Impact of the atmospheric boundary layer on mountain waves

Interaction between mountain waves and the atmospheric boundary layer (BL) has been investigated using a mesoscale model with BL parameterizations and analytical BL models. The impact of the BL diagnosed from the mesoscale model simulations is in qualitative agreement with the analytical BL models. In general, for stratified flow over a mesoscale ridge, the influence of the BL tends to shift wave patterns upstream, decrease the wave drag by up to 60% and the momentum flux above the BL top by up to 80%, and significantly delay the onset of wave breaking. It has been demonstrated that the BL effect is governed by four nondimensional parameters, namely, the nondimensional BL depth, BL shape factor, nonhydrostatic parameter, and frictional adjustment parameter. The BL effect is stronger over a rougher surface, which creates a deeper BL with a smaller shape factor (i.e., slower BL flow). The scale dependence of the BL effect is governed by the nonhydrostatic and frictional adjustment parameters. For narrow terrain, the BL reduces mountain drag through rapid inertial adjustment and behaves like an inviscid shear layer, and the momentum reduction across the BL is negligible. The attenuation of the momentum flux across the BL is more substantial over a wider ridge because of the increasing importance of frictional adjustment. The BL effect decreases with increasing ridge height and for 3D terrain.