Diurnal Cycle of Supercritical Along-Coast Flows

Abstract

In this study, a three-dimensional hydrostatic mesoscale model is used to address the transient behavior of supercritical along-coast flow. A control experiment and several sensitivity tests are performed in order to investigate the diurnal cycle of flow characteristics. An idealized representation of the northern California terrain is used, and the model results are interpreted within the reduced-gravity shallow water concept. In two preceding studies by the authors, this theory appeared to be violated since the flow accelerated along the coastline upstream of the change in coastline orientation, even though the flow was supercritical. Here, it is shown that the criticality of the flow for typical summertime conditions along the California coast actually varies diurnally. The gradual acceleration of the flow along the upstream coastline is established during a subcritical phase of the simulation; thus, the shallow water concept is not violated. Because the along-coast jet is primarily driven by the cross-coast baroclinicity, there will be a continuous variation in the strength of the jet. This in turn will affect the flow criticality and thus the flow is not only spatially, but also temporally, transcritical. The results here suggest that observations of quasi-steady-state supercritical flow in reality are not likely; transcritical flow along mountainous coastlines should be more prevalent.