WAVE BREAKING SIGNATURES IN SODIUM DENSITIES AND OH NIGHTGLOW .2. SIMULATION OF WAVE AND INSTABILITY STRUCTURES

Measurements of atmospheric structure and dynamics near the mesopause were performed using a sodium lidar, an MF radar, and a nightglow CCD camera during the CORN campaign performed in central Illinois during S eptember 1992. The major features of the observed structure on Septemb er 27/28 include a low-frequency, large-scale wave accounting for pers istent overturning of the temperature and sodium density fields, super posed higher-frequency motions, small-scale transient ripples in the n ightglow images suggestive of instability structures, and large-scale wind shear near the height of apparent instability. We describe four s imulations of wave breaking with a three-dimensional model designed to assist in the interpretation of these observations. Two simulations a ddress the instability of a low-frequency wave in a background shear f low with and without higher-frequency modulation. These show higher-fr equency motions to be important in assigning the spatial and temporal scales of instability structures. Two other simulations examine the in stabilities accompanying a convectively unstable inertia-gravity wave with and without higher-frequency modulation without mean shear. These show the instability structure to remain aligned in the direction of wave propagation, with only weak influences by the high-frequency moti on. Our results suggest that instability due to a superposition of wav es accounts best for the nightglow features observed during the CORN c ampaign and that streamwise convective instabilities observed due to w ave breaking at higher intrinsic frequencies continue to dominate inst ability structure for internal waves for which inertial effects are im portant.