Abstract:

On the evening of May 17, 2019, local extremely heavy precipitation, thunderstorms gales, and hail occurred in Tongzhou District of Beijing. A detailed analysis of this weather process was conducted using various observation data from surface intensive automatic stations, radars, microwave radiometers, and European Center Mesoscale Weather Forecast reanalysis data (ERA-interim) with a temporal resolution of 6 h. The results indicated that a high precipitation supercell is a direct system that caused the severe convective weather, and this typical characteristic can be identified from the radar echo. In the afternoon, a small thermal and low-pressure system occurs near the surface and at the ultra-low altitudes in the southern part of Beijing, which makes the southeasterly wind significantly strengthened. On the one hand, it provides abundant water vapor and forms an unstable stratification of upper dry and lower wet. Meanwhile, the interaction between the low-level southwesterly flow and the upper-level westerly jet enhances vertical wind shear and produces vertical circulation. The low-level temperature inversion layer exists for a long period, and the energy continues to accumulate, making it possible to increase the explosiveness of thunderstorms in the afternoon. The outflow of the surface cold pool flow confronts the southerly wind, forming a dense zone of surface pseudo-equivalent potential temperature. The cold pool was maintained steadily for a long time, and a moist-warm convergence line of the boundary layer occurred ahead of the cold pool. The frontogenesis causes the warm and humid airflow to be continuously uplifted, and the convective system is strengthened and in a quasi-static state. In a favorable environment, combined with sufficient water vapor supply and strong boundary layer convergence and uplift, thunderstorms strongly develop into a high precipitation supercell. Then the storm splits, and the storm on the right develops into a supercell, passing through eastern Tongzhou again and forming the rainstorm center.

Key words: High precipitation supercell, Low-level southeasterly wind, Temperature inversion layer, Cold pool