Stratified Flow past a Hill: Observations during MATERHORN Campaign

As a part of the second MATERHORN field campaign on atmospheric flow in mountainous terrain, the interaction between an isolated hill and an approaching [undisturbed] stably stratified flow was investigated with emphasis on the dividing streamline concept. The hill was located northwest of and close to the Granite Mountain, and was approximately 60m in height. A suite of (smoke) flow-visualization, remote sensing and in-situ measurement assets were deployed. At small Froude numbers (Fr<1), a stratified flow approaching the hill either possesses sufficient kinetic energy to pass over the summit or else flow around the sides, with the dividing streamline separating the two scenarios. By applying a logarithmic approach velocity profile to the well-known Sheppard's formula based on simple energetics, an explicit representation for the dividing streamline height was derived and a new set of parameters were identified to determine the dividing streamline height. The analysis shows that there will always be a dividing streamline for real atmospheric stratified shear flows. Incoming flow velocities and temperature profiles were measured using an array of sonic anemometers mounted on a 32m tower, a sodar and a microwave radiometer, while measurements near the hill were taken with portable weather stations. During the first visualization experiment, sufficiently strong stratification was developed around 3:00AM GMT, with the Froude numbers within (classical) theoretical range required for a dividing streamlines to exist. In the first trial, suitably placed red smoke releases were used while in another trial white smoke was released from a 25m crane. Well-defined dividing streamlines were observed in both cases, and its vertical location was at a height about half of the mountain height, consistent with modified theoretical results based on Sheppard's formula. The second visualization experiment was conducted under neutral atmospheric conditions, serving as a control case with no dividing streamline. This latter case was signified by clear vortex shedding and flow separation at the top of the hill. The observations were compared with simulations conducted with the Weather Research and Forecasting (WRF) model.