Properties of Negative Initial Leaders and Lightning Flash Size in a Cluster of Supercells

Abstract

Properties of negative initial leaders (NILs) and flash size in a cluster of supercells with generally inverted charge structure in Oklahoma on 10–11 May 2010 are examined, primarily using Lightning Mapping Array data. A method to identify NILs from Lightning Mapping Array source is proposed and helps to reveal the multiple NILs properties and their distributions. The NILs in the supercell cluster have smaller speed (median 3‐D displacement speed: 0.65 × 105 m/s), relative to the previous reports in normal thunderstorms. Furthermore, median NIL speeds initially decrease with increasing height but begin increasing above 12 km. The NILs tend to decelerate during the early stage. The parameters characterizing flash duration and spatial size are also investigated. It is found that they all follow lognormal distributions and the spatial flash size is relatively small on average (median horizontal distance: 5.54 km). Most flashes (83.18%) extend primarily in the horizontal direction. Flash area shows an inverse relationship with flash density at their fast changes during storm evolution. Although large flash initiation density (FID) generally occurs in regions with small flash size, the smallest flash size is nearly not collocated with large FID value. In the regions with large FID, average flash duration roughly increases with increasing FID, while average flash area changes little. We proposed that the pattern of charge pockets and variation of charge density dominated by the strong kinematics are responsible for some new findings about the properties of NIL and flash size in the supercell cluster.

Initial Leader Properties During the Preliminary Breakdown Processes of Lightning Flashes and Their Associations With Initiation Positions

“…It should be noted that the parameters including vertical distance, 3‐D distance, vertical speed, 3‐D speed, vertical step length, and 3‐D step length are all associated with the displacement but the length of an IL; that is, the tortuousness of an IL is ignored, which follows the previous studies (Bitzer et al, ; Shi et al, ; Wu et al, ; Zheng et al, ).…”

“…The negative relationships of the speed and pulse rate of ILs with flash initiation altitude were also reported by Wu et al (). Zheng et al () further noted that the speeds of ILs changed their decreasing trend at approximately 12 km in both their investigated Oklahoma supercell cluster and a New Mexico supercell. They speculated that it might be associated with the common effects of the ambient electric field and air density on the IL speed.…”

“…In contrast to most of the previous studies that reported the median or mean values of IL durations to be typically only several milliseconds (Marshall et al, ; Nag & Rakov, ; Stolzenburg et al, ; Wilkes et al, ; Zhang et al, ; Zhu et al, ), the median or mean IL durations in this study seemed to be larger. However, there were indeed some other studies, especially the studies associated with the ILs of IC flashes, that also documented durations of IL mainly larger than 10 ms (Liu & Krehbiel, ; Shao & Krehbiel, ; Wilkes et al, ; Zheng et al, ). We will provide further discussion in section .…”

“…In some flash cases, speeds on the order of 10 6 m/s were also obtained (Tran & Rakov, ; Wilkes et al, ). On the other hand, Zheng et al () documented the lower average speed of the flashes in a supercell cluster, with a median (mean) 3‐D speed of 0.65 (0.84) × 10 5 m/s. They suggested that IL speed might be impacted by the charge distribution patterns of supercells, which we will further discuss in section .…”

“… Duration : The difference in the time between the last and the first LPDEs of the IL. Vertical distance : The vertical displacement of the IL, which is equal to the height difference of the first and last LPDEs of the IL. 3‐D distance : The displacement of the IL in space, which is equal to the distance between the first and last LPDEs of the IL. Vertical speed: The average displacement speed of the IL in the vertical direction, which is equal to the quotient of vertical distance divided by duration. 3‐D speed : The average displacement speed of the IL in space, which is equal to the quotient of 3‐D distance divided by duration. In the calculation of the IL speed, we actually neglect the possible variation of IL speed during the PB process, which has been noted by previous studies (Behnke et al, ; Stolzenburg et al, ; Zheng et al, ). Angle from the vertical direction : The angle between the 3‐D displacement direction of the IL and the vertical direction. Pulse rate : Its original definition is the number of PB pulses in unit time according to the electric field change waveform. Here, it is defined as the number of located LPDEs in a unit time.…”