Lightning optical pulse statistics from storm overflights during the Altus Cumulus Electrification Study

The Altus Cumulus Electrification Study (ACES) was conducted during the month of August 2002 in an area near Key West, Florida. One of the goals of this uninhabited aerial vehicle (UAV) study was to collect time resolved optical pulse data from thunderstorms. During the month long campaign, we acquired 5256 lightning generated optical pulses. Most of these observations were made while quite close to the top of the storms. We divided our data into two amplitude groups based on prior NASA U2 aircraft optical data and the pulse characteristics. The group of strong pulses with radiance greater than 2.1 mW m(-2) sr(-1) had mean and median 10-10% optical pulse widths of 770 and 740 mu s, respectively, 50-50% pulse widths of 399 and 355 mu s, respectively, and 10-90% risetimes of 292 and 260 mu s. These values are very similar to the previous U2 based optical results The other group of pulses consisting of slightly more than a quarter of the total pulses observed had radiances less than the minimum values detected in the U2 study. These weak pulses were narrower than the strong pulses with 50-50% mean and median values of 199 and 160 lis, respectively. Only 12% of the flashes observed contained only weak pulses. The source of these weak pulses is unknown, but we suspect that some are artifacts of the close proximity of the aircraft to cloud top. (c) 2005 Elsevier B.V. All rights reserved.