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广州地区雷暴过程云-地闪特征及其环境条件
Thunderstorm Cloud-to-ground Lightning Characteristics in the Contiguous Guangzhou City and the Influences of Local Environmental Conditions
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摘要: 应用雷电定位系统和高空观测资料并结合雷达回波资料, 对广州地区雷暴过程云-地闪特征进行分析, 并就有、无云-地闪出现的两组不同对流天气过程的环境条件进行了比较研究。结果表明:广州地区的雷暴过程以负的云-地闪为主, 负云-地闪所占比例在90%以上。云-地闪发生频率与雷暴系统强度演变有直接联系, 对于同一系统来说, 随着系统回波强度的增强, 云-地闪发生的频率也增高。但不同系统中, 云-地闪发生频率有很大不同, 回波强(弱)的对流系统并不意味着云-地闪发生的频率就高(低)。有云-地闪记录的对流天气过程具有更大的垂直切变、更高的相对风暴螺旋度以及更多的对流抑制能量, 云-地闪现象更易于出现在更加有组织和更强的对流系统中。研究还发现广州及周边城市区域对雷暴系统回波强度及云-地闪现象可能有影响, 两个典型个例分析表明, 雷暴系统移经城市区域时回波强度减弱, 云-地闪发生频率减小, 雷暴移过城市区域后, 强度可重新加强, 云-地闪发生频率增大。Abstract: The possibility of a relationship between thunderstorm and cloud-to-ground(CG)lightning activity suggeststhat the real-time lightning data provided by local lightning detectors may be useful in the nowcasting of some severe local thunderstorms.In fact, in recent years, increased interest in severe storms associated with lightninghas been led to by such potential application and the desire to understand the cloud electrification mechanisms responsible for the lightning behavior.In the current study, bsaed on available data sources such as CG flashrecords, radar detection data and conventional sounding data, the CG lightning properties of thunderstorms inthe contiguous Guangzhou city and the influences of local environmental conditions are explored.Based on theweather reports and the lightning activities, 10 lightning storms and 10 non-lightning storms are selected and divided into two groups to identify the CG lightning properties and favorable environmental conditions.In additionto general statistical analysis, sounding data from the nearby Qingyuan station are analyzed to identify systematicdifferences in the local mesoscale environment of lighting and non-lightning storms.And two typical lightningstorms are investigated to study the lightning behavior in severe and non-severe thunderstorms.It shows that over 90% negative polarity cloud-to-ground(CG)lightning is produced by most thunderstorms around Guangzhou city.During a 3-hour period surrounding the most lightning activity and calculatedwithin an analysis range of 150 km, of the ten selected storms with lightning records, the largest storm-averageflash density is 0.13 km-2·h-1, the maximum mean negative current is 28.4 kA, the peak current of the firststroke is 183 kA, and the largest multiplicity(the number of strokes per flash)is 14.CG flash rates are foundto be associated with storm intensity.Within the same system, when the storm develops stronger, it tends toproduce higher CG flash rates.When compared among different storms, however, the CG flash rates are quitedifferent.Stronger(weaker)storm does not necessarily mean that it produces higher(lower)CG flash rates.Bystudy of the differences of local environmental conditions between the two groups of selected lightning and nonlightning storms, at least, it can be inferred that storms with CG flash phenomena are supported in environmentwith stronger vertical wind shear, higher storm-relative helicity and convective inhibition(CIN). CG flash isproved more likelihood to occur in stronger convective systems with more organized features.Analysis of typicalcases reveals that urban environment may have possible impact on the evolution of thunderstorms and subsequentlightning activity.It is found that when moving thunderstorm approaches the urban areas of Guangzhou, its intensity and CG flash rates tend to decrease, and after it travels through the city, thunderstorm intensity andflash rates increase again.Testing with more observational case studies is needed for this primary result, andmore studies are necessary before general assumptions can be made regarding the typical thunderstorm lightningbehavior in this local area.
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图 1 云-地闪活动(各时次每10 min发生频数最大的云-地闪分布)及广州雷达回波特征(a)2005年3月22日10:00,(b)2005年3月22日11:00,(c)2005年3月22日12:00, (d)2004年6月20日19:00,(e)2004年6月20日20:00,(f)2004年6月20日21:00
(图中阴影区为雷达回波大于25 dBz的回波区; “ +”为正云-地闪记录, “-”为负云-地闪记录)
Fig. 1 Cloud-to-ground(CG)lightning activity(the hourly maximum 10-min CG flash rate)and the associated echo intensity as depicted by Guangzhou radar(a)10:00 on March 22, 2005,(b)11:00 on March 22, 2005,(c)12:00 on March 22, 2005, (d)19:00 on June 20, 2004,(e)20:00 on June 20, 2004,(f)21:00 on June 20, 2004
(shaded areas denote the echo intensity more than 25 dBz; a plus sign(+)and minus sign(-)indicate the position of a positive and negative CG flash)
图 2 雷暴系统随时间演变的示意图(a)2005年3月22日10:00—13:00,(b)2004年6月20日19:00—22:00
(图中阴影区为每间隔20 min强度大于35 dBz雷达回波区的移动情况, 灰度等级表示不同时间; 圆圈为对回波强度和云-地闪发生记录进行统计的范围)
Fig. 2 Sketch map for thunderstorm evolution (a)10:00—13:00 on March 22, 2005,(b)19:00—22:00 on June 20, 2004
(Radar reflectivity(>35 dBz)echo are showed every 20 minutes with shaded areas; analysis rings to calculate the echo intensity and CG flash records are also showed)
图 3 雷暴过程中每10 min平均的云-地闪发生频率及系统强度随时间的演变(a)2005年3月22日10:00—13:00,(b)2004年6月20日19:00—22:00
Fig. 3 Time series of the 10-min average CG lightning flash rate and system intensity for lightning thunderstorm cases (a)10:00—13:00 on March 22, 2005,(b)19:00—22:00 on June 20, 2004
表 1 2003—2005年3—6月广州地区部分雷暴过程及其云-地闪特征
Table 1 Characteristics of cloud-to-ground(CG)lightning in several thunderstorms over Guangzhou areas from March to June during 2003—2005
表 2 有、无云-地闪记录对流过程环境条件对比
Table 2 Comparison of environmental properties for lightning storms and non-lightning storms
表 3 不同强度有云-地闪记录雷暴过程环境条件对比
Table 3 Comparison of environmental properties for severe and non-severe lightning storms
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