Evidence for synchronicity of lightning activity in networks of spatially remote thunderstorms

Visual observations by space shuttle astronauts have described a phenomenon in which spatially distant thunderstorm cells seem to reciprocally "ignite" lightning flashes in a semi-cyclic sequence. Lightning occurring in one cell is immediately followed by lightning in other cells, separated by tens or hundreds of kilometers. We present quantitative analysis of lightning observations conducted within the framework of the MEIDEX-sprite campaign on board the space shuttle Columbia in January 2003 (Yair et al., 2003). Video footage of 6 storm systems with varying flash rates, which occurred over Africa, South America, Australia and the Pacific Ocean were analyzed. It is found that when the storm flash rate was high, lightning activity in horizontally remote electrically active cells became clustered, with bursts of nearly simultaneous activity separated by quiet periods. The recurrence time was ~2.5 seconds, close to the previously reported time delay between consecutive ELF transient signals in the Schumann resonance range (Füllekrug, 1995). We propose that this behavior is similar to the collective dynamics of a network of weakly coupled limit-cycle oscillators (Strogatz, 2000). Thunderstorm cells embedded within a mesoscale convective system (MCS) constitute such a network, and their lightning frequency is best described in terms of phase-locking of a globally coupled array (Kourtchatov et al., 1995). Comparison of basic parameters of the lightning networks with predictions of random-graph models reveals that the networks cannot be described by the classical random-graph model (Erdos and Renyi, 1960), but are more compatible with generalized random-graphs with a prescribed degree distribution (Newman et al., 2001) that exhibit a high clustering coefficient and small average path lengths. Such networks are capable of supporting fast response, synchronization and coherent oscillations (Lago-Fernandez et al., 2000). Several physical mechanisms are suggested to explain this phenomenon. 1. Introduction The proximity in time and space of consecutive lightning flashes in thunderstorms was first studied by Mazur (Mazur, 1982) who termed the phenomenon "associated lightning discharges". These flashes were reported to occur in multi-cell storms, where several electrically active cells (EACs) co-exist in the same mesoscale region. VHF radar observations showed that lightning echoes from discharges follow one another within a time interval typical of a multistroke cloud-to-ground (CG) flash, but are spaced in range by several km. Mazur (1982) chose the maximum time interval of 200 ms between single echoes in analogy with that for the between-stroke interval of multistroke flashes, and used statistical methods to disprove the null …