Insights into Columbia Gorge Gap Flow through High Resolution Simulations of Actual and Idealized Events

Justin Sharp[1] and Dr. Clifford F. Mass

University of Washington, Seattle, Washington

Upstream of its final section between Portland, Oregon and the Pacific Ocean, the Columbia River has cut a gorge through the heart of the Cascade Mountains that provides the only near sea-level conduit between the dry continental climate to the east and the moist maritime climate to the west.  Gap flow through the Columbia Gorge is common and plays a profound role in determining the weather and climate both within and adjacent to the Gorge, including the city of Portland.

Results from an observational and high-resolution (444 m inner domain) modeling study of a gap flow event in December 2000 will briefly be reviewed.  Major conclusions from 1) a model resolution and parameterization sensitivity study, and 2) a dynamical analysis of the event will be presented.  It will be shown that the MM5, run at sufficiently high resolution, can realistically recreate Gorge gap flow.  Evidence that, for the December 2000 case, the flow through the Gorge was analogous to the hydraulic responses predicted by shallow water theory will be discussed. 

However, most of the presentation will focus on results from an ongoing comprehensive investigation of the parameter space affecting Columbia Gorge gap flow events.  This is being achieved using idealized simulations in which initial and boundary conditions are user defined.   These simulations are being run using the MM5 with three nested domains, where a large inner domain with a grid spacing of 1 km covers the entire Columbia Gorge and surrounding terrain.  First, it will be shown that the gap flow of the December 2000 case can be approximately reproduced using idealized initial conditions based upon a single point sounding on each side of the Cascades.  Numerous other sounding profiles are then used to systematically analyze the gap flow that develops within the Gorge for different upstream and downstream conditions.  Results of this “phase space” investigation along with the dynamical insights they reveal will be discussed.