Twelve Years of Cloud-to-Ground Lightning Characteristics, 1989-2000: Small Scale Results

The National Lightning Detection Network recorded over 250 million cloud-to-ground lightning flashes from 1989 to 2000. Analysis of lightning flash density and polarity reveal significant small-scale geographical variations. These variations can occur as broad trends over several hundred kilometers, or as sharp contrasts over as little as a few kilometers. This study focuses on four regions of the United States: the Pacific Coast, Rocky Mountains, Appalachian Mountains, and the Houston Metropolitan area. In the Pacific Coastal region, an interesting geographical variation is found in the percentage of positive lightning. A higher than normal percentage of positive lightning discharges (up to five times the national average) dominates the coastline, but drastically decreases about a hundred kilometers inland. In the Rocky and Appalachian Mountains, observations of lightning flash density reveal two distinct and opposite patterns, despite similar terrain. Although located at about the same latitude, the southern Rockies display a significant increase in flash density over regions of high terrain, while the southern Appalachians show a significant decrease in flash density as elevations increase. Further analysis shows that the pattern in the Southern Rockies is a result of a large number of days with low flash counts, while the pattern in the Appalachians results from a few large events. The top 5 percent of lightning producing days accounted for 28.6 percent of all flashes in the high terrain of Rockies, but 56.9 percent of the total flashes in the Appalachians. Our final area of research concerns the lightning enhancement over the Houston, TX region. Fine scale analysis of this area suggests two leading hypotheses to explain this phenomenon. The first explanation is that the complex coastline of Galveston Bay in combination with the urban heat island alters the wind flow in the Houston area, initiating new thunderstorms directly over the city. The second possibility is that pollutants from an industrial city like Houston can act to increase the electrical charge separation processes within storms. The culmination of these three studies will lead to a better understanding of lightning, one of nature's most incredible displays of power.