TIDES IN THE EXTENDED UGAMP GENERAL-CIRCULATION MODEL

In this paper the effect of tides on the zonal mean flow in the Extend ed UK Universities Global Atmospheric Modelling Programme General Circ ulation Model (EUGCM) is examined. Previous modelling studies have sho wn that omitting the diurnal cycle of solar radiation gives rise to ch anges in the zonal wind of the order of 10-20 m s-1 near the mesopause , and to larger changes in the lower thermosphere. In the EUGCM, which has a top level near the mesopause, turning off the diurnal cycle giv es rise to model zonal winds in July that are up to 65 m s-1 stronger near the equatorial mesopause. It is shown that such zonal wind change s are reduced by up to 20 m s-1 when the model horizontal diffusion is reduced by a factor of 100, but are increased by up to 10 m s-1 when the model vertical diffusion is switched off above the lower stratosph ere. Such large differences in zonal wind between diurnal and non-diur nal runs cannot be attributed to excessive model tidal amplitudes, sin ce they are found to be in good general agreement with both observatio ns and other modelling studies. An exception is the diurnal tide in th e subtropics, which decreases with height above about 0.01 mb instead of increasing to the model top. It is shown that this behaviour is ins ensitive to changes in the model diffusion, but it is suggested that i t could be caused by spurious reflections at the model top.