Estimates of recoil leader peak currents based on high-speed video observations

During the summer season of 2012, a network of high-speed cameras, called RAMMER network, recorded two bipolar flashes produced by a thunderstorm that occurred on March 13. Data from BrasilDAT network provided location and peak current information for the events. Additional RAW data from the sensors, allowed more detailed analyses. Both bipolar flashes presented multiple negative return strokes, and there was intense recoil leader activity bellow cloud base. The objective of this work is the evaluation of the peak currents of those recoil leaders. The bipolar nature of these flashes allowed a unique way to calibrate the camera luminosity with the stroke peak currents estimated by the local lightning location system (LLS). Recoil leaders are commonly visible below cloud base during positive flashes development, but have not yet been observed during negative flashes in a similar fashion. The bipolar flashes observed had one positive return stroke and multiple negative return strokes that were used in the calibration process. In order to evaluate the recoil leaders peak current, the luminosity levels of each video were calibrated using the unsaturated pixel brightness levels during the negative return strokes and the data from the BrasilDAT LLS. The same high-speed camera used during the observation campaign was tested in laboratory to determine its CMOS sensor response curve. Through a scatter diagram of luminosity (estimated from the pixel intensity values) versus estimated peak current (provided by the BrasilDAT network), it was shown that both physical quantities are related. The calibrated pixel values were used to create equations to calculate the recoil leader peak currents. A total of 281 recoil leaders were identified, 191 in flash #1 and 90 in flash #2. Results show that recoil leader estimates are closely related to previous observations of cloud discharges by LLS. The geometric mean value was 1.06 kA, with a maximum of 7.7 kA. Less than 14% presented peak currents comparable to weak return strokes (> 2 kA). Keywords—recoil leaders; peak current estimates; high-speed videos; lightning location system; luminosity