Genesis of Typhoon Fengshen (2008) from Vortex Superposition: PALAU Field Experiment and a Global Cloud-Resolving Simulation

Although to understand a mechanism governing tropical cyclogenesis is one of the greatest remaining challenges in meteorology, it has been recognized that a key to understanding the mechanism is to explain the transformation of a preexisting tropical disturbance into a tropical cyclone. Since the circulation of a tropical cyclone develops in axisymmetric gradient-wind and hydrostatic balance, it is plausible that an incipient vortex with upright cylindrical circulation, rather than a tilted vortex with height, is preferable for tropical cyclogenesis. In this sense, what is important in understanding tropical cyclogenesis is to clarify how the coherent vertical structure of a vortex from the lower to upper troposphere can be established until the designation as a tropical storm. A research group of JAMSTEC carried out the PALAU field campaign using Doppler radars in the western tropical Pacific in June 2008, and succeeded to capture the structure of an incipient cyclonic circulation that subsequently developed into a category-3 typhoon “Fengshen”. In addition, the formation and intensification of the typhoon was numerically simulated using a state-of-the-art global cloud-resolving model (NICAM) with 3.5 km of horizontal grid spacing in a whole globe. In this study, synoptic-scale and mesoscale processes leading the typhoon formation were analyzed using the observational data and simulation results.

The highlight of observation is that a Doppler radar captured two separated cyclonic circulations in the lower and middle troposphere in the genesis stage. Each of the vortices was embedded in an individual cloud system, and satellite images show that the typhoon was formed after the merger of these cloud systems. This observational evidence gives us a speculation that a superposition of the mid-level cyclonic circulation with the preexisting surface vortex caused a deep upstanding vortex with weakened vertical shear, which subsequently developed into a typhoon.

In the numerical simulation with 3 days of lead time, the westward propagation of the mid-level cyclonic circulation and the intensification of the surface vortex after the superposition were reproduced well. Just after the vortex superposition, the fall of surface pressure and the formation of a warm core occurred in the vicinity of a partial eyewall, which was transformed from a mesoscale convective system. The environment of the convective system during its transformation was marked by an increase in the inertial stability between the lower and middle troposphere, which plays a crucial role in determining the storm's response to latent heating. These results clearly show that the simulated typhoon was generated from a superposition of two synoptic-scale vortices. We propose these processes as an example of tropical cyclogenesis from a tropical depression (TD)-type disturbance in the western tropical Pacific. The present study also suggests an importance of reproducing synoptic-scale disturbances in an initial condition for better simulation of a tropical cyclogenesis in this basin.