Spatiotemporal Analysis of Total Lightning Data for Assessing their Utility in Warning of Pulse-Type Thunderstorm Severity
A subset of total lightning data from the Earth Networks Total Lightning Network were analyzed for the daily timeframe of 2-9 pm from May 1 through August 31, 2012 within a spatial domain that approximates the County Warning Area of the Blacksburg, Virginia National Weather Service Forecast Office (NWSFO) (75,630 km2). The two-dimensional (x-y) location and the time of occurrence of each lightning strike were used to identify clusters of strikes in space and time using a single linkage clustering technique, with each cluster representing a discrete thunderstorm cell or multi-cell complex. Each lightning-defined thunderstorm was characterized in terms of its duration, areal extent, rate of motion, and two-dimensional geometric shape, with the idea that pulse-type thunderstorms are relatively short-lived, of small spatial extent, slow-moving, and non-linear. Numerical values for these variables were used as inputs to create a "Pulse Storm Index" (PSI). The PSI was generated using an analytic hierarchical process approach that was informed through a survey of forecaster perspectives regarding the relative importance of the various characteristics of pulse storms when operationally evaluating convective mode. The PSI was then used to identify the thunderstorm days during the 2012 study period that were most typified by individual storms that appeared and behaved like pulse storms. Atmospheric sounding data were then used to cross-validate these days, focusing on the strength of low-level wind shear as an additional indicator of a pulse storm environment. The time series of total lightning associated with each individual thunderstorm identified as a likely pulse-type storm was analyzed using an established “lightning-jump” algorithm. Lightning jumps, or rapid increases in flash rates, were analyzed as indicators of subsequent storm severity, as evidenced by local storm reports, to assess the viability of using the tool to help forecast severe pulse-type thunderstorms specifically.