The role of the cold pool in a mountain-to-plain and plain-to-mountain squall line case in the south of the Yangtze River in China

A squall line event that occurred in the south of the Yangtze River in China on 4 May 2020 was simulated using the WRF model and 3DVAR data assimilation. The radar radial wind, reflectivity, and conventional observation data were assimilated in a cycling interval of 15 min. The main findings from the examination of the development mechanisms of the squall line are as follows. The simulation effectively reproduced the development and evolution of the squall line. The squall line strengthened from the Wuling Mountains to the Dongting Lake Plain, moved eastwards and developed to its strongest stage over the plains, before weakening from thereon to the Mulianjiu Mountains. When the squall line moved from the mountains to the plains, the convergence of the airflow in front of the squall line increased because of the strengthening of the near-surface easterly airflow over the plains. Concurrently, the intensity of the cold pool behind the squall line decreased, while the low-level vertical wind shear over the plains increased. This reduced the gap between the cold pool and vertical wind shear. The low-level upward airflow in front of the cold pool tended to be vertical, which was conducive to the development and maintenance of the squall line. When the squall line moved eastwards from the plains to the mountains, the convergence of the airflow in front of the squall line weakened because of the weakening of the surface easterly airflow over the mountains. Concurrently, the intensity of the cold pool behind the squall line increased, while the low-level vertical wind shear in front of the squall line significantly declined. The intensity of the cold pool significantly exceeded the low-level vertical wind shear, inducing a tendency for the low-level upward airflow in front of the cold pool to tilt towards the cold pool, which was not conducive to the development and maintenance of the squall line. This was a reason why the squall line moved eastwards faster after going up the mountains. Based on these observational and simulation results, physical conceptual models for the enhancement of squall lines from the mountains to the plains and their weakening from the plains to the mountains have been preliminarily developed.