Cloud-to-ground flash patterns for Atlanta, Georgia (USA) from 1992 to 2003

We analyzed the patterns of cloud-to-ground (CG) lightning flashes around Atlanta, Georgia (USA), a region that has undergone an intense conversion from natural to anthropogenic land uses. For the 12 yr period from 1992 to 2003, annual average CG flash densities of 6 to 8 flashes km^-2 emerged around Atlanta. These values are 50 to 75% higher than in the surrounding rural areas, and comparable to flash densities along the Atlantic coast of Georgia. High flash densities extended over a large swath of Atlanta, and into Gwinnett County, a heavily suburbanized, rapidly growing county to the northeast. Urban flash production peaked during the summer (May through June) and exhibited more night and early morning activity (18:00 to 06:00 h) than in surrounding rural areas. Atlanta's higher flash densities do not result from isolated flash production over the city; rather they develop when the large scale atmospheric setting favors widespread lightning throughout the region. Maps of flash counts by interval classes also revealed where flash density maxima emerge in different county regions around the city. A large area of reduced positive polarity flashes developed along the arc of Atlanta's loop highway, Interstate 285. This area also trended south along the corridor of Interstate Highway 75 into central Georgia. This pattern suggests that automobiles may be a source of particulate matter, which is hypothesized to reduce the percentage of positive flashes.