A Case Analysis of Extreme and Severe Convection Initiation in Trumpet-Shaped Topography at Eastern Foot of Taihang Mountains
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Abstract:
Convection initiation (CI) directly affects the early warning of severe convective weather, while the mechanism of CI with obvious regional characteristics is complex. Based on multi-source high-resolution observation data and ERA5 reanalysis data, the characteristics and causes of the extreme severe convection triggered at the trumpet-shaped terrain in the eastern foot of the Taihang Mountains on 17 May 2023 are explored, combined with a similar case. The results show that under the unstable stratification of the 500 hPa shallow trough carrying weak cold air from the middle layer invading the surface thermal low pressure, the extreme convection caused by a supercell storm evolved from a mesoscale convective cell triggered at the 400 m height of the trumpet-shaped terrain in the eastern foot of the Taihang Mountains. Under the environment of continuous high temperature and weak vertical wind shear, a convective cell was triggered near Wangkuai Reservoir on the warm side of the temperature front below 850 hPa. The 2-m temperature difference between the north and south sides of the convective cell was 3-4 ℃, and the water vapour was close to saturation. Meanwhile, the convergence line of southeast wind and northerly wind extended from the ground to 850 hPa, and the thickness of easterly wind with a maximum wind speed of 6-8 m/s near 0.5 km was about 1 km. Before convection initiation, the new banded convective cumulus clouds at the 200-600 m slope of the eastern foot of the Taihang Mountains were consistent with the mountain direction, which was related to the boundary convergence line and the thermal forcing on the mountain. The convection was triggered on the side of the convergence line near the mountain. Froude number (Fr) , terrain vorticity and terrain divergence quantitatively characterised the terrain uplift effect. The Fr number decreased from the mountain to the plain with the peak value at the height of 400 m where the convective cell was triggered. Before CI, the Fr number escalated to about 1, indicating the airflow climbing effect, and the cyclonic terrain vorticity and terrain horizontal divergence were enhanced, then the convective cell was triggered.
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Supported by:Beijing E-Tiller Technology Development Co., Ltd.
Supported by:Beijing E-Tiller Technology Development Co., Ltd.