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 case study of a gap flow event in December 2000 will briefly be reviewed along with conclusions from a model resolution and parameterization sensitivity study of the same case. An accompanying poster presentation provides a more detailed examination of December 2000 event. This oral presentation will focus on a dynamical analysis of high-resolution MM5 model simulations. First, an analysis of a 444-meter simulation of the December 2000 event (using the NCEP Eta model for initialization and boundary conditions of the coarse domain) will be presented, showing the detailed three-dimensional structure of the airflow through the Gorge and at its exit. Evidence that, for this case, the flow through the Gorge is analogous to the hydraulic responses predicted by shallow water theory will be discussed. For example, calculations of Froude number indicate critical flow at constrictions with supercritical “shooting” flow downstream in wider parts of the gap. Features that exhibit some of the characteristics expected in a hydraulic jump accompany transition back to sub-critical flow within the Gorge. Fields from 1-km idealized simulations will then be shown. The gap flow of the December 2000 case is approximately reproduced using idealized initial conditions based upon a sounding on each side of the Cascades. This principle is then extended to investigate the relationship between different sounding profiles and the gap flow that develops within the Gorge. Preliminary results of this “phase space” investigation will be given.