The GCM response to current parameterizations of non-orographic gravity wave drag

A number of non-orographic gravity-wave drag (GWD) parameterization schemes have been developed for use in middle atmosphere general circulation models (GCMs). Central to each scheme are assumptions regarding how best to account for the nonlinear breakdown and turbulent dissipation of the wavefield. In principle, each scheme differs only in the method by which dissipation is applied. Debate within the community regarding the validity and appropriateness of each scheme has been vigorous and ongoing. Here we consider the more practical issue of identifying and understanding differences in the response that arise solely from the method of dissipation employed in each scheme. We accomplish this by building the dissipation mechanisms from three of the most widely used schemes (Hines 1997, Alexander and Dunkerton 1999, and Warner and McIntyre 2001) into one parameterization so that no other factors enter into the comparison.

GCM simulations reveal differences in the climatological response to the three dissipation mechanisms. These differences are documented for both tropopause and surface launch elevations of the parameterized waves. The simulations also reveal systematic differences in the height at which momentum is deposited. This behaviour is investigated further in a set of experiments designed to reduce these systematic differences, while leaving the details of the dissipation mechanisms unaltered. These sensitivity experiments demonstrate that it is possible to obtain nearly identical responses from all three mechanisms, which indicates that the GCM response is largely insensitive to the precise details of the dissipation mechanisms. The results of this study suggest that climate modelling efforts would potentially benefit more from the refinement of other aspects of the parameterization problem, such as the properties of the launch spectrum, than they have benefited from the refinement of dissipation mechanisms.