Physical modeling of an outflow event in Howe Sound, British Columbia

Outflow winds occur when differing air masses are separated by a coastal mountain barrier. In extreme cases the cross-barrier pressure gradient and the high degree of stratification (often approaching a distinct layered structure) result in channel winds which exhibit hydraulic features. We present a study of outflow winds in Howe Sound, British Columbia. A field investigation, aimed specifically at locating and quantifying hydraulic effects, was undertaken during the winter months of 1992/1993. Microbarographs positioned in the region recorded pressure changes at discrete locations in the streamwise direction. The pressures obtained during a severe outflow wind event, which occurred from December 27, 1992 to January 1, 1993, show a highly variable lower-layer depth suggestive of hydraulic control. Experiments were conducted with a three-dimensional physical model that is geometrically and kinematically similar to Howe Sound. Synoptic conditions recorded during the outflow wind event in Howe Sound in December 1992 were used to determine appropriate model flow forcing. The expanse of supercritical flow area was observed to be relatively sensitive to changes in along-channel pressure gradient and downstream depth, when compared to changes in discharge. Channel sinuosity and local topography appeared to force critical conditions at specific locations. For example, a channel bend combined with headlands was observed to force a situation where subcritical and supercritical streams flow side by side. Flow separation, resulting in lateral sheer discontinuities, produced similar conditions. These effects are discussed and put into context with field observations. Field and model results show good agreement.