The interaction between mountain waves and an evolving synoptic-scale flow

In the 1970s Doug Lilly led several efforts to observe mountain waves and their associated momentum fluxes over the Colorado Rockies. Data collected in situ on January 11, 1972 stands to this day as the largest amplitude mountain wave ever documented. Doug was also among the first to examine the influence of parameterized mountain wave drag in a general circulation model.

This talk will review some of Doug's ground-breaking mountain-wave research and then present recent results demonstrating how slow changes in the cross-mountain flow (over a two-day period) can produce mountain waves whose momentum fluxes differ dramatically from the predictions of steady-state theory. The feedback of breaking mountain waves on a slowly evolving barotropic jet is examined, and it is demonstrated that even a perfect knowledge of the time evolution of the vertical momentum flux profile cannot be used in the context of current gravity wave drag parameterizations to yield an accurate estimate of the large-scale (balanced) flow response. Future parameterizations might do better to estimate the potential vorticity perturbations generated by the breaking waves.