Gap Winds in a Fjord. Part II: Hydraulic Analog

A simple shallow-water model of gap wind in a channel that is based upon hydraulic theory is presented and compared with observations and output from a 3D mesoscale numerical model. The model is found to be successful in simulating gap winds. The speed and depth of gap wind flow is strongly controlled by topography. Horizontal or vertical channel contractions can act to force strong, shallow supercritical flow downwind and light, deep subcritical flow upwind. Force-balance analysis of the hydraulic model output confirms mesoscale model results and indicates that the prime force balance in gap wind is between external pressure gradient and friction for supercritical flow and between external pressure gradient and height pressure gradient for subcritical flow. This force balance changes near channel controls when the balance is between advection and height pressure gradient. Sensitivity analyses show positive sensitivity of gap wind speed to changes in discharge and external pressure gradient, negative sensitivity to changes in friction and boundary layer height at the channel exit, and mixed sensitivity of gap wind speed to changes in reduced gravity.