THE OHIO VALLEY WAVE-MERGER CYCLOGENESIS EVENT OF 25-26 JANUARY 1978 .1. MULTISCALE CASE-STUDY

The long-standing observational view of cyclogenesis involves an inter action between tropopause- and surface-based finite-amplitude disturba nces. There are, however, instances where more than one upper-level di sturbance contributes to low-level development. A subset of these even ts involves wave (or trough) merger, which has been defined as the ama lgamation of two or more distinct 500-hPa vorticity maxima. An example of this phenomenon involving a case of very intense continental cyclo genesis (25-26 January 1978) over the eastern United States is selecte d to elucidate the origin and evolution of precursor disturbances, fro m the planetary scale to the mesoscale. The analysis of this event rev eals that the two well-defined tropopause-based disturbances that cont ribute to cyclogenesis have distinctly different origins and are broug ht together by confluent planetary-scale flow. One of these disturbanc es originates over the western North Pacific Ocean 10 days prior to cy clogenesis and tracks eastward toward western North America. The other disturbance originates over Siberia, tracks over the North Pole, and then southward through central Canada. As the upper disturbances come together, a surface cyclone deepens 43 hPa in 24 h, reaching a minimum sea level pressure of 955 hPa over southern Ontario, and establishing all-time low sea level pressure records for a large portion of the Oh io Valley and southern Ontario. Wave merger in this case consists of a close approach of the two upper-level precursor vorticity centers, ra ther than an amalgamation. The upper-level disturbances are found to t ranslate conservatively (in terms of potential vorticity) and to attai n maximum amplitude prior to cyclogenesis. Both disturbances are highl y anomalous, with the dynamic tropopause locally depressed to near 800 hPa. Diabatic effects appear to alter the upper-level disturbance tha t originated over the Pacific late in the life cycle of the cyclone. T he surface response to the upper-level disturbances is a multiple-low- center configuration, with the main cyclone forming and deepening in r elatively colder air behind a prominent preexisting depression. On the basis of this observation, the importance of surface precursor distur bances is questioned in cases characterized by strong upper-level dyna mics. A highly amplified thermal wave accompanies the cyclone at the g round and tropopause. At the ground, the wave results from a large-sca le rotation of preexisting arctic and coastal/warm frontal boundaries, which follow closely the tracks of the two upper-level disturbances. The main surface cyclone develops in the zone between these preexistin g fronts. The tropopause thermal wave consists primarily of contributi ons from the highly localized precursors and, thus, cannot be viewed a s the result of the nonlinear amplification of a wave or wave packet o n a background potential temperature held. The foregoing results sugge st a generalization of conceptual and theoretical models of cyclogenes is to include multiple upper-level precursor disturbances, a flexible upper boundary (tropopause), and ''one way'' baroclinic development in which upper-level disturbances attain maximum amplitude prior to surf ace cyclogenesis.