Intraregional Comparisons of the Near-Storm Environments of Storms Dominated by Frequent Positive Versus Negative Cloud-to-Ground Flashes

We gridded 11 years of cloud-to-ground (CG) flashes detected by the U.S. National Lightning Detection Network during the warm season in 15 km × 15 km × 15 min grid cells to identify storms with substantial CG flash rates clearly dominated by flashes lowering one polarity of charge to the ground or the other (+CG flashes vs. −CG flashes). Previous studies in the central United States had found that the gross charge distribution of storms dominated by +CG flashes included a large upper negative charge over a large middle level positive charge, a reversal of the usual polarities. In each of seven regions spanning the contiguous United States (CONUS), we compared 17 environmental parameters of storms dominated by +CG flashes with those of storms dominated by –CG flashes. These parameters were chosen based on their expected roles in modulating supercooled liquid water content (SLWC) in the updraft because laboratory experiments have shown that SLWC affects the polarity of charge exchanged during rebounding collisions between riming graupel and small ice particles in the mixed phase region. This, in turn, would affect the vertical polarity of a storm's charge distribution and the dominant polarity of CG flashes. Our results suggest that the combination of parameters conducive to dominant +CG flash activity and, by inference, to anomalous storm charge structure varies widely from region to region, the lack of a favorable value of any particular parameter in a given region being offset by favorable values of one or more other parameters.