Influences of surface heat flux on twin cyclone structure during their explosive development over the East Asian marginal seas on 23 January 2008

The formation of explosively developing twin extratropical cyclones is investigated for background conditions involving a transition from double jets to a single jet. Upper-level double jets produced a latitudinally wide baroclinic zone before formation of the twin cyclones. In the early development stage, those double jets merged into the single jet with a strong potential vorticity (PV) streamer that moved to the Japan Sea and coupled with lower-level PV of the northernmost of the twin cyclones. Subsequently, the area of high PV associated with the single jet migrated to the northwestern Pacific, and the upper-level trough coupled with the southernmost of the twin cyclones. The northern low over the Japan Sea then merged with the developing southern low over the Pacific. For this case, sensitivity experiments suggest that sea surface turbulent heat flux before formation of the cyclones is crucial to formation of the twin cyclone structure, via development of a latitudinally elongated trough. Both Eady growth rate and water vapor mixing ratio near the surface were maintained by a strong sea surface turbulent heat flux and contributed to the development of the latitudinally elongated trough around Japan, where diabatic PV production strengthening the two lows separated into two distinct areas. The twin cyclone structure was formed when the lower-level trough extended from the humid Pacific region was developed by the surface turbulent heat flux. While the upper-level PV coupled with the northernmost low over the Japan Sea, the southernmost low developed independently in the lower-level Pacific region.