Abstract:

Based on the data of FY-4A satellite infrared and water vapor cloud image, the NCEP (National Centers for Environmental Prediction) analysis field, variable pressure, variable temperature, gale, and precipitation from the ground automatic weather stations, the occurrence, and development mechanism of a strong squall line in South China occurring on March 4, 2018, and the main causes of the persistent gale were analyzed by calculating the vortex of the isentropic surface.The results show that the convective cell is triggered by a low-level convergence system.A thunderstorm sinking airflow forms a ground convergence line with the southwest airflow in front of it, and the water vapor, static instability, and vertical wind shear conditions in the moving area are favorable to the development of severe convective storms and cause a convective storm to occur.The area of the extreme gale is mainly distributed at the top of the bow structure in the squall line.The falling area corresponds to the gradients of the brightness temperature and the variable pressure, which increases the variable pressure wind.The moving direction of the squall line is consistent with that of the guiding airflow and almost perpendicular to that of the low-level Southwest jet.The speed of the squall line is significantly accelerated, and the momentum downward effect is enhanced, which may be the important cause of an extreme gale on the ground.The potential vortex (PV) distribution indicates that the intrusion of high PV has a certain correlation with the formation of bow-shaped in the low PV region.The dry intrusion zone has a good correspondence with the high PV region on the isentropic layer.The dark zone at the back of the squall line is the front of dry intrusion.The downward propagation of the high potential vortex at the upper level is the cause of the continuous development of the squall line.

Key words: Extreme gale, FY-4A satellite, Infrared image, Water vapor cloud images, Dry intrusion