Resolved Gravity Wave Transport in WACCM-SE-RR
Gravity waves are one of the key processes through which the atmosphere adjusts to disturbances. They can be generated by convective heating, weather fronts, flow over hills and mountains, and even by other gravity waves when they break, and they are a dominant contributor to the global structure of the atmosphere through vertical transport of momentum, heat, and chemical species. At the relatively coarse ~100km resolution of climate models, much of the gravity wave spectrum must be parameterized. This means that the climate and variability of the middle and upper atmosphere are highly sensitive to the assumptions and simplifications of the gravity wave schemes. An NCAR Strategic Capability (NSC) project was established to study resolved gravity wave dynamics and transport within CESM2(WACCM6) using the regionally-refined Continental United States (CONUS) mesh with 100 km global resolution and 10 km refined resolution.
Figure 1. Gravity wave vertical momentum flux spectrum at 0.0001 hPa (the lower thermosphere) at (left) all longitudes and (right) in the CONUS regional refinement. Gray shading indicates areas of the spectrum unresolved by the simulation and the mixed space-time spectral analysis. Red lines indicate the theoretical maximum gravity wave phase speed, while dotted lines indicate periods of 1, 3, 6, and 12 hr.
Because gravity waves are a fast adjustment process, output was saved every 3.75 min model timestep, resulting in more than 1 TB/simulated day of model output across several year-long simulations, for a total of more than 1 PB of output. A postprocessing suite was created that converts this expansive output to zonal mean gravity wave fluxes and level-by-level gravity wave variance, momentum, and constituent transport spectra. Early results indicate that the regional refinement resolves more of the mesoscale (short wavelength) gravity wave spectrum, but also resolves more energy at nearly all wavelengths and phase speeds throughout the spectrum. Results from this project are guiding the development of a scale-adaptive gravity wave scheme that can operate on non-uniform meshes with regional refinement.