The marine layer off northern California: an example of supercritical channel flow

Abstract

During the spring and summer, northerly winds driven by the North Pacific high pressure system are prevalent over the Northern Califonia continental shelf, fixed station and aircraft observations were made to describe the temporal and spatial structure of the lower atmosphere, and their relation to the strong upwelling of coastal waters. The surface wind can be everywhere weak, it can blow at large speeds in a uniform pattern or finally the structure of the northerly surface wind can be complex, with large changes in the wind speed and corresponding changes in the surface pressure over short spatial scales. The latter pattern, which occurs with generally northerly winds, is characterized by a strong low-level inversion which acts as a material interface, and the marine layer behaves as a supercritical channel flow, when the Froude number is greater than one: oblique expansion waves and hydraulic jumps, associated with changes in the orientation of the coastline, account for the observed spatial structure of the flow. -from Authors