Simulations of the winter stratopause and summer mesopause at varying spatial resolutions

Abstract

The high‐altitude analysis from the Navy Operational Global Atmospheric Prediction System‐Advanced Level Physics High Altitude (NOGAPS‐ALPHA) forecast/assimilation system is used to initialize a series of 10 day forecasts at different spatial resolutions without any assumed gravity wave drag parameterization. As the spatial resolution is increased from 2.25° (spectral truncation at 79 total wave numbers, T79) to 0.75° (T239) and finally, to 0.375°(T479), the model simulation progressively improves compared with the analysis. The model vertical momentum fluxes are significantly greater at T479 relative to T239, consistent with resolving a greater fraction of the gravity wave spectrum. However, even at 0.375° resolution, a residual cold bias at the winter stratopause and warm bias at the summer mesopause remain, suggesting a continuing need for parameterized gravity wave drag. In addition, the zonal mean temperature solution is sensitive to the assumed value of spectral hyperdiffusion, thus highlighting an additional parameter that must be properly tuned in high‐resolution middle atmosphere models.