Life Cycle of a Linear Coastal-Trapped Disturbance

Abstract

A recent climatology of observed coastal-trapped disturbances in the marine atmospheric boundary layer along the United States west coast motivates the detailed examination, for a specific form of imposed forcing, of a linear shallow-water boundary layer model. The model is forced by a time-dependent pressure field, imposed at a fixed level above the boundary layer, that is an idealized representation of the climatological synoptic evolution: a low pressure center translates westward across the coastal boundary, corresponding to the observed offshore extension of a continental thermal trough. The alongshore structure of the model disturbance is characterized by enhanced northerly flow, a depressed marine layer, and low surface pressure to the north; and southerly flow, a raised marine layer, and high surface pressure to the south. Initially, the marine-layer thickness along the coast responds predominantly to convergence of the ageostrophic cross-shore flow driven by the imposed cross-shore pressure gradient and to convergence (to the south of the low pressure center) and divergence (to the north) of the geostrophic cross-shore flow balanced by the imposed alongshore pressure gradient, lifting in the central and southern parts of the forcing region and failing north of the forcing region. For the parameter values considered here, the amplitude of the coastal-trapped thickness response to the geostrophic cross-shore flow is roughly three times as large as that due to the ageostrophic cross-shore flow, but this ratio is likely to be sensitive to the cross-shore/alongshore aspect ratio of the pressure forcing. The coastal-trapped alongshore velocity disturbance is dominated by the response to the alongshore pressure gradient. There is no alongshore propagation in thickness disturbance during the initial stage of the event, while the alongshore velocity and surface pressure exhibit only weak propagation. In the later stages of the event, when the imposed coastal pressure gradients relax (as the low translates offshore), the cross-shore flow weakens, and the response at the coast is controlled by the convergence and divergence of the alongshore flow. The thickness disturbance, alongshore velocity reversal, and surface pressure perturbation propagate northward along the coast essentially as a Kelvin wave in the later stages of the event. Although both the model and the imposed pressure forcing are highly idealized, the model response is qualitatively and, to some degree, quantitatively consistent with many aspects of existing observations of coastal-trapped wind reversals along the United States west coast.