A Nonhydrostatic Mesoscale Simulation of the 10–11 June 1994 Coastally Trapped Wind Reversal

Abstract

During the summer months, the California coast is under the influence of persistent northwesterly flow. Several times each summer, this regime is disrupted by coastally trapped wind reversals (CTWR) in which the northwesterly flow is replaced by southerlies in a narrow zone along the coast. Controversy exists as to the physical mechanisms responsible for initiation and maintenance of CTWRs. While it is clear that coastal terrain is important in creating the trapped response, the precise role played by terrain is unclear. In the present study, these issues are investigated using a nonhydrostatic mesoscale model to simulate the 10–11 June 1994 CTWR event. The results show that the model successfully reproduces many of the observed features of this event, including anomalous vertical structure involving the relatively shallow boundary layer with a warm, nearly neutral layer above; the northward propagation of southerly flow in advance of a tongue of coastal stratus/fog; and a substantial reduction in propagation speed due to the sea breeze. Of the several mechanisms that have been proposed in the literature to characterize these events, these results are most consistent with a topographically trapped gravity current. Further investigation, required to verify this hypothesis, is ongoing. Two sensitivity studies are used to examine the role of terrain in producing and maintaining the CTWR. In the first sensitivity study, the coastline from Pt. Conception to Pt. Reyes is replaced with a straight line and a uniform 840-m-high ridge is placed adjacent to the coast. This simplification permits better isolation of the terrain influence on the mesoscale pressure field and the forcing of the CTWR by the pressure distribution. The results show that adiabatic warming associated with flow over the coastal terrain is required to produce the alongshore pressure gradient, which forces ageostrophic southerly flow, and that, in the absence of bays and gaps in this terrain, southerly flow extends to the location of the minimum pressure. In a second sensitivity study, the height of the ridge along the coast is set to zero. In this simulation there is no mesoscale organization of the southerly flow. Moreover, the structure of the marine boundary layer near the coast is altered by removal of downslope flow and the gravity current characteristics seen in the control and first sensitivity study are absent.