Characteristics of Katabatic Flows along a Gentle Slope of Granite Mountain during MATERHORN X-1

The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program is a Multidisciplinary University Research Initiative (MURI) designed to improve weather predictability in mountain terrain. Here we present some results from the first MATERHORN field experiment, a 30-day intense field campaign conducted during September 25-October 25, 2102 at the Granite Mountain Atmospheric Science Testbed (GMAST) of the US Army Dugway Proving Grounds (DPG). Although the thermal circulation at the Granite Peak is considered nominally simple, data from sonic anemometers as well as slow sensors mounted up to seven levels on 4 towers deployed along a main gentle slope (α ≈ 2-3 degrees) of GMAST show that canonical downslope flows existed only for short time, overshadowed by those arriving from nearby mountains and basins. Tethered balloon (traversed up to 400 m agl) and LiDAR measurements operated during Intense Operational Periods (IOPs) confirmed basin-scale interactions causing the downslope flows to be rather intermittent while fine wire thermocouples provided additional information on the flow thermal structure close to the ground. When focused on the initial onset and developments 2-3 hours after sunset, measurements revealed a distinct development of shallow downslope currents at all four towers. Nose-shaped jet-like wind speed profiles were observed, with maximum values of 2-3 m/s occurring at heights around 4-5 m agl. During this period, the downslope flow exhibited low frequency oscillations (around 10^-3 Hz) in both temperature and velocity traces. Analysis suggests the presence of a multi-layer structure in terms of wave activity, stratification and wind velocity profiles. Upper level measurements adumbrated along slope (critical) oscillations with frequency Nsinα, N being the buoyancy frequency, while long internal waves appear to dominate the bottom, more stable stratified layer. Evidence of downward transport of heat suggests enhanced mixing by breaking internal waves. Mean and turbulent characteristics associated to these downslope currents were analyzed and compared with existing and new theoretical concepts in order to investigate dominant mixing mechanisms and their relation to internal wave dynamics of downslope flows. Outcomes from the above analyses are expected to yield better parameterizations for mixing in mesoscale numerical models and improve the ability of models in capturing katabatic flows.