Evidence of damping and overturning of gravity waves in the Arctic mesosphere: Na lidar and OH temperature observations

Lidar observations of the mesospheric sodium (Na) layer have been made at Poker Flat Research Range, Chatanika, Alaska (65°N,147°W) over a 4-year period. Long-period oscillations have been observed routinely in the bottomside of the Na layer. Simultaneous hydroxyl airglow temperature measurements have confirmed that these oscillations are associated with upwardly propagating gravity waves. These lidar observations have yielded statistically significant measurements of upwardly propagating gravity waves on 24 occasions. A gravity-wave model with pseudo-steady-state chemistry is used to determine the characteristics of the waves. These 24 waves have an average observed period of 6.9 h , average vertical wavelength of 14.2 km , average temperature amplitude of 8 K , and average horizontal velocity amplitude of 30 m/ s . These waves appear to be damped over the altitude of the Na layer with a growth length of 216 km . The waves do not appear to be damped by viscous dissipation or linear instabilities in the waves themselves. However, 20 of the 24 wave events are accompanied by overturning structures in the bottomside of the Na layer. These wave-overturning events have the same characteristics as those observed at Urbana, IL (40°N,88°W), that were interpreted as convective instabilities arising from the superposition of large- and small-scale waves. The current observations suggest that such convective instabilities are a relatively common feature at this high-latitude site and contribute to the damping of large-scale gravity waves.