Radar measurements of critical-layer absorption in mountain waves

Abstract

The conditions controlling the vertical propagation of mountain waves in the troposphere and lower stratosphere have been examined using radar observations at 46.5 MHz of vertical-velocity perturbations at Aberystwyth (52.4°N, 4.1°W) and radiosonde observations from Aberporth, some 50 km to the south-west. Attention has been paid to the influence of the variation with height of the mean wind vector (U), the Brunt–Väisälä frequency and the Scorer parameter. Four case-studies show the absorption of mountain waves at heights where the mean wind component normal to the wave front vanishes; this corresponds to U = 0 if the wind direction does not change with increasing height, and to the wind vector being orthogonal to the wave vector if the wind rotates with increasing height. For two cases, enhanced turbulence is found in the upper few hundred metres of the mountain-wave field, directly below the height at which the waves disappear; for a third case, the enhancement extends over a broader range of heights, and for the fourth no clear identification of the enhancement is possible. The frequency power-spectra of the vertical-velocity oscillations show a ωn dependence, with n approaching −5/3 for regions of mountain-wave activity below the critical layers, and n = 0 for heights above the critical layers. Statistically, mountain waves generated by low-level easterly winds are often confined to tropospheric heights. An analysis of a large sample of radiosonde data indicates that critical-layer absorption associated with rotation of the background wind in addition to wave reflection and trapping, plays an important role in this confinement. The results are discussed in terms of the momentum flux associated with gravity waves, their turbulent breakdown near the critical layer, and the implications of the frequency spectra below and above such layers.