Coordinated 3-D Lightning Mapping, Balloon-borne Electric Field, and Polarimetric Radar Observations of Triggered Lightning Flashes at Camp Blanding, Florida

During July – August in 2013 and 2014, coordinated observations were made with a balloon-borne electric field meter, a balloon-borne particle imager, a mobile 5-cm wavelength polarimetric radar, a small-baseline Lightning Mapping Array (LMA), and the extensive observing facilities for triggered lightning at the International Center for Research and Testing (ICLRT) at Camp Blanding, Florida. This experiment was the first to provide vertical profiles of the electric field relative to radar data in Florida storms. Furthermore, mapped three-dimensional lightning structure, surface measurements of lightning current, and multiple-station electric fields and electric field derivatives were provided by the ICLRT for triggered flashes.

For example, on 1 August, strong convection reaching up to 14 km MSL occurred over the ICLRT. Radar data showed that the storm was weakening as the balloon carrying an electric field meter was launched at 1915 UTC. Four attempts were made to trigger lightning in this storm just before and during the balloon flight. A natural flash apparently interfered with the first trigger attempt, made approximately 1 minute before the balloon was launched. The last three attempts were successful, producing lightning channels that propagated horizontally just above the melting level of the storm. The second attempt produced a relatively long (571 ms) initial stage (IS) discharge followed by a single leader/return stroke and 6 ms of continuing current. The third attempt produced a 532 ms IS discharge followed by 5 leader/return strokes. Continuing current ranging from 10 ms to >40 ms followed return strokes 1, 4 and 5. The fourth trigger attempt, which occurred when the electric field meter was near the melting level, produced a relatively short IS (205 ms) and no subsequent return strokes. Just above the melting level, the vertical component of the in situ electric field had a large negative gradient with height spanning the mapped horizontal channel structure of the lightning triggered by the second and third attempts. The 1-dimensional approximation of Gauss's Law, appropriate for the relatively stratified structure of the weakening storm, indicates that the discharge propagated horizontally through a layer of negative charge.

Electric field meters also were launched into two other weakening storms in north Florida on different days in 2013. In all three observed cases, the vertical structure of the inferred charge consisted of a vertical stack of alternating charge polarities, with the lowest region of large charge density (│ρ│≥0.5 nC m-3) being a negative layer near or just above the melting layer. A fourth electric field meter launched near the deep convection of a mature storm indicated a more complex vertical distribution of charge, and the polarity of the charge region just above the melting level was positive.