Models of coastally trapped disturbances

During the spring and summer, the climatological northerly flow along the U .S. west coast is occasionally interrupted by transitions to southerly how that have a limited offshore scale and appear to be manifestations of marin e-layer flow that is rotationally trapped by the coastal mountains. Existin g climatological and observational studies suggest that a synoptic-scale of fshore flow initiates these coastally trapped disturbances (CTDs). Using id ealized simulations produced with a 3D nonhydrostatic model, the authors fi nd that an imposed offshore how will produce CTDs in idealized coastal envi ronments. The imposed flow first weakens the prevailing northerly flow in t he marine layer and lowers the pressure at the coast. The marine-layer flow around this low pressure evolves toward geostrophic balance, but is retard ed as it encounters the coastal mountains to the south of the low and subse quently deepens the marine layer in this region. The elevated marine layer then begins progressing northward as a Kelvin wave and later may steepen in to a bore or gravity current, this progression being the CTD. Many observed features accompanying CTDs are found in the numerical simulations, includi ng the formation of a mesoscale pressure trough offshore and deep southerli es in the CTD at the coast. Stability in the atmosphere above the marine la yer can give rise to topographically trapped Rossby waves and stronger CTD winds. In these stable conditions, propagation of wave energy away from the disturbance does not preclude strong, quasi-steady, propagating CTDs.