Thursday, 26 January 2012: 2:45 PM
Developing An Improved Understanding of Steep Slope Evening Transition Processes
Room 339 (New Orleans Convention Center )
Transitory processes in the lower levels of the atmospheric boundary layer are poorly understood compared to their steady counter parts and have proven quite difficult to model. This is especially the case for processes occurring over very steep slopes where theory and numerical work are limited by a paucity of experimental data. In this work, we focus on collecting field data over steep slopes during evening transitions dominated by local thermally generated winds when the build up of stable layers rapidly influences pollutant mixing and air quality. Data were collected as part of a hydrological observation project in the Val Ferret region of the Swiss Alps near the Grand St. Bernard pass. A relatively regular slope with grades ranging from 20-45% was instrumented along a linear transect with four towers (including a surface energy budget station and a 10 m tower with sonic anemometers), surface temperature measurement stations and a tethered balloon system to capture the complex interplay between the surface and atmosphere. In this work, a phenomenological description of a “shading front” that governs the extremely strong and rapid transition from upslope to downslope flow will be presented in the context of an analysis of the momentum, energy and turbulent kinetic energy balances. The analysis supports the notion that during these types of transition periods, classic surface layer similarity breaks down as the air above the surface responds to local energy balance processes leading to the initiation of downslope winds.