Nonhydrostatic Effects in the Parameterization of Non-Orographic Gravity-Wave Drag

In this study a parameterization of the drag associated with non-orographic gravity waves is developed for use in a general circulation model. In general, for computational efficiency current parameterization schemes make the hydrostatic approximation. The main consequence of this approximation is the neglect of the physical process of back-reflection. Here, an efficient implementation of the non-hydrostatic parameterization scheme suggested by Warner and McIntyre (1996) is developed and used in fully interactive multi-year climate simulations. In this way we identify qualitative and quantitative effects of including back-reflection in the parameterization of non-orographic gravity waves.

The results indicate that the momentum flux launched into the stratosphere at mid-latitudes can be reduced by as much as 75% due to the process of back-reflection. The momentum flux launched at tropical latitudes, however, is relatively unaffected and, so, is effectively larger in the non-hydrostatic scheme. This has important consequences for the forcing of tropical oscillations such as the SAO and QBO in the model. It is also found that the new scheme improves middle-atmosphere wind biases present in the model when compared to the climate produced by the default hydrostatic non-orographic GWD parameterization used currently.