MESOSPHERIC VHF ECHOING LAYERS - AN INTERPRETATION OF CERTAIN OBSERVATIONS IN TERMS OF WAVE SCAVENGING

Excellent observations, obtained with the MU radar in Japan, have exhibited the existence on one occasion of three layers of enhanced VHF echoing from the mesosphere in association with a nearly monochromatic gravity-wave wind profile there. The heights of the layers did not correlate with levels of minimum Richardson number induced by the wave, however, and so an interpretation in terms of wave instability was initially rejected. Here it is shown that smaller-scale waves, unresolved by the radar, would be expected to have been approaching critical layers, and so to have been rendered unstable, at just those heights from which the strongest echoes were obtained. The same mechanism-the scavenging of wave energy for the production of turbulence-is likely to be operative on other occasions and at other heights.