The Relationship between Severe Storm Reports and Cloud-to-Ground Lightning Polarity in the Contiguous United States from 1989 to 1998

Abstract

The majority (61%) of severe storm reports (i.e., large hail and tornado) during the 1989?98 warm seasons (April?September) were associated with predominantly (>90%) negative cloud-to-ground (PNCG) lightning. Across the contiguous United States, only 15% of severe storm reports were characterized by predominantly (>50%) positive CG (PPCG) lightning activity. However, significant regional variability occurred in the relationship between warm season severe storm reports and CG lightning polarity. In the eastern United States, a significant fraction (81%) of severe storm reports occurred nearby PNCG lightning while only 2% of severe storms were associated with PPCG lightning. The CG lightning behavior was quite different over the northern plains; only 28% of severe storm reports were linked with PNCG lightning while 43% were characterized by PPCG lightning. Although the direct physical relationship is still not evident, this regional variability appears to be at least partially explained by differences in the meteorological environment of severe storms producing PPCG and PNCG lightning. The locations of the monthly frequency maxima of severe storms that produced PPCG and PNCG lightning were systematically offset with respect to the climatological monthly position of the surface ?e ridge on severe outbreak days. Severe storms that produced PPCG lightning generally occurred west and northwest of the ?e ridge in the upstream ?e gradient region. Severe storms generating PNCG lightning were located southeast of the PPCG lightning maxima, closer to the axis of the ?e ridge in higher mean values of ?e.