Extratropical transition of western North Pacific tropical cyclones: An overview and conceptual model of the transformation stage

Extratropical transition (ET) in the western North Pacific is defined here in terms of two stages: transformation, in which the tropical cyclone evolves into a baroclinic storm; and reintensification, where the transformed storm then deepens as an extratropical cyclone. In this study, 30 ET cases occurring during 1 June-31 October 1994-98 are reviewed using Navy Operational Global Atmospheric Prediction System analyses; hourly geostationary visible, infrared, and water vapor imagery; and microwave imagery. A brief climatology based on these cases is presented for the transformation stage and the subsequent cyclone characteristics of the reintensification stage. A three-dimensional conceptual model of the transformation stage of ET in the western North Pacific Ocean is proposed that describes how virtually all 30 cases evolved into an incipient, baroclinic low. The three-step evolution of the transformation of Typhoon (TY) David (September 1997) is described as a prototypical example. Four important physical processes examined in each of the three steps include (i) environmental inflow of colder, drier (warm, moist) air in the western (eastern) quadrant of David's outer circulation that initiates an asymmetric distribution of clouds and precipitation, and a dipole of lower-tropospheric temperature advection; (ii) the interaction between TY David and a preexisting, midlatitude baroclinic zone to produce ascent over tilted isentropic surfaces; (iii) systematic decay and tilt of the warm core aloft in response to vertical shear; and (iv) an evolution of David's outer circulation into an asymmetric pattern that implies lower-tropospheric frontogenesis. The beginning and end of the transformation stage of ET in the western North Pacific is defined based on the interaction of the tropical cyclone circulation with a preexisting, midlatitude baroclinic zone. In particular, cases that complete the transformation stage of ET become embedded in the preexisting, midlatitude baroclinic zone, with the storm center in cold, descending air. Cases that begin transformation but do not become embedded in the baroclinic zone fail to complete transformation and simply dissipate over lower sea surface temperatures and in an environment of vertical wind shear. Use of the conceptual model, together with satellite imagery and high-resolution numerical analyses and forecasts, should assist forecasters in assessing the commencement, progress, and completion of the transformation stage of ET in the western North Pacific, and result in improved forecasts and dissemination of timely, effective advisories and warnings.