An observational study of extreme winds triggered by a squall line crossing the mountain and intensifying

Based on dual-polarization radar observations, ground data and ERA5 reanalysis product, a large-scale extreme wind event in Hubei province triggered by squall line is studied. Results show that in the environment with typical thunderstorm temperature and humidity profiles (wet downburst), the squall line originating in Southwest Henan province significantly enhanced after crossing Tongbai mountain, and triggered a Derecho event in Hubei Province. The direct reason for the enhancement of the squall line is that several storms on the south side merged into the squall line. Further analysis reveals that the key mesoscale systems for the enhancement of the squall line included a thin cold outflow from another squall line, an elevated boundary-layer jet by the topography and the cold pool outflow of the squall line. The topographic effects include the blocking of cold pool outflow, the gap penetration, and the orographic uplift, which triggered isolated storms and provided a mesoscale ascending environment for storm development. After the squall line crossed the mountain, extreme winds in Guangshui were mainly caused by downward momentum transfer and divergence of strong downdrafts. Inside the squall line, convective cells were composed of graupels or small hails above the melting layer, and many small solid particles melted into large water droplets or water-covered ice cores near the melting layer. Strong evaporation under the melting layer significantly reduced the diameter of raindrops and liquid water content. This indicates that strong melting and evaporation are the main mechanisms for the formation of strong downdrafts in the storm. The results enhance our understanding of the effects of mesoscale topography on storms and physical processes of the formation of extreme winds.