Interpretation of the structure of mesospheric turbulence layers in terms of inertia gravity waves

Abstract

MU radar observations with good time-height resolution have found that inertia gravity waves play an important role in producing turbulence layers in the mesosphere. When the Richardson number modified by the inertia gravity wave was larger than 1, the inertia gravity wave confined the altitude region with relatively small Richardson number, where smaller scale perturbations superposed on the wave seemed to make the Richardson number smaller than the critical values for instabilities, and produce turbulence. Scattering layers showed a descending motion at a vertical phase velocity of a monochromatic inertia gravity wave, and appeared at the altitude of the minimum Richardson number modified by the wave. A mixture of several layers with different descending motions was observed when wind fields consist of several gravity waves with different vertical wavelengths and phase velocities. On the other hand, when the Richardson number modified by the inertia gravity wave became as small as 0, the wave itself seemed to be dissipated through instabilities, and produce thicker turbulence layer than that in other cases. The radial wind velocities showed large fluctuations with a period of 9 min, which showed a phase reversal near the altitude of minimum Richardson number. The fluctuation seemed to be attributed to the shear instability induced by the saturated inertia gravity wave.