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The characteristics for explosive developing extratropical cyclones in
the northwestern Pacific region were analyzed using a global
objectively analyzed data set (GANAL) provided by the Japan Meteorological
Agency (JMA). We classified these cyclones into three types by the
positions of formation and by the positions of most deepening. The first type originated
over the eastern Asian Continent and developed over the Sea of Japan
or Sea of Okhotsk (OJ type). The second one also originated over the Asian
Continent and developed over the northwestern Pacific Ocean (PO-L
type). The last one originated and developed over the northwestern Pacific
Ocean (PO-O type). Statistical analyses suggest that: OJ type
cyclones frequently appeared in late fall and their deepening rates
were the smallest among the three types; PO-L type cyclones had medium
deepening rates and mostly occurred in early and late winter; and PO-O
type cyclones had the largest deepening rates and mostly occurred in
mid-winter. Two kinds of composite analyses were conducted to
understand their structures and mechanisms of development. The first
composite analysis was the geographically fixed composite
analysis. The results suggest that the atmospheric favorable
condition for each type cyclone is highly connected to the formation of
the cold air mass over the Asian Continent. These conditions are largely
related to the seasonal variation over the area. The other analysis was
a composite for the cyclone structure at maximum deepening. The
analyses suggest that: the OJ type cyclone had a short wave
upper-level trough accompanied with jet streak and a stronger shallow
cold front; the PO-L type cyclone, associated with a zonally
stretched strong narrow jet stream, had a remarkable front system in
the mid-level and the warm front was clearly defined than the cold
front at the surface; and the PO-O type had a short and wide strong jet
streak, also a distinct front system in the mid-level, and strong
precipitation around the cyclone center. The analyses using the Zwack
- Okossi development equation suggest that the crucial mechanism for
the development was vorticity advection for OJ type cyclones,
temperature advection for PO-L type, and latent heat release for PO-O
type, respectively. We concluded that the difference of cyclone
structures between the three types affected the distribution of the
vorticity, temperature advections, and latent heat release within
the cyclone systems and caused their characteristic deepening rates.
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