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

Abstract

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 TY David reinforced the dynamical support for the coupling of David with the midlatitude trough, which resulted in the development of an intense extratropical cyclone. During the transition of Opal, maximum rotational frontogenesis occurred over the region where Opal interacted with the preexisting midlatitude cyclone. This weakened the coupling between Opal and the midlatitude trough and prevented the development of a separate extratropical cyclone. One of the unresolved aspects of forecasting extratropical transition is to define when transition has occurred. Although the final extratropical cyclone characteristics may vary greatly from case to case, increased warm frontogenesis seems to be consistent during the initial change from tropical to extratropical characteristics. Therefore, evolution of a frontogenesis parameter is calculated for each case from before transition, through transition, and after transition. In both cases, the rate of increase in frontogenesis peaks at a time that may be defined as the transition time.