Characterization of the internal gravity wave field generated by a katabatic flow in a deep valley

Atmospheric circulation over complex terrain is governed by both synoptic forcing and thermal circulation induced by radiative heating or cooling of the ground surface. In the case of a weak synoptic forcing, the dynamics of the atmospheric boundary layer in a deep valley either at night or in winter is dominated by katabatic (down-slope) flows. Indeed, as soon as the ground surface is sloping, a horizontal temperature gradient is created at night between the air just above that surface and the ambient air, because of the differences between thermal capacity of the air and of the ground.

As predicted theoretically (e.g. Fleagle, 1950; McNider, 1982) and verified with in situ measurements (e.g. Helmis and Papadopoulos, 1996; Van Gorsel et al., 2003; Bastin and Drobinski, 2005), oscillations in katabatic winds do occur along the slope if the atmospheric layer is stably stratified. For this along-slope oscillations, the angular frequency is proportional to the Brunt-Väisälä frequency N of the ambient atmosphere and to the sine of the slope angle.

Such an unsteady katabatic flow must generate internal gravity waves because of the stable stratification within the valley. These waves are usually not resolved in mesoscale models. Whilst breaking, they will induce mixing, which might need to be parameterized in such models. A first high-resolution numerical investigation in the stably stratified atmosphere of a valley has suggested that the pulsation of these waves is equal to about 0.8 N (Chemel and Staquet, 2007, submitted), and so, is independent of the slope angle of the topography (unlike the pulsation of the katabatic wind). Theoretical work (Voisin 2007) and laboratory experiments are consistent with this finding.

The aim of this study is to extend the characterization of the oscillations in the katabatic flow and of the internal gravity wave field emitted by this flow to a large range of stratification and slope angle values. To proceed, we have performed numerical simulations in an idealized topography of a deep valley with the ARPS meteorological model using a high resolution in space and time (down to 50 m horizontally, 4 m vertically, and 0.2 s). In this talk, we will discuss the mechanisms responsible for the along-slope oscillations and for the emission of the waves and we will clarify the dependance of the pulsation of the waves upon the stratification.

References :

Bastin S. and Drobinski P. 2005: Temperature And Wind Velocity Oscillations Along a Gentle Slope During Sea-Breeze Events. Boundary-Layer Meteorology, 114(3): 573.

Chemel C. and Staquet C. 2007 Generation of internal gravity waves by a katabatic wind in an idealized alpine valley. Submitted to Meteorology and Atmospheric Physics.

Fleagle, R., 1950 : A theory of air drainage. J. Meteor., 7, 227-232.

Helmis C.G. and Papadopoulos K.H. 1996: Some aspects of the variation with time of katabatic flow over simple slope, Quart. J. Roy. Meteor. Soc., 122, 595-610.

McNider R.T. 1982: A note on velocity fluctuations in drainage flows. J. Atmos. Sci., 39 (7), 1658-1660.

van Gorsel E., Vogt R., Christen A. and Rotach M. 2003: Low frequency temperature and velocity oscillations in katabatic winds. International Conference on Alpine Meteorology, Brig, May 19 to 23.

Voisin B. 2007: Added mass effects on internal wave generation. Fifth International Symposium on Environmental Hydraulics, Tempe, AZ, USA, 4--7 decembre 2007.