INVERTED TROUGHS AND CYCLOGENESIS OVER INTERIOR NORTH-AMERICA - A LIMITED REGIONAL CLIMATOLOGY AND CASE-STUDIES

A limited regional climatology of cyclones with and without inverted t roughs that form in the Colorado region is presented along with case s tudy 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 c onditions prevail at the surface across south-central Canada and the n orthern plains states beneath a confluent flow aloft. Although both ty pes of cyclones form in response to a progressive trough crossing the Rockies, a composite analysis shows that an inverted trough is most li kely when a band of meridionally oriented ascent in the lower and midd le troposphere persists along the eastern slopes of the Rockies beneat h confluent flow aloft. Cyclones without an inverted trough tend to oc cur 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 attendent cold-air damming cannot be sustai ned. The life cycle of both cyclones departs significantly from the si mple conceptual ideas illustrated in the Norwegian cyclone model. Four principal air masses are estimated to be involved in the cyclone evol ution: 1) warm moist air from the Gulf of Mexico, 2) older modified po lar air returning poleward behind a retreating surface anticyclone, 3) subsided Pacific air crossing the southern Rockies, and 4) a new pola r or arctic air mass moving southward east of the Rockies. The inverte d 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 s econdary cold front, originating as a northerly wind surge along the e astern slopes of the Rockies and appearing as a bent-back cold front, separates 3 from 4. The secondary cold front eventually becomes the do minant cold front as the primary front weakens. In the January 1975 ca se a third cold front, marking the leading edge of arctic air, eventua lly overtakes the second cold front. Although the '' catch-up '' of th e warm front by the cold front as envisioned in the Norwegian cyclone model occurs in both storms, the results depict a rich life cycle tape stry that depends upon the interaction of orographically induced mesos cale circulations with synoptic-scale transient disturbances. For ''am ple, in the April 1986 case the older modified polar air mass wraps cy clonically westward and then southward against the colder air to the w est (the inverted trough is acting as the primary warm front) creating a warm-air extrusion near the cyclone center between the highly baroc linic 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.