A CHARACTERISTIC LIFE-CYCLE OF UPPER-TROPOSPHERIC CYCLOGENETIC PRECURSORS DURING THE EXPERIMENT ON RAPIDLY INTENSIFYING CYCLONES OVER THE ATLANTIC (ERICA)

This paper documents a characteristic life cycle of upper-tropospheric precursors to surface cyclogenesis observed during the field phase of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ER ICA, December 1988-February 1989). This life cycle begins with the dev elopment of an elongated region of lower dynamic tropopause that forms in association with an intensifying midtropospheric jet/front over ce ntral North America. The elongated disturbance subsequently compacts i nto a more circular configuration prior to crossing the east coast of North America and frequently is associated with rapid surface cyclogen esis offshore. A representative example of the life cycle outlined abo ve is documented through a detailed case study of the upper precursor associated with the second ERICA intensive observation period (IOP 2) cyclone. Emphasis is placed upon (i) description of the tropopause str ucture and evolution during the upper-precursor life cycle, (ii) diagn osis of mechanisms leading to the development and intensification of a midtropospheric cyclonic vorticity maximum and frontal zone, (iii) an alysis of the role of transverse jet-front circulations in deforming t he dynamic tropopause, (iv) documentation of the influence of the low- and high-frequency flow components on the upper-precursor life cycle, and (v) isolation of dynamic and thermodynamic factors that render thi s life cycle especially conducive to rapid surface cyclogenesis. Confl uence downstream of the axis of a low-frequency (i.e., periods greater than 120 h), troposphere-deep ridge over western North America facili tates the organization of a midtropospheric jet/front over central Nor th America. As this precursor disturbance approaches the inflection be tween the western ridge and a downstream trough, tilting, in the prese nce of cold advection along the midtropospheric frontal zone, becomes an important vorticity generation and frontogenesis mechanism in the u pper precursor. Transverse circulations accompanying the jet/front ste epen and lower the dynamic tropopause prior to surface cyclogenesis. C ompaction of the initially elongated upper precursor is shown to invol ve deformation in the high-frequency component of the upper-tropospher ic flow. The compacted upper-precursor configuration, lowered tropopau se, and reduced static stability in the offshore environment lead to s trong vertical coupling and vigorous surface cyclogenesis as the upper precursor passes offshore. The foregoing results suggest that the lif e cycle of a common class of cyclogenetic precursors is closely relate d to midtropospheric frontogenesis. A favored location for the develop ment of midtropospheric jet/fronts is over central North America durin g northwesterly flow episodes. Production of vorticity in the midtropo spheric jet/front and subsequent compaction of this vorticity feature suggest a link between midtropospheric frontogenesis and mobile upper- trough genesis. This link may explain the existence of a maximum in th e upper-trough-genesis distribution over central North America documen ted by Sanders.