Factors Affecting Lightning Behavior in Various Regions of the United States

March 06, 2014

Brody Fuchs

Committee: Steve Rutledge (advisor), Sue van den Heever, Timothy Lang (Affiliate), Richard Eykholt (Physics)

Abstract

Lightning activity varies greatly on a global scale. Global maps of total flash density show a strong tendency for lightning to favor continental areas over the open ocean, even in regions with similar instability. Previous studies have attributed the difference to thermodynamic and aerosol differences over continental regions, but the exact cause is still elusive. While this is not a global study, we attempt to characterize lightning activity in 4 different regions of the United States with high resolution Lightning Mapping Array (LMA) networks over one warm season. The regions of study are Washington, D.C. (DC), northern Alabama, central Oklahoma and northeast Colorado. A wide spectrum of environmental characteristics is afforded by these regions. Lightning characteristics include storm total flash rates, positive cloud-to-ground (+CG) strikes and intra-cloud (IC) to CG ratio (IC:CG). This is accomplished by using the CSU Lightning, Environmental, Aerosol and Radar (CLEAR) framework, first developed by Lang and Rutledge (2011), to objectively analyze large amounts of storm data. Lightning activity is provided by a new flash clustering algorithm, which produces total flash rates and IC flash rates when combined with NLDN CG data.

The results have shown that lightning behavior has high variability throughout the regions of study. Median total storm flash rates range from approximately 1 flash min^-1 in Alabama and DC to near 8 flashes min^-1 in Colorado. Positive CG flash fractions exhibit a similar relationship with 10% of all CG flashes being positive polarity in Alabama and DC up to 40% in Colorado. The anomalous nature of the Colorado region is evident in all lightning metrics. Colorado is also characterized by an anomalous environment with high cloud base storms and coincident shallow warm cloud depths. Examination of all storms simultaneously has shown that relationships exist between total flash rate and environmental parameters. The similarity of these results to other studies on global scales is striking and provides evidence for the robustness of these relationships. Examination of relationships between radar and lightning intensity metrics are also performed. Similar behaviors between these intensity metrics are observed in all regions.