Numerical Simulations of a Landfalling Cold Front Observed during COAST: Rapid Evolution and Responsible Mechanisms

Colle, Brian A.; Smull, Bradley F.; Yang, Ming-Jen
August 2002
Monthly Weather Review;Aug2002, Vol. 130 Issue 8, p1945
Academic Journal
This paper identifies mechanisms that led to the observed rapid evolution of a landfalling weak cold front along the steep mountainous northern California coast on 1 December 1995. This event was simulated down to 3-km horizontal grid spacing using the Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5). The MM5 simulation reproduced the basic features such as the timing, location, and orientation of the cold front and associated precipitation evolution, as well as the tendency for enhanced precipitation to extend ∼50–100 km upwind of the coastal barrier, with the heaviest amounts occurring over the windward slopes (0–20 km inland); locally, however, the model underestimated the magnitude of the prefrontal terrain-enhanced flow by as much as 30% since the simulated low-level static stability was weaker than observed. The MM5 simulations illustrate the complex thermal, wind, and precipitation structures in the coastal zone. Upstream flow blocking by the steep coastal terrain led to the development of a mesoscale pressure ridge and prefrontal terrain-enhanced winds exceeding 25 m s[sup -1] . Because of the irregular coastline and highly three-dimensional terrain, the low-level winds were not uniform along the coast. Rather, prefrontal southerly flow was significantly reduced downwind of the major capes (viz. Mendocino and Blanco), while there were localized downgradient accelerations adjacent to regions of higher topography along uninterrupted stretches of coastline. Terrain–front interactions resulted in a slowing of the front as the system made landfall, and blocking contributed to a “tipped forward” baroclinic structure below 800 mb. The MM5 was used to investigate some of the reasons for the rapid intensification of the frontal temperature gradient and banded precipitation in the coastal zone. During this event the large-scale vertical motions increased in an environment favorable for moist...


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