The impact of conservation of momentum in gravity wave drag parameterization on planetary wave structure and variability

We examine the impact of conserving gravity wave momentum flux in the Canadian Middle Atmosphere Model on the modelled planetary waves. The model is run for 40 years with two model lid heights: 10 hPa and 0.001 hPa. When the lid is placed at 0.001 hPa the planetary wave heat and momentum fluxes are in good agreement with observations and there is little sensitivity to conservation. When the lid is placed at 10 hPa there is a significant degradation of the planetary waves which are deflected equatorward near the model lid leading to a cold bias which is most severe when momentum is not conserved. In winter in the northern hemisphere, momentum conservation ensures that the amplitude, phase, variability (both in terms of the meridional heat flux and stratospheric sudden warmings) are in reasonable agreement with the high-lid model. When momentum is not conserved, the spurious deflection is much larger and this negatively impacts the planetary wave phase, amplitude and variability. In the southern hemisphere in winter, we find a large impact of the low lid on the planetary wave behaviour. In particular, the stronger Antarctic jet leads to more planetary wave deflection and larger differences between the low and high-lid models even when momentum is conserved. However, overall momentum conservation always leads to a closer agreement between the low and high-lid models. The results have implications for low-lid models which try to capture the dynamical response to ozone depletion in the southern hemisphere.