The effect of back-reflection in the parameterization of non-orographic gravity-wave drag

Abstract

An efficient method of solution is developed for the Warner and McIntyre parameterization of the drag associated with nonhydrostatic non-orographic inertia-gravity waves. The scheme is sufficiently fast as to enable, for the first time, fully interactive multi-year climate simulations that include the effects of rotation and nonhydrostatic wave dynamics in the parameterization of non-orographic gravity wave drag. It is found that the new scheme alleviates much of the middle-atmosphere wind biases that occur in the Canadian Middle Atmosphere Model when either of its two operational hydrostatic non-orographic gravity wave drag parameterizations are used. The addition of the nonhydrostatic process of back-reflection has an important impact on the amount momentum flux launched into the stratosphere by the parameterization scheme. This quantity is a free parameter in the problem and it is specified here to be independent of time and geographic location. However, due to back-reflection, the net momentum flux that actually enters the stratosphere undergoes a systematic seasonal and latitudinal variation which is a consequence of the seasonal and latitudinal variation of the winds and temperatures in the middle atmosphere. This strong influence results in a characteristic latitudinal distribution for the net momentum flux entering the stratosphere in winter and summer. During these seasons, mid- to high-latitude launch momentum flux that is directed oppositely to the mesospheric jet can be reduced by as much as 75% due to back-reflection. The momentum flux launched into the stratosphere at tropical latitudes, however, is relatively unaffected by back-reflection. This has important implications for the forcing of tropical oscillations such as the semi-annual oscillation and the quasi-biennial oscillation in general circulation models.