Distribution and environmental characteristics of severe thunderstorm gusts associated with the Northeast China Cold Vortex

To improve the analysis and forecast ability of severe thunderstorm gusts (speed≥24.5 m/s) under the background of Northeast China Cold Vortex (NCCV), it is necessary to study the distribution and environmental characteristcs of severe thunderstorm gusts under the background of NCCV. Based on 8 a observations collected at 17677 automatic weather stations, 1717 severe thunderstorm gust events and 49803 ordinary gust events (17.2 m/s≤speed＜24.5 m/s) under the background of NCCV are identified. The thunderstorm gust events are classified into four quadrant groups according to their relative position to the center of the NCCV. Observational characteristics of severe thunderstorm gusts in different quadrants of the NCCV and comparisions of environmental conditions for thunderstorm gusts of different intensities as well as key environment parameters associated with severe thunderstorm gust occurrences in different quadrants are studied. The results show that severe thunderstorm gusts occur most frequently in the southeast quadrant, followed by the southwest, northeast and northwest quadrants. Severe thunderstorm gusts occur predominantly in plain areas. Heilongjiang is the province with the highest frequency of severe thunderstorms gusts under the background of NCCV. Severe thunderstorm gusts occur most frequently in the southwest quadrant in Henan, Jiangsu and Anhui, while they are most frequent in the southeast quadrant in other provinces. Most severe thunderstorm gusts occur in July and during the afternoon. Monthly distributions of severe thunderstorm gusts in different quadrants exhibit the characteristics of a main peak in the summer and a secondary peak in the spring. All severe thunderstorm gust events are analyzed by establishing a new coordinate system with the center of the NCCV as the origin and the radius of NCCV as the unit distance. Major results are as follows. (1) 48.7% of severe thunderstorm gusts are concentrated in the bottom of NCCV with the azimuth angle of 135°—210° and within 1.2—2.5 times of the NCCV radius. This area is a high occurrence zone of severe thunderstorm gusts at the bottom of the NCCV. Severe thunderstorm gusts in Northeast China and North China mainly occur on the southeast side of the high occurrence area at the bottom of NCCV, while severe thunderstorm gusts in East China largely occur on the southwest side. Compared to ordinary thunderstorm gusts, especially in the southwestern quadrant, 0—6 km wind vector difference, 0—3 km storm helicity and average wind speed in the storm bearing layer of severe thunderstorm gusts are larger in spring and autumn, and the convective available potential energy and downdraft convective available potential energy of severe thunderstorm gusts are higher in summer. The above-mentioned physical prameters are key environment parameters used to distinguish thunderstorm gusts of different intensities. The areas of large-value key environment parameters are predominantly concentrated in the high occurrence region at the bottom of the NCCV, which accounts for the frequent occurrence of severe thunderstorm gusts in this region. (2) There is another high occurrence area with the azimuth angle of 235°—245° and within 1.5—2 times of the NCCV radius in the southwest quadrant, which is smaller than the high occurrence area at the bottom of the NCCV. Severe thunderstorm gusts in summer in Northeast China mainly occur in this area, where there is a low-level convergence line accompanied by a dryline. Compared to ordinary thunderstorm gusts, the convergence characteristics and dew point gradient in the convergence line corresponding to severe thunderstorm gusts are stronger. The observational characteristics and environmental conditions of severe thunderstorm gusts in different quadrants of NCCV can provide a reference for improving the forecast ability of severe thunderstorm gusts.