Structure and Intensity Change of Typhoon Songda (0418) Undergoing Extratropical Transition

Typhoon Songda (0418), which damaged much of the Japan Islands with strong winds, underwent extratropical transition (ET) and reintensified while it moved northeastward in the Sea of Japan. The features associated with Songda during ET are investigated by using data of various satellite-borne sensors. The interaction of the cyclone with the midlatitude environment is also diagnosed and discussed by using the gridded dataset of regional analysis (RANAL) produced by the Japan Meteorological Agency. The satellite observations reveals dissipation of the eyewall, formation of a convective rainband accompanied by warm frontogenesis, and some features characteristic of a wind field around a tropical cyclone undergoing ET, e.g., a “horseshoe” gale/storm-force winds shape and an axis of minimum winds extending northeastward from the cyclone center. The satellite data also suggest that a disturbance that originates in the upper tropospheric westerly interacts with the tropical cyclone at the ET stage. The representation of Songda in RANAL is generally consistent with the satellite observations. Diagnoses of upper-tropospheric flows and the quasi-geostrophic omega equation suggest baroclinic development of Songda due to its interaction with a disturbance that originates in the upper troposphere in the midlatitude westerly. Finally it is pointed out that the interaction between the tropical cyclone and the baroclinic disturbance may cause ambiguity of the cyclone center.