Structure and dynamics of katabatic flows: results from MATERHORN X-1

The flow structure and turbulence characteristics of katabatic flows are investigated using measurements collected during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program, a Multidisciplinary University Research Initiative designed to improve weather prediction in mountain terrain. The first MATERHORN field experiment (MATERHORN X-1) was a 30-days field campaign conducted during September-October 2102 at the Granite Mountain Atmospheric Science Testbed (GMAST) of the US Army Dugway Proving Grounds. It mainly dealt with thermally driven flows that occur when the synoptic winds are weak and thermal forcing is strong. We have analyzed data from sonic anemometers as well as slow sensors mounted up to seven levels on 4 towers deployed along a main lower slope (α ≈ 2-3 degrees) of GMAST. Measurements reveal several occurrences of downslope flows during quiescent periods. Slope flow develops rapidly after sunset, and usually persists for few hours. After that, strong interactions between slope flows and the circulation in the valley weaken the katabatic flow, leading to more complex flow scenarios. When focused on the initial onset and the following 2-3 hours, wherein the flow structure resembles a “pure” katabatic flow, clear evidence of low frequency oscillations (around 10^-3 Hz) within the katabatic flow was found at all four measurement sites. Oscillations in the bottom layer of the katabatic flow suggest coexistence of two dominant oscillatory systems, in contrast to what is observed at the upper measurement levels which are dominated by a system of along slope (critical) oscillations with frequency Nsinα, N being the buoyancy frequency. The former resembles to long internal waves trapped in the thin, lower stratified layer. A detailed investigation of such oscillations and their interactions with both the mean flow and the turbulence is presented, while attempting to identify dominant mixing mechanisms and their relation to internal wave dynamics of katabatic flows. The observations are also compared with existing and new theoretical concepts.