Simulation of stably stratified flow in complex terrain: flow structures and dividing streamline

The advanced research version of the weather research and forecasting model was employed to simulate Intense Operational Periods of the field campaigns of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program with a 0.5 km horizontal resolution. The focus was on synoptically dominated stably stratified periods, with the mean flow approaching a strongly non-symmetrical rugged topography—the Granite Mountain of the US Army Dugway Proving Ground, Utah. The model was validated against a comprehensive set of MATERHORN data. The special in-house developed software enabled calculations of energetics and pressure anomalies along individual streamlines and tracing of spatial trajectories of streamlines. Owing to complexities of natural flows, for example, the directional shear (skewed vertical velocity profiles) and irregular topographic shape, the flow patterns depend on multiple parameters, although in idealized cases the flow is described by a single parameter (Froude number or a variant). Streamlines at different altitudes of a given location diverged rapidly, making it difficult to study the dividing streamline concept. The new software allowed identification of the dividing streamline passing over the highest crest (summit) of the mountains and its trajectory. The upstream height of the dividing streamline did not follow the well-known Sheppard's formula. Three cases of flow patterns are presented, identified based on the presence of lee waves, flow separation, horizontal vortex shedding and hydraulics jumps.