Unusual wave clouds occur infrequently along the California coast, primarily during summer months. These wave clouds, generally most prominent to the west of the Monterey Peninsula, typically appear near midday and are absent the next morning. They are not typical boundary layer roll vortices as they are align more nearly perpendicular than parallel to the prevailing upstream wind. Further, they are neither lee waves as they occur upwind of the orography of the Monterey peninsula, nor do they exhibit the character of ordinary Kelvin-Helmholtz billows. And, they are not confined to a narrow, Kelvin wedge characteristic of the ship bow wave solution. We use the nonhydrostatic Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), nested down to an inner mesh having 1/3 km grid spacing, to produce a real data forecast on a day when the wave clouds were observed (Fig. 1). The NCAR EC130Q aircraft, flown during the Coastal Waves experiment, made measurements of the wave clouds that are used for comparison with COAMPS results.
Near 2200 UTC (1500 LT) the COAMPS forecast winds strengthen and a remarkable feature appears in the boundary layer wind field. The flow approaching the coast undergoes an abrupt directional and speed change along an arc extending either side of the Monterey Peninsula. This flow structure resembles an attached, oblique shock. The blocking and deflection of the wedge-shaped Monterey Peninsula forces the approaching supercritical flow to decelerate, and the Froude number becomes subcritical across the jump.
Shock angle computations for a range of incoming Froude numbers indicate an oblique shock angle of ~700 is to be expected, which is close to the angle formed by both the modeled shock and the observed cloud lines relative to the peninsula’s ridge axis. Downwind of this compression jump, the model forecasts extensive cellular structures showing quadrature between the temperature and vertical velocity fields. These long, lineal variations appear to be trapped internal waves and their model-forecast wavelength agrees well with the ~4 km wavelength of the observed wave clouds.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics