SENSITIVITY OF THE EXTENDED UGAMP GENERAL-CIRCULATION MODEL TO THE SPECIFICATION OF GRAVITY-WAVE PHASE SPEEDS

Two simulations made using the Extended UGAMP General Circulation Model (EUGCM) are compared. The first uses a gravity-wave parametrization scheme that only includes orographically forced gravity waves with zero phase speeds, whilst the second uses a similar scheme, except that it includes a gravity wave with a phase speed of 20 m s-1. Results from the first simulation show that wintertime westerlies are reasonably estimated, but summertime and equatorial easterlies are excessive owing to an absence of gravity-wave drag in these regions. This absence of gravity-wave drag also means that the summertime mean meridional circulation is too weak, and summer mesospheric model temperatures are accordingly warmer than observations by up to 35 degC. Inclusion of gravity waves with phase speeds of 20 m s-1 reduces summer and equatorial easterlies and decreases summer mesosphere temperatures because, unlike zero phase-speed waves, these waves can propagate into the summer and equatorial middle atmosphere and thus transport momentum into those regions. Inclusion of the 20 m s-1 gravity wave also improves the simulation of the semiannual oscillation both at the stratopause and at the mesopause.