Modelling atmospheric vertical coupling: role of gravity wave dissipation in the upper atmosphere

Recently, an increasing number of modelling and observational studies have looked at the signatures of small-scale waves, such as gravity waves (GWs) of meteorological origin, in the upper atmosphere. General circulation models require an appropriate GW parameterisation to reproduce a realistic global circulation. Existing schemes implemented into GCMs lack a physically proper wave dissipation mechanism above the turbopause. For more self-consistent wave propagation in GCMs, and to estimate their dynamic and energetic importance in the upper atmosphere, it is crucial to account for realistic upper atmospheric dissipation processes. The UCL Coupled Middle Atmosphere-Thermosphere-2 (CMAT2) general circulation model was developed in order to investigate atmospheric vertical coupling. This model is an updated version of its predecessor CMAT. Three GW parameterisations of different characteristics have been implemented into CMAT2. While they all, to some extent, reproduce the necessary mesosphere and lower thermosphere (MLT) dynamics, artificial dissipation above the turbopause is a problem in all these schemes. Therefore, an extended spectral non-linear GW parameterisation, which incorporates more physically realistic GW dissipation, has been developed. Using an offline column model, the sensitivity of GW propagation and dissipation in the thermosphere-ionosphere has been studied. Considerable GW propagation into the upper atmosphere is demonstrated. The associated wave forcing and heating are significant. This new extended scheme has been implemented in CMAT2. Its dynamical effects on the circulation of the upper atmosphere have been investigated. Results demonstrate the importance of GW momentum flux divergence in the thermosphere-ionosphere. The extended scheme improves the model simulations markedly with respect to an empirical model