Onset of genesis of Typhoon Chanchu (2006) from the vertical alignment of a westward-tilted vortex

Recent studies of tropical cyclogenesis (TCG) suggest that large, updraft-dominated mesoscale convective systems (MCSs) often precede the formation of tropical cyclones (TCs). In this study, we present a cloud-resolving simulation of the genesis of Chanchu (2006) during the MJO, in which such a giant MCS develops on the downtilt-right side of a tilted but increasingly upright vortex. Results indicate that the MCS is an important aspect of the vortex, the evolution of which comprises TCG.

The MCS initiates as a cluster of convection and takes on a comma shape during the onset of TCG (i.e. sustained surface pressure falls), which occurs simultaneously with the vertical alignment of the low- and midlevel circulation centers. An auxiliary feature of the MCS is an area of largely precipitation-free subsidence adjacent to the comma-shaped area of precipitation. The minimum surface pressure falls most rapidly at the interface of the MCS updraft and clear-air subsidence, which descends cyclonically inward from above 500 hPa to the surface.

While the simulation results do not suggest an obvious “trigger” for genesis, they instead reveal a series of events associated with the life cycle of the comma-shaped MCS, in the context of a tilted vortex, which can be conceptualized as a pathway to genesis. The results appear consistent with previous observational studies of TCG that are naturally limited to only select portions of the multiscale TCG process. Though the processes discussed are from a single case study, they appear testable with future case studies and observational investigations, at least in the case of the MJO.