Analysis of characteristics of an strengthened elevated convection downhill and the forecast deviation of thunderstorm winds in Beijing

Using multi-source observational data including radiosonde, Doppler Weather Radar, Automatic Weather Stations and wind profiler, combined with a thermodynamic analysis data derived from RMAPS-NOW model, this study was conducted on the convective characteristics, the intensification mechanisms that the storm was moving from the mountains to plain and the deviation in urban thunderstorm wind forecasts in Beijing on 13 May, 2025. The results show that: First, This severe convection process experienced two successive phases of convection moving from mountains to plain in Beijing, earlier in the northern region and then in the western region, resulting in rare large - scale heavy hail across Beijing in spring and localized thunderstorm gales in the northern areas.Second, in the first phase, a boundary layer convergence line existed at the foot of the mountains in the northern region, which was conducive to the enhancement and development of convection moving from the mountains, and the formation of a large - scale cold pool in the plain areas. In the second phase, elevated convection formed over the cold pool in the western region. The favorable dynamic triggering and enhancement conditions were as follows: the low - to - middle level southwest jet stream climbed above the cold pool, and the northwest wind at the rear of the storm converged with the southeast wind in the plain at the front of the storm. These factors jointly led to deep - seated convergence and vertical ascending motion above 1.5 km altitude. Third, convection has caused a cold pool with large-scale expansion within the boundary layer in plain areas. After the convection descended the mountains in the second stage, the temperature gradient and pressure difference were small, leading to weak cold pool outflow (density current). Meanwhile, the westerly-northwesterly inflow jet at the rear turned into ascending rather than descending airflow within the storm, with no downward momentum transfer. The combined effect of these two factors resulted in the absence of the expected large-scale thunderstorm gales in downtown Beijing after the elevated convection descended the mountains in the second stage.