Interaction of a Warm-Season Frontal System with the Coastal Mountains of the Western United States. Part II: Evolution of a Puget Sound Convergence Zone

Abstract

Observational analyses and numerical simulations are used to investigate a Puget Sound convergence zone (PSCZ) event that occurred in the lee of the Olympic Mountains of Washington State. The PSCZ, which develops when low-level airstreams are forced to converge over Puget Sound by the regional orography, is frequently associated with a mesoscale swath of clouds and precipitation across the central Puget Sound that stretches eastward over the western side of the Cascade Mountains. It was found that latent heat release enhances the PSCZ circulation and associated precipitation. Both the Olympic and the Cascade Mountains are important in the formation of the PSCZ. 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 of the Olympics; in addition, a lee trough, which develops to the east of this barrier, induces convergence over the central Puget Sound. The Cascades deflect low-level flow over northern Puget Sound into a more northerly direction and generate a windward ridge over the southern sound that contributes to a northward deflection of the flow. In a series of sensitivity experiments, the Froude number (Fr) was varied by changing the height of the Olympic Mountains. When the height of the Olympics is reduced by half (Fr ? 1), the lee trough is weaker than in the control run (Fr = 0.4) and the low-level flow tends to move over the Olympics with less deflection and converges farther downstream. When the height of the Olympics was doubled (Fr ? 0.19), most of low-level air is forced to flow around the mountains, producing weakened convergence over the south-central sound. In the control simulation, a pair of vortices appear to the north and the south of the convergence zone; in the double Olympics run, the vortices occur farther downstream than in the control. Lee vortices do not appear in the half Olympics simulation, and in a simulation without the Cascades, the vortices are weakened and more symmetrical.