Potential Method for Warning the First Lightning Flash of Isolated Thunderstorm Cells over South China

Abstract This study improved the first lightning flash (FLF) warning method of isolated thunderstorm cells in the warm season over South China. The S-band polarimetric radar and three independent lightning location systems provided 57 thunderstorm and 39 nonthunderstorm cells. The results indicated the median value of horizontal reflectivity below the melting layer when the cloud detected by the radar first was a good indicator for forecasting the FLF with the longest lead time (threshold value ≤ 15 dB Z in this study). The algorithm produced 0.96–1 probability of detection (POD), 0.32–0.38 false alarm ratio (FAR), 0.63–0.66 critical skill index (CSI), and 12–21-min average lead time. Methods of minimum echo (20 dB Z ) top height extending to −10°C and the 35 dB Z at −10°C were more accurately for warning the FLF but with a shorter lead time; the former algorithm resulted in 1 POD, 0.08–0.29 FAR, 0.71–0.92 CSI, and 5–9-min average lead time. The latter algorithm resulted in 1 POD, 0–0.06 FAR, 0.94–1 CSI, and 3–5.5-min average lead time. The fluctuation of lead time depended on the FLF occurrence time. The differential reflectivity Z DR column and the height of graupel were not as effective as the reflectivity information on warning the FLF in this study. The results are expected to improve the FLF forecasting in considering accuracy and lead time within warm-season thunderstorm cells over South China based on the radar. Significance Statement It is difficult to forecast the first lightning flash (FLF) occurrence. While many methods are provided based on weather radars, the threshold values for hitting the start of lightning activity are dominated by mixed-phase ice microphysics, especially associated with graupel particles; therefore, to improve the lead time of the FLF nowcasting, usage of knowledge of warm-phase microphysics is possible. Fortunately, the latest discovery indicates the difference is recognizable in warm-phase microphysics during the first radar echo between thunderstorms and nonthunderstorms based on the weather radar. This study utilized the finding to construct a method for the FLF warning. The early lead time and accuracy are considered comprehensively in this method. It is important for the FLF warning.