Interaction of frontal systems with the coastal mountains of the western U.S

Abstract

Observational analyses and numerical simulations are used to investigate the interaction between frontal systems and the complex terrain of the western U.S. for both warm and cold seasons. The different characteristics of mesoscale phenomena resulting from the interaction for each synoptic condition are explored and compared.For the warm season event, synoptically driven offshore flow was associated with the northward development of an inland thermal trough. Strong thermal and pressure gradients across the coast, coupled with the subsequent weakening of the offshore flow, resulted in the onshore push of cool marine air approximately 12 h before frontal passage. Surface fluxes are important in the formation of the thermal trough and the prefrontal onshore push, and damming on the coastal mountains results in a mesoscale coastal ridge and alongshore southerlies.After frontal passage, a Puget Sound Convergence Zone (PSCZ) occurred to the east of the Olympic Mountains. The Olympics deflect the low-level onshore flow into two branches, one along the Strait of Juan de Fuca and another around the southern flank, and contribute to the formation of a lee trough which induces convergence over the central Puget Sound. The Cascades deflect the northern and southern air streams into a more northerly and a more southerly directions, respectively. The two air streams converge and result in the PSCZ. It was also found that latent heat release enhances the PSCZ circulation and associated precipitation.For the cold season event, the front was associated with large temperature gradient and sharp windshifts offshore, especially along its northern portion. Before frontal landfall, strong coastal southerlies were enhanced by the coastal mountains as a result of down-gradient acceleration within a Rossby radius from the coast. Coastal ridging and lee troughing were evident on the windward and the lee sides of the coastal mountains. During landfall, the front was delayed and weakened by the Olympics, while it could advance inland to the north and the south. In addition, the over-water path along the Strait of Juan de Fuca allowed the front to maintain its integrity at low levels, in contrast to the distinct weakening over land.