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Comparison of Two Damaging Wind Events Caused by Strong Downbursts
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摘要: 利用多普勒天气雷达、常规探空和地面观测数据、1 min降水量和5 min间隔加密自动气象站观测数据, 对比分析了2017年6月2日和8月6日山东两次强下击暴流风暴(简称6·2超级单体和8·6强单体)雷达特征及地面致灾大风的成因。研究表明:两次致灾大风过程在强天气尺度和有利中尺度环境下分别形成超级单体和强单体风暴并触发系列下击暴流, 最强下击暴流发生时垂直积分液态水含量先跃增后骤降, 6·2超级单体伴随中气旋顶和底高度的剧烈下沉。两次强下击暴流触地前均出现强反射率因子核的快速下降、底层高径向速度和强辐散、中层径向辐合和高空强辐散特征。6·2超级单体旋转特性强、中气旋深厚, 低层伴随弧形入流缺口和勾状回波。8·6强单体中低层辐合特征显著, 风暴前端低层伴有由雷暴出流和前侧入流形成的辐合带。两次强下击暴流引起地面致灾大风的过程中负浮力效应基本相当, 6·2超级单体冷池密度流效应更明显, 8·6强单体动量下传效应更显著。潍坊南孙站位于风暴移动方向正前侧, 前侧辐散气流与同向快速移动的风暴叠加, 是导致37 m·s-1极端大风的重要原因。Abstract: Multi-source observations are used to comprehensively analyze the Doppler weather radar characteristics of strong storms and the formation mechanisms of surface damaging wind induced by two strong downbursts, occurred in Shandong on 2 June 2017 and 6 August 2017. It's found that two damaging wind events occurs under strong synoptic scale weather system forcing and favorable meso-scale environmental conditions, the relatively isolated supercell storm on 2 June (6·2 supercell) and strong single storm on 6 August (8·6 strong single storm) fiercely develop into series of downbursts, which leads to the occurrence of large-scale damaging winds. As two strong downbursts induced by 6·2 supercell storm and 8·6 strong single storm evolves, the vertically integrated liquid water content radar parameter first rises and then quickly plunges down. During 6·2 supercell storm, a powerful downburst descends sharply, causing the mesocyclone's top and bottom to shoal and its thickness to decrease. The occurrence of two strong downbursts are accompanied by obvious radar characteristics including reflective factor core rapid decline, high value area of radial velocity at low elevation, strong divergence at bottom, remarkable mid altitude radial convergence and severe divergence at upper-level. 6·2 supercell storm is characterized by intense rotation. Its mesocyclone lasts for a long time and extends deeply both upwards and downwards. Additionally, there are arc-shaped inflow notches and hook echoes at low levels of 6·2 supercell storm. 8·6 strong single storm is characterized by significant low-level convergence. Besides, there is a convergence zone at or near the surface formed by the outflow of the strong downburst and the inflow in front of 8·6 strong single storm. Among all the formation mechanisms of two damaging winds induced by two strong downbursts, the negative buoyancy effect of two storms is basically equivalent, but the cold pool outflow effect is more evident for 6·2 supercell storm, and the downward transport momentum effect is more significant for 8·6 strong single storm. The Nansun Station of Weifang locating right ahead of 8·6 strong single storm's approaching direction, therefore the front divergence flow from the strong downbursts is preferable superimposed on the fast moving homodromous storm, indicating the speed of the front divergence flow better superimposes on the speed of the storm itself, which is crucial for the occurrence of 37.0 m·s-1 extreme wind.
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图 1 2017年6月2日17:00—3日02:00(a)和2017年8月6日18:00—7日02:00(b) 山东8级(不小于17.2 m·s-1) 以上大风分布
Fig. 1 Distributions for the wind speed no less than 8 degree (no less than 17.2 m·s-1) in Shandong from 1700 BT 2 Jun to 0200 BT 3 Jun(a) and from 1800 BT 6 Aug to 0200 BT 7 Aug(b) in 2017
图 2 2017年6月2日济南雷达17:00(a), 17:53(b), 18:39(c), 20:01(d)和8月6日潍坊雷达17:01(e), 18:00(f), 18:58(g), 19:40(h)组合反射率因子
Fig. 2 Composite reflectivity of Jinan Radar at 1700 BT(a), 1753 BT(b), 1839 BT(c), 2001 BT(d) on 2 Jun 2017 and that of Weifang Radar at 1701 BT(e), 1800 BT(f), 1858 BT(g), 1940 BT(h) on 6 Aug 2017
