Orographic modulation of precipitation and rainfall rate in the mountains of western Maine

Datasets from two tipping-bucket rain gauges located in the mountains of Western Maine are compared to determine the orographic effects on both total precipitation and rainfall rate during the warm season (May-October). The dataset from the lower station, in Rangeley Plantation on the shore of Mooselookmeguntic Lake at an elevation of 453 meters, consists of nearly a complete record of full rate data for the period 1984-2003 and establishes a reliable precipitation and rainfall rate climatology for this location. The data from the upper station, in Township D at an elevation of 640 meters, consists of total precipitation data for two years plus full rate data since October 2001. Consistent orographic enhancement at the upper station relative to the lower is evident in the monthly totals separated by logarithmic rate channels (1-2 mm/hr, 2-4 mm/hr, etc) as well as in both the total amounts and median rainfall rates of significant rain events. The amount of orographic enhancement varies according to month and the overall enhancement during the warm season appears to have three peaks in the three rate channels 1-2 mm/hr, 8-16 mm/hr, and 32-64 mm/hr, apparently due to three different processes. Possible mechanisms of the orographic enhancement such as moisture scavenging, augmentation of stratiform mesoscale circulations and inducement of convection are discussed.