Towards the implementation of a transient gravity wave drag parameterization in atmospheric models

The aim of the presented work is to improve the parameterization of subgrid-scale gravity wave (GW) effects on the resolved flow in atmospheric models in a large altitude range from the upper troposphere to the mesopause (~85km). State of the art GW parameterization schemes are using the steady-state approximation for the wave field and therefore assume an instantaneous GW propagation neglecting direct interactions between the GW field and the resolved flow within the whole altitude range mentioned above. As such, these schemes rely on dissipative processes - GW breaking and critical layer filtering - as the only mechanism to accelerate/decelerate the resolved flow. In contrast to this, by dropping the steady-state assumption a contribution to the mean-flow forcing emerges in the form of direct GW-mean-flow interactions. Several idealized studies show that, besides dissipative effects, direct GW-mean-flow interactions contribute to GW dynamics in an important extent (Bölöni et al., 2016, J. Atmos. Sci.}, 73, 4833-4852, Wilhelm et al., 2018, J. Atmos. Sci., 75, 2257-2280, Wei et al., 2019, J. Atmos. Sci., 76, 2715-2738). This motivates the implementation of a transient GW model (MS-GWaM: Multi Scale Gravity Wave Model) to UA-ICON, the upper atmosphere version of ICON (Borchert et al., 2019, Geosci. Model Dev., 12, 3541-3569) which does not rely on the steady-state assumption and thus includes direct GW-mean-flow interactions. As a reference and a representative of currently available GW parameterization schemes a steady-state version of MS-GWaM (ST-MS-GWaM) has been implemented to UA-ICON as well, which shares the treatment of all possible components (wave sources and wave saturation scheme) with the transient MS-GWaM scheme and differs from it "only" in the treatment of propagation, i.e. excluding direct GW-mean-flow interactions and thus transience. Both MS-GWaM and ST-MS-GWaM reproduce the observed wind and temperature climatology (e.g. URAP data: Swinbank, R. and D. A. Ortland, 2003, J. Geophys. Res., 108, D19, 4615) reasonably well, but the transient propagation makes a robust difference in the circulation in perpetual runs. The transient propagation in MS-GWaM substantially contributes to an increase of the GW intermittency in the whole altitude range, giving a better comparison with super-pressure balloon observations (e.g. Hertzog et al., 2012, J. Atmos. Sci., 69, 3433-3448), whereas the lack of transience prevents any occurrence of higher GW momentum flux values than the launch magnitude itself. This is explained by the fact that the direct GW-mean-flow interactions involve a highly transient evolution of the wave action and the vertical group velocity, which often leads to increased pseudo-momentum fluxes as compared to the launch magnitude.