The origin of stationary planetary waves in the upper mesosphere

Satellite observations indicate that quasi-stationary planetary waves often exist at least 100 km in the winter mesosphere. Waves are also seen in the summer upper mesosphere. A three-dimensional numerical model was used to simulate these waves and to diagnose the physical processes involved. The waves simulated in the model closely resemble observed waves. Several model runs that isolate specific processes are used to determine the relative importance of two forcing mechanisms. In the model, planetary waves that propagate from below are significantly damped at the altitude where gravity wave drag becomes large (about 75 km in the winter midlatitudes) or below if a reversal in the mean wind is encountered. Momentum forcing associated with breaking gravity waves that have been filtered by planetary-scale wind variations below acst to generate planetary waves in the middle and upper mesosphere. The amplitude from in situ forcing by gravity wave breaking exceeds the amplitude from the upward-propagating Rossby wave above 80 km.