An Orographic Mechanism for Rapid Frontogenesis

Observations of the New England coastal front show that the extraordinary temperature gradients are caused by ageostrophic wind deformation, that is, deformation in the vertical plane. The phenomenon is therefore an instance of rapid (non-classical) frontogenesis, a process broadly characterized by a lack of geostrophic balance between the temperature gradient and vertical wind shear. The primary cause of the deformation has thus far been difficult to identify. Furthermore, the relevance of a secondary vertical circulation comparable to that driven by slight mass-momentum imbalance in large-scale fronts has not been established. Local sea-surface heating (in itself frontogenetical) and the surface roughness contrast at the coast have been proposed elsewhere as the necessary catalysts for the frontal collapse. The alternative explored in the present study is that the frontogenesis arises primarily from an interaction between an initially balanced baroclinic flow and an inland mountain barrier. The advantage of this purely orographic theory is that it can be shown to be consistent with the systematic, long-term steepening of an upwind gradient. It thus becomes a more robust mechanism under certain environmental conditions. The likelihood of the orographic mechanism is investigated both analytically and numerically. The analysis proceeds from a canonical study of linear line-source disturbances in an unbounded, stable, baroclinic atmosphere. The results are used to identify a plausible feedback mechanism to account for the virtually unlimited frontogenesis observed in nature. The feedback is found to involve a secondary response to increased stratification in the vertical, rather than the horizontal. By requiring upward motion in a stably-stratified flow, the barrier plays an analogous role to the classical mechanism of horizontal wind deformation, which requires geostrophic adjustment. The feedback is significant only in the case of a narrow (high Rossby number) mountain profile.