Sensitivity of middle atmosphere GW forcing to vertical model resolution

Several current general atmospheric circulation models provide sufficiently high resolutions to resolve important parts of the internal gravity wave spectrum allowing for numerical experiments without GW drag parameterizations. GWs start to be well resolved from horizontal wavelengths of about 7 times the horizontal grid spacing. How much does the resolved wave spectrum and its forcing on the mean circulation depend on the vertical resolution?

−1,The middle atmosphere summer hemisphere provides a suitable background to investigate this question. The mean stratospheric and mesospheric circulation is characterised by prevailing easterlies which prevent planetary wave propagation upwards and represents a mean state driven by IGWs. The sensitivity of the forcing by IGWs is analysed on the basis of the Eliassen-Palm (EP) flux divergence, which describes the forcing on the circulation by resolved eddies.
Model simulations are performed using the upper atmosphere version of the ICON (ICOsahedral Nonhydrostatic) general circulation model, UA-ICON (Borchert et al. 2019, GMD). The simulations start in October and run for an extended austral summer season until March with a horizontal grid spacing of roughly 20 km. The top of the model atmosphere is located at 150 km. Three different model configurations are used with 90, 180, and 360 vertical model layers. The mean vertical grid spacing ranges from roughly 1300 m (90 layers) to 320 m (360 layers) at stratospheric levels, and from roughly 2300 m to 500 m at mesospheric levels. Gravity wave drag parameterizations (orographic and non-orographic) are turned off. The resolved forcing on the mean state due to the EP flux divergence is decomposed into contributions of different scales with respect to horizontal wave numbers. For contributions of IGWs wave numbers above 20 are considered.

The stratospheric and mesospheric easterlies appear stronger in the lower resolution from October to the end of the austral summer season. Westerlies occur above the mesopause. This strong vertical gradient in the zonal mean zonal wind amplifies in the lower resolution. At the beginning of the simulation period, differences between the mean states are weak, of the order of 5 ms⁻¹ , and strengthen during the summer season. The forcing due to internal GWs appears stronger in the lower resolution at higher altitudes and amplifies in the region of the strong vertical gradient of the zonal mean zonal wind. Furthermore, wave spectra are discussed. In accordance with previous studies, an increased vertical resolution results in a reduction of the IGW forcing close to strong zonal mean zonal wind gradients in the upper mesosphere/lower thermosphere.