Frontal Interaction with Isolated Orography

Abstract

The interaction of a three-dimensional cold front and an isolated orographic ridge is examined by means of primitive equation model simulations. The front evolves as part of a developing nonlinear baroclinic wave and propagates southward toward the ridge. Many of the features in this interaction, such as the anticyclonic distortion of the front, divergence and frontolysis on the windward slope, convergence and frontogenesis in the lee, and the frontogenetical forcing associated with tilting, have previously been captured by simulations of a passive scalar traversing a ridge. It is shown that the ridge decelerates the cold postfrontal air and creates a high pressure anomaly on the windward slope. If this anomaly is strong enough, it accelerates air over the ridge peak in a shallow ageostrophic flow that possesses many features found in a gravity current. This current provides relatively strong surface frontogenesis through the convergence term, but cannot transport enough mass across the peak to weaken the anomalous high pressure. The cold air and pressure anomaly propagate eastward in a manner similar to a topographic Rossby wave. When the east ridge end is reached, the anomalous pressure gradient accelerates the flow into the lee, where frontogenesis occurs from shearing. The motion behind the front as it propagates over and around the ridge is distinctly unbalanced. Blocking, as measured by the ratio of the mass flux around the ridge end to that over the peak, is determined by a Froude number that depends on the propagation speed of the front (i.e., the strength of the baroclinic wave) and the mountain height. Higher mountains or weaker waves tend to produce total blocking of the front, resulting in flow only around the east ridge end. Lower mountains and stronger waves produce frontogenesis patterns and frontal distortions that more closely resemble the passive scalar simulations.