Effects of coastal orography on landfalling cold fronts. Part I: Dry, inviscid dynamics

In this study, the interaction of cold fronts with idealized coastal terrai n typical of the western United States and Canada is considered. Two issues are examined. First, what are the factors that determine the strength of t he coastal winds, and second, what are the orographic effects on the fronta l evolution? To address these issues, the authors utilize a two-dimensional , Boussinesq terrain-following coordinate numerical model in which a unifor m prescribed flow is forced to move over a plateau. The resultant across-mo untain velocities are characterized by a zone of strongly decelerated flow upstream of the windward slope and a train of inertia-gravity waves downstr eam. A barrier-jet oriented parallel to the mountain is produced by the Cor iolis force. The variations of the magnitude of the upstream deceleration a nd the barrier jet over a wide range of Froude numbers and Rossby numbers o re described. Steady, linear theory applied to flow over a plateau shows th at the upstream deceleration is determined largely by the shortwave charact eristics 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 fron t upstream of the coast indicate that the motion of fronts can be significa ntly retarded along the coast. Across-frontal circulations induced by front ogenesis or frontolysis caused by the mountain are small compared to the ch anges in the mountain circulation caused by the stability perturbations ass ociated with the front. The strength of the along-mountain winds in the coa stal zone during frontal passage are approximately determined by a superpos ition of thr 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.