Sensitivity of a nocturnal squall line to atmospheric conditions
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摘要: 利用CM1数值模式，以2017年8月7日发生在长江三角洲地区的一次夜间飑线过程为例，开展弱切变背景下中层相对湿度、低层风切变和对流有效位能的敏感性试验。结果表明：中层相对湿度升高，有利于夜间飑线雷达回波面积、回波强度和地面降温幅度增大。湿度降低，虽导致夜间飑线的雷达回波宽度变窄，但有利于夜间飑线结构和强度的维持。中层相对湿度的改变对夜间飑线成熟阶段的地面最大风速的影响并不十分明显，但是中层相对湿度的降低会增大地面最大风速的波动；低层风切变的增大使夜间飑线雷达回波强度增强、面积增大、移速变慢，也使飑线冷池强度增强，而对成熟飑线的冷池厚度和地面最大风速影响不大，但是更弱的环境风垂直切变更容易出现脉冲风暴地面强风。低层风切变的减小不利于夜间飑线的发展以及成熟夜间飑线结构和强度的维持；对流有效位能越大，越有利于夜间飑线雷达回波强度和回波面积以及冷池强度和厚度的增大，也有利于夜间飑线地面降温幅度和地面最大风速的增大。中等大小的对流有效位能更有利于成熟夜间飑线强度和结构的维持。低对流有效位能不利于夜间飑线发展，但在中层湿环境条件下依然能发展成为成熟的夜间飑线。该研究揭示了中层相对湿度、低层风切变和对流有效位能等大气环境条件对夜间飑线发生、发展的影响机制，为夜间飑线的预报提供了参考依据。Abstract: The influence of atmospheric conditions in the form of changing midlevel relative humidity, low-level wind shear and convective effective potential energy (CAPE) on the 7 August 2017 nocturnal squall line in the Yangtze River Delta is investigated using CM1 numerical model. The idealized simulation results show that the increase in midlevel relative humidity is conducive to increases in radar echo area, echo intensity and surface temperature deficit of the nocturnal squall line. The humidity decrease will narrow the radar echo of the nocturnal squall line, but it is favorable for the squall line to maintain its structure and intensity. The results also show that the influence of midlevel relative humidity on maximum surface wind speed is not very obvious while the decrease of the relative humidity increases the fluctuation of maximum surface wind speed. The increase in low-level wind shear results in radar echo enhancement and area increase of the nocturnal squall line, slows its propagation, and also strengthens the cold pool intensity while has little effect on maximum surface wind speed and cold pool depth, but the weak low-level wind shear is prone to produce strong surface winds caused by pulse storms. The decrease in low-level wind shear is not favorable for the development of nocturnal squall line and maintenance of the structure and intensity of the mature nocturnal squall line. A larger CAPE value is more conducive to increases in radar echo intensity and area as well as cold pool intensity and depth of the nocturnal squall line. It can also lead to larger surface temperature deficit and maximum wind speed. A moderate CAPE value is more beneficial to the maintenance of the intensity and structure of the mature nocturnal squall line, while a low CAPE value is not conducive to the development of the nocturnal squall line, but initial convective storm with the low CAPE value can still develop into a nocturnal mature squall line in midlevel moisture condition. This study reveals the influence mechanism of atmospheric environmental conditions such as midlevel relative humidity, low-level wind shear and CAPE value on the occurrence and development of nocturnal squall lines, which provides a scientific basis for the forecasting of nocturnal squall lines.
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Key words:
- Nocturnal squall line /
- Midlevel relative humidity /
- Low-level wind shear /
- CAPE /
- Idealized simulation
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图 1 常州雷达 （a. 18时12分，b. 19时57分，c. 20时32分） 和南汇雷达 （d. 21时22分， e. 22时17分，f. 23时30分） 探测到的飑线系统发展演变的雷达回波 （PPI，0.5°仰角，色阶，单位：dBz）
Figure 1. PPI of reflectivity factor （shaded，unit：dBz） at 0.5° elevation angle from Changzhou radar at （a） 18：12 BT，（b） 19：57 BT，（c） 20：32 BT，and from Nanhui radar at （d） 21：22 BT，（e） 22：17 BT，（f） 23：30 BT
图 2 WRF模式模拟的“格点”探空曲线 （a） 与2017年8月7日20时宝山站探空曲线 （b） 的比较
Figure 2. Sounding profiles from （a） "grid" sounding simulated by WRF model and （b） Baoshan station at 20：00 BT 7 August 2017
图 3 CM1初始场“格点”探空曲线 （a） 与冷池和热泡位温强迫扰动 （b） 示意
