A Review of Cold Fronts with Prefrontal Troughs and Wind Shifts

The conceptual model of a classical surface-based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface. Many cold fronts, however, do not conform to this model—time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front (hereafter called a prefrontal trough and prefrontal wind shift, respectively). Although many authors have recognized these prefrontal features previously, a review of the responsible mechanisms has not been performed to date. This paper presents such a review. Ten disparate mechanisms with different frontal structures have been identified from the previous literature. These mechanisms include those external to the front (i.e., those not directly associated with the cold front itself): synoptic-scale forcing, interaction with lee troughs/drylines, interaction with fronts in the midand upper troposphere, and frontogenesis associated with inhomogeneities in the prefrontal air. Mechanisms internal to the front (i.e., those directly associated with the structure and dynamics of the front) include the following: surface friction, frontogenesis acting on alongfront temperature gradients, moist processes, descent of air, ascent of air at the front, and generation of prefrontal bores/gravity waves. Given the gaps in our knowledge of the structure, evolution, and dynamics of surface cold fronts, this paper closes with an admonition for improving the links between theory, observations, and modeling to advance understanding and develop better conceptual models of cold fronts, with the goal of improving both scientific understanding and operational forecasting.