Lightning flash counts
Estimating trends and seasonality in Australian monthly lightning flash counts
We present the results of a statistical analysis of lightning characteristics in mainland Australia for the period from approximately 1988 to 2012, based on monthly lightning flash count (LFC) series obtained from a network of 19 CIGRE-500 sensors. The temporal structures of the series are examined in terms of detecting and characterizing seasonal cycles, long-term trends and changes in seasonality over time. A generalized additive modeling approach is used to ensure that the estimated structures are determined by the data, rather than by the constraints of any assumed mathematical form for the trends and seasonal cycle. Results indicate strong seasonality at all sites, the presence of long-term trends at 16 sites, and interactions between trend and seasonality (corresponding to changes in seasonality over time) at 13 sites. The most systematic change corresponds to a progressive deepening of the seasonal cycle (i.e. an ongoing decline in winter lightning flash counts), and is most noticeable across southern Australia (south of 30° S). These results are consistent with previous analyses that have detected decreasing atmospheric instability during the austral winter since the mid 1970s. This is associated with increasing mean sea level pressure and declining rainfall.