Investigation into the high percentage of positive CG lightning along the west coast of the United States

Abstract

For more than a decade, the U.S. National Lightning Detection Network (NLDN) has been recording cloud-to-ground (CG) lightning across the contiguous United States. In recent years, the focus of NLDN data analysis has been in the area of lightning climatology (e.g. Orville and Silver 1997). These climatological studies have revealed significant lightning patterns across the United States. One feature that has been observed for many years is a high annual percentage of positive polarity CG lightning along the west coast of the United States. Along the west coast, annual percent positive values average around 40 to 50%, while the average value of the U.S. is only around 10%. Full analyses of the annual CG lightning characteristics were conducted to document the percent positive anomaly. In addition, seasonal and thunderstorm analyses were performed to determine the reasons behind the annual percent positive anomaly. Through the seasonal and monthly storm analysis, it was determined that the high annual percent positive along the coast was due to the low variability in total CG flashes throughout the year and a high number of positive CG flashes during the winter season. These CG lightning characteristics can be directly attributed to the climate of the Pacific Coast. The land elevation was determined to affect the CG lightning characteristics in the region by restricting the inward extent of the coastal climate. The secondary goal of the study was to determine how certain meteorological variables affect the dominant CG lightning polarity in a storm. The three environmental variables investigated included: 1) the tilting of the charge regions by strong windshear, 2) the variation of the charge region heights and their dependence on specific temperatures, and 3) the strength of the convective updraft in the thunderstorms. Analysis showed that the electrically weakest thunderstorms produced the majority of positive CG lightning flashes. These storms are assumed to correspond to convectively weak storms. Statistical analysis showed that the height of the -10⁰C temperature level affects the dominant CG polarity in a storm, while the windshear did not significantly contribute.