Frontogenesis and frontal motion due to confluent deformation with a translating dilatation axis

Abstract

AbstractA two‐dimensional model of primitive‐equation frontogenesis is formulated in which the forcing is due to a basic‐state field of confluent deformation with a dilatation axis that translates in the cross‐front direction. Although the translation speed and direction of motion of the basic‐state dilatation axis have no effect on frontal structure when the total deformation is spatially uniform, they can influence considerably the motion of the front. At the surface, this motion may be separated into two components: (i) internal motion, directed towards the warm air, which is associated with the secondary ageostrophic circulation of the front itself; and (ii) external motion, either towards cold or warm air, which is associated with the initial location and translation speed of the basicstate dilatation axis. the latter component ultimately dominates, and thus the motion of a mature front may be thought of as being controlled by the large‐scale flow. However, the former may dominate early on in the frontal evolution, resulting in significantly different frontal trajectories depending on the initial location of the basic‐state dilatation axis and on whether this axis is moving towards the cold or the warm air. the relative locations of various conventional frontal measures are examined in the special case of a uniformly translating dilatation axis. of these measures the axes of maximum baroclinicity and vorticity become collocated at the surface as the front intensifies. However, the basic‐state dilatation axis and the confluence axis, the latter being defined as the location of zero total wind in the cross‐front direction, may be displaced considerably from this merger point, with this displacement depending on the initial location and subsequent motion of the basic‐state dilatation axis. Potential extensions of the model are discussed with regard to improved understanding of the difference in structure and motion between cold and warm fronts.