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.

Limited observations have resulted in few studies examining mesovortices within quasi-linear convective systems (QLCSs) in China, especially in Northeast China, where the polarimetric signatures of tornadic versus nontornadic vortices remain unclear. This study examined 12 mesovortices (including one that produced a tornado) within a QLCS that occurred on 18 August 2020 over the Liaohe Plain, Northeast China, using conventional observations, reanalysis data, and Doppler weather radar observations. The QLCS was associated with notable conditional instability and moderate-to-strong low-level shear under the influence of a cold vortex. Observed extreme winds, reaching up to 41.5 m s⁻¹, were located north of the rear-inflow jet (RIJ) core, a pattern consistent with mesovortices along the gust front rather than those at the bow echo apex. The tornadic mesovortex exhibited faster propagation speeds and stronger low-level rotation compared to its nontornadic counterparts. Notably, dual-polarization radar parameters indicated a marked increase in mid-level specific differential phase (K_DP) and lower average differential reflectivity (Z_DR) for the tornadic mesovortex. Hydrometeor identification (HID) analysis further revealed that above the wet-bulb zero (WBZ) level, the tornadic mesovortex contained a higher proportion of hail, whereas below the WBZ, it exhibited a greater concentration of smaller raindrop particles. The Eulerian vorticity budget analysis identified the stretching term as the largest contributor to vortex intensification, especially in its initial stage. The tornadic mesovortex originated as a bookend vortex at the intersection of two QLCS segments, with a transient northern anticyclonic vortex observed in its immediate vicinity during the convective merger process. These findings provide valuable observational context and conceptual models on the structure and evolution of mesovortices within QLCSs, offering guidance for improving the forecasting of tornadic and nontornadic mesovortices in QLCSs over Northeast China.

To improve the analysis and forecast ability of thunderstorm gusts (TGs) in Northeast China, climatic characteristics of TGs in Northeast China and their relation with Northeast China Cold Vortex (NCCV) are studied. TGs are identified based on comprehensive analysis of data collected at automatic weather stations and from lighting locators and Himiwari-8 satellite. In addition, the ERA5 reanalysis dataset is used to calculate the centers and radius of NCCV. The results show that TGs in Northeast China mainly occur in the regions from the Mongolian plateau to the west of the Greater Khingan mountains, Northeast China plain and the coastal area of Liaoning. About 50.6% of TGs are caused by NCCV and Liaohe plain is the area with the highest occurrence frequency of TGs caused by NCCV. The diurnal variation of TGs occurrence shows a high frequency in the afternoon, and the TGs caused by NCCV are more frequent in the late night and morning, accounting for 75%. The number of stations with TGs in one hour is generally less than 10, while the cases of regional TGs occurring at more than 10 stations are mostly related to NCCV and account for 56.5%. Compared with TGs not caused by NCCV, TGs caused by NCCV occur in a drier and colder environment with strong temperature lapse rate between 850 hPa and 500 hPa, strong vertical wind shear and storm bearing layer. The percentages of TGs occurring in different quadrants of NCCV in the NCCV TGs are 73.5% in the southeast quadrant, 17.5% in the southwest quadrant, 7.5% in the northeast quadrant and 1.5% in the northwest quadrant in descending order. The frequency of TGs occurring in the periphery of NCCV is more than that in the NCCV itself, and they are concentrated within 0.5—2 times of NCCV radius from the southeast quadrant of NCCV to the center of NCCV. In this area, there is a dry layer in the middle atmosphere, a large vertical temperature lapse rate in the lower atmosphere, a medium or higher vertical wind shear and a larger wind speed in the storm carrying layer, and they are more vulnerable to the impact of low-level shear line. This is the reason why NCCV TGs are concentrated in this area. The centers of NCCV that can cause TGs are concentrated over 45°—55°N and 111°—128°E, and the centers of NCCV that can cause regional TGs are concentrated at 116°E and 122°E with a meridional distribution. The geopotential height at the central and outermost areas of NCCV that can cause regional TGs is lower than that of NCCV responsible for local TGs, and the difference is more obvious in spring and autumn. Furthermore, the radius of NCCV that can cause local TGs in August slightly higher than that of NCCV that can cause regional TGs in spring and autumn, and vice versa. The results of this study have proved that NCCV is the most important system that causes TGs in Northeast China. The spatial distribution of TGs in different quadrants of NCCV and characteristics of NCCV that can generate regional TGs provide a reference for improving the forecast and early warning ability of TGs caused by NCCV.

