Value added of regionally refined earth system models in simulation of extreme wind events

Extreme and impactful wind events in midlatitudes are often associated with cold-season extratropical cyclones that feature upper tropospheric jet streaks, vertically coupled ageostrophic circulations, and unstable conditions in the lower troposphere. Even though they are embedded in synoptic-scale circulations, it remains challenging to capture such events accurately in coarse resolution earth system models and to assess their sensitivity to climate change. We provide evidence that it is important to resolve mesoscale circulations with strong gradient winds to reliably capture these events, their large-scale vertically coupled signatures, and their sensitivity to climate change. We find that regional refinement of the horizontal grid of a global earth system model (NCAR's Variable Resolution Community Earth System Model - VR-CESM) from a standard resolution scale of 100 km to a fine scale of 7 km, separately tested over several domains centred on different urban regions of Canada, realistically captures the vertically coupled structure of extreme wind events in the refined regions. Furthermore, model projections of extreme winds under climate change in the refined regions reveal a consistent increase in strength of the strongest wind extremes, in response to climate change, that is largely absent or of mixed sign in standard-resolution simulations. An important driver of the strengthening response is enhanced turbulent mixing of momentum associated with boundary layer destabilization arising from surface amplified warming. This effect can compete with or reinforce other large-scale drivers, depending on the region. The strengthening response is a specific value-added (significant, consistent, resolution-dependent, explainable) effect that should be a target of investigation in other earth-system models. These findings add to growing evidence of the benefits of dynamical downscaling using regional refinement as a key frontier area of work with global earth system projections.

Morris, M., P. J. Kushner, G. W. K. Moore, and O. Mercan, 2023: Atmospheric circulation patterns associated with extreme wind events in Canadian cities. J. Climate, 36, 4443–4460, https://doi.org/10.1175/JCLI-D-22-0719.1.

Morris, M., P. J. Kushner, G. W. K. Moore, and O. Mercan, 2024: Resolution Dependence of Extreme Wind Speed Projections in the Great Lakes Region. J. Climate, 37, 3153–3171, https://doi.org/10.1175/JCLI-D-23-0547.1.