The generation of mesospheric planetary waves by zonally asymmetric gravity wave breaking

A semispectral numerical model is used to study the influence of a longitudinally varying gravity wave source on the general circulation of the winter mesosphere. The gravity wave source consists of stationary (topographic) waves with a longitudinally varying amplitude distribution that is approximated by the first two terms in a zonal harmonic expansion (i.e., the zonal mean plus planetary wavenumber 1). The computed zonal mean circulation in the mesosphere is nearly the same as that computed for a zonally symmetric gravity wave source of equal amplitude. However, the asymmetric source excites a strong stationary wavenumber 1 disturbance near the level of gravity wave breaking (equal to about 71 km). This disturbance has a zonal wind maximum about 1/4-cycle upstream from the gravity wave drag maximum. It is concluded that vertically propagating gravity waves produced in the troposphere are a possible source for mesospheric planetary waves.