The urban area of Shenyang was affected by an intense rainfall system with low-level meso-γ-scale vortices on 16 August 2019. This system caused record-breaking hourly precipitation (102 mm) since the observations started in 1951. In order to improve the analysis and forecasting ability of heavy rain caused by such vortices, multi-source observations and ERA5 reanalysis data are comprehensively used to analyze the characteristics of the low-level meso-γ-scale vortices in this process, the environment for their generation and their roles in the formation of the torrential rain. The results show that during this process, Shenyang was located in the front of the Northeast Cold Vortex at 500 hPa and in the water vapor conveyor belt on the west side of the residual vortex of typhoon "Likima" at low levels. Moreover, a low-level nearly dry adiabatic lapse rate prevailed in the urban area of Shenyang on the afternoon of 16 August with lower lifted condensation level and increasing vertical wind shear. At 16:00 BT (Beijing Time), the meso-γ-scale convergent wind field in the urban area of Shenyang triggered the local storm, and the cold vortex storm then entered Shenyang. The storm group merged in the area with anticyclonic rotation of the local storm. The merged storm strengthened precipitation, and a meso-γ-scale vortex pair with a lifetime of 30 min appeared in lower levels, followed by a rare phenomenon of the strengthening of the converging anticyclonic vortex. Compared with the statistical characteristics of mesocyclones in China, this low-level shallow vortex demonstrated a short life span with small scale, slow moving speed and strong vertical vorticity. All the automatic weather stations with precipitation exceeding 10 mm in 5 min after the vortex were located in the area between the vortex pair, and the record-breaking intense hourly precipitation occurred since observations started in 1951. The strength and extent of the heavy precipitation can be characterized by the strength of the vortex's rotation, the height of its extension, and the distance between the two vortices. The occurrence of extreme precipitation events requires strong rainfall intensity and long duration time. The merged storm in this process had the characteristics of warm clouds and low centroid echo in radar observations. The early local storm precipitation also reduced the difference between ground temperature and dew point, which ensured high precipitation efficiency. Furthermore, the updraft near the ground generated by low-level vortex promoted the growth and collision of raindrops, thereby enhanced the rain intensity. The strong rotation of the vortex caused an updraft near the ground, which was conducive to the re-development of storms and prolonged the precipitation duration time. The reason for the emergence and strengthening of the low-level anticyclonic vortex is likely attributed to the following factors. The low-level quasi-linear outflow boundary of the cold vortex storm formed a horizontal vortex tube from north to south. Under the downward twisting action of the downdraft in the initial precipitation, meso-γ-scale vortex pair appeared near the ground, and since the environmental wind shear vectors rotated counterclockwise with altitude, the clockwise rotation was more conducive to the strengthening of anticyclonic storms. This is consistent with the theoretical research. In addition, this anticyclone was closer to the strong updraft area in the merged storm, and it could also be reinforced under its stretching. Finally, the formation mechanism of this low-level meso-γ-scale vortices and the rainstorm model are summarized, which provide a reference for future weather analysis and forecasting research.

The Northeast China cold vortex (NCCV) is one of the main synoptic-scale systems causing short-duration heavy rainfall (SDHR) in Northeast China. Environmental conditions (e.g., water vapor, instability, and vertical wind shear) are known to be distinctly different over the four quadrants of NCCVs, rendering prediction of the SDHR related to NCCVs (NCCV_SDHR) more challenging. Based on 5-yr hourly rainfall observations from 3196 automatic weather stations and ERA5 reanalysis data, 10,232 NCCV_SDHR events were identified and divided into four quadrant groups according to their relative position to the center of the NCCV (CVC). The results show that the southeast quadrant features the highest frequency of SDHR, with stronger intensity, longer duration, and wider coverage; and the SDHR in different quadrants presents different formation mechanisms and varied temporal evolution. A new coordinate system is established relative to the CVC that uses the CVC as the origin and the radius of the NCCV (r_CV) as the unit distance. In this new coordinate system, all of the NCCV_SDHR events in the 5-yr study period are synthesized. It is found that the occurrence frequency of NCCV_SDHR initially increases and then decreases with increasing distance from the CVC. The highest frequency occurs mainly between 0.8 and 2.5 times r_CV from the CVC in the southeast quadrant. This can be attributed to the favorable conditions, such as convergence of the low-level shear line and abundant water vapor, which are concentrated in this region. Furthermore, high-frequency NCCV_SDHR larger than 50 mm (NCCV_SDHR50) is observed to be closer to the CVC. When NCCV_SDHR50 occurs, the NCCV is in closer proximity to the subtropical high, resulting in stronger low-level convergence and more abundant water vapor. Additionally, there are lower lifting condensation levels and stronger 0–6- and 0–1-km vertical wind shears in these environments. These findings provide a valuable reference for more accurate prediction of NCCV_SDHR.