THE LIFE-CYCLE OF AN EXTRATROPICAL MARINE CYCLONE .1. FRONTAL-CYCLONEEVOLUTION AND THERMODYNAMIC AIR-SEA INTERACTION

The Experiment on Rapidly Intensifying Cyclones over the Atlantic was carried out over the western North Atlantic Ocean to provide temporall y continuous comprehensive datasets from which to document the life cy cle of extratropical marine cyclones. The most intense cyclogenetic ev ent occurred on 4-5 January 1989 over the warm (>20-degrees-C) Gulf St ream current; the cyclone's central sea level pressure decreased by 60 mb in 24 h, from 996 to 936 mb. This study presents the synoptic-scal e and mesoscale life cycle of this cyclone in two parts. Part I, prese nted here, describes the 24-h frontal-cyclone evolution through 6-h an alyses of observations taken by specially deployed observing systems f rom air, land, and sea. The analyses of temperature, wind, and pressur e about the incipient cyclone first illustrate the precursor signature s to cyclogenesis. The 850- and 500-mb temperature evolutions show a s ignificant departure from the Norwegian frontal-cyclone model. In part icular, the 850-mb analyses document 1) a storm-relative westward deve lopment of the warm front as a bent-back front into the polar airstrea m, and 2) the formation of a warm-core frontal seclusion in the post-c old-frontal coot air at the southwestern tip of the bent-back front. A nalyses of sea level pressure provide a detailed account of cyclone in tensification along the bent-back front. Infrared satellite imagery sh ows the evolution and immense size (approximately 5000 km) of the cycl one's cloud signature, and a 250-km-scale comma-cloud system in the vi cinity of the warm-core seclusion situated at the southwestern tip of the large-scale comma head. Thermodynamic air-sea interaction diagnost ics reveal large upward fluxes of heat and moisture from the sea surfa ce into the marine boundary layer of the evolving cyclone. The maximum of combined upward flux approached 3000 W m-2, several times larger t han that typically observed in both extratropical and tropical cyclone s. These fluxes exhibited extreme spatial variability, reflecting the mesoscale characteristics of the cyclone circulation.