Tides in the Extended UGAMP General Circulation Model

Abstract

AbstractIn this paper the effect of tides on the zonal mean flow in the Extended UK Universities Global Atmospheric Modelling Programme General Circulation Model (EUGCM) is examined. Previous modelling studies have shown that omitting the diurnal cycle of solar radiation gives rise to changes 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 gives 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 changes 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 stratosphere. Such large differences in zonal wind between diurnal and non‐diurnal runs cannot be attributed to excessive model tidal amplitudes, since they are found to be in good general agreement with both observations and other modelling studies. An exception is the diurnal tide in the subtropics, which decreases with height above about 0.01 mb instead of increasing to the model top. It is shown that this behaviour is insensitive to changes in the model diffusion, but it is suggested that it could be caused by spurious reflections at the model top.