Gravity Waves over Mt. Blanc

Mt. Blanc, the highest peak in Europe (4808m), is located in the western Alps near the common borders of France, Switzerland and Italy. On November 2, 1999, southwesterly flow over Mt. Blanc generated a field of vertically propagating mountain waves that was simultaneously observed by four research aircraft from the Mesoscale Alpine Programme. Three of these aircraft were equipped with dropsondes and two with downlooking Lidar, making this one of the most carefully measured atmospheric gravity wave systems to date. The strategy of flying repeat legs (up to eight legs for the UK C-130) gave indisputable evidence that the wave system was in steady state.

Due to the relatively small amplitude of the observed wave, the authors have chosen linear theory to provide the primary interpretation of the event. An extended formulation of linear theory is used, including three dimensions, a three-layer background state and nonhydrostatic effects. Perhaps because of the unusually well known background state in this event, a remarkable degree of agreement is found between the theory and aircraft observations of vertical air motion and a lidar-surveyed lenticular cloud. The theory is used in hierarchical fashion to investigate the role of non-hydrostatic dispersion, reflection from the wind shear aloft and absorption by a critical level. Further interpretation was carried out using the COAMPS model. Three paradoxes arose in this investigation.

First, in spite of the great height of Mt. Blanc, the observed waves are rather small. This paradox can be understood by considering the observed stagnant layer of air below 3250 meters abmsl. Only the top kilometer or so of the peak penetrated into the wind, reducing the amplitude of the generated waves. The stagnant layer itself is believed to be caused by blocking from the large number of nearby lower mountains in the western Alps.

Second, in spite of the strong forward wind shear, satisfying the "Scorer Condition", no trapped lee waves were found. This can be explained by the top of the stagnant layer acting as an absorbing critical level for waves that have been reflected downward by the windshear aloft. Thus, the ground cannot provide the lower reflector of the waveguide, as it usually does in linear lee wave theory. This absorbing behavior can be easily incorporated into linear theory. Vertically propagating waves are damped and slightly dispersed by the windshear, but manage to reach the lower stratosphere.

When the waves reach the lower stratosphere, a strong shear layer there seems to destroy the waves and generate an unsteady train of trapped waves or instabilities. We try to simulated this non-linear transformation using the Coamps model. The results indicate that the pre-existing wind shear is a factor in wave breakdown and that it involves a transformation to other wave types, rather than to direct turbulence generation.