Characterization of oscillatory motions in the stable 1 atmosphere of a deep valley

In a valley sheltered from strong synoptic effects, the dynamics of the valley 6 atmosphere at night is dominated by katabatic winds. In a stably stratified atmosphere, these 7 winds undergo temporal oscillations, whose frequency is given by N sinα for an infinitely 8 long slope of constant slope angle α , N being the buoyancy frequency. Such an unsteady 9 flow in a stably stratified atmosphere may also generate internal gravity waves (IGWs). 10 The numerical study by Chemel et al. (Meteorol Atmos Phys 203:187-194, 2009) showed 11 that, in the stable atmosphere of a deep valley, the oscillatory motions associated with the 12 IGWs generated by katabatic winds are distinct from those of the katabatic winds. The IGW 13 frequency was found to be independent of α and about 0.8 N. Their study did not consider 14 the effects of the background stratification and valley geometry on these results. The present 15 work extends this study by investigating those effects for a wide range of stratifications and 16 slope angles, through numerical simulations for a deep valley. The two oscillatory systems 17 are reproduced in the simulations. The frequency of the oscillations of the katabatic winds 18 is found to be equal to N times the sine of the maximum slope angle. Remarkably, the IGW 19 frequency is found to also vary as Cw N, with Cw in the range 0.7 – 0.95. These values for 20 Cw are similar to those reported for IGWs radiated by any turbulent field with no dominant 21 frequency component. Results suggest that the IGW wavelength is controlled by the valley 22 depth. 23