Interaction of an Intense Extratropical Cyclone with Coastal Orography

Abstract

Observational analyses and numerical simulations are used to investigate the interaction of an intense extra-tropical cyclone with the coastal orography of the Pacific Northwest. Known as the ?Inauguration Day cyclone,? the system made landfall upon the Washington State coast on 20 January 1993, producing one of the most damaging wind storms in Pacific Northwest history. The strongest winds accompanying the storm were associated with an intense low-level pressure gradient that was concentrated along a bent-back front. Mesoscale pressure perturbations produced by the time-dependent interaction of the cyclone and bent-back front with the coastal orography were isolated using numerical simulations. The simulations showed that during the period of peak winds over Puget Sound, there was only a minor enhancement of the local pressure gradient by troughing to the lee (east) of the Olympic Mountains. Gradual amplification of this Olympic Mountain lee trough over a period of 2?3 h extended the period of strong winds by enhancing the pressure gradient over Puget Sound as the bent-back front moved out of the region. The influence of orographically induced coastal ridging and pressure surges was also investigated. It was found that the evolution of coastal ridging was closely connected to the progressive northward development of onshore flow behind the bent-back front. There was no evidence that a self-propagating feature, such as a Kelvin wave or gravity current, was triggered during the landfall of the cyclone and its attendant fronts. The momentum budget in the coastal zone following passage of the bent-back ftont is also discussed.