GAP WINDS IN A FJORD .2. HYDRAULIC ANALOG

A simple shallow-water model of gap wind in a channel that is based up on hydraulic theory is presented and compared with observations and ou tput from a 3D mesoscale numerical model. The model is found to be suc cessful in simulating gap winds. The speed and depth of gap wind flow is strongly controlled by topography. Horizontal or vertical channel c ontractions can act to force strong, shallow supercritical Row downwin d and light, deep subcritical flow upwind. Force-balance analysis of t he hydraulic model output confirms mesoscale model results and indicat es that the prime force balance in gap wind is between external pressu re gradient and friction for supercritical flow and between external p ressure gradient and height pressure gradient for subcritical flow. Th is force balance changes near channel controls when the balance is bet ween advection and height pressure gradient. Sensitivity analyses show positive sensitivity of gap wind speed to changes in discharge and ex ternal pressure gradient, negative sensitivity to changes in friction and boundary layer height at the channel exit, and mixed sensitivity o f gap wind speed to changes in reduced gravity.