Effects of surface friction on downslope wind and mountain waves

A nonlinear nonhydrostatic model developed at National Taiwan University and Purdue University is applied to study the effects of surface friction on downslope wind and hydraulic jump for the windstorm of 11 January 1972 Boulder, Colorado, USA. The model reproduces strong downslope wind and hydraulic jump for both free-slip surface and viscous surface simulations. However, simulated hydraulic jump propagates downstream with an inviscid free-slip boundary, whilst the jump becomes stationary with a more realistic no-slip boundary condition. Furthermore, a returning flow (u<0) and trapped waves also develop in the lower atmosphere downstream from the stationary hydraulic jump in the viscous surface case. These lee waves decay with distance due to surface friction. With the inclusion of surface friction, the model is able to generate a realistic turbulence field consistent qualitatively with aircraft observations at the time.