Detecting summer rainfall enhancement within metropolitan Atlanta, Georgia USA

This paper presents a methodology for detecting summer rainfall enhancement within a metropolitan area. The chosen study domain is the Atlanta, Georgia USA metropolitan area, and the time period covers summer (i.e. June-August) seasons from 1949 to 2005. In order to determine if observed heavy-rainfall frequencies at nine candidate stations in the metropolitan area are enhanced, predicted frequencies are derived from heavy-rainfall frequencies at seven reference stations located more than 50 km from downtown Atlanta. Both parametric and non-parametric tests are used to test for significant differences between observed frequencies and predicted frequencies within 29 overlapping 30 year periods (i.e. climatological periods). Norcross, which was similar to 30 km northeast of downtown Atlanta and thus was typically downwind of urbanized Atlanta on heavy-rainfall days at all candidate stations, was the only station to exhibit significant rainfall enhancement. The final ten climatological periods exhibited enhancement, with observed frequencies of >= 30 mm days for those periods being at least 20% larger than predicted frequencies. It is hoped the methods employed in this paper can be used in other metropolitan areas. Copyright (C) 2007 Royal Meteorological Society.