High percentage of positive lightning along the USA west coast



[1] Analyses of the cloud-to-ground (CG) lightning characteristics recorded by the US National Lightning Detection Network (NLDN) along the west coast reveal an average annual percentage of positive CG flashes around 40%, while the average value for the USA is approximately 10%. The primary goal of the study was to document and suggest reasons for this positive CG anomaly. Through seasonal and monthly storm analysis, it was determined that the high annual percent positive along the coast is the result of the low variability in total CG flashes throughout the year coupled with a high number of positive CG flashes during the winter season which can be directly attributed to the climate of the Pacific Coast. The topography of the region was determined to affect the areal distribution of the percent positive anomaly by restricting the inward extent of the coastal climate. The secondary goal of the study was to determine whether the following meteorological variables were related to the dominant CG polarity in a storm: 1) the variation of charge region heights using the −10°C level as a proxy and 2) the tilting of the charge regions by strong windshear. Statistical analysis showed that the height of the −10°C temperature level is related to the dominant CG polarity in a storm, while the windshear did not show a significant relationship. In addition, analyses showed that the thunderstorms that produced few CG flashes (<6 flashes) contributed most to the total number of positive CG flashes.