Observation and simulation of the genesis of Typhoon Fengshen during the PALAU-2008 field experiment

Fengshen (2008) is a typhoon that formed near the Palau islands (near 7.5N, 134.5E) in June and made a direct hit on the Philippines, leaving severe damage and resulted in more than 1,300 deaths. The formation of an incipient vortex and its rapid growth into Typhoon Fengshen were successively observed using ground-based and ship-borne Doppler radars to the east of Philippines during the PALAU-2008 field experiment. In addition, the growth of incipient vortex into typhoon was numerically simulated using a global cloud-system-resolving model (NICAM). To understand the role of large-scale forcing and mesoscale processes in the genesis of the typhoon, the evolution of a large-scale background flow and mesoscale cyclone structure are examined using the observational and simulation datasets. The incipient vortex formed over a monsoon trough when a monsoon westerly flow in the equatorial side became strong in association with