Physical process studies and the representation of non-orographic gravity waves in
the middle atmosphere of the Met Office global model

Andrew Bushell* and Neal Butchart
Met Office

For current state-of-art Earth system models (ESMs), parameterization of small-scale gravity waves is essential to represent the middle atmosphere momentum budget to an acceptable accuracy. Key features such as the mean strength of polar night jets or quasi-biennial oscillation of the equatorial stratosphere are captured in ESMs with non-orographic gravity wave schemes able to represent realistic zonal mean budgets. Such models, however, characteristically underestimate middle atmosphere tropical variability, with quasi-biennial and semi-annual oscillations that appear more regular than is suggested by observational evidence. The Met Office model (MetUM) operates seamlessly from the relatively coarse horizontal resolutions favoured by ESMs to O[10km] scales for global numerical weather prediction (NWP) and O[1km] in regional configurations which can resolve fine-scale gravity waves that would require parameterization in ESMs.

MetUM simulations have recently been run (Bushell et al., 2015) to test a new simple parametrization of gravity wave source amplitudes based on a relationship with total precipitation, derived empirically from analysis of results with a convection-permitting configuration of the same model. The new source parameterization replaces the previous invariant source within the existing spectral scheme. Consequences of the introduction for model behaviour include, in a climate change context, the ability for alterations in pattern and occurrence of convection arising from future global warming to affect gravity wave generation and hence feedback on large-scale phenomena such as the Brewer-Dobson Circulation. In addition to source flux amplitudes, other aspects of the spectral scheme that might be reassessed are the saturation /dissipation processes and assumption of purely vertical propagation. As model resolutions are progressively improved and shorter wavelength waves can be sustained, new issues are raised such as whether appropriate levels of dissipation will be achievable in the absence of the really fine vertical structure required to create realistic critical layers.


*email: andrew.bushell@metoffice.gov.uk
*Preference: Oral