occurs in the humid zone between the enhanced continental high pressure and typhoon Lekima. Typhoon provides abundant water vapor and strong unstable environment to generate large convective available potential energy (CAPE) and convergence of northeast airflow during the pre-TC squall line event. Under the weather background of middle-level dry air and low-level wet air thermal unstable stratification and the low-level strong vertical wind shear, the pre-TC squall line is generated. (3) After the pre-TC squall line enters Fujian, the significantly enhanced unstable structure of middle-level dry air and low-level wet air, the abnormal enhancement of low-level vertical wind shear, and the convective storm generated by the pre-TC squall line incorporates into the pre-TC squall line and promotes the development and enhancement of the pre-TC squall line, which are favorable for ground wind increase. (4) The enhanced northeast airflow guides the pre-TC squall line to move toward the southwest, which has a large angle with the guidance air flow. Their moving directions are consistent with each other, which causes the high moving speed and ground wind increase. (5) The peak values of thermodynamic convection parameters (K, TT and A index) calculated from the ground-based microwave radiometer data have a good correspondence with the extreme values of the gale caused by pre-TC squall line. The indices K≥37 ℃ and A≥11 ℃ have certain reference value for severe convection weather warning.