This paper presents an observational and numerical study of an intense wintertime cold front that made landfall along the northwest coast of the United States during IOP 2 (3-4 Dec 1993) of the Coastal Observation and Simulation with Topography experiment. Observations suggest that the offshore frontal zone was associated with two transitions: The first characterized by a substantial temperature gradient, a nearly vertically oriented wind shift from southwesterly ahead to westerly behind, and a convective rainband, while the second transition had a slantwise wind shift zone from westerly ahead to northwesterly behind. The frontal zone was quite narrow (∼5 km wide) and nearly vertical below 850 mb, and its width increased by 1-2 orders of magnitude above 850 mb. Well before the frontal landfall, low-level flow to the west of the Olympics was associated with geostrophic balance in the cross-shore direction and downgradient acceleration in the alongshore direction, which contributed to the formation of strong coastal southerlies roughly within ∼130 km off the coast. The front started weakening approximately 80 km upstream from the coast. As the front moved closer to the coast, the westerly wind component decreased toward the coastline, which was contributed by both an offshore-directed pressure gradient force and friction as suggested by the force balance result. During landfall, the thermal evolution indicated that the low-level front was delayed by the Olympics, while it could advance farther inland to the north and to the south. Over the water of the Strait of Juan de Fuca the front maintained its integrity at low levels and its thermal gradient even increased as a result of tilting effect, in contrast to the distinct weakening over land. After the frontal landfall, strong northwesterly flow behind the front was greatly modified by the mountains: Winds over the ocean were forced to turn into more westerly, and winds over the barrier were substantially disturbed.
|Number of pages||22|
|Journal||Monthly Weather Review|
|Publication status||Published - 2001 May|
ASJC Scopus subject areas
- Atmospheric Science