Cloud-to-Ground Lightning: Climatological Characteristics and Relationships to Model Fields, Radar Observations, and Severe Local Storms

Abstract

Data for nearly 2 million lightning flashes recorded during the 1985?86 warm seasons by the National Severe Storm Laboratory's (NSSL's) lightning strike locating network were evaluated to determine some of the climatological characteristics of cloud-to-ground lightning. Among the characteristics studied were the seasonal, diurnal, and spatial variations Of Positive and negative lightning strike activity, including flush rates, signal strength, and flash multiplicity. The lightning data were also compared to manually digitized radar data, reports of tornadoes, large hail, and damaging winds, and to analyzed 0000 UTC fields obtained from operational numerical models. An examination of the diurnal distribution of lightning revealed that peak rates occurred later than in other sections of the country, reflecting the prevalence of nocturnal convection within much of the NSSL network. An analysis of the spatial variations in lightning activity also confirmed the existence of distinct climatological regimes within the network. A study of the diurnal variations in signal strength revealed that first return strokes lowering negative charge have higher signal strengths at night and in the early morning hours, when flash rates are normally decreasing. In addition, positive flashes were found to exhibit three distinct peaks in signal strength, two of which are associated with late afternoon and nocturnal maxima in fish activity. A good correspondence between lightning frequency and radar echo intensity was found for the region within the effective range of the Oklahoma City WSR-57 radar. Both positive and negative flashes were found to be strongly correlated with the low-level moisture flux and circulation, as characterized by favorable moisture convergence, cyclonic relative vorticity, and strong upward vertical motions in the boundary layer. Contrary to expectations, freezing level height and wind shear were not nearly as important as the boundary layer fields in determining thunderstorm formation and subsequent positive lightning activity. A significant correlation was also found between the occurrence of severe local storm and elevated rates of 30 or more positive flashes per hour within 48 km grid blocks.