A Numerical Study of Gravity Wave Breaking and Impacts on Turbulence and Mean State

Abstract

A model system is established that includes three interactive components: a dynamics model, a turbulence model, and a chemistry model. The dynamics model solves the two-dimensional, nonlinear, nonhydrostatic, compressible, and viscous flow equations, and the turbulence model is adapted from the 2.5-level Mellor?Yamada turbulence model with minor adjustments. The dynamics and the turbulence models are coupled with a chemistry model to study the mesoscale impacts of gravity wave breaking on the atmospheric compositional structures. The present study focuses on the local distribution of atomic oxygen and ozone. The model system is used to study the gravity wave propagation, growth, breakdown, and its impacts on the mean state in the middle and upper atmosphere. The inclusion of a turbulence model makes it possible to study the long-term evolution of the gravity wave after wave breaking and in the presence of nonuniform turbulence, as well as the interaction between a breaking wave and turbulence. The turbulence model parameterizes the three-dimensional mixing due to the flow instability and it eliminates the unrealistically strong supersaturation observed in previous two-dimensional simulations. The modeling result suggests that the induced acceleration due to convective instability may lead to strong shear, which causes dynamical instability at lower altitudes. The result reveals the interdependence of waves and turbulence and shows that the turbulence energy density due to instability has similar temporal and spatial characteristics to previous radar observations. The result is also compared with the linear saturation theory, and it is found that the eddy diffusion coefficients in the wave-breaking region are nonuniform, and the average values are less than those obtained from the linear saturation theory. The result also suggests that the inclusion of the turbulence model could be a valid approach to study the averaged two-dimensional gravity wave and turbulence features after wave breaking. More adjustments of the turbulence model parameters, according to upper-atmosphere observations and turbulence physics studies using large eddy simulation and direct numerical simulation methods for three-dimensional gravity wave?breaking processes, are necessary to improve the model performance in future studies.