An Eta-model output study of frontogenesis conditions favoring development of a troposphere-spanning front

Abstract

The study of frontal structure and behavior is a subject which has received considerable attention during the past century. During this period our atmospheric observing network, as well as our understanding of various atmospheric phenomena, improved dramatically. The conceptual model of the polar front evolved from a continuous boundary extending from the surface to the tropopause to one which featured independent upper- and lower-tropospheric fronts. This evolution notwithstanding, studies by various authors indicate the existence of the case in which a troposphere spanning front develops. Neither the development nor the implications of this sort of vertically extensive frontal system have received significant attention, as noted by Frank and Barber (1977). Two cases that, after initially featuring an upper-level front, underwent varying magnitudes of baroclinic development were chosen for inclusion in the current study. In an effort to determine a set of meteorological conditions underlying the formation of a troposphere spanning front, tilting, confluent and shear deformation terms of the Petterssen and other applicable frontogenesis equations were evaluated utilizing numerical model output of the Eta model. The outcome of this study indicates the importance of a cooperative relationship between the frontogenesis terms noted above. Upstream of the 500 mb trough, the tilting process of differential vertical motion ants in a positive sense and results in the formation of an upper-tropospheric front. As the jet-front system advents toward and around the base of the trough, confluent and shear processes are observed to play important roles in determining the total frontogenesis. In combination, frontogenetical confluence and frontolytic shear deformation and tilting processes allow a merged front to survive into the downstream portion of the baroclinic wave system. Significant differences in the inter-relationship between the frontogenesis inputs are noted between cases l and 2. These results, coupled with the discussion of superposition in Hoskins et al (1985), imply a relationship between frontal merger and intense surface cyclogenesis. This relationship is illustrated by the intense (case 1) and less intense (case 2) surface cyclogenesis seen in this study.