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

Abstract

Abstract 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 ...