A Portable, PC-Based System for Making Optical and Electromagnetic Measurements of Lightning

Abstract

A digital video camcorder, a four-channel analog-to-digital (A/D) converter, and a GPS time/position receiver have been interfaced to a portable personal computer (PC) to provide a mobile data-collection platform for making precise time-synchronized optical and electromagnetic measurements of lightning with correlated video imagery. Optical waveforms and the associated electric field signatures are digitized with 14-bit resolution at a sampling frequency of up to 2.5 MHz, and entire flashes can be captured and stored with less than 280 ms of dead time between events. Digital video (DV) data can be recorded either on Mini-DV tape within the camera or directly in the PC using an IEEE 1394 ?Firewire? serial data link. The video format has a standard 30 interlaced frames per second (33.3 ms per frame) and 720 ? 480 pixels per frame, and in postprocessing the frames are deinterlaced to double the time resolution (to 16.7 ms). The system was tested on natural lightning during the summer of 2001 near Tucson, Arizona. Video images are shown of cloud-to-ground (CG) flashes, attempted leaders, and vertical and horizontal air discharges, and several examples of correlated optical and electric field waveforms are given. An analysis of 249 CG flashes shows an average of 1.46 strike points per CG flash, and shows that flashes with a visible attempted leader are more likely (by a factor of 2) to produce multiple strike points and less likely (by 1/3) to have a long continuing luminosity, but the statistics are limited. A subset of these data shows that the flash and stroke detection efficiencies (DE) of the U.S. National Lightning Detection Network (NLDN) near Tucson are about 61% and 36%, respectively, with a 24-ms coincidence criterion, and 71% and 41%, respectively, with a 33-ms criterion. The latter DE values are similar to DEs measured near Albany, New York, and agree with model predictions.