Small-scale waves, big implications: a regionally refined perspective with the Whole Atmosphere Community Climate Model

State-of-the-art global chemistry-climate models such as WACCM cannot practically resolve the small-scale gravity waves (GWs) that are important in the mesosphere and lower thermosphere (MLT, ≈ 70-120km). A solution is the use of parametrizations that represent subgrid dissipating GWs (see e.g. Garcia et al., 2007). To reproduce key MLT features such as mesospheric jet reversals, pole-to-pole circulation and the summer mesopause, models rely on such schemes (McLandress, 1997; Holton & Alexander, 2000), though more development is needed. For example, WACCM tends to underestimate observed mesospheric densities of O, O₃ and NO, and overestimate observed densities of the Na and Fe layers produced from cosmic dust ablation. Increasing evidence suggests a reason for this is a missing vertical transport from subgrid propagating GWs, and a solution has recently been achieved when these effects were included in the WACCM GW scheme (Guarino et al., 2023). In the current work, we resolve subgrid waves natively using WACCM with Regional Refinement (WACCM-RR). WACCM-RR provides the unprecedented opportunity to model the global climate up to altitudes of 140 km, and resolve individual regions down to as far as 1/32° at a low computational cost compared to global high resolution models. Trends from a model using a 1/8° grid over the Continental US  (1° elsewhere), when compared to a global 1° model, are consistent with comparisons of standard WACCM models, to models using our updated GW scheme. For example, mesospheric densities of O, O₃ and CO₂ are increased, as predicted. A surprising contrast is a globally warmer atmosphere, likely due to sensitivity of the meridional circulation to GW activity in the refined region. The results point to the applicability of WACCM-RR for detailed investigations of wave-transport processes, and their impact on MLT dynamics and composition. We point out remaining questions and challenges.