Precipitation Climatology in the Pittsburgh Urban Area During Late Spring and Summer

Southeastern Geographer Vol. XXIX, No. 2, November 1989, pp. 75-91 PRECIPITATION CLIMATOLOGY IN THE PITTSBURGH URBAN AREA DURING LATE SPRING AND SUMMER M. Shawn Rosenberger and Philip W. Suckling INTRODUCTION. Some climatic changes generated by urban environments are obvious and easy to measure, while others are more subtle and difficult to detect. Some of the more evident effects of urban areas on atmospheric elements include reduced visibility from air pollutants, generally lower windspeeds due to increased friction, and higher temperatures as a result of the urban heat island. (J) Less conclusive are urban effects upon moisture formation in terms ofatmospheric humidity, fog, cloudiness, and precipitation. Although there is general consensus that relative humidity values are lower in cities, due primarily to the existence of warmer urban temperatures, (2) and that the cities are relatively moist at night and dry during the day compared to the surrounding countryside, (3) there is considerable debate over the meaning of other measures of moisture. (4) With respect to urban effects on precipitation, many inconsistencies exist in the literature. Landsberg has stated that "of all meteorological elements in the urban environment, precipitation still confronts us with the most puzzles." (5) Perhaps part of this inconsistency is due to the fact that precipitation is inherently less coherent spatially when compared to temperature or pressure. (6) When assessing the urban impact upon precipitation, additional difficulties arise in trying to separate urban effects from topographic effects. Urban modification of precipitation was first noted in the United States during the 1950s when Landsberg presented evidence for increased precipitation at Tulsa. (7) Significant increases in warm season precipitation, including increases in the frequency of rain days, thunderstorm days, and hail days, downwind of the Chicago-Gary urban-industrial complex were later presented by Changnon. (8) Known as the La Porte anomaly (since the increases were observed near La Porte, InMr . Rosenberger is a 1988 M.A. graduatefrom the University ofGeorgia and is currently employed by the Pennsylvania Department of Environmental Resources . Dr. Suckling is Sandy Beaver Associate Professor of Geography at the University of Georgia in Athens, GA 30602. 76Southeastern Geographer diana), these findings received considerable national attention but were also met with much skepticism. The arguments and counter-arguments stressed the need for more study and evaluation of inadvertent urban weather modification. (9) Recent climatological studies of other cities have sought to determine the extent of urban effects on precipitation. One of the most significant to date was the Metropolitan Meteorological Experiment (METROMEX ) conducted over five summers in the St. Louis area. (JO) Its results showed anomalously high rainfall amounts about 20 km northeast (downwind) of St. Louis around Edwardsville, Illinois. The Edwardsville anomaly was a consistent feature, receiving roughly 30% more rainfall than the network mean. A survey of the literature suggests that there is little consensus regarding urban influences on precipitation. Some studies have provided evidence for urban-enhanced precipitation totals and/or frequency of rain days. (JJ) Other studies have found no evidence of urban effects upon precipitation for certain cities. (12) In cases where urban effects have been detected, there is disagreement as to whether the city initiated rainfall or simply enhanced an existing situation. Some studies present the higher frequency of heavy rain events as evidence that urban areas enhance rather than initiate the precipitation process. (J3) Yet other studies, including METROMEX, offer evidence that urban areas initiate as well as intensify existing regional precipitation events. (14) The main objective of the present study is to determine whether an area ofenhanced precipitation exists downwind ofthe Pittsburgh, Pennsylvania , urban area. Previous studies have suggested that the summer season represents the greatest potential for urban effects. (15) Therefore, this study will focus on the late spring and summer months of May, June, July, and August for 1958-1987. Precipitation will be studied in terms of monthly totals as well as frequency measures—number of days with a trace or more of precipitation, number of days with 12.5 mm or more of precipitation, and number of days with 25 mm or more of precipitation . If evidence is found for urban enhancement as measured by monthly totals of precipitation, then the frequency analyses will address the...