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

Fang-Ching Chien, Clifford F. Mass
1997 Monthly Weather Review  
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
more » ... at 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 ϳ 1) , the lee trough is weaker than in the control run (Fr ϭ 0.4) and the lowlevel flow tends to move over the Olympics with less deflection and converges farther downstream. When the height of the Olympics was doubled (Fr ϳ 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.
doi:10.1175/1520-0493(1997)125<1730:ioawsf>2.0.co;2 fatcat:7g5ptx7jhrbhpd5ipsydjudpta