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The characteristics of explosively developing extratropical cyclones in the northwestern Pacific region are analyzed using the global objectively Analyzed dataset (GANAL) provided by the Japan Meteorological Agency (JMA). In the present paper, these cyclones are classified into three types, depending on positions of formation and of rapid development: OJ cyclones originate over the eastern Asian continent and develop over the Sea of Japan or the Sea of Okhotsk; PO-L cyclones are also formed over the Asian continent and develop over the northwestern Pacific Ocean; and PO-O cyclones are formed and develop over the northwestern Pacific Ocean. Statistical analyses suggest that OJ cyclones frequently appeared in late fall and had the smallest deepening rates of the three types; PO-L cyclones had medium deepening rates and frequently occurred in early and late winter; and PO-O cyclones mainly occurred in midwinter and had the largest deepening rates. Two kinds of composite analyses were conducted to understand the structures and the mechanisms of development. The first composite analysis used geographically fixed coordinates. The results suggest that the favorable atmospheric conditions for the development of each type of cyclone are closely connected to the presence and extension of the cold air mass over the Asian continent. In addition, these conditions are closely related to seasonal variations across the area. The other analysis of cyclone mesoscale structure, using cyclone-relative coordinates at the maximum deepening rate, suggests that OJ cyclones had a short-wave, upper-level jet streak and a strong baroclinic zone in the lower level. PO-L cyclones, associated with a zonally stretched jet stream, had a remarkable midlevel baroclinic zone. PO-O cyclones with a strong jet streak also had a distinct baroclinic zone in the midlevel, and a large water vapor budget (precipitation minus evaporation) appeared around the cyclone center. These cyclone structures reflected vorticity, temperature, and moisture advection, that is, larger-scale atmospheric conditions that affected cyclone development. [ABSTRACT FROM AUTHOR]