Warm Occlusions, Cold Occlusions, and Forward-Tilting Cold Fronts

Abstract

The classical definitions of warm and cold occlusions are updated in light of recent observations and numerical modeling of occluded fronts within midlatitude cyclones. Mesoscale model simulations of occluded fronts show that they are first-order discontinuities in potential temperature, with broad zones of fairly uniform horizontal potential temperature gradient and static stability on either side of the frontal surface. Under this assumption, an analysis of the relationship between the slope of an occluded front and the potential temperature distribution on either side of it yields a ?static stability rule,? namely, that an occluded front slopes over the statically more stable air, not the colder air. The static stability rule leads to modifications of the classical definitions of warm and cold occlusions, in which the term ?colder air? is replaced by ?statically more stable air.? Examples are given of occluded fronts in which this distinction is important, and the static stability rule is used to examine possible reasons why warm occlusions are the predominant type of occlusion in midlatitude cyclones. The static stability rule also leads toa better understanding of forwardtilting cold fronts, including cold fronts aloft in the central United States. The static stability rule, and its implications for occluded and other frontal structures, suggests that greater emphasis be placed on the effects of horizontally nonuniform static stability in theoretical and modeling studies of frontogenesis, frontal interactions, and the occlusion process?an emphasis that has been largely absent from such studies in the past.