A Model Investigation of the Dynamics of a Coastally Trapped Disturbance

Abstract

Numerical simulations of the 15?17 May 1985 coastally trapped disturbance (CTD) event along the west coast of North America are compared with the schematic model of CTD evolution developed by Skamarock, Rotonno, and Klemp (SRK), which was based upon more idealized simulations. It is shown that the general evolution of the simulated May 1985 CTD is consistent with the SRK schematic model. It is further shown that secondary effects not contained in the SRK simulations, such as diurnal radiation variations and mesoscale topographic variations, can account for the variable CTD initiation and propagation observed both in nature and in the present numerical simulations. Diurnal radiation variations, coupled with differential heating of land and ocean, appear to play an important role in setting up the alongshore temperature gradient necessary for CTD formation and evolution. The modeled CTD is found to change dynamical characteristics from an initial Kelvin wave/bore similar to that discussed by Ralph, Nieman, Persson, Bane, Cancillo, and Wilczak to a gravity current, and this change is consistent and coincident with a sharp change in translation speed of the disturbance.