Air Mass Exposure as a Control of Elevation Dependent Warming

Changes in several controls of the surface energy budget have been proposed to explain the recent elevation dependent warming (EDW) observed in mountain ranges around the globe. These controls include snow cover, cloud cover, latent heat release during cloud formation, absorbing and non-absorbing aerosols, integrated water vapor, soil moisture, and atmospheric circulation patterns (e.g., Ruckstuhl et al. 2007; Pepin and Lundquist 2008; Rangwala and Miller 2012). In this presentation, we propose a new integrating control on EDW: air mass exposure. The mid to high elevations of many mountain ranges experience considerable time in both the boundary layer and the free troposphere. This air mass variability occurs at the upper elevations for some mountain ranges, at mid-elevations for others, and is largely dependent on elevation relative to the surrounding lower elevations and local drivers of boundary layer depth (e.g., solar angle, surface type, Bowen ratio, albedo, synoptic scale pattern). EDW can occur from different warming rates of the boundary layer and free tropospheric air masses (e.g., Pepin and Seidel 2005). EDW can result from changes in the frequency of exposure to each air mass and from the aforementioned energy budget controls that are often dependent on air mass: cloud cover, cloud type, and integrated water vapor. This presentation will discuss the theory of air mass exposure as an important factor of EDW and use observations from Mount Washington, New Hampshire (1917 m asl; 44.27°N 71.32°W) and boundary layer heights derived from the NOAA snow-level radar in Plymouth, New Hampshire (152 m asl; 43.76°N 71.69°W) to demonstrate its potential impact.