Measurement of Momentum Fluxes near the Summer Mesopause at Poker Flat, Alaska

Abstract

Observations of the motion field near the summer mesopause using the Poker Flat MST radar in a symmetric six-beam configuration during 8 days of July 1986 were used to examine the mean structure, the wave variances, and the momentum fluxes due to gravity wave and tidal motions. Our results reveal a mean horizontal wind structure generally consistent with previous observations, but with considerable daily variability and large mean shears. Particularly significant is a mean downward (Eulerian) vertical velocity of ∼0.3 m s⁻¹, which implies a significant upward flux of wave energy. Horizontal and vertical velocity variances are found to be ∼1500 and 6 m² s⁻² and to remain nearly constant with height. Momentum flux measurements reveal a largely zonal mean flux of ∼5 to 15 m² s⁻² that achieves a maximum just below the height of wind reversal. Daily mean values, on the other hand, exhibit large variability, with maxima as large as ∼30 to 40 m² s⁻². Hourly momentum flux profiles were found to achieve maximum values of ∼60 m² s⁻² and to exhibit variations that appear to correlate with the background wave environment. These observations imply significantly stronger forcing of the mean flow in this region than has been inferred at lower latitudes.