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

The overwhelming majority of severe storms throughout the contiguous United States 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 CG lightning behavior.  This repeated observation led to the hypothesis that the local mesoscale environment indirectly influences CG lightning polarity by directly controlling storm structure, dynamics, and microphysics, which in turn control storm electrification.  According to the 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, and increased frequency of positive CG lightning.  A handful of studies have explored the relationship between the local mesoscale environment and the CG lightning behavior of severe storms.  Since it is difficult to obtain representative soundings, further study is warranted.

 

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.  We identified one non-severe negative (23 May), four severe negative (24 May; 4, 12, 15 June), and four severe positive (23, 24 May; 15, 19 June) storm systems on six different days during IHOP.   From hundreds of IHOP soundings, we carefully selected roughly fifty inflow proximity soundings that best represented the mesoscale environment of the nine storm systems.  Consistent with past results, deep layer (0-6 km) shear had little control on CG lightning polarity.  In the figure below, we show the relationship between the median CAPE (Convective Available Potential Energy), LCL (Lifting Condensation Level), low-level (0-3 km) shear, and dominant CG lightning polarity.  At odds with past studies, we found little difference in CAPE between positive and negative severe storms.  The size of the circle and the interior value below indicate the strength of the 0-3 km shear (bigger circle=bigger low-level shear).  Consistent with past work, positive storms had noticeably higher mean low-level shear than negative storms (statistically significant to the 1% level).   Interestingly, the average LCL for positive storms (2079 m) was 1.9 times higher than for negative storms (1121 m) (statistically significant to the 0.1% level).  Our results appear to support the suggestion in Williams and Stanfill (2002, C. R. Physique, 3, 1277-1292) that cloud base height must be considered when evaluating the effect of CAPE on updraft intensity, cloud electrification, and lightning behavior.  Further analysis and interpretation of sounding data along with implications of these preliminary results will be presented.