Impact of model resolution and turbulence scheme on the representation of mountain waves and turbulence

Simulating mountain waves and associated turbulence in the upper troposphere and lower stratosphere (UTLS) remains a challenge in numerical weather prediction (NWP). We investigate how the representation of mountain-wave dynamics and turbulence in the ICOsahedral Nonhydrostatic (ICON) model depends on model resolution and turbulence parameterization. ICON simulations were performed in NWP mode (ICON-NWP) with varying horizontal (2, 1 km, 500 m) and vertical (400, 200, 100 m) resolutions, using the operational turbulent kinetic energy scheme (ICON-TKE) and the newly developed two-energy turbulence scheme (ICON-2TE). The simulations were evaluated against high-frequency in situ observations from the Deep Propagating Gravity Wave Experiment (DEEPWAVE) over New Zealand on 12 July 2014, as well as a nested large-eddy simulation (ICON-LES) at 130 m resolution. The results show reasonable agreement with observations: ICON-LES more closely captures wavelength and phase, while ICON-NWP better reproduces wave amplitude. Near-convergence of the primary mountain wave and of turbulence structures requires horizontal grid spacings of 1 km or finer and vertical spacings in the UTLS of 200 m or finer, whereas trapped lee waves and downstream wave structure remain resolution-sensitive at these resolutions. Area-averaged, bulk measures allow this convergence behavior to be characterized more systematically: the low-level gravity-wave momentum flux continues to increase with increasing resolution from 1 km to 500 m. A key finding is that both turbulence schemes yield similar wave structures, despite large differences in simulated turbulent kinetic energy. This discrepancy is attributed to the empirical horizontal-shear source term of the operational TKE-scheme configuration, which produces spurious TKE at km-scale resolution. These results provide guidance on the resolution and turbulence representation needed for reliable simulations of small-scale mountain waves and turbulence in the UTLS.