Statistical frequency analysis of lightning producing storms during STEPS 2000

Abstract

Most cloud-to-ground (CG) lightning lowers negative charge to ground, but roughly 10% of flashes are reversed and transfer positive charge to ground. A small number of storms produce predominately (greater than 50%) positive CG lightning, and recent studies have associated the occurrence of tornadoes, hail, and microbursts with these "positive" storms. Much of this work has been centered on case studies. The use of case studies, however, is limited; the nature of case selection is subjective and possibly susceptible to researcher bias. This research presents a new method for addressing how a thunderstorm "looks" from a statistical perspective and is based on information readily available to researchers and operational forecasters alike. A statistical analysis of High Plains thunderstorms during the summer of 2000 was conducted as part of the Severe Thunderstorm Electrification and Precipitation Study (STEPS). WSR-88D NEXRAD and National Lightning Detection Network (NLDN) data sets were used to produce statistical radar reflectivity distributions based on cloud-to-ground (CG) lightning flash densities. Comparisons were made based on flash polarity, geographical location and storm type. The main goal of this research was to better understand relationships between storm structure and microphysical processes (inferred from radar reflectivity). Consistent with previous findings, statistical results show that for high flash densities, (above 0.1 flashes km-2 hr-1) positive storms are five times more likely than negative storms to produce reflectivities between 55-70 dBZ. Further, these results provide evidence that low flash density storms (defined to produce less than three CGs in thirty minutes) are more likely to contain upper-level reflectivity maximums if the CGs are positive. This result suggests that positive CGs are favored when a storm contains strong updrafts and contains large particles suspended aloft. These conditions are generally associated with developing convection, prior to the onset of heavy precipitation.