Extratropical Transition of Tropical Cyclones over the Western North Pacific. Part I: Evolution of Structural Characteristics during the Transition Process.

The development of extratropical cyclone structural characteristics that resulted from the extratropical transition of Typhoon (TY) David (1997) and TY Opal (1997) over the western North Pacific is examined. David moved poleward ahead of a midlatitude trough that was moving eastward as the dominant midlatitude circulation feature over the western North Pacific. During the transition, David coupled with the midlatitude trough, which led to the evolution of an intense cyclone that became the primary circulation over the North Pacific. Although Opal also moved poleward ahead of a midlatitude trough, the principal midlatitude feature over the western North Pacific was a preexisting stationary cyclone over the Kamchatka peninsula. During transition, Opal weakened and became a secondary cyclone to the preexisting primary North Pacific cyclone. The structural characteristics of the evolving extratropical cyclone with respect to each case are examined in the context of the interaction between a vortex and a baroclinic zone using vector-frontogenesis diagnostics for the Lagrangian rate of change of the magnitude and direction of the horizontal gradient of potential temperature. In this framework, total frontogenesis is divided into components that define the magnitude and rotation of the potential temperature gradient. The initial evolution of extratropical cyclone features for both cases was dominated by warm frontogenesis due to the large amount of warm advection on the east side of the decaying tropical cyclone and the deformation field defined by the poleward movement of the tropical cyclone. However, large differences between the components of rotational frontogenesis for David and Opal are observed that are related to the subsequent reintensification of David and weakening of Opal. The differences are attributed to the different midlatitude circulation characteristics into which each tropical cyclone moved. The pattern of rotational frontogenesis associated with... [ABSTRACT FROM AUTHOR]