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 evolv es into a baroclinic storm; and reintensification, where the transformed st orm then deepens as an extratropical cyclone. In this study, 30 ET cases oc curring during 1 June-31 October 1994-98 are reviewed using Navy Operationa l Global Atmospheric Prediction System analyses; hourly geostationary visib le, infrared, and water vapor imagery; and microwave imagery. A brief clima tology based on these cases is presented for the transformation stage and t he subsequent cyclone characteristics of the reintensification stage. A three-dimensional conceptual model of the transformation stage of ET in t he western North Pacific Ocean is proposed that describes how virtually all 30 cases evolved into an incipient, baroclinic low. The three-step evoluti on of the transformation of Typhoon (TY) David (September 1997) is describe d 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 circula tion that initiates an asymmetric distribution of clouds and precipitation, and a dipole of lower-tropospheric temperature advection; (ii) the interac tion between TY David and a preexisting, midlatitude baroclinic zone to pro duce ascent over tilted isentropic surfaces; (iii) systematic decay and til t of the warm core aloft in response to vertical shear; and (iv) an evoluti on of David's outer circulation into an asymmetric pattern that implies low er-tropospheric frontogenesis. The beginning and end of the transformation stage of ET in the western Nort h Pacific is defined based on the interaction of the tropical cyclone circu lation with a preexisting, midlatitude baroclinic zone. In particular, case s that complete the transformation stage of ET become embedded in the preex isting, midlatitude baroclinic zone, with the storm center in cold, descend ing air. Cases that begin transformation but do not become embedded in the baroclinic zone fail to complete transformation and simply dissipate over l ower sea surface temperatures and in an environment of vertical wind shear. Use of the conceptual model, together with satellite imagery and high-reso lution numerical analyses and forecasts, should assist forecasters in asses sing the commencement, progress, and completion of the transformation stage of ET in the western North Pacific, and result in improved forecasts and d issemination of timely, effective advisories and warnings.