Identifying time-clustering structures in lightning sequences

Two years of lightning data, measured during 2002 and 2003 in an area of southern Italy, have been analyzed in order to reveal scaling behaviour in their time dynamics. We used the Allan Factor method to evidence the presence of time-clustering in the lightning data. We found that i) the sequence of lightning time-occurrences is characterized by two scaling regions, which reveal intracluster (inside an individual thunderstorm) and inter-cluster (among successive thunderstorms) time-clustering properties of lightning data. A crossover timescale is identified, which is connected with the mean duration of a thunderstorm. We investigated intra- and inter-cluster time-dynamics for the whole, sequence as well as the sequence of only positive and only negative flashes. The negative lightning show a time-clustering feature stronger than that of the positive lightning.