Based on dual-polarization radar observations, surface data and ERA5 reanalysis product, an extensive propagation high 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 resulted in a Derecho event in Hubei province. The direct reason for the enhancement of the squall line is that several isolated 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 shallow cold outflow from another squall line, an boundary-layer jet forced by the topography and the cold pool outflow of the squall line. The topographic effects include the blocking of cold pool outflow, the valley penetration of outflow, and the orographic uplift, which triggered isolated storms and provided a mesoscale ascending environment. After the squall line crossed the mountain, extreme winds in Guangshui were mainly caused by downward momentum transfer and divergence of strong downdrafts. The intense convective cells in the squall line 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. Significant evaporation under the melting layer significantly reduced the diameter of raindrops and liquid water content. This indicates that significant 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.

To improve the understanding of the atmospheric environmental characteristics of severe storms from the Taihang mountains to the North China plain, the environmental elements of 568 samples of MTPSS in the eastern foothills of the Taihang mountains during the warm seasons from 2011 to 2020 are investigated. The study is based on ERA5 reanalysis data and radiosonde observations. Results suggest that MTPSS events in North China are closely associated with significantly enhanced environmental instability, a more substantial moisture layer, and a greater pseudo-equivalent potential temperature gradient between 850 hPa and 500 hPa, all of which markedly surpass the warm season average (WSA) in North China. Additionally, the common environmental conditions during WSA and MTPSS events are characterized by higher downdraft convective available potential energy (DCAPE) and marked mid-tropospheric dryness, indicating that the environmental characteristics of MTPSS events are similar to those of thunderstorm high winds in North China. Statistical analysis of synoptic-scale system forcing, assessed by Q-vector divergence, reveals that intensified synoptic-scale forcing increases the likelihood of MTPSS events successfully moving down from the mountains. The dominant environmental factors influencing the MTPSS evolution differ with the intensity of the forcing: dynamic factors such as wind vertical shear prevail under strong forcing (SF), while thermodynamic factors such as CAPE dominate under weak forcing (WF). When synoptic-scale forcing types are not distinguished, no significant differences in environmental parameters between downhill and non-downhill events are observed. However, the differentiation of forcing types reveals significant environmental differences under SF. In an environment with larger vertical wind shear, pronounced mid-tropospheric dryness, and higher DCAPE values, most MTPSS successfully propagate to the plain, indicating that organized storms with robust downdrafts are more likely to reach the plain. Under WF, downhill events exhibit larger pseudo-equivalent potential temperature differences and higher low-level moisture content compared to non-downhill events, suggesting that MTPSS are more likely to move down and influence the plain in an environment conducive to the formation of wet downbursts.

Supercells are the most severe and long-lasting type of highly organized convective storms, with the greatest potential for producing extreme weather events and causing significant disasters. This article provides a comprehensive overview and recent highlights of supercell research, including the unique structure, environmental characteristics, and the formation and maintenance mechanisms of the mesocyclone. Buoyancy instability is a necessary ingredient in the supercell's environment, whereas dynamic factors such as vertical wind shear and low-level storm relative helicity are more sensitive parameters for distinguishing supercells from non-supercells. The near-storm environmental parameters derived from multi-sensor observations are expected to enhance high-resolution nowcasting of supercell storms. Different types of supercells, including those producing distinct hazardous weather, exhibit unique reflectivity morphology and dynamical/microphysical structures, e.g., tornadic supercells have a strong low-level meso-cyclone while severe hail supercells feature a strong and deep mesocyclone. Mesocyclones associated with damaging winds are accompanied by significant mid-level radial convergence, while those responsible for heavy precipitation are typically located at low levels. The vertical vorticity of the mesocyclone is generated through the tilting of environmental horizontal vorticity by storm-related intense updrafts. The horizontal vorticity that tilts into the mid-level mesocyclone originates from the environmental vertical wind shear, which produces the horizontal vorticity along the inflow to the storms. In contrast, the horizontal vorticity contributing to the low-level mesocyclone derives from two distinct mechanisms, i.e., environmental vertical shear in the boundary layer and gust front-induced baroclinicity. It remains unclear which mechanism is more dominant. Moreover, the maintenance and enhancement mechanisms of mesocyclones are complex and vary across different scenarios, particularly when embedded within heavy precipitation, during storm mergers, or in proximity to surface mesoscale boundaries (e.g., fronts, drylines, gust fronts, and their associated convergence lines). In recent years, based on super high-resolution numerical experiment results, the physical conceptual models of the supercell tornadogenesis have been updated. The newly revealed micro-physical and dynamic characteristics from polarimetric Doppler radar observations enable more accurate hail size detection. However, the refined physical conceptual model of severe hail growth still requires improvement, and our understanding of the formation mechanisms behind extreme wind gusts and flash floods associated with supercells remains limited.

