Blocking and Frontogenesis by Two-Dimensional Terrain in Baroclinic Flow. Part I: Numerical Experiments

Abstract

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 Ϫ1 . 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.

Mechanisms of Intense Alpine Rainfall

“…A separate issue concerns the source of the cold air flowing along the mountain ridge. Garner (1999), also within the infinitely long-ridge framework, investigates the effect of a diffuse cross-ridge temperature gradient (in thermal wind balance with an along-ridge wind shear) carried by the across-ridge mean flow and finds that, with temperature decreasing toward the ridge, the baroclinicity allows low-level air to stagnate on the upwind slope of the ridge. In this model the static stability does not vary along the ridge and hence the blocking mechanism is qualitatively different from that discussed here and summarized in Fig.…”

“…16. Garner (1999) briefly considers the effect of a weak unbalanced temperature gradient in the along-ridge direction (with colder air on the right, facing with the mean flow) together with the across-ridge temperature gradient and remarks on the enhancement of the upwind cold pool due to cold advection along the ridge (p. 1504). In the present case, we have only the along-ridge baroclinicity due to a small along-ridge variation of in the mean flow.…”

A Multiwinter Analysis of Channeled Flow through a Prominent Gap along the Northern California Coast during CALJET and PACJET

“…For example, a remnant cold pool can remain trapped against the mountains following the passage of a warm front aloft (e.g., Bjerknes and Solberg 1921;Garner 1999). Orographically forced ascent of a stably stratified airstream yields a dome of adiabatically cooled air and windward ridging (e.g., Smith 1979).…”

Formation of a coastal barrier jet in the Gulf of Mexico due to the interaction of cold fronts with theSierraMadreOriental mountain range

“…The influence of topography on the propagation of a CF (and associated processes) in Mexico has been investigated by Mosiño () and Osorio Tai (). Other studies of CFs and topography in the world include regions of the Alps (Smith, ; Hoinka and Volkert, , ; Hoinka et al , ; Kurz, ; Egger and Hoinka, ), the Rocky Mountains (Colle and Mass, ; Neiman and Wakimoto, ), the Appalachians (Wesley et al , ; O'Handley and Bosart, ; Schumacher et al , ; Garner, ), the coast of Alaska (Colle et al , ; Loescher et al , ), and the Andes (Marengo et al , ). Some of the processes that have been analysed in these studies include the acceleration or retardation of the CF, frontogenesis or induced frontolysis, modification of pre‐frontal and post‐frontal flow, blocking, cold air damming, and the formation of a barrier jet.…”