Brandon R. Vincent* NOAA/National Weather Service, Newport/Morehead City, NC

Lawrence D. Carey* Department of Atmospheric Sciences, Texas A&M University

Doug Schneider, Kermit Keeter and Rod Gonski NOAA/National Weather Service, Raleigh, NC

Forecasting the initiation of lightning activity is important for the protection of human life and property. Lightning results from a charge separation that most likely occurs during rebounding collisions between ice crystals and large ice hydrometeors such as graupel and hail that remain suspended in the mixed phase zone by the updraft of a growing thunderstorm. WSR-88D radar reflectivity can be used to indirectly identify this electrification process within a growing thunderstorm because graupel and hail return large reflectivity echoes. This study examined a sample of 50 central North Carolina thunderstorm cases using three different characteristics of WSR-88D data (e.g., reflectivity threshold [dBZ] at a given environmental temperature [°C] for a specified number of volume scans [# Vol]) that were organized into 8 different sets of criteria for determining the cloud-to-ground (CG) lightning potential.

Preliminary results showed that the best lightning prediction algorithm was associated with either the 1 Vol /40 dBZ/–10°C or 1 Vol /40 dBZ/–15°C criterion. Based on the Critical Success Index (CSI), the 1 Vol /40 dBZ/–10°C criterion performed the best with a 100% Probability of Detection (POD), a 37% False Alarm Rate (FAR) and a 63% CSI. The 1 Vol /40 dBZ/–15°C criterion closely followed with an 86% POD, a 30% FAR and a 62.5% CSI. Lead times for these criteria were 14.7 minutes and 11.0 minutes, respectively. If lead time is a high priority and a slight reduction in CSI can be tolerated, the 1 Vol /35 dBZ/–15°C or 1 Vol /35 dBZ/–10°C criterion may be a better choice. The 35 dBZ criteria resulted in lead times 2-3 minutes longer than with 40 dBZ.

In addition, an analysis of vertical reflectivity lapse rates between the 0°C and –20°C isotherm heights in both detection and false alarm cases showed that vertical reflectivity lapse rates for false alarms (–2.04 dBZ/kft) were much larger than for detections (–0.69 dBZ/kft). This suggests that the lightning prediction algorithm could be further improved by including vertical reflectivity lapse rates into the detection criteria. The results show that it is possible to use WSR-88D reflectivity to reasonably predict the onset of CG lightning in the central North Carolina region using criteria similar to that used in previous studies of thunderstorms in other regions.

The purpose of this research is to present operational meteorologists with a method of predicting the first lightning strike in a developing convective cell by utilizing WSR-88D reflectivity data to indirectly sample the electrification process and to discuss methods of refining the detection algorithm to improve its performance.