An analysis of two EF1 weak tornadoes that occurred in Weichang and Longhua including the pre-storm environmental conditions and fine structures is conducted based on observations of radars on different bands and wind profile radars. The results show that the tornado occurred under the background of northwesterly flows behind a 500 hPa cold vortex, and the tornadic supercells were triggered by the dryline and its associated surface convergence. The supercell tornadoes occurred in the pre-storm environment with strong convective available potential energy (CAPE) and 0—6 km vertical wind shear, while water vapor at the low level and 0—1 km vertical wind shear significantly increased. The thunderstorm activity in the afternoon improved low-level moisture. The reflectivity of these two tornadic supercells was overall less intense with that of the Weichang tornado less than 40 dBz. The area was small and the precipitation near the tornado was less than 2 mm. The rotating updraft corresponding to the mesocyclone and the hook echo head were separated from the precipitation echo. Both of these two tornadic storms exhibited low-precipitation (LP) supercell characteristics. In this paper, the pre-storm environment and structural evolution of the two LP supercell storms are recorded. Moderate mesocyclone at low levels and tornadic vortex signatures (TVS) were identified, indicating the X-band radar can capture the LP supercell tornado. The occurrence of the TVS corresponded well with the occurrence of the Weichang tornado, and the Longhua TVS was detected 8 min in advance, indicating that the leading time for the LP supercell tornado warning was limit.

Supercells are the most severe and long-lasting type of highly organized convective storms, with the greatest potential for producing extreme weather events and causing significant disasters. This article provides a comprehensive overview and recent highlights of supercell research, including the unique structure, environmental characteristics, and the formation and maintenance mechanisms of the mesocyclone. Buoyancy instability is a necessary ingredient in the supercell’s environment, whereas dynamic factors such as vertical wind shear and low-level storm relative helicity are more sensitive parameters for distinguishing supercells from non-supercells. The near-storm environmental parameters derived from multi-sensor observations are expected to enhance high-resolution nowcasting of supercell storms. Different types of supercells, including those producing distinct hazardous weather, exhibit unique reflec-tivity morphology and dynamical/microphysical structures, e.g., tornadic supercells have a strong low-level meso-cyclone while severe hail supercells feature a strong and deep mesocyclone. Mesocyclones associated with damaging winds are accompanied by significant mid-level radial convergence, while those responsible for heavy precipitation are typically located at low levels. The vertical vorticity of the mesocyclone is generated through the tilting of environmental horizontal vorticity by storm-related intense updrafts. The horizontal vorticity that tilts into the mid-level mesocyclone originates from the environmental vertical wind shear, which produces the horizontal vorticity along the inflow to the storms. In contrast, the horizontal vorticity contributing to the low-level mesocyclone derives from two distinct mechanisms, i.e., environmental vertical shear in the boundary layer and gust front-induced baroclinicity. It remains unclear which mechanism is more dominant. Moreover, the maintenance and enhancement mechanisms of mesocyclones are complex and vary across different scenarios, particularly when embedded within heavy preci-pitation, during storm mergers, or in proximity to surface mesoscale boundaries (e.g., fronts, drylines, gust fronts, and their associated convergence lines). In recent years, based on super high-resolution numerical experiment results, the physical conceptual models of the supercell tornadogenesis have been updated. The newly revealed micro-physical and dynamic characteristics from polarimetric Doppler radar observations enable more accurate hail size detection. However, the refined physical conceptual model of severe hail growth still requires improvement, and our understanding of the formation mechanisms behind extreme wind gusts and flash floods associated with supercells remains limited.

The frontogenesis, atmospheric diabatic heating and net water vapor budget are analyzed to reveal the development and maintenance mechanism of the synoptic scale system of the Henan province extreme heavy rainfall from 19 July to 21 July in 2021 using automatic weather station observations and the fifth-generation European Center for Medium-Range Weather Forecasts atmospheric reanalysis data. Results show that Henan is located in the saddle area between the Northwest Pacific Subtropical High (NPSH) and the continental high. In this situation, the convergence in low levels is collocated with the divergence at high levels, which is conducive to the development and maintenance of the low-pressure system at 500 hPa during the extreme heavy rainfall. The frontogenesis mainly occurs in the lower troposphere and is consistent with the θ_se intensive region. The horizontal divergence term and horizontal deformation term play equally important leading roles in the frontogenesis. The horizontal distribution of the apparent heat source $⟨Q_1⟩$ and moisture sink $⟨Q_2⟩$ coincide with the area of heavy rainfall. However, vertical distributions show significant differences between Q₁ and Q₂. Q₁ features an obvious single peak structure with the heating center in the middle and upper troposphere, while the large value area of Q₂ is evenly distributed at 850—400 hPa. Q₁ is larger than Q₂ in the middle and upper troposphere. The potential temperature vertical transport term dominates in Q₁ and the specific humidity horizontal transport term dominates in Q₂, indicating that the release of regional-scale strong condensational latent heat has a positive feedback effect on the precipitation. With the continuous strengthening of the easterly air flow from the north side of typhoon In-Fa (2106), the strong wind shear and orographic uplift lead to an abnormally strong convergence of water vapor in the boundary layer. The net inflow of water vapor from the east and west dominates the total water vapor budget. The extremely strong water vapor transport in the boundary layer from the east plays a key role in maintaining and strengthening the extreme rainfall process.