Effects of Coastal Orography on Landfalling Cold Fronts. Part I: Dry, Inviscid Dynamics

Abstract

In this study, the interaction of cold fronts with idealized coastal terrain typical of the western United States and Canada is considered. Two issues are examined. First, what are the factors that determine the strength of the coastal winds, and second, what are the orographic effects on the frontal evolution? To address these issues, the authors utilize a two-dimensional, Boussinesq terrain-following coordinate numerical model in which a uniform prescribed flow is forced to move over a plateau. The resultant across-mountain velocities are characterized by a zone of strongly decelerated flow upstream of the windward slope and a train of inertia-gravity waves downstream. A barrier-jet oriented parallel to the mountain is produced by the Coriolis force. The variations of the magnitude of the upstream deceleration and the barrier jet over a wide range of Froude numbers and Rossby numbers are described. Steady, linear theory applied to flow over a plateau shows that the upstream deceleration is determined largely by the shortwave characteristics of the orography while the barrier-jet strength is related to the longwave characteristics of the orography. Simulations that include an initially steady, geostrophically balanced front upstream of the coast indicate that the motion of fronts can be significantly retarded along the coast. Across-frontal circulations induced by frontogenesis or frontolysis caused by the mountain are small compared to the changes in the mountain circulation caused by the stability perturbations associated with the front. The strength of the along-mountain winds in the coastal zone during frontal passage are approximately determined by a superposition of the southerly barrier jet and the frontal jets (e.g., a southerly prefrontal jet and/or northerly postfrontal jet). This result implies that a barrier jet forming ahead of a front can combine with a prefrontal jet to produce very strong winds in the coastal zone prior to frontal passage.