The Dynamics of Wave Clouds Upwind of Coastal Orography

Abstract

The Naval Research Laboratory?s Coupled Ocean?Atmosphere Mesoscale Prediction System (COAMPS) is used in conjunction with satellite observations and data from the Coastal Waves 1996 experiment to investigate the dynamics of unusual wave clouds that occur upwind and offshore of orographic features along the California coast. Results indicate that supercritical flow within the marine boundary layer, interacting with blocking coastal orography, is forced to decelerate and an atmospheric bow shock forms. The location and orientation of the COAMPS forecast shock matches well with the leading edge of the wave clouds in satellite imagery, and the modeled jump in boundary layer depth across the shock is in good agreement with the aircraft observations. In the parameter space of Froude number and jump strength that develops within the flow (observed and modeled), the shock manifests itself as an undular bore. On the innermost grid (?x = ? km), long, lineal variations in the wind, temperature, and moisture fields are forecast to develop on the subcritical side of the shock front and the modeled wavelength of these perturbations is close to the observed ?4 km wavelength of the cloud lines. Their cellular structure and the quadrature between the vertical velocity and potential temperature fields strongly suggest that these are trapped internal gravity modes. Further, solutions to the Taylor?Goldstein equation for stationary waves, using a model-computed Scorer parameter profile, provide a comparable estimate of ?3 km for a trapped, resonant wavelength. The subkilometer forecasts presented are the highest-resolution real data forecasts with COAMPS to date. Time-dependent outer boundary conditions are supplied to COAMPS by the Naval Operational Global Atmospheric Prediction System. The nonhydrostatic nature of the COAMPS model is essential to forecasting these nonhydrostatic, trapped waves.