NUMERICAL SIMULATIONS OF THE EFFECTS OF GRAVITY-WAVES ON NOCTILUCENT CLOUDS

We have used a two-dimensional numerical model of mesospheric cloud fo rmation to simulate the effects of internal gravity waves on the forma tion and appearance of ice clouds at the mesopause. We have used gravi ty wave parameters (amplitude, wavelength, period, etc.) consistent wi th measurements of gravity waves in the high-latitude mesosphere, incl uding observations of band and billow structures in noctilucent clouds (NLCs). We have investigated both the dynamical effects due to wave-i nduced winds and the microphysical effects due to wave-induced tempera ture perturbations. We find that the structures observed in NLC from t he ground are predominantly caused by dynamical processes. Since the t ime required for ice crystal growth at the mesopause is much longer th an the observed wave periods and ice crystal sublimation is much more rapid than deposition growth, the primary effect of wave-induced tempe rature perturbations is to decrease the albedo of the clouds. The fact that gravity waves with periods less than a few hours are ubiquitous near the high-latitude, summertime mesopause imposes an additional con straint on the formation of visible NLC. We find that the mean mesopau se temperature must be about 5 K colder for NLC to form when gravity w aves are present than when they are not present. Long-period (> 10 hou rs) waves may temporarily enhance the brightness of existing NLC but w ill not induce the formation of NLC.