Structures and Environment of Explosively Developing Extratropical Cyclones in the Northwestern Pacific Region

A. Yoshida* and Y. Asuma

(IUGG2003 JSM14 018498-1)


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.