Idealized Three-Dimensional Simulations of Fronts Interacting With the U.S. West Coast Topography

During the past several years a number of field studies (i.e., COAST, CALJET, PACJET) have investigated the structural evolution of landfalling fronts along the U.S. West coast during the cool season. The observational and numerical studies from these field experiments have revealed many interesting features near steep coastal terrain, such as barrier jets, enhanced upstream frontogenesis, and forward tilting frontal structures over terrain. Unfortunately, our understanding of landfalling storms is currently limited to these handful of case studies. In order to increase our understanding of the full range of frontal-terrain interactions along the U.S. West Coast, a suite of three-dimensional idealized simulations are necessary.

This paper describes some modeling results using the Penn State/NCAR mesoscale model (MM5) in an idealized configuration to investigate the structural evolution of landfalling systems. A solitary baroclinic wave is initialized well upstream of the coast in a zonal basic state. The three-dimensional idealized setup is designed to control the strength of the background temperature gradient, the horizontal scale and strength of the initial pressure perturbation, vertical tilt of the initial wave, and initial lapse rate. The MM5 is nested down to 6-km grid spacing around the northern California coast in order capture the structural changes of the front approaching the coast during the 48-h simulation.

It will be shown that the idealized setup is able to capture many of the observed features of landfalling baroclinic waves such as the northward deflection of the incipient cyclone and cyclolysis during landfall, development of terrain-enhanced flow near the coast, and upstream retardation and intensification of the front approaching the coast. It is found that the terrain-induced frontogenesis is increased for a wide plateau geometry rather than a narrow (100 km) ridge, since wide terrain favors a deeper and wider barrier jet response. The impacts of diabatic effects are also investigated.