Trapped Lee Wave Interference in Presence of Surface Friction

Trapped lee waves form downwind of a mountain ridge under conditions conducive to wave energy confinement within the lower troposphere. Placing a secondary ridge downwind of the first significantly affects the lee wave field. As shown in our earlier study of lee wave resonances under free slip conditions, constructive and destructive interference can occur, and their appearance is governed by the ratio of the intrinsic trapped lee wave wavelength and the ridge separation distance. In this study we investigate trapped lee wave interference over twin peaks in the presence of a frictional boundary layer, under conditions that can lead to boundary-layer separation and formation of rotors, with the attendant recirculating regions forming underneath the wave crests. Idealized high-resolution two-dimensional simulations with the NRL COAMPS model are used to investigate both the linear and nonlinear flow regime, and the effect of trapped lee wave interference on formation and characteristics of lee-wave rotors.

The results show that the interference pattern for all mountain heights examined is consistent with the predictions of linear interference theory and is therefore governed by ratio of valley width to intrinsic wavelength. The valley flow is unaffected by the presence of the second ridge up until h=600m, whereupon the wave amplitudes and rotor flow is diminished compared to the single ridge values. Rotor strength is insensitive to further increase in mountain height which suggests an existence of a limit to the rotor strength within the valley. The wave response is found to be most sensitive to interference in the lee of the downstream ridge, where changes in the wave amplitude due to interference affect the rotor strength most significantly. Constructive and destructive interference are shown not to be symmetric phenomena, where destructive interference significantly reduces the rotor strength, while constructive interference fails increase it beyond the single ridge value. Only for the strongest nonlinear regimes examined (h=1000 and 1500m) is the rotor strength amplified by constructive interference beyond that obtained in the lee of a single ridge.