Large-Scale Structure of the June–July 1996 Marine Boundary Layer along California and Oregon

Abstract

Data from surface stations, profilers, long-range aircraft surveys, and satellites were used to characterize the large-scale structure of the marine boundary layer off of California and Oregon during June and July 1996. To supplement these observations, June?July 1996 averages of meteorological fields from the U.S. Navy?s operational Coupled Ocean?Atmospheric Mesoscale Prediction System (COAMPS) model were generated for the region. Model calculations show a broad band of fast northerly surface winds exceeding 7 m s?1 extending along the California?Oregon coast. Buoy-measured peaks of 7.1 m s?1 off Bodega Bay, 7.2 m s?1 off Point Piedras Blancas, and 8.8 m s?1 near Point Conception were reported. Mean winds at the buoys located 15?25 km offshore are generally faster than those at coastal stations, and all station winds are faster in the afternoon. The aircraft and station observations confirm that an air temperature inversion typically marks the top of the marine boundary layer, which deepens offshore. Along the coast, the marine boundary layer thins between Cape Blanco and Santa Barbara. The inversion base height is at its lowest (195 m) at Bodega Bay in northern California and at its highest at Los Angeles and San Diego (416 m). The inversion strength is strongest between Bodega Bay and Point Piedras Blancas, exceeding 10.8°C. The June?July 1996 marine boundary layer depth from COAMPS shows a gradual deepening with distance offshore. The model-averaged flow within the marine boundary layer is supercritical (Froude number > 1) in a region between San Francisco and Cape Mendocino that extends offshore to 126.4°W. Smaller isolated supercritical areas occur in the lee of every major cape, with the peak Froude number of 1.3 in the lee of Cape Mendocino. This is consistent with aircraft flights of Coastal Waves ?96, when extensive regions of supercritical flow off central California and downwind of major capes were recorded with highest Froude numbers around 1.5?2.0. A broad, wedge-shaped area of nearly critical flow (Froude number > 0.8) extends from Cape Blanco to Point Piedras Blancas and offshore to about 128.5°W in the model output. The model wind stress has a broad maximum exceeding 0.3 N m?2 between Cape Mendocino and San Francisco with the highest values found within 100 km of the coast. Stress calculated directly from low aircraft legs is highest in the lee of large capes with peak values exceeding 0.7 N m?2. Overall aircraft magnitudes are similar to the model?s, but a direct comparison with the 2-month average from the model is not possible due to the lesser space and time coverage of the flights. The stress maxima along the California coast shown in the model results are spatially consistent with the region of coldest sea surface temperature observed by satellite.