Interaction between gravity waves and the 2-day wave in the Whole Atmosphere Community Climate Model

NCAR's Whole Atmosphere Community Climate Model, version 2 (WACCM2), is a General Circulation Model extending from the surface to ~ 150 km. It includes fully interactive chemistry and a suite of physical parameterizations for the middle atmosphere (UV heating shortward of 200 nm, NLTE infrared transfer, molecular diffusion, and ion drag). The effects of mesoscale gravity waves are calculated using a Lindzen-type parameterization, with a source spectrum specified in the troposphere.

We use WACCM2 to examine the interaction between gravity waves and the 2-day wave. We find that the amplitude of the 2-day wave is excited with large amplitude in the model during the solstice seasons; at other times of the year the amplitude of the wave is negligibly small. Model calculations indicate that baroclinic instability of the summer jet in the mesosphere accounts for the large amplitude of the 2-day wave during solstice. This unstable wind distribution is the result of mesoscale gravity wave drag, which reverses the summer easterlies above about 80-90 km. In the time-mean, the model's summer climatology results from a balance between the destabilizing effects of mesoscale gravity wave drag and the EP flux divergence due to the 2-day wave, which acts to remove the instability.