WACCM‐RR: A Regionally‐Refined Version of the Whole Atmosphere Community Climate Model

We introduce the Whole Atmosphere Community Climate Model with Regional Refinement (WACCM‐RR) and study the impact of directly resolving gravity waves (GWs) in the mesosphere and lower thermosphere (MLT; 70–140 km). Two simulations of 2010 are compared: “Non‐RR” is a standard WACCM case with a horizontal resolution of ∼1° (111 km) globally; and “RR” is a WACCM‐RR case with a ∼1/8° (14 km) nested grid over the contiguous United States (CONUS) and a 1° grid outside the nest. More GWs are resolved in RR and can be connected to tropospheric sources. Enhancements of downward fluxes of heat, O, NO, and CO are seen near the RR mesopause, doubling those of Non‐RR, which typically reduce these quantities in the lower thermosphere and increase them in the mesosphere. Temperature increases of 10–15 K, O by factors of 1.5–3, NO by 1.5–2.5, occur around 10⁻³ hPa (95 km), and a 10%–20% CO decrease is seen as much of it is transported below 10⁻² hPa (80 km). Enhanced upward CO₂ fluxes are observed, with CO₂ increasing by 50% at 10⁻⁵ hPa (125 km). Additionally, O₃ mixing ratios double at the secondary maximum where a doubling of O is seen. Compared to vertical heat flux estimates from Na LiDAR within CONUS, larger RR fluxes remain smaller than observed. Despite this, WACCM‐RR enables MLT GW studies and may address long‐term discrepancies between models and observations at significantly lower cost than global high‐resolution simulations.