Evolution and Mechanism of Strong Squall Line Occurred in Zhejiang on 25 March 2024

Based on conventional upper-air and surface observations, S-band dual-polarization radar data, cloud ground lightning positioning data,ERA5 reanalysis data (0.25°×0.25°), environmental conditions, evolution characteristics, structure and mechanism of the strong squall line process in Zhejiang on 25 March 2024 are analyzed. Main conclusions are as follows: With a trough at 500 hPa, V-shaped shear at 850-925 hPa level and strong cold front on the ground, the initial convection is triggered within the strong convergence zone on the ground convergence line before cold front. Under favorable conditions such as strong upper-level westerly jet stream, strong low-level southwest jet stream, moderate unstable layering conditions and strong vertical wind shear of 0-3 km and 0-6 km, multiple isolated convective cells develop strongly and organize into the squall line. The primary cause for the development and maintenance of this squall line is its self-organizing structure, characterized by strong thunderstorm-induced high pressure and a cold pool. In the mature stage, the β mesoscale squall line merges with supercell A, resulting in the formation of the α mesoscale squall line. The surface cold outflow formed by the squall line confront the southwest wind ahead, creates a frontogenesis which continuously lifts the warm and humid airflow in front of it, stacks the convergence and uplift effect, and leads to the development of cells into supercells. At the same time, there is a significant angle between low-level vertical wind shear and the squall line, which facilitates the continuous uplift of low-level vertical airflow and supports the long-term development and maintenance of supercells. During the phase transition of water vapor within supercells, significant heat absorption leads to a sharp cooling of the atmosphere and the formation of a strong downdraft. Combined with the descending airflow generated by the inflow from the rear, the surface temperature decreases significantly, thereby intensifying the cold pool at the ground. The strengthening of cold pool facilitates the formation and development of the inflow jet stream from the rear, thereby promoting the emergence and strengthening of squall lines. Extreme winds exceeding level 12 primarily occur during the mature stage of squall lines. Supercell A experiences extreme winds of level 12 or above both before merging into the squall line and during its weakening phase. The former is mainly caused by strong downdrafts with features such as mid-level echo overhang and bounded weak echo area, while the latter is induced by combined effects of strong downdrafts and momentum transfer mechanisms with features disappearing.