The Genesis of Typhoon Chanchu (2006)
Hogsett, Wallace Andrew
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The phenomenon of tropical cyclogenesis (TCG), defined as the processes by which common tropical convection organizes into a coherent, self-sustaining, rapidly-rotating, and potentially destructive tropical cyclone (TC), consistently headlines research efforts but still remains largely mysterious. TCG has been described by a leading TC scientist as "one of the great remaining mysteries of the tropical atmosphere." This dissertation was motivated by a specific case of TCG: the near-equatorial formation of a well-organized synoptic cyclonic disturbance during the active West Pacific Madden-Julian Oscillation (MJO). At very high resolution, the Weather Research and Forecasting (WRF) mesoscale atmospheric model proves capable of reproducing the multiscale interactions that comprise the TCG of Typhoon Chanchu. In the first part of the dissertation, the synoptic observations of the incipient disturbance (i.e., weak cyclonic vortex) are compared with the results from the WRF simulation. It is found that the disturbance tilts westward with height, and as a consequence of the vertical tilt, large-scale ascent (and thus precipitation) is dynamically favored on the downtilt-right side of the vortex. A major result is that the precipitation to the north of the tilted vortex serves as an attractor to the vortex through its generation of vorticity, thereby serving to dually diminish the vertical tilt of the vortex and deflect the incipient storm northward. Observations and the model simulation both indicate that TCG commences when the storm becomes vertically upright. In the second part of the dissertation, it is shown that the simulated downtilt-right precipitation takes the form of several mesoscale convective systems (MCSs), which spawn midlevel vortices that merge with and intensify the midlevel vortex. The precipitation during the several days prior to TCG generally serves to precondition the near-vortex environment by raising the low- and midlevel humidity. Apparently, the onset of TCG can be characterized as the transition of a tilted cold-core vortex into a vertically-erect warm-core vortex. The final part of the dissertation addresses the storm-scale processes during the day of TCG. The primary point of emphasis is the vertical wind shear, which is often dismissed as detrimental to the prospect of TCG. On the contrary, it is found that the inherent vertical shear of the tilted vortex is actually the critical variable in understanding the TCG of Chanchu through its role in the vorticity tilting term. More specifically, the clockwise turning (i.e., veering) of the shear vector with height suggests a new mechanism for the rapid generation of system-scale vorticity, which current hypotheses posit as simply a stochastic amalgamation of vorticity sufficient to trigger self-intensification. The near-surface spin-up mechanism is analogous to that which occurs in supercell thunderstorms. The missing link in TCG understanding may be the dynamic implications of the veering wind profile in which the convection develops.