Hilly and mountainous terrain exert an important influence on the Earth's atmosphere and affects atmospheric transport and mixing at a wide range of temporal and spatial scales. The vertical scale of this transport and mixing is determined by the height of the atmospheric boundary layer, which is therefore an important parameter in air pollution studies, weather forecasting, climate modeling, and many other applications. It is recognized that the spatio-temporal structure of the daytime convective boundary layer (CBL) height is strongly modified in hilly and mountainous terrain compared to flat terrain, but detailed observations of the structure and understanding of the underlying processes are still limited. Furthermore, the determination of CBL height in non-flat terrain is often ambiguous since vertical mixing usually originates from both turbulence and organized terrain induced motions.

In this presentation, I will show results from an investigation of CBL heights during 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 during the experiments. During a few days, radiosondes were released. We find a wide range of boundary layer behaviors over the mountain top location, which can be roughly organized in two types: a convective boundary layer evolving independently from the boundary layer over the valley and slope (‘local' boundary layer) and convective boundary layers advected over the mountain top site (‘regional' boundary layer). In this presentation, I will show examples of these two types of CBL behaviors, discuss the atmospheric and surface conditions affecting these behaviors and discuss implications for air pollutant concentrations in mountainous regions.