An observational and numerical study of an intense landfalling cold front along the northwest coast of the United States during COAST IOP 2

This paper presents an observational and numerical study of an intense wint ertime cold front that made landfall along the northwest coast of the Unite d States during IOP 2 (3-4 Dec 1993) of the Coastal Observation and Simulat ion with Topography experiment. Observations suggest that the offshore fron tal zone was associated with two transitions: the first characterized by a substantial temperature gradient, a nearly vertically oriented wind shift f rom southwesterly ahead to westerly behind, and a convective rainband, whil e the second transition had a slantwise wind shift zone from westerly ahead to northwesterly behind. The frontal zone was quite narrow (similar to5 km wide) and nearly vertical below 850 mb, and its width increased by 1-2 ord ers of magnitude above 850 mb. Well before the frontal landfall, low-level flow to the west of the Olympic s 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 similar to 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 c omponent decreased toward the coastline, which was contributed by both an o ffshore-directed pressure gradient force and friction as suggested by the f orce balance result. During landfall, the thermal evolution indicated that the low-level front was delayed by the Olympics, while it could advance far ther 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 therm al gradient even increased as a result of tilting effect, in contrast to th e distinct weakening over land. After the frontal landfall, strong northwes terly flow behind the front was greatly modified by the mountains: winds ov er the ocean were forced to turn into more westerly, and winds over the bar rier were substantially disturbed.