Investigating the Potential of Using Radar Echo Reflectivity to Nowcast Cloud-to-Ground Lightning Initiation over Southern Ontario

Abstract

The potential for using radar echo reflectivity to forecast cloud-to-ground (CG) lightning initiation in the 0?1-h time frame was investigated in southern Ontario, Canada. The main purpose of this investigation was to determine a reflectivity threshold at an isothermal altitude and a threshold for echo tops that best predict CG lightning initiation. The study examined lightning, radar, and upper-air sounding data for only airmass-type convection during the summer of 2008. The best predictor of the onset of CG lightning was found to be a 40-dBZ reflectivity level detected at an altitude with an environmental temperature of ?10°C, with an average lead time of 17 min. Echo tops reaching or exceeding 7 km were a necessary condition prior to or at the time of the first CG lightning occurrence. Also, certain differences were observed depending on the polarity of the initial lightning flashes. Positive lightning flashes, when compared to negative ones, tended to deliver stronger electric currents and to be farther away from the locations of highest reflectivity on maximum reflectivity (MAXR) radar products. Lead times were observed to be shorter for positive lightning, which might suggest that positive-lightning-producing storm clouds became strongly electrified faster than their negative counterparts. Findings indicate the potential to develop a lightning nowcast algorithm suitable for Canadian forecast operational use.