Aspects of the surface—and boundary layer structure over a mountain top

Mountain top research stations have traditionally been set up to monitor background concentrations of atmospheric constituents. While boundary layer effects on mountain top measurements have long been recognized, a detailed investigation of boundary layer dynamics in these locations has largely been ignored. For example, mountain top stations such as Mount Washington, Jungfraujoch, Storm Peak Lab, and Mauna Loa do not routinely measure energy balance components or aerosol layer height. Monitoring these variables at mountain top locations will help understand the effects that mountains exert on the atmosphere and the measured diurnal variation of atmospheric constituents. This understanding is important in assessing the suitability of a mountain top station for the measurement of background concentration and in evaluating air pollutant transport and air quality in mountainous terrain.

In this presentation, we will provide a short historical overview of surface- and boundary layer structure measured at various mountain top locations around the world and present results of a few field studies carried out on and around a mountain top location in the Blue Ridge Mountains of Virginia, in the eastern part of the Appalachian mountain range. A 17-m walk-up tower at the mountain top location is equipped with a suite of meteorological instruments including temperature, humidity, wind, and radiation sensors, sonic anemometers, and gas analyzers for the measurement of water and carbon dioxide concentration and fluxes. A portable eye-safe UV aerosol lidar is housed in a shed adjacent to the tower and monitors boundary layer heights and aerosol loading of the atmosphere. We find a wide range of boundary layer behaviors over the mountain top location, which can be roughly organized in two ways: days on which a convective boundary layer evolves independently from the boundary layer over the valley and slope (‘local' boundary layer) and days on which convective boundary layers are advected over the mountain top site (‘regional' boundary layer). At night, wind speeds increase significantly at the mountain top site and the lidar measurements provide evidence of the presence of residual layers that decrease in height during the nighttime. The results demonstrate the need for vertical structure information to assess the representativeness of mountain top measurements.