PARAMETERS OF TRIGGERED-LIGHTNING FLASHES IN FLORIDA AND ALABAMA

Channel base currents from triggered lightning were measured at the NA SA Kennedy Space Center, Florida, during summer 1990 and at Fort McCle llan, Alabama, during summer 1991. Additionally, 16-mm cinematic recor ds with 3- or 5-ms resolution were obtained for all flashes, and strea k camera records were obtained for three of the Florida flashes. The 1 7 flashes analyzed here contained 69 strokes, all lowering negative ch arge from cloud to ground. Statistics on interstroke interval, no-curr ent interstroke interval, total stroke duration, total stroke charge, total stroke action integral (integral i2dt), return stroke current wa ve front characteristics, time to half peak value, and return stroke p eak current are presented. Return stroke current pulses, characterized by rise times of the order of a few microseconds or less and peak val ues in the range of 4 to 38 kA, were found not to occur until after an y preceding current at the bottom of the lightning channel fell below the noise level of less than 2 A. Current pulses associated with M com ponents, characterized by slower rise times (typically tens to hundred s of microseconds) and peak values generally smaller than those of the return stroke pulses, occurred during established channel current flo w of some tens to some hundreds of amperes. A relatively strong positi ve correlation was found between return stroke current average rate of rise and current peak. There was essentially no correlation between r eturn stroke current peak and 10-90% rise time or between return strok e peak and the width of the current waveform at half of its peak value . Parameters of the lightning flashes triggered in Florida and Alabama are similar to each other but are different from those of triggered l ightning recorded in New Mexico during the 1981 Thunderstorm Research International Program. Continuing currents that follow return stroke c urrent peaks and last for more than 10 ms exhibit a variety of wave sh apes that we have subdivided into four categories. All such continuing currents appear to start with a current pulse presumably associated w ith an M component. A brief summary of lightning parameters important for lightning protection, in a form convenient for practical use, is p resented in an appendix.