Environmental control of cloud-to-ground lightning polarity in severe storms

The overwhelming majority of severe storms throughout the contiguous U.S. generate primarily (>75%) negative ground flashes (so-called negative storms). However, a certain subset of severe storms produces an anomalously high (> 25%) percentage of positive ground flashes (so-called positive storms). The frequency of these "anomalous" positive storms varies regionally and seasonally. In some regions (e.g., central and northern plains) and months, these positive storms are common, representing 30% or more of all severe storms. Several past studies have noted that severe storms passing through similar mesoscale regions on a given day exhibit similar cloud-to-ground (CG) lightning behavior. This repeated observation led to the idea that the local mesoscale environment indirectly influences CG lightning polarity by directly controlling storm structure, dynamics, and microphysics, which in turn control storm electrification. Although a few studies have explored this relationship, the exact conditions favoring positive storms are poorly understood. The purpose of this study is to conduct a systematic comparison of the mesoscale environments for positive and negative storms, set in the framework of a testable hypothesis. According to our hypothesis, intense updrafts and associated high liquid water contents in positive storms lead to positive charging of graupel and hail via the non-inductive charging mechanism, an enhanced lower positive charge (or inverted-polarity), and increased frequency of positive CG lightning. We have utilized abundant environmental soundings taken during the International H₂O project (IHOP, May-June 2002) to document the relationship between mesoscale environment and dominant CG lightning polarity in the central plains. From hundreds of IHOP soundings, we carefully selected roughly fifty inflow proximity soundings that best represented the mesoscale environment of five (four) negative (positive) storm systems. Mean convective available potential energy (CAPE) estimated in the electrically important mixed phase zone (-10 to -40 degrees Celsius) was significantly higher in positive storms (1210 J kg^-1) than in negative storms (957 J kg^-1). Positive storms (14.7 m s^-1) had noticeably higher mean low-level (0-3 km) shear than negative storms (10.7 m s^-1). Interestingly, the mean lifting condensation level (LCL) for positive storms (2079 m) was 1.9 times higher than for negative storms (1121 m). The environmental freezing level (FL) was also lower in positive storms (3777 m) than in negative storms (4070 m). As a result, the mean warm cloud depth (FL-LCL) was dramatically larger in negative storms (2949 m) than in positive storms (1699 m). According to parcel theory, higher mixed phase CAPE directly leads to stronger updrafts and higher liquid water contents in positive storms. Larger low-level shear in positive storms aids in the development of intense low-to-mid level updrafts and enhanced liquid water contents through dynamic forcing. Higher LCL or cloud base height, which is associated with increased parcel size and decreased entrainment of dry air, in positive storms results in more efficient conversion of CAPE into kinetic energy and hence enhanced updraft strength and liquid water content. Reduced warm cloud depth in positive storms may decrease the amount of liquid water that is lost through the collision-coalescence and rainout process in a rising air parcel below the mixed phase zone, effectively increasing the amount of supercooled cloud water that is available for cloud electrification.