An Intercomparison of Model-Predicted Wave Breaking for the 11 January 1972 Boulder Windstorm

Abstract

Two-dimensional simulations of the 11 January 1972 Boulder, Colorado, windstorm, obtained from 11 diverse nonhydrostatic models, are intercompared with special emphasis on the turbulent breakdown of topographically forced gravity waves, as part of the preparation for the Mesoscale Alpine Programme field phase. The sounding used to initialize the models is more representative of the actual lower stratosphere than those applied in previous simulations. Upper-level breaking is predicted by all models in comparable horizontal locations and vertical layers, which suggests that gravity wave breaking may be quite predictable in some circumstances. Characteristics of the breaking include the following: pronounced turbulence in the 13-16-km and 18-20-km layers positioned beneath a critical level near 21-km, a well-defined upstream tilt with height, and enhancement of upper-level breaking superpositioned above the low-level hydraulic jump. Sensitivity experiments indicate that the structure of the wave breaking was impacted by the numerical dissipation, numerical representation of the horizontal advection, and lateral boundary conditions. Small vertical wavelength variations in the shear and stability above 10 km contributed to significant changes in the structures associated with wave breaking. Simulation of this case is ideal for testing and evaluation of mesoscale numerical models and numerical algorithms because of the complex wave-breaking response.

The Effects of Surface Friction on Downslope Wind and Mountain Waves

“…1. The same initial condition has been used in Doyle et al (2000). Hsu and Sun (2001), Chen and Sun (2001), and others discussed in Doyle et al (2000).…”

“…The domain consists of 471 grids in the x-direction with ∆ x = 1 km and 400 levels in the z-direction with ∆ z = 75 m, except the first level above the ground surface where ∆ z = 25 m. The mountain profile is a witch of Agnesi curve, z x h x a s ( ) /( / ) = + 1 2 2 , where h =2 km and a = 10 km, which are typical values for the Colorado Front Range (Doyle et al 2000). The initial condition (same for both free-slip and no-slip boundaries) is horizontally homogeneous and based upon the upstream 1200UTC 11 January 1972 Grand Junction, Colorado, sounding shown in Fig.…”

“…The same initial condition has been used in Doyle et al (2000). Hsu and Sun (2001), Chen and Sun (2001), and others discussed in Doyle et al (2000). It is noted that a semi-implicit scheme is used to filter acoustic waves in Chen and Sun (2001), which is quite different from the model presented here.…”

“…Using the Grand Junction, Colorado sounding of 11 January 1972, many scientists (Klemp and Lilly 1975;Clark and Peltier 1977;Durran and Klemp 1983;Ikawa 1988;Doyle et al 2000;Hsu and Sun 2001;Chen and Sun 2001) have applied numerical models to simulate the mountain waves, the hydraulic jump, and the strong downslope wind observed on the lee of the Rocky Mountains (Lilly and Zipser 1972;Lilly 1978).…”

A nonhydrostatic unstructured-mesh soundproof model for simulation of internal gravity waves

“…In particular, unstructured-mesh simulations of internal gravity waves are scarce. In the Earth's atmosphere these waves are both ubiquitous and intricate, as their occurrence and form depend on a relative magnitude and structure of ambient flow, density/entropy stratification and forcing (Smith, 1979;Wurtele et al, 1996;Doyle et al, 2000). Consequently, their numerical simulation constitutes a canonical aspect of weather-prediction and climate models.…”