Return-stroke multiplicity of negative cloud-to-ground lightning flashes

The percentage of negative lightning flashes composed of a single stroke is examined. This percentage is estimated from data reported by the U. S. National Lightning Detection Network (NLDN) for Florida and New Mexico and is compared with that determined from electric field and optical observations in these two states. The latter observations allowed a very accurate stroke count and, therefore, were used as the ground truth in the comparison. The percentage of negative single-stroke flashes reported by the NLDN is a factor of 2-3 higher than from the accurate-stroke-count studies in Florida (44% vs 17%) and is a factor of 3-4 higher in New Mexico (51% vs 14%). The observed discrepancies suggest that many small subsequent strokes are missed by the NLDN because these strokes fail to exceed the system's trigger threshold level so that only one stroke per flash is recorded in many multiple-stroke flashes. The percentage of negative single-stroke flashes reported by the Austrian lightning detection network is 40%, similar to the percentages reported by the NLDN for Florida and New Mexico. Percentages of single-stroke flashes determined from accurate-stroke-count studies in Sweden and Sri Lanka, which represent additional meteorologically distinct regimes, are 18% and 21%, respectively, in fair agreement with the Florida and New Mexico accurate-stroke-count studies. From comparison of the NLDN-reported and ground-truth data, it is possible to estimate the NLDN stroke and flash detection efficiencies. If the NLDN stroke detection efficiency were the same for both first and subsequent strokes, the percentage of single-stroke flashes and number of strokes per flash reported by the NLDN for Florida (44% and 2.4, respectively) would correspond to a stroke detection efficiency of about 40% and a flash detection efficiency of about 78%. A similar approach to the New Mexico data would yield a stroke detection efficiency of about 20% and a flash detection efficiency of about 62%.