Abstract
The passage of a frontal zone over a two‐dimensional triangular ridge is investigated using a semigeostrophic model. The model atmosphere consists of three layers of uniform density and potential vorticity. The densest (coldest) layer represents the airmass behind the cold front, the next layer the frontal zone itself, and the least dense (warmest) layer represents the flow ahead of and above the frontal zone. At ground level, the frontal zone is contained between two surface fronts. A prescribed pressure field imposes translation at a speed U, and confluence with a deformation rate α. A one‐layer version of the model has also been considered. The main conclusions are that (a) the mountain‐induced shift of the position of a front does not depend on U or α; it is mainly negative on the windward slope and the upper part of the leeward slope, but is positive in the downstream plain and on the lower part of the leeward slope (there are no effects far‐afield), and (b) frontal collapse must occur for α > 0. The collapse is delayed over most of the mountain, compared to the no‐mountain case, but occurs earlier downstream from the mountain. Prior to the collapse, there is up‐slope frontogenesis when part of the frontal zone is in the plain. Frontolysis prevails if all of the frontal zone is on the up‐slope and if α is not too large. Frontogenesis occurs on the down‐slope and in the plain.