Mixing Induced By Slope and Valley Flow Collisions in Complex Terrain

Abstract:

A fascinating observation made during the fall 2012 field campaign of the Mountain Terrain Atmospheric Modeling and Observations Program (MATERHORN) was the collision of slope and valley flows that led to powerful but spasmodic turbulence generating events. The contributions of these collisions to overall basin (meso) scale mixing were studied using the MATERHORN field data as well as laboratory measurements conducted under controlled conditions using counter flowing gravity currents. The collisions cause localized shear layers and Kelvin Helmholtz billowing, which, together with turbulence generated by impingement of fronts on one another, generate a turbulence field that decays rapidly under local stable stratification. Buoyancy fluxes measured using these events are parameterized in terms of a suitably defined Richardson number and dimensionless geometric parameters. The time and lengthscales of initial turbulence as well as its decay time scale are measured and quantified using governing parameters. Mechanisms responsible for the recurrence of collision events are elicited based on data from a large number of flux towers, remote sensors and infrared cameras located in the domain of observations.