Effects Of Internal Gravity Waves Research Articles

Permanent monitoring of the upper mesosphere and lower thermosphere wind fields (prevailing and semidiurnal tidal components) obtained from LF D1 measurements in 1991 at the Collm Geophysical Observatory

The wind field of the upper mesosphere and lower thermosphere region (85–105 km) over Central Europe (52°N, 15°E) has been continually and reliably recorded by regular daily D1 radio wind measurements in the LF range (177, 225 and 270 kHz) using commercial radio transmitters. These measurements show the prevailing winds, the tidal wind components and the effects of internal gravity waves, as well as the seasonal and irregular variations of these parameters. The height of the wind measurements is determined by measuring the travel time differences between corresponding modulation bursts in the sky wave and in the ground wave. Using a quasi-online calculation procedure, the results are available immediately. Therefore they are useful for monitoring the upper atmospheric circulation with regard to upper atmosphere meteorology in the future. Vertical profiles of the wind field parameters can be derived with the aid of the combined wind and height measurements. Height-time cross-sections of the monthly mean prevailing winds and semidiurnal wind components have been calculated almost continuously for the last 10 years. The present paper deals with recent results for the year 1991.

Numerical simulations of the effects of gravity waves on noctilucent clouds

We have used a two‐dimensional numerical model of mesospheric cloud formation to simulate the effects of internal gravity waves on the formation and appearance of ice clouds at the mesopause. We have used gravity wave parameters (amplitude, wavelength, period, etc.) consistent with measurements of gravity waves in the high‐latitude mesosphere, including observations of band and billow structures in noctilucent clouds (NLCs). We have investigated both the dynamical effects due to wave‐induced winds and the microphysical effects due to wave‐induced temperature perturbations. We find that the structures observed in NLC from the ground are predominantly caused by dynamical processes. Since the time required for ice crystal growth at the mesopause is much longer than the observed wave periods and ice crystal sublimation is much more rapid than deposition growth, the primary effect of wave‐induced temperature 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 constraint on the formation of visible NLC. We find that the mean mesopause temperature must be about 5 K colder for NLC to form when gravity waves are present than when they are not present. Long‐period (>10 hours) waves may temporarily enhance the brightness of existing NLC but will not induce the formation of NLC.

Effects of Internal Gravity Waves on Energy Budgets and the Vertical Transport of Angular Momentum Over Mountainous Terrain

High-resolution pictures obtained with a handheld camera during the earth-orbiting photographic mission of the Apollo 9 spacecraft provided a unique opportunity to map the areal extent of gravity waves over the southwestern United States. The gravity waves were manifested in thin layers of middle- and high-cloud patterns over the rugged terrain of Arizona, Colorado, and New Mexico. Zonal shearing stress gradients in stably stratified air flowing over rugged mountain terrain indicated very large subgrid-scale vertical fluxes of zonal momentum. Maximum stress values of several tens of dynes·cm−2 (10−3 mb) were calculated for the lower tropospheric layers where the air flowed over the high Rocky Mountains of central New Mexico. The vertical fluxes of zonal momentum were directed upward to tropopause levels when extensive areas of gravity waves covered eastern Arizona, New Mexico, and western Texas. The potential energy contained in the wave population attained values through the troposphere as high as 9.3× 105 J·m−2; this potential energy was a substantial part of the atmospheric energy budget. The residual “dissipation” term in the kinetic energy budget indicated a subgrid-scale flux of energy downward from the lower stratosphere concurrent with the upward flux of zonal momentum in the troposphere. Atmospheric turbulence exhibited a maximum during the wave occurrence, diminishing gradually thereafter.