Figure 3. Initial conditions for the idealized simulations （a. "grid" sounding profile，b. potential temperature perturbations （shaded） for cold pool and 2 bubbles in the CM1 model domain）
图 4 CM1积分 （a） 20、（b） 90、（c） 130、（d） 180、（e） 240、（f） 300 min后模拟飑线2 km高度的雷达回波
Figure 4. 2 km CAPPI of the reflectivity simulated by the CM1 at （a） 20 min，（b） 90 min，（c） 130 min，（d） 180 min，（e） 240 min and （f） 300 min
图 7 各试验 （a） 地面最大风速和 （b） 地面最大降温随模式积分时间的变化
Figure 7. Time series of （a） maximum surface wind and （b） maximum surface temperature deficit for various schemes
图 8 各试验 （a） 平均冷池强度和 （b） 平均冷池厚度随模式积分时间的变化
Figure 8. Time series of （a） domain mean cold pool intensity and （b） domain mean cold pool depth for various schemes
图 10 RH1 （a） 和RH2 （b） 试验模式积分290 min时垂直速度加速度 （a1、 b1）、气压梯度力项 （a2、 b2）、浮力项 （a3、 b3） 和凝结物项 （a4、 b4） 的垂直剖面 （黑色等值线，间隔0.01 m/s2）
Figure 10. Cross sections of （a1， b1） PWDT，（a2， b2） VPGA，（a3， b3） BUOY and （a4， b4） LOAD （black contour，interval of 0.01 m/s2） along cross line for RH1 scheme （a） and RH2 scheme （b） at 290 min of the simulation
图 11 各试验平均C/ΔU随模式积分时间的变化
Figure 11. Time series of averaged C/ΔU along the cross line for various schemes
图 12 各试验 （a. Ctl，b. SHR1，c. SHR2，d. SHR3） CM1模式积分240 min模拟的2 km高度雷达回波
Figure 12. 2 km CAPPI of reflectivity simulated by （a） Ctl，（b） SHR1，（c） SHR2 and （d） SHR3 schemes at 240 min
图 13 各试验 （a） 地面最大风速和 （b） 地面最大降温随模式积分时间的变化
Figure 13. Time series of （a） maximum surface wind and （b） maximum surface temperature deficit for various schemes
图 14 各试验 （a） 平均冷池强度和 （b） 平均冷池厚度随模式积分时间的变化
Figure 14. Time series of （a） domain mean cold pool intensity，（b） domain mean cold pool depth averaged along the cross line for various schemes
图 15 各试验 （a. Ctl，b. SHR1，c. SHR3） 时空平均的垂直速度 （蓝色等值线，间隔0.5 m/s） 和冷池降温 （色阶，单位：℃） 垂直剖面
Figure 15. Cross sections of line-averaged vertical velocity （blue contour，intervals of 0.5 m/s） and cold pool temperature deficit （shaded，unit：℃） along the cross line for （a） Ctl，（b） SHR1 and （c） SHR3 schemes （the values are computed by averaging over 320—360 min of the simulations）
图 16 各试验平均 C/ΔU随模式积分时间的变化
Figure 16. Time series of averaged C/ΔU along the cross line for various schemes
图 17 模式积分275 min时（a） SHR1、（b） SHR2试验垂直速度加速度 （黑实线，间隔0.01 m/s2） 的垂直剖面以及 （c） SHR1、（d） SHR2试验的冷池前沿上升气流质量通量 （黑实线，间隔1.0 kg/（m2•s））、冷池 （黑虚线，间隔−1℃） 和风速的垂直剖面
Figure 17. Cross sections of PWDT （black solid contour，interval of 0.01 m/s2） for （a） SHR1 and （b） SHR2 schemes，updraft flux （black solid contour，interval of 1.0 kg/（m2•s）） with cold pool （black dotted contour，interval of −1℃） and wind for （c） SHR1 and （d） SHR2 schemes along cross line at 275 min of the simulation
图 18 各试验 （a. Ctl，b. CAPE1，c. CAPE2，d. CAPE2+RH） CM1模式积分340 min模拟飑线2 km高度的雷达回波
Figure 18. 2 km CAPPI of radar reflectivity simulated by （a） Ctl，（b） CAPE1，（c） CAPE2，and （d） CAPE2+RH schemes at 340 min of the simulation
图 19 各试验模拟的 （a） 地面最大风速和 （b） 地面最大降温幅度随模式积分时间的变化
Figure 19. Time series of （a） maximum surface wind and （b） maximum surface temperature deficit for various schemes
图 20 各试验模拟的 （a） 平均冷池强度和 （b） 平均冷池厚度随模式积分时间的变化
Figure 20. Time series of （a） domain mean cold pool intensity，（b） domain mean cold pool depth averaged along the cross line for various schemes
图 22 各试验平均C/ΔU随模式积分时间的变化
Figure 22. Time series of C/ΔU averaged along the cross line for various schemes
图 23 CAPE2试验 （a—c） 和CAPE2+RH试验 （d—f） 模式积分340 min时垂直速度加速度 （a、d）、气压梯度力项 （b、e） 和浮力项 （c、f） 的垂直剖面 （黑色等值线，间隔0.01 m/s2）
Figure 23. Cross sections of （a，d） PWDT，（b，e） VPGA，（c，f） BUOY （black contour，interval of 0.01 m/s2） along cross line for CAPE2 scheme （a—c） and CAPR2+RH scheme （d—f） at 340 min of the simulation
表 1 CM1模式（V19.9）参数配置
Table 1. Summary of CM1 （V19.9） model settings for simulations
模式参数 配置 水平格点 X：400，Y：350 水平分辨率（Δx，Δx） 1 km 垂直层次 50 模式顶高 20 km 积云参数化方案 无 微物理方案 Thompson 辐射参数化方案 无 陆面过程参数化方案 无 侧边界条件 东西侧：开放辐射，南北侧：周期性 模拟时间 6 h 模拟结果输出时间间隔 5 min 
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