Mechanism analysis of a rare squall line process in Yunnan Province

doi:10.11755/j.issn.1006-7639-2025-02-0265

Abstract

Abstract:

Based on ground automatic station, multi-band weather radar and sounding data, and the fifth generation atmospheric reanalysis ERA5 from the European Centre for Medium-Range Weather Forecasts (ECMWF), the environmental conditions, mesoscale characteristics and maintenance mechanisms of a rare squall line weather process in Yunnan Province on 7 July 2022 were analyzed. The results show that this squall line occurred within the convergence zone of the continental high and subtropical high, with environmental conditions characterized by strong convective available potential energy, moderate vertical wind shear, and significant intrusion of high-level dry and cold air. Changes in C-band radar reflectivity and radial velocity were closely associated with the occurrence of strong surface winds and hail. The squall line exhibited distinct features, with a pronounced inflow jet near the surface layer, accompanied by velocity ambiguity and gust front characteristics. The hail cells exhibited characteristics such as three-body scattering, midlevel convergence, and storm-top divergence. High spatial and temporal resolution observations from the X-band dual-polarization phased array radar showed that mature hail clouds had a strong reflectivity factor exceeding 55 dBZ. At the same time, a prominent differential reflectivity (Z_DR) column was observed near the strong updraft, with vertical extension exceeding the height of the wet-bulb temperature 0 ℃ layer. Dual-polarization parameters further indicated that precipitation occurred during the hailfall process. The analysis found that the long-term maintenance of ground convergence lines, the coexistence of strong updrafts and tilted downdrafts within the storm, and the coupling of low-level convergence with high-level divergence was the main mechanisms sustaining this squall line.

Key words: squall line, mesoscale features, dual-polarization quantity, ground convergence line

摘要： 基于地面自动站、多波段天气雷达、实况探空及欧洲中期天气预报中心（European Centre for Medium-Range Weather Forecasting，ECMWF）第5代再分析资料ERA5，对2022年7月7日云南省一次罕见飑线天气过程的环境条件、中尺度特征及维持机制进行了分析。结果表明，此次飑线过程发生在大陆高压与副热带高压辐合区内，环境场具有较强对流不稳定能量、中等强度垂直风切变及明显的高层干冷空气入侵特征。C波段雷达反射率和径向速度的变化与地面大风、冰雹的发生有很好的对应关系，飑线特征明显，近地层入流急流明显，伴随速度模糊和阵风锋特征；降雹单体具有三体散射、中层辐合和风暴顶辐散特征；X波段双偏振相控阵雷达的高时空分辨率观测显示，成熟冰雹云的强反射率因子超过55 dBZ，强上升气流附近存在明显的差分反射率（Z_DR）柱，垂直伸展至高于湿球温度0 ℃层的高度；双偏振参数还表明，冰雹降落过程中伴有降水。分析认为，地面辐合线的持续维持、风暴内部上升气流与倾斜下沉气流共存，以及低层辐合与高层辐散配合，是此次飑线得以维持的主要机制。

关键词: 飑线, 中尺度特征, 双偏振量, 地面辐合线

CLC Number:

P458.3

YANG Fangyuan, YANG Suyu, ZHEN Tingzhong, YANG Zhuyun, LI Xiaopeng, HU Yonghua, SHI Baoling. Mechanism analysis of a rare squall line process in Yunnan Province[J]. Journal of Arid Meteorology, 2025, 43(2): 265-276.

杨芳园, 杨素雨, 甄廷忠, 杨竹云, 李晓鹏, 胡勇华, 石宝灵. 云南省一次罕见飑线过程发生机制分析[J]. 干旱气象, 2025, 43(2): 265-276.

Figures/Tables 10

Fig.1 Temporal and spatial evolution of strong convective weather from 14：00 to 23：00 on 7 July in Yunnan Province

Fig.2 The wind field (wind vectors, Unit: m·s-1), geopotential height field (black contours, Unit: dagpm) and temperature field (red dashed lines, Unit: ℃) at 500 hPa (a, b), and shear lines (brown lines), water vapor flux (arrow vectors, Unit: g·cm-2·hPa-1·s-1) and water vapor flux divergence (the color shaded, Unit: 10-5 g·cm-2·hPa-1·s-1) at 700 hPa (c, d) at 08:00 (a, c) and 20:00 (b, d) on 7 July 2022 （The red thick solid line rang is Yunnan Province, the black thick line is convergence zone, the letter D is the low pressure center）

Fig.3 T-ln P diagram of Kunming (a), Pu'er (b) souding stations at 08:00 on 7 July 2022 (The blue line represents stratification curve, the green line represents dew point temperature and the red line represents state curve)

Fig.4 The composition reflectivity factors (a, Unit: dBZ) and radial velocity on the elevation of 0.5° (a), 1.5° (b), 2.4° (c) (Unit: m·s-1) at 14:58 of Kunming radar on 7 July 2022 (The red box indicates velocity ambiguity area， the same as below)

Fig.5 Reflectivity factors (a, b) (Unit: dBZ) and radial velocity (c, d) (Unit: m·s-1) on 0.5° elevation at 19:25 (a, c) and 20:01 (b, d) of Pu'er radar on July 7, 2022 (The red ellipse indicates the position of gust front)

Fig.6 Reflectivity factor (a, Unit: dBZ) and radial velocity (b, Unit: m·s-1) on 3.3° elevation, and the radar vertical section of reflectivity factor (c, Unit: dBZ) and radial velocity (d, Unit: m·s-1) of Kunming radar on 7 July 2022 (The red ellipse indicates the mid-altitude radial convergence area)

Fig.7 The ZH (a, Unit: dBZ), CC (b), KDP (c, Unit: °·km-1), ZDR (d, Unit: dB) on 1.5° elevation of Kunming X-band radar at 14:19, and rainfall in the main city from 14:00 to 15:00 (e, Unit: mm) on 7 July 2022 (The black box indicates the research area, the black line indicates the path along which dual-polarization variables are extracted for vertical cross-section analysis)

Fig.8 The vertical profile of ZH (Unit: dBZ), CC, KDP (Unit: °·km-1), and ZDR (Unit: dB) along the black line in fig.7 of Kunming X-band radar on July 7, 2022 at 13:57， 14:09 and 14:45 (The number 1 and 2 represent convective cells)

Fig.9 The surface temperature (the color shaded, Unit: ℃), hourly average wind field (streamlines) in the first 2 minutes and instantaneous maximum wind speed (red bars) (Unit: m·s-1) of Yunnan at 13:00 (a), 15:00 (b), 17:00 (c) and 19:00 (d) on 7 July 2022

Fig.10 The composition reflectivity factors (a, Unit: dBZ) and the radar vertical section of reflectivity factor (b) and radial velocity (c, Unit: m·s-1) of Pu'er radar at 20:01 on 7 July 2022 (The red line indicates the position of the profile along the moving direction of the squall line, the black arrows indicate the direction of air flow in fig. c)

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