Investigating Orographic-Modification of Precipitation in Hubbard Brook Experimental Forest Using Water Stable Isotopes

Floods and related deaths and property damage have increased in recent decades in the Northeastern U.S. Many of these floods occur in mountain valleys or river valleys downstream from mountain headwaters. Observations in these sparsely-populated headwaters are scant and radar-based quantitative precipitation estimates are frequently inaccurate due to radar beam blocking by terrain or being too high above the surface. As a result, orographically-modified precipitation processes at the mountain-valley scale remain poorly understood, except in more idealized landscapes. This study examines spatial precipitation patterns in high precipitation events in Hubbard Brook Experimental Forest (HBEF), New Hampshire to develop a conceptual model of these orographic precipitation processes.

            Forty-five years of daily precipitation data from a network of precipitation gauges maintained by the United States Forest Service at HBEF were studied to better understand climate scale changes and spatial precipitation patterns. A ~2-km latitudinal gap in this network, located in the lowest elevations, was filled with 12 4-inch diameter standard gauges to monitor precipitation events exceeding 25 mm during June-November 2015. Rainwater was collected from these 12 gauges for stable isotope (ẟ¹⁸O and ẟD) analysis to help determine the atmospheric processes causing the observed spatial precipitation patterns.

            Mean annual precipitation has increased 11%, entirely as a result of an increased frequency of >30 mm day^-1 events. The highest precipitation events (>50 mm day^-1) are 78% more common than they were 45 years ago. In high precipitation events (>25 mm) with a southerly low-level wind, precipitation averaged higher in the center of the valley and decreased with elevation on both the north- and south-facing slopes – contrary to basic theory and expectations. Rainwater ẟ¹⁸O varied strongest with precipitation amount; least-fractionated isotopes were collocated with the highest precipitation amounts. The location of the highest precipitation and least-fractionated ẟ¹⁸O shifted slightly northward and their inverse elevational-gradients strengthened as the speed of the southerly wind increased.

            The spatial distribution of ẟ and precipitation amount support three major processes impacting the orographic modification of precipitation: 1) a seeder-feeder process, 2) orographic cloud and precipitation drift by the low-level wind, and 3) enhanced collision-coalescence by shear-induced turbulence. The lifting of stable, moist air over the southern ridge of HBEF forms orographic cloud that is advected downstream over the valley with sufficiently strong southerly winds (>5 m s^-1). Falling seeder hydrometeors collect orographic cloud droplets to enhance precipitation over the valley. Shear-induced turbulence may also contribute to enhance precipitation over the HBEF valley in the favorable shear layer just above the mountain ridges. A radiosonde launch during one high precipitation events indicated a moist-adiabatic layer just above the ridge suggestive of orographic cloud and vertical mixing.