The Structure and Evolution of a Continental Winter Cyclone. Part I: Frontal Structure and the Occlusion Process

Abstract

The frontal structure and occlusion process in a cyclone of moderate intensity that affected the central United States in January 1995 is examined. The deep warm-frontal zone associated with this cyclone had a lateral extension to the southwest of the sea level pressure minimum that, although characterized by cold-air advection near the surface, had many of the characteristics of a warm front aloft. In fact, this feature had a structure similar to the so-called bent-back fronts previously documented only in association with explosively deepening maritime cyclones. The development of a warm-occluded structure was investigated with the aid of a numerical simulation of the event performed using the University of Wisconsin?Nonhydrostatic Modeling System. The development of the warm-occluded structure was asynchronous in the vertical; occurring first at midtropospheric levels and later near the surface, in contrast to the classical occlusion process. Near the surface, the warm-occluded front was formed as the warm front was overtaken by the frontogenetically inactive portion of the historical cold-frontal zone. At midtropospheric levels, the warm occluded structure formed as a result of the cold-frontal zone approaching, and subsequently ascending, the warm-frontal zone in accord with a component of the classical occlusion mechanism. The observed asynchronous evolution of the occluded structure is proposed to result from the vertical variation in vortex strength associated with the upper-level potential vorticity (PV) anomaly that controls the cyclogenesis. It is suggested that the occlusion process begins aloft, where the associated vortex strength is greatest, and gradually penetrates downward toward the surface during the cyclone life cycle. Additionally, a characteristic?treble clef? shape to the upper-level PV anomaly is shown to be a sufficient condition for asserting the presence of a warm-occluded structure in the underlying troposphere.