A CASE-STUDY DIAGNOSIS OF THE FORMATION OF AN UPPER-LEVEL CUTOFF CYCLONIC CIRCULATION OVER THE EASTERN UNITED-STATES

The synoptic-scale evolution during the formation phase of a midtropos pheric cutoff cyclonic circulation over the eastern United States is d iagnosed within the potential vorticity framework using the GALE (Gene sis of Atlantic Lows Experiment) case of 18-19 January 1986. The study examines 1) the precursor flow evolution prior to cutoff cyclone form ation; 2) the wind, mass, and potential vorticity evolution during the 2-day period encompassing cutoff formation; and 3) the relative contr ibution of upper- versus lower-tropospheric forcing on the quasigeostr ophic height tendency field prior to and during cutoff formation. The primary large-scale features prior to cutoff cyclone formation are an amplifying ridge over the western United States and eastern North Paci fic and a diffluent trough over the central United States. The primary smaller-scale feature prior to cutoff formation is a short-wave troug h-jet streak system that propagates through the longer-wave-amplifying ridge, and then intensifies upon arriving in northwesterly flow downs tream of the ridge axis. The intensification of this shorter-wavelengt h system is associated with increases in stratospheric potential vorti city at levels considered to be well within the middle and upper tropo sphere. Major midtropospheric cyclogenesis then ensues as the jet prop agates toward the base of the diffluent trough while further intensify ing. The circulation then ''closes off'' at 500 hPa within the base of the amplifying trough as stratospheric potential vorticity values des cend to near 620 hPa, and become increasingly confined to the base of the trough. The subsequent intensification of the cutoff circulation i s accompanied by sustained potential vorticity and temperature increas es well above the level of the extruded tropopause. This intensificati on phase is also accompanied by an increasingly isolated distribution of stratospheric potential vorticity, and by the formation of an isola ted warm pool, in the mid- and upper troposphere above the circulation center. These features are consistent with calculations showing that the primary mass loss required to support the formation and subsequent intensification of the cutoff circulation is confined to the upper tr oposphere. A quasigeostrophic height tendency diagnosis suggests that the advection of potential vorticity at and above the 500-hPa level dr ives the process of upper-level trough amplification and cutoff cyclog enesis in this case. The quasigeostrophic height tendency patterns are also entirely consistent with the observed mass and wind-field tenden cies, and with previous observational and theoretical analyses regardi ng the invertibility principle of potential vorticity.