Surface Potential Gradients and NEXRAD Radar Reflectivities Before the Onset of Lightning at the KSC-ER

Measurements provided by Next Generation Weather Radar and operational thunderstorm monitoring instruments at the Kennedy Space Center and the Air Force Eastern Range have been examined to determine the initial electrical development of 13 isolated, air mass thunderstorms. The same instruments were used to examine the surface potential gradient prior to and following 13 long, horizontal discharges that propagated into the area. The motivation and primary objective for this work was to evaluate the safety of the existing lightning-related launch constraints associated with surface potential gradient and precipitation radar measurements. The onset of cloud electrification as seen by a large-area surface electric field mill network was detected 3.7–14.6 min before the first lightning discharge, with lead-times that depended on proximity to the storm. In 11 of 13 cases, the first detectable field change was a positive excursion in potential gradient close to the storm, likely indicating initial development of lower positive charge. Lead-times for the radar-derived cloud tops (0 dBZ) reaching −20°C were longer than those for early electrification in all but two cases. Surface potential gradients above +500 V/m or below −100 V/m “warning thresholds” were exceeded before the first lightning flash in all cases. Radar reflectivities >35 dBZ above the −10°C level provided 3–14 min of lead time for lightning. Potential gradients just prior to and near long horizontal discharges exceeded 3 kV/m at most sites and were typically positive. Measurements at a single field mill site would not have provided adequate warning.