In the western North Pacific, tropical cyclones (TCs) that complete extratropical transition (ET) can become powerful, mid-latitude storms capable of causing significant damage to coastal and maritime interests. Furthermore, ET often produces dangerous storms that are poorly forecast by numerical weather prediction models. The process of ET is a complex, four-dimensional problem in which meteorological features of different horizontal and vertical scales must interact over a variety of spatial and temporal scales. Any effort that improves the understanding, description, and definition of ET would provide forecasters with better tools for diagnosing and forecasting ET, and lead to dissemination of effective advisories and warnings to maritime and coastal interests that may be affected.
Extratropical transition is defined in terms of two stages: transformation, where the TC evolves into a baroclinic, extratropical cyclone; and re-intensification, where the transformed storm then re-deepens. In this presentation, detailed studies of selected cases of ET occurring in the western North Pacific during 1 June through 31 October 1994-98 will be used to explain the process of transformation. These studies are based on analyses produced by the Navy Operational Global Atmospheric Prediction System at 1 degree lat./long. resolution, hourly geostationary visible, infrared, and water vapor imagery, microwave imagery, and NSCAT and ERS-1/2 scatterometer data. A sequence of events that results in transformation of a TC is described in terms of relevant physical mechanisms as well as typical patterns in the satellite imagery. Diagnostic tools are used to examine the systematic decay of the TC warm core, changes in the TC vertical motion patterns, and associated patterns of rainfall and deep convection. Also, the evolution of the TC lower-tropospheric outer circulation into an asymmetric circulation pattern is described in terms of frontogenesis effects. A conceptual model is proposed that explains and describes a common sequence of events during transformation that is the same for nearly every case of ET that occurs over the ocean in the western North Pacific.