Inverted Troughs and Cyclogenesis over Interior North America: A Limited Regional Climatology and Case Studies

Abstract

A limited regional climatology of cyclones with and without inverted troughs that form in the Colorado region is presented along with case study results from two major cyclone events in which an inverted trough plays a prominent role in the life cycle of the storm. Typically, the inverted trough separates a polar or arctic air mass dammed up along the eastern foothills of the Rockies from an older modified polar air mass over the plains. Inverted troughs are favored when anticyclonic conditions prevail at the surface across south-central Canada and the northern plains states beneath a confluent flow aloft. Although both types of cyclones form in response to a progressive trough crossing the Rockies, a composite analysis shows that an inverted trough is most likely when a band of meridionally oriented ascent in the lower and middle troposphere persists along the eastern slopes of the Rockies beneath confluent flow aloft. Cyclones without an inverted trough tend to occur when the synoptic-scale ascent region moves rapidly eastward away from the mountains, so that surface pressure falls immediately to the east of the mountains with attendant cold-air damming cannot be sustained. The life cycle of both cyclones departs significantly from the simple conceptual ideas illustrated in the Norwegian cyclone model. Four principal air masses are estimated to be involved in the cyclone evolution: 1) warm moist air from the Gulf of Mexico, 2) older modified polar air returning poleward behind a retreating surface anticyclone, 3) subsided Pacific air crossing the southern Rockies, and 4) a new polar or arctic air mass moving southward east of the Rockies. The inverted trough separates 2 from 4, and a weak warm front delineates 1 from 2. The primary cold front marks the boundary between 1 and 3, while a secondary cold front, originating as a northerly wind surge along the eastern slopes of the Rockies and appearing as a bent-back cold front, separates 3 from 4. The secondary cold front eventually becomes the dominant cold front as the primary front weakens. In the January 1975 case a third cold front, marking the leading edge of arctic air, eventually overtakes the second cold front. Although the ?catch-up? of the warm front by the cold front as envisioned in the Norwegian cyclone model occurs in both storms, the results depict a rich life cycle tapestry that depends upon the interaction of orographically induced mesoscale circulations with synoptic-scale transient disturbances. For example, in the April 1986 case the older modified polar air mass wraps cyclonically westward and then southward against the colder air to the west (the inverted trough is acting as the primary warm front) creating a warm-air extrusion near the cyclone center between the highly baroclinic inverted trough and the much weaker occluded front to the east. The conventional surface warm front plays only a secondary role in the storm life cycle.