Adjustment of the Marine Atmospheric Boundary Layer to a Coastal Cape

Abstract

During summer, significant changes in marine atmospheric boundary layer (MABL) speed and depth occur over small spatial scales (<100 km) downstream from topographic features along the California coast. In June and July 1996, the Coastal Waves 96 project collected observations of such changes at capes with an instrumented aircraft. This paper presents observations from the 7 June flight, when the layer-averaged speed increased 9 m s?1 and depth decreased by 500 m over a 75-km downwind from Cape Mendocino, accompanied by enhanced surface fluxes and local cloud clearing. The acceleration and thinning are reproduced when the flow is modeled as a shallow transcritical layer of fluid impinging the bends of a coastal wall, leading to the interpretation that they are produced by an expansion fan. Model runs were produced with different coastlines and imposed pressure gradients, with the best match provided by a coastline in which the cape protruded into the flow and forced a response in the subcritical region upstream of the cape. A hydraulic jump was produced at a second bend, near where the aircraft's lidar observed the MABL height to increase. Light variable winds observed within Shelter Cove were replicated in model flows in which the flow separated from the coastline. Though highly idealized, the shallow-water model provided a satisfactory representation of the main features of the observed flow.