Climatology of the middle atmosphere in LMDz: Impact of source-related parameterizations of gravity wave drag
Gravity wave (GW) parameterizations control the mean state and variability of the middle atmosphere in present-day climate models. The most recent parameterizations relate the GWs to their nonorographic sources (fronts and convection), which impacts the annual cycle of the GW drag, and makes the GWs respond to the changing climate. These issues are addressed using the Laboratoire de Météorologie Dynamique Zoom (LMDz) climate model, showing first a climatology of the middle atmosphere in the presence of nonorographic GW sources. The model performance is comparable with that documented in earlier model versions, illustrating that there are no major difficulties in including nonorographic GW sources in models. A twin experiment where the parameterization of GWs has no link with the nonorographic sources is also performed. Provided that in the twin experiment the launched GW stress is very intermittent, its climatology compares reasonably well with the experiment with sources. This illustrates that GW intermittency is a key factor in GW dynamics, but also that the dynamical filtering of the waves by the background flow strongly modulates the significance of the sources. Some impacts of having GW sources on the annual cycle of the zonal mean circulation of the middle atmosphere are nevertheless evident. In a changing climate, the impact of introducing GW sources also seems to be substantially mitigated by the dynamical filtering. The experiments and diagnostics are nevertheless limited in time and to the averaged climatology, respectively, calling for longer tests to measure the impacts on the atmospheric low frequency variability.