The Ohio Valley Wave-Merger Cyclogenesis Event of 25–26 January 1978. Part I: Multiscale Case Study

The long-standing observational view of cyclogenesis involves an interaction between tropopause- and surface-based finite-amplitude disturbances. There are, however, instances where more than one upper-level disturbance contributes to low-level development. A subset of these events involves wave (or trough) merger, which has been defined as the amalgamation of two or more distinct 500-hPa vorticity maxima. An example of this phenomenon involving a case of very intense continental cyclogenesis (25–26 January 1978) over the eastern United States is selected to elucidate the origin and evolution of precursor disturbances, from the planetary scale to the mesoscale. The analysis of this event reveals that the two well-defined tropopause-based disturbances that contribute to cyclogenesis have distinctly different origins and are brought together by confluent planetary-scale flow. One of these disturbances originates over the western North Pacific Ocean 10 days prior to cyclogenesis 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 Ohio Valley and southern Ontario. Wave merger in this case consists of a close approach of the two upper-level precursor vorticity centers, rather than an amalgamation. The upper-level disturbances are found to translate conservatively (in terms of potential vorticity) and to attain maximum amplitude prior to cyclogenesis. Both disturbances are highly anomalous, with the dynamic tropopause locally depressed to near 800 hPa. Diabatic effects appear to alter the upper-level disturbance that originated over the Pacific late in the life cycle of the cyclone. The surface response to the upper-level disturbances is a multiple-low-center configuration, with the main cyclone forming and deepening in relatively colder air behind a prominent preexisting depression. On the basis of this observation, the importance of surface precursor disturbances is questioned in cases characterized by strong upper-level dynamics. A highly amplified thermal wave accompanies the cyclone at the ground and tropopause. At the ground, the wave results from a large-scale 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 preexisting fronts. The tropopause thermal wave consists primarily of contributions from the highly localized precursors and, thus, cannot be viewed as the result of the nonlinear amplification of a wave or wave packet on a background potential temperature field. The foregoing results suggest a generalization of conceptual and theoretical models of cyclogenesis 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 surface cyclogenesis.