Climatological lightning characteristics of the Southern Rocky and Appalachian Mountain chains, a comparison of two distinct mountain effects

Abstract

This study presents a high-resolution lightning climatology for southern portions of both the Rocky Mountains and the Appalachian Mountains. Data from the National Lightning Detection Network (NLDN) are analyzed to produce maps of average annual lightning flash density, positive flash density, percent positive flashes, median peak current, and multiplicity. Three-hourly increments are used to demonstrate the annual average diurnal evolution of flash density. Data are also divided into seasonal averages for the same three-hourly increments to describe the daily evolution of flash density for each of the four seasons: December-January-February, March-April-May, June-July-August, and September-October-November. The flash density analyses reveal opposite mountain-valley effects. In the Rocky Mountains, flash density enhancements occur over and near mountains and flash density minima occur in the valleys. In the Appalachians, the enhancements occur in the valleys, while minimums are noted over the mountains. The eastern edge of the Appalachian lightning suppression is determined to be a result of faster propagation of mountain-initiated convection. Weaker mountain breezes in the Appalachians are theorized to be the catalysts for this. The western edge of the suppression is the cumulative effect of consistent flash density gradients at the Appalachian's western slopes. A theory is presented which links this gradient to observations of high median peak currents. Statistical tests on flash density means show that the Appalachian suppression is significant. Multiple regressions predict lightning flash density from terrain characteristics. Vertical wind and thermodynamic profiles, horizontal temperature differences at summit levels, and average annual precipitation complete the study. From these data, a conceptual model is presented to describe the nature of the lightning evolution in each region, and explain the processes that lead to the end state. This study concludes that the differences between the patterns of lightning characteristics in the Southern Rockies and the Southern Appalachians are the cumulative effects of subtle differences in the diurnal evolution patterns. Furthermore, the Appalachian lightning suppression is a product of lightning propagation and storm evolution, rather than a suppression of convective initiation.