Simulations of the winter stratopause and summer mesopause using NOGAPS-ALPHA at varying resolutions

It is well understood that at high latitudes, the warm winter stratopause and cold summer mesopause are essentially dynamical features. Both are consequences of breaking gravity waves which deposit their momentum on the zonal flow and drive the atmosphere away from a radiative equilibrium condition. Because gravity waves typically are manifest on spatial scales smaller than typical theoretical model grid sizes, their effects in global models must be accounted for by parameterizations, rather than via explicit resolution. However, with recent increases in computing power, three dimensional global models of the middle atmosphere are developing the capacity to resolve at least a portion of the gravity wave spectrum. The implication is that, with sufficient spatial resolution, gravity wave parameterizations would no longer be necessary. We evaluate the ability of our model, NOGAPS-ALPHA, to simulate both the warm winter stratopause and cold summer mesopause via explicitly resolved gravity waves by performing simulations with varying vertical and horizontal resolutions. Our “typical” middle atmospheric forecast-assimilation cycle is at T79. We show this to be too coarse to resolve the waves necessary to capture these dynamical features. As the model resolution is increased to T239 and then T479, progressive improvements are seen in comparison with the analysis. We also test the effects of either increasing the horizontal or the vertical resolution. Preliminary comparisons with our analysis suggest that calculated extratropical forecast temperature fields are most sensitive to the horizontal resolution.