Orographic Impact on Precipitation Amount in
Hubbard Brook Experimental Forest
It is well known that mountains have a significant affect on atmospheric processes. They disturb air flow causing turbulence, induce cloud formation, and alter cloud microphysical processes. Perhaps one of the most impactful effects of mountains is the modification of precipitation amount. Typically, the precipitation amount increases with elevation; as flow is forced up mountain slopes the air cools and water vapor condenses into clouds and precipitation. Orographic modification of precipitation varies widely with mountain height and width, wind speed, and atmospheric stability. These complexities make it challenging for meteorologists to observe and forecast, especially in the rugged terrain of the White Mountain National Forest. The purpose of this study is to gain a better understanding of the precipitation processes in high precipitation events that impact mountain forest hydrology and often lead to flooding in the White Mountain National Forest and at downstream population centers.
Recent research performed with precipitation data from the Hubbard Brook Experimental Forest, a 3,160 hectare reserve in the White Mountain National Forest, suggests that in high precipitation events with a southerly low-level wind flow, the precipitation maxima is located in the valley – contrary to theory and observations in most other places. The leading hypothesis is that a lee-side rotor focuses precipitation in the valley. However, there is approximately a 2 km latitudinal gap in the current ombrometer network centered over the lowest part of the valley.
A research collaboration between Mount Washington Observatory and Plymouth State University's Eric Kelsey and PSU Masters student Matthew Cann is filling that gap to investigate the magnitude and location of the precipitation maxima, and to better understand the orographic modification of precipitation in the White Mountain National Forest. In May 2015, they located sites with large forest canopy openings for new precipitation gauges to fill the latitudinal gap and validate where in the valley the highest precipitation amounts typically occur.
A total of 12 precipitation gauges were installed in early June and will be used to measure precipitation for a three month period (June-August 2015). Cann, Kelsey, and other volunteers will measure the precipitation from all 12 gauges after every storm that produces ≥15 mm of rain with a low-level southerly wind.