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 Mode l (EUGCM) are compared. The first uses a gravity-wave parametrization scheme that only includes orographically forced gravity waves with zer o phase speeds, whilst the second uses a similar scheme, except that i t includes a gravity wave with a phase speed of 20 m s-1. Results from the first simulation show that wintertime westerlies are reasonably e stimated, 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 circu lation is too weak, and summer mesospheric model temperatures are acco rdingly warmer than observations by up to 35 degC. Inclusion of gravit y waves with phase speeds of 20 m s-1 reduces summer and equatorial ea sterlies 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 re gions. Inclusion of the 20 m s-1 gravity wave also improves the simula tion of the semiannual oscillation both at the stratopause and at the mesopause.