An intercomparison of model-predicted wave breaking for the 11 January 1972 Boulder windstorm

Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, winds torm, obtained from 11 diverse nonhydrostatic models, are intercompared wit h special emphasis on the turbulent breakdown of topographically forced gra vity waves, as part of the preparation for the Mesoscale Alpine Programme f ield phase. The sounding used to initialize the models is more representati ve of the actual lower stratosphere than those applied in previous simulati ons. Upper-level breaking is predicted by all models in comparable horizont al locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the bre aking include the following: pronounced turbulence in the 13-16-km and 18-2 0-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpos itioned above the low-level hydraulic jump. Sensitivity experiments indicat e that the structure of the wave bi caking was impacted by the numerical di ssipation, numerical representation of the horizontal advection, and latera l boundary conditions. Small vertical wavelength variations in the shear an d stability above 10 km contributed to significant changes in the structure s associated with wave breaking. Simulation of this case is ideal for testi ng and evaluation of mesoscale numerical models and numerical algorithms be cause of the complex wave-breaking response.