Blocking and frontogenesis by two-dimensional terrain in baroclinic flow. Part I: Numerical experiments

The shallow atmospheric fronts that develop in the early winter along the east coast of North America have been attributed, in various modeling and observational studies, to the land-sea contrasts in both surface heating and friction. However, typical synoptic conditions are such that these "coastal" fronts could also be a type of upstream influence by the Appalachian Mountain chain. Generalized models have suggested that relatively cold air can become trapped on the windward side of a mountain range during episodes of warm advection without a local contribution from differential surface fluxes. Such a process was proposed decades ago in a study of observations along the coast of Norway. Could coastal frontogenesis be primarily a consequence of a mountain circulation acting on the large-scale temperature gradient? A two-dimensional, terrain-following numerical model is used to find conditions under which orography may be sufficient to cause blocking and upstream frontogenesis in a baroclinic environment. The idealized basic flow is taken to have constant vertical shear parallel to a topographic ridge and a constant perpendicular wind that advects warm or cold temperatures toward the ridge. Land-sea contrasts are omitted. In the observed cases, the mountain is "narrow" in the sense that the Rossby number is large. This by itself increases the barrier effect, but the experiments show that large-scale warm advection is still crucial for blocking. For realistic choices of ambient static stability and baroclinicity, the flow can be blocked by a range like the northern Appalachians if the undisturbed incident wind speed is around 10 m s(-1). Cold advection weakens the barrier effect. The long-term behavior of the front in strongly blocked cases is described and compared to observations. Because of the background rotation and large-scale temperature advection, blocked solutions cannot become steady in the assumed environment. However, the interface between blocked and unblocked fluid can settle into a balanced configuration in some cases. A simple argument suggests that, in the absence of dissipation, the frontal slope should be similar to that of the ambient "absolute momentum" surfaces.