Orographic Impact on Precipitation Amount in the Hubbard Brook Experimental Forest, New Hampshire

Extreme precipitation amounts in the Hubbard Brook Experimental Forest (HBEF) and encompassing White Mountain National Forest in New Hampshire often lead to downstream flooding in some of New Hampshire's population centers. Our analysis of precipitation data between 1968-2013 from HBEF, a bowl-shaped orographic formation, indicates that in high precipitation events (>15mm) with a southerly low-level wind regime, the precipitation maxima are located in the valley – contrary to theory and observations in many other places. High precipitation events (15, 25, and 50 mm thresholds) at HBEF have been increasing steadily, and 15 mm events occur approximately 25% more frequently than 45 years ago. On average, the precipitation data suggests these maxima may occur within a 2-km latitudinal gap in data between the existing north- and south-facing ombrometer networks. In early June, five rain gauges were deployed to fill the gap in precipitation data and an additional seven gauges were deployed near existing rain gauges to make catchment derived corrections on new gauges. Precipitation data were recorded for June-October to capture a variety of convective and stratiform events; the fall season is the climatological peak of high precipitation events in New Hampshire. High precipitation events during this time period were investigated to test the hypothesis of a valley precipitation maxima and to gain a better understanding of the precipitation processes in high precipitation events that often lead to flooding in the White Mountain National Forest and downstream in population centers. Additionally, rain water oxygen and hydrogen isotopes were analyzed to help identify any spatial variability in moisture source and/or the atmospheric processes that cause the highest precipitation amounts in the lowest elevations.