Wa. Bristow et al., ON THE SEASONAL DEPENDENCE OF MEDIUM-SCALE ATMOSPHERIC GRAVITY-WAVES IN THE UPPER-ATMOSPHERE AT HIGH-LATITUDES, J GEO R-S P, 101(A7), 1996, pp. 15685-15699
Seasonal dependence of medium-scale atmospheric gravity waves is exami
ned in SuperDARN HF radar data and through analysis of the gravity wav
e dispersion relation. We found that the probability of gravity wave o
bservation in the HF radar data is highest in winter months and lowest
in summer months. The winter probability of observation in a given 2-
hour period with sufficient amounts of ground backscatter peaks at abo
ut 0.8, while in the summer the probability remains near 0.4. Examinat
ion of the dispersion relation shows that there is a seasonal dependen
ce to the altitude profile of the gravity wave's vertical wavelength t
hat may lead to a seasonally dependent reflection of the waves resulti
ng from the mesospheric temperature gradient. The dispersion relation
predicts that for waves of a given horizontal wavelength, the minimum
wave period that may be transmitted through the mesosphere is longer i
n the summer than in the winter; the longer the wavelength, the longer
the minimum period. Thus some waves that are transmitted through the
mesosphere in the winter may not be transmitted in the summer. This se
asonal dependence predicted from the dispersion relation is consistent
with the HF radar observations.