Evidence of Gravity Wave–Tidal Interaction Observed near the Summer Mesopause at Poker Flat, Alaska

An analysis of gravity wave-tidal interaction observed near the mesopause by the MST radar at Poker Flat in July of 1986 is presented. Our observations revealed daily mean wind maxima of ∼60 m s−1 westward and 20 m s−1 southward with daily mean momentum fluxes, contributed by gravity waves with periods less than 1 hour, of ∼4–5 m2 s−2 eastward and ∼1–2 m2 s−2 northward. Considerable hourly height variability was found to exist for both winds and momentum fluxes. A significant modulation of the fluxes by tidal winds was observed, characterized by out-of-phase correlations over a number of heights. Diurnal and semidiurnal winds and momentum fluxes exhibited very similar height variations, with amplitudes of ∼5–30 m s−1 and ∼1–5 m2 s−2, meridional winds leading zonal winds by ∼90°, and maximum fluctuating winds and momentum fluxes at lower levels. The observed variations in the amplitude and phase of winds and momentum fluxes provide some support to the gravity wave-tidal interaction model proposed by Fritts and Vincent and developed in this paper, and largely conform to theoretical expectations and other observations.