Effective vertical diffusion by atmosphere gravity waves

Abstract:

The vertical transport of momentum, heat and atmospheric species by small scale waves plays an important role in determining the wind, temperature and compositional structures of the atmosphere, including the thermosphere and ionosphere. Quantification of the transport in global models has been challenging due to the limited model resolutions, and is currently parameterized based on linear theories of gravity wave breaking and gravity wave dissipation. The parameterization schemes suffer from over-simplification or unphysical assumptions with regard to wave source, wave propagation and wave impact, and they do not usually account for the effects of wave ensemble, transience, or nonlinearity. Comparisons with observed compositional structure and effective eddy diffusion derived from various measurements suggest underestimation by the parameterized diffusion used in the model. In this study, a new approach is explored to quantify the effective eddy diffusion by using a high-resolution WACCM simulation and scale invariance. The WACCM simulation can partially resolve the mesoscale gravity wave spectrum down to ~250 km horizontal wavelength, and the heat flux and the effective vertical eddy diffusion by these waves are calculated directly. The effective vertical diffusion by the smaller-scale waves, which are under-resolved/unresolved, is then deduced based on scale invariance, following the method outlined by Liu (2019) in quantifying gravity wave momentum flux and forcing. The effective vertical diffusion obtained is generally larger than that obtained from parameterization, and is comparable with that derived from observations in the mesosphere and lower thermosphere region.