Positive Leaders Propagate Slower at Higher Altitudes: Experimental Evidence and Theoretical Explanation

Abstract

Leader propagation velocity is one of the most important properties to characterize both intracloud (IC), cloud-toground (CG) and artificially triggered lightning discharges. Increasing observation results implicate that the lightning leader speed may decrease with the increase of altitude of leader propagation. The positive leader in rocket triggered lightning observed in New Mexico with an altitude of 3,200 m developed more slowly than those in low altitude regions (Edens et al., 2012). The vertical velocity of initial negative leaders in the preliminary breakdown of IC flashes shows a decreasing trend with the increase of initiation altitude (Wu et al., 2015). Recently, similar trend is also found in the propagation of positive leader in IC flashes, where the velocity of positive leader decreases with the increase of initiation altitude (Wu et al., 2019). Nevertheless, the effect of reduced air density on lightning leader velocity is difficult to recognize in numerous observations of CG lightning flashes, because the lightning leader velocity can be affected by multiple factors, such as the ambient electric field, injected current, humidity, etc. For instance, although a clear increasing trend in speed was observed for positive leaders as they approach to ground in +CG flashes (Kong et al., 2008;Saba et al., 2008;Wang & Takagi, 2011), the increase of the electric field between the downward leader and the ground may be more crucial to the acceleration of lightning leaders. Due to difficulties in decoupling of the influence of multiple factors on the leader velocity in field observations, the effect of reduced air density at high altitude on the lightning leader velocity has not been fully understood yet.During the past two decades, numerical simulation was developed to investigate the formation of leader discharge at reduced air density conditions (

Observation of a narrow thermal channel connected to the anode during dark periods in long positive sparks

“…Therefore, the relatively high current density and weak thermal convection may sustain the weak ionization and temperature of the narrow channel. 32) Based on the above analysis, it is rationally inferred that the narrow channel corresponds to the weak luminosity with ionization activity on optical images. The residual channel with high temperature and low conductivity, and the narrow channel with lower temperature but intense ionization activities, form the evolution of the discharge channel during dark periods, as shown in Fig.…”

Temperature measurements of long sparks in air using time-resolved moiré deflectometry

“…Both c p and k T are functions of the gas temperature. When an initial Dirac temperature distribution is set in equation (11), the heat spreads out from the origin in a Gaussian profile. The radial temperature profile can be expressed as…”

“…For instance, the magnitude and distribution of gas temperature determines geometric size of the spark discharge heating area, which directly affects the ignition stability of the combustible mixture [10]. Meanwhile, the formation of thermal sparks is a fundamental concern of air insulation breakdown and lightning physics [11][12][13]. In these cases, the long spark is at non-local thermodynamic equilibrium (non-LTE).…”