Analysis of the characteristics of a strengthened elevated convection downhill and the forecast deviation of thunderstorm gales in Beijing

Using multi-source observational data including radiosonde, Doppler weather radar, automatic weather stations and wind profilers, combined with a thermodynamic analysis data derived from RMAPS-NOW 4D-Var data assimilation model, this study investigated the development and evolution characteristics, the intensification mechanisms after moving from the mountains to plain, and the deviation in urban thunderstorm gales forecasts for the first severe convective process in Beijing on May 13, 2025. The results are as follows: (1) 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 and localized thunderstorm gales in the northern areas in the spring seasons. (2) 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 movement 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 climbing above the cold pool, and the northwest wind at the rear of the storm converging with the southeast wind in the plain at the front of the storm. These factors jointly led to deep convergence and vertical ascending motion above 1.5 km altitude. (3) Convection caused a cold pool with large-scale expansion within the boundary layer in plain areas. After the convection descended the mountains in the second phase, the temperature gradient and pressure difference were small, leading to weak cold pool outflow (density current). Meanwhile, the northwesterly inflow jet at the rear turned into ascending rather than descending airflow within the storm, with no downward momentum transfer. The combined effects of these two factors resulted in the absence of the expected large-scale thunderstorm gales in downtown Beijing after the descent of the elevated convection in the second phase.