Aspects of gravity wave breaking predictability for the 11 January 1972 windstorm

The basic theory for inviscid mountain waves forced by air flow over a two-dimensional obstacle in a stratified atmosphere has been established for several decades. As vertically propagating internal waves amplify, in part due to the decrease of atmospheric density with altitude and nonlinear processes, the mountain waves may overturn and lose energy due to turbulent breakdown. The observations of the 11 January 1972 windstorm and wave breaking event in the lee of the Colorado Front Range, which were analyzed by Doug Lilly, Ed Zipser, and Joe Klemp, have inspired a generation of mountain meteorologists. Remarkably, these observations are still one of only a few cases of in situ measurements of mountain wave breaking and are widely considered classical. Although numerical models have been able to successfully simulate the general characteristics of gravity wave events such as the 11 January 1972 case, basic questions still exist regarding the predictability of gravity waves and wave breaking.

Multiple regions of complex upper-level wave breaking develop in the 11 January 1972 event and represent a challenging test for modern numerical models. The goal of this study is to examine the variability in simulated gravity wave breaking associated with varying initial conditions and model formulations that differ by small amounts, rather than focusing on replication of the fine-scale wind storm aspects. A set of random perturbations to the terrain and basic state wind profile is used to explore uncertainty in the initial state. Variations in the vertical mixing parameterization and horizontal advection are used to represent typical model formulation uncertainties. This ensemble of two-dimensional simulations is used to provide insight into the predictability of mountain waves and wave breaking for the 11 January 1972 event. The distribution of the wave-breaking regions is particularly complex in the stratosphere, and it follows that one focus is on the lower-stratospheric flow. The results indicate that although the general prediction of wave breaking is robust, considerable spread exists among the various realizations. The low-level wave breaking that is present just above the shooting flow region near the surface and the breaking in the lower stratosphere are particular sensitive to small variations in the initial state and vertical mixing parameterization, suggesting that the fine-scale features associated with wave breaking may be less predictable than the larger scale wave breaking, even in a two-dimensional framework. Additionally, the strength of the windstorm appears to be quite sensitive to the initial state as well, which points to basic predictability limits.