Boundary Layer Exposure on Mount Washington Project
Mountain ecosystems provide sustaining water, natural resources, and numerous economic benefits for more than half the world’s population. Climate change threatens the stability of these mountain ecosystems and the economies they support. In New England, Mount Washington and Mount Mansfield are warming more slowly than the lower elevations (Fig. 1). This project hypothesizes that Mount Washington’s variable exposure to two distinct horizontal layers in the atmosphere, the boundary layer and the overlying free troposphere, drives a significant portion of this elevation-dependent warming.
How Boundary Layer Height Changes Can Impact Temperature Trends
Two factors that impact the energy budget and, therefore, elevation-dependent temperature trends, are atmospheric water vapor content (Ruckstuhl et al. 2007) and cloudiness (Rangwala and Miller 2012). While previous mountain climate studies have focused on relating moisture variables, radiation, and surface temperature, no published studies have tested these relationships within the context of changing exposure to boundary layer and free tropospheric air. The boundary layer is the lowest layer of the atmosphere, typically 1-3 km deep, is often turbulent, and responds quickly to diurnal solar heating, friction as wind blows over the Earth’s surface, and nocturnal radiational cooling (Fig. 2). The free troposphere is a deeper, more laminar atmospheric layer that lies directly above the boundary layer. The temperature, humidity, cloud, and water vapor isotopic characteristics of these two air masses are typically distinct. As a result, if the frequency with which high elevations are exposed to boundary layer air masses is changing, large changes in seasonal temperature, humidity and cloud patterns might result, with significant impacts on temperature trends and mountain ecosystems.
Summer-Fall 2016 Boundary Layer Project
To test this hypothesis, a necessary first step is to develop a robust method for measuring the air mass type at the summit of Mount Washington. This August and September, MWO, Plymouth State University and collaborators from the Appalachian Mountain Club and Dartmouth College will test a novel methodology to determine summit air mass type and the processes driving boundary layer height changes on and around Mount Washington. First, water vapor isotopes will be measured at the summit of Mount Washington for 5 weeks during August and September. Significant changes in the summit isotopes compared with our understanding of the synoptic scale atmospheric conditions will inform us when the summit is transitioning between the boundary layer and the free troposphere. In the second component of the project, vertical profiles of temperature, humidity, wind and stable isotopes of water will be measured during two different days to assess the diurnal evolution of the boundary layer. Weather balloons will be launched throughout these two days, from prior to sunrise to just after sunset (Fig. 3). Concurrently, a water vapor isotope analyzer will be driving up and down the Mount Washington Auto Road.
This unique multivariate approach should yield a robust determination of the air mass type at the summit of Mount Washington. The test of this methodology will be the launching pad for future research to directly address the hypothesis that variable boundary layer exposure at high elevations significantly impacts elevation-dependent warming in mountainous regions.
This project is funded by a Plymouth State University research grant. Dr. Eric Kelsey (MWO Director of Research) would like to thank Dr. Heidi Asbjornsen (University of New Hampshire) for loaning her water vapor isotope analyzer for this project, and intellectual contributions from collaborators Dr. Adriana Bailey (Dartmouth) and Georgia Murray (Appalachian Mountain Club). This project would not be possible without the generous logistical support of the Mount Washington Auto Road.
Dr. Eric Kelsey, Director of Research
Figure 1: Minimum temperature trends for low elevation New Hampshire sites (black circles; First Connecticut Lake, Hanover, Durham, Keene, Bethlehem, Hubbard Brook), Mount Washington (red circles), and Mount Mansfield, VT (orange circles). Open circles indicate statistically significant Sen’s slope trends (p<0.05). Some low elevation temperature trend data are from Wake et al. (2014a, 2014b) and all low elevation trends are for 1970-2012.
Figure 2: Schematic of the daily evolution of the boundary layer under clear skies. From MetEd UCAR.
Figure 3: Students preparing to launch a weather balloon at the base of the Mount Washington Auto Road.
Rangwala, I., and J.R. Miller, 2012: Climate change in mountains: A review of elevation-dependent warming and its possible causes. Climatic Change, 114, 527-547, doi:10.1007/s10584-012-0419-3.
Ruckstuhl, C., R. Philipona, J. Morland, and A. Ohmura, 2007: Observed relationship between surface specific humidity, integrated water vapor, and longwave downward radiation at different altitudes. J. Geophys. Res., 112, D03302, doi:10.1029/2006JD007850.
Wake, C.P., C. Keeley, E. Burakowski, P. Wilkinson, D. Hayhoe, A. Stoner, and J. LaBrance, 2014a: Climate Change in Northern New Hampshire. Past, Present, and Future. The Sustainability Institute, 78 pp.
Wake, C.P., E. Burakowski, P. Wilkinson, K. Hayhoe, A. Stoner, C. Keeley, and J. LaBranche, 2014b: Climate Change in Southern New Hampshire. Past, Present, and Future. The Sustainability Institute, 86 pp.