Locations of positive and negative cloud-to-ground lightning flashes can be distinctly regionalized within some mesoscale storm systems. This phenomenon, observed as a bipolar pattern, was documented and analyzed for the year 1988 using archived lightning data of the Eastern United States. The feature was found in all seasons and in all geographical areas studied, although it is more common in summer than in winter and has somewhat different characteristics. The region of negative flashes was determined to be between south and west of the positive flash region in nearly all of the 91 bipolar cases found. The ratio of positive to negative flash density for the pattern ranged from 0.025 to 0.30, and the feature is most common in the middle half of the storm's lightning activity.

Detailed analysis using synoptic meteorological data has shown that cloud top height and radar echo intensity are generally lower over the positive flash region. It is hypothesized that the source charge of the positive flashes is generated above the main convective area and the area of negative lightning. It appears that positive charge could be advected downshear by the upper-level winds to the location of the positive lightning in 60 to 75 minutes. This result agrees with many of the characteristics of the bipolar pattern, within current understanding of the electrical structure of thunderstorms, and with recent studies of anvils and stratiform clouds. Evaluation of satellite imagery and more precise radar data should reveal whether this is a valid explanation.

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*First-place winner of 1990 Father James B. Macelwane Annual Award