图 3 6·2超级单体风暴(a)和8·6强单体风暴(b)结构演变
Fig. 3 Evolution of 6·2 supercell storm(a) and 8·6 strong single storm(b) structures
图 4 2017年6月2日18:45济南雷达2.4°仰角反射率因子(a)和0.5°仰角(b)、6.0°仰角(c)、9.0°仰角(d)径向速度(蓝色虚线、实线、点线分别为45、55 dBZ和60 dBZ等值线)
Fig. 4 Reflectivity at 2.4° elevation(a) and radial velocity at 0.5° elevation(b), 6.0° elevation(c), 9.0° elevation(d) of Jinan Radar at 1845 BT 2 Jun 2017 (the blue dashed line, the blue solid line and the blue dotted line denote 45, 55 dBZ and 60 dBZ isolines of reflectivity, respecitvely)
图 5 2017年8月6日18:58和19:04潍坊雷达1.5°仰角反射率因子和0.5°、6.0°以及14.6°仰角径向速度(蓝色虚线、实线分别为45、55 dBZ等值线) (a)18:58 1.5°仰角反射率因子，(b)19:04 1.5°仰角反射率因子, (c)18:58 0.5°仰角径向速度，(d)19:04 0.5°仰角径向速度，(e)18:58 6.0°仰角径向速度, (f)19:04 6.0°仰角径向速度，(g)18:58 14.6°仰角径向速度，(h)19:04 14.6°仰角径向速度
Fig. 5 Reflectivity factor at 1.5° elevation and radial velocity at 0.5°, 6.0°, 14.6°elevation of Weifang Radar at 1858 BT and 1904 BT on 6 Aug 2017 (the blue dashed line and the blue solid line denote 45 dBZ and 55 dBZ isolines of reflectivity factor, respecitvely) (a)1.5° elevation reflectivity factor at 1858 BT, (b)1.5° elevation reflectivity factor at 1904 BT, (c)0.5° elevation radial velocity at 1858 BT,(d)0.5° elevation radial velocity at 1904 BT,(e)6.0° elevation radial velocity at 1858 BT,(f)6.0° elevation radial velocity at 1904 BT, (g)14.6° elevation radial velocity at 1858 BT,(h)14.6° elevation radial velocity at 1904 BT
图 6 2017年6月2日济南雷达反射率因子和径向速度沿236.6°和2017年8月6日潍坊雷达反射率因子和径向速度沿24°的垂直剖面(黑色、红色和蓝色水平实线分别为0 ℃层，-10 ℃层和-20 ℃层高度) (a)6月2日18:33济南雷达反射率因子垂直剖面，(b)6月2日18:39济南雷达反射率因子垂直剖面, (c)6月2日18:45济南雷达反射率因子垂直剖面，(d)6月2日18:33济南雷达径向速度垂直剖面, (e)6月2日18:39济南雷达径向速度垂直剖面，(f)6月2日18:45济南雷达径向速度垂直剖面, (g)8月6日18:53潍坊雷达反射率因子垂直剖面，(h)8月6日18:58潍坊雷达反射率因子垂直剖面, (i)8月6日19:04潍坊雷达反射率因子垂直剖面，(j)8月6日18:53潍坊雷达径向速度垂直剖面, (k)8月6日18:58潍坊雷达径向速度垂直剖面，(l)8月6日19:04潍坊雷达径向速度垂直剖面
Fig. 6 Cross-sections of horizontal reflectivity factor and radial velocity along 236.6° radial direction of Jinan Radar on 2 Jun 2017 and along 24° radial direction of Weifang Radar on 6 Aug 2017 (black, red and blue horizontal solid lines denote heights of the 0 ℃ layer, -10 ℃ layer and -20 ℃ layer, respectively) (a)reflectivity factor cross-section of Jinan Radar at 1833 BT 2 Jun,(b)reflectivity factor cross-section of Jinan Radar at 1839 BT 2 Jun,(c)reflectivity factor cross-section of Jinan Radar at 1845 BT 2 Jun,(d)radial velocity cross-sections of Jinan Radar at 1833 BT 2 Jun, (e)radial velocity cross-section of Jinan Radar at 1839 BT 2 Jun,(f)radial velocity cross-section of Jinan Radar at 1845 BT 2 Jun, (g)reflectivity factor cross-section of Weifang Radar at 1853 BT 6 Aug,(h)reflectivity factor cross-section of Weifang Radar at 1858 BT 6 Aug, (i)reflectivity factor cross-section of Weifang Radar at 1904 BT 6 Aug,(j)radial velocity cross-section of Weifang Radar at 1853 BT 6 Aug, (k)radial velocity cross-section of Weifang Radar at 1958 BT 6 Aug,(l)radial velocity cross-section of Weifang Radar at 1904 BT 6 Aug
图 7 2017年6月2日18:30(a)、18:40(b)、18:45(c)和8月6日18:50(d)、19:00(e)、19:05(f)气温(等值线，单位：℃) 及其临近时次济南(18:28、18:39、18:45) 和潍坊(18:47、18:58、19:04) 雷达组合反射率因子和风暴中心位置分布(绿色实心圆分别代表 6·2超级单体和8·6强单体风暴路径位置)
Fig. 7 Spatial distribution of surface temperature (the isoline, unit:℃) at 1830 BT(a), 1840BT(b), 1845 BT(c) on 2 Jun 2017 and 1850 BT(d), 1900 BT(e),1905 BT(f) on 6 Aug 2017 and composite reflectivity factor and storm locations of Jinan Radar (1828 BT, 1839 BT, 1845 BT) and Weifang Radar (1847 BT, 1858 BT, 1904 BT) (green solid circles denote locations of 6·2 supercell storm and 8·6 strong storm paths, respectively)
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