An observational and numerical study of a cold front interacting with the olympic mountains during COAST IOP5

This paper documents the three-dimensional flow and precipitation structures associated with a weak cold front interacting with the Olympic Mountains and the subsequent development of a Puget Sound convergence zone. This study utilizes data collected during COAST IOP5 (the fifth intensive observing period of the Coastal Observation and Simulation with Topography field experiment) that took place on 11-12 December 1993. One of the most important data sources was a NOAA P-3 aircraft, which provided flight-level data, radar reflectivity, and Doppler winds as it circumnavigated the Olympics. Initially, frontal passage along the western foothills of the Olympics was accompanied by a 2°-3°C temperature drop, a rapid wind shift to northwesterlies, and an intense line of precipitation (35-45 dBZ); however, the wind shift and associated precipitation structures attenuated when the front began to ascend the windward slopes of the Olympics. Surface and P-3 observations document the deformation of the front around the Olympics. The front accelerated and became more intense and shallow to the northeast of the Olympics where it encountered prefrontal downslope warming and strong southerlies. This portion of the front and the associated precipitation band moved southward toward central Puget Sound and eventually became stationary to the east of the Olympics, where a Puget Sound convergence zone developed and subsequently dissipated after a few hours. This case was simulated down to 3-km horizontal resolution using the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5). Frontogenesis calculations using the model data suggest that prefrontal flow splitting around the Olympics intensified the front by enhanced stretching deformation as it approached the barrier. A simulation with half-height Olympics showed that reduced flow splitting resulted in a weaker front approaching the windward foothills. As observed by P-3 observations and in accordance with previous theoretical studies, frontolysis occurred as the modeled front ascended the windward slope. Sensitivity experiments showed that diabatic effects were important in maintaining the front as it rounded the north side of the Olympics and pushed southward toward central Puget Sound.