Supercritical flow interaction within the Cape Blanco-Cape Mendocino Orographic Complex

Supercritical flow interaction occurring in the marine boundary layer betwe en closely spaced coastal capes is investigated with a mesoscale numerical prediction model. As an extension of previous work, the U.S. Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) is used to perform i dealized model simulations with marine layers of varying upstream Froude nu mber to elucidate the different flow responses for a single convex bend. Th e impact upon the supercritical flow of introducing a series of closely spa ced coastal bends is then investigated. The expansion fan is significantly reduced in magnitude and size by the formation of a compression wave at a b locking, concave bend approximately 150 km downstream. Building upon the id ealized marine layer response, real-data forecasts are then examined for se veral time periods of supercritical flow interaction between Cape Blanco, O regon, and Cape Mendocino, California. Observations from the Coastal Waves 1996 (CW96) field program were collecte d in the vicinity of these capes on several days during June-July of 1996. Aircraft measurements on three CW96 flights provide model validation and sh ow ample evidence of supercritical phenomena, while buoy data along the Ore gon and California coastline indicate substantial diurnal variability in th e marine environment. GOES-9 satellite imagery reveals preferred regions of clearing in the coastal stratus deck downwind of convex coastal bends, whi ch is consistent with supercritical expansion fan dynamics. Real-data COAMPS forecasts of summertime marine layer flow between these ma jor capes indicate that the supercritical flow features, and their degree o f interaction, vary diurnally. Diurnal oscillations in the upstream Froude number and flow direction driven by the sea-land-breeze circulation enhance or diminish the expansion fan in the lee of Cape Blanco, thereby altering the flow conditions encountering the concave turn at Cape Mendocino. In a m anner similar to that produced in the idealized simulations, a compression jump forms due to the impact of highly supercritical flow within the Cape B lanco expansion fan upon the Cape Mendocino terrain. The compression wave b ecomes detached and propagates northward during the afternoon in response t o a reduction in upstream Froude number. This propagating compression wave occurred in all three days of the study. The findings presented here demons trate that supercritical flow responses about several closely spaced coasta l bends cannot be analyzed independently.