The structure and dynamics of Columbia Gorge gap flow revealed by high-resolution numerical modeling

Abstract

The Columbia Gorge is a topographic feature providing the only near sea-level conduit through the Cascade Mountains. Gap flow is therefore common in the Gorge and plays a profound role in defining the climate of the surrounding region, including the city of Portland.A case study of a Gorge gap flow event compares surface, aircraft and radar observations to a high-resolution mesoscale model (MM5) simulation. Gap flow is accurately simulated when sufficient horizontal (1.33 km grid spacing) and vertical (44 levels) resolution is used. Observations and model data reveal that blocking upstream of the Cascade crest enhances pre-existing pressure differences across the mountains. Weak acceleration occurs as air approaching the Cascade crest converges into the narrow Gorge. However, most flow acceleration is found west of the crest in association with large pressure gradients created by subsidence. Correlation of acceleration with channel geometry changes suggests that a hydraulic mechanism is important in the gap flow dynamics.The case study is generalized by performing semi-idealized simulations with the MM5. Real terrain is used, but initial and boundary conditions are idealized, allowing the parameters influencing Gorge gap flow to be investigated systematically. Over 100 simulations are performed to assess the relative importance of atmospheric stratification, initial wind speed, vertical shear, and cross-Cascade thermal contrasts.Qualitative similarities emerge between the gap flow structures found in the case study and structures produced by a broad range of atmospheric conditions examined in semi-idealized simulations. However, the magnitude of responses to topographic controls is a strong function of atmospheric conditions. Vertical wind shear and atmospheric stability are particularly important variables governing the gap flow strength.Discrepancies are found in the hypothesis that acceleration is primarily due to a hydraulic mechanism. In particular, the structure of low and high Froude number flow is qualitatively similar. Acceleration is strongest at the sides of the gap where downslope flow is occurring on surrounding terrain. Changes in gap flow response produced by different initial flow speeds, vertical shears and stabilities indicate that wave dynamics are likely the most important factor influencing the gap flow strength.