Three cases of explosively developing extratropical cyclones over eastern Asia are analyzed using ERA-Interim data. The morphological characteristics of the upper-tropospheric potential vorticity (PV) were examined. The common feature of all of these three cases is a hook-shaped high-PV streamer wrapping counterclockwise around the center of surface cyclones on the southern and eastern sides and an arch-shaped low-PV tongue that wrapped the high-PV hook head from the north. The hook-shaped high-PV tongue overlaps with the maximum centers of both the relative vorticity and static stability parameter, indicating the stratospheric nature of the PV source inside the hook-shaped high-PV tongue.
The analysis indicates that there existed a deep tower of high PV above the surface cyclone at the time when these cyclones underwent explosive cyclogenesis. The high PV in the upper troposphere originates from the polar stratospheric PV reservoir associated with the tropopause-folding process. The high PV in the lower troposphere, however, is associated with the latent heat release, as nearly 70%–90% of the high-PV values in the lower troposphere reside in the region where the rainfall is the heaviest.
The synoptic situation and mesoscale structure of an explosive extratropical cyclone over the Northwestern Pacific in March 2007 are investigated by using the weather station observations and reanalysis data. The cyclone locates beneath the poleward side of the exit of 200 hPa jet, which is a strong divergent region aloft. At mid-level, the cyclone lays on the downstream side of a well-developed trough, where strong ascent motion frequently happens. The cross-section analyses with weather station data show that the cyclone has a warm and moist core. A "nose" of the cold front, characterized by a low-level protruding structure in the equivalent potential temperature field, is observed when the cyclone moves offshore. It is hypothesized that this "nose" structure may be caused by the heating effect of the Kuroshio Current. Also, two low-level jet streams are identified on the western and eastern sides of the cold front. The western jet conveys the cold and dry air at 800-900 hPa. The northern part is northeasterly wind and the southern part is featured by northwesterly wind. For the eastern jet, on the contrary, it carries the warm and moist air into the cyclone system with northward ascending from 900 hPa to 600-700 hPa. The southern part is dominated by the southerly wind and the wind in the northern part is southwesterly. The eastern and western jets 2 significantly increase the air temperature and moisture contrast in the vicinity of the cold front, which could play an important role in favoring the rapid cyclogenesis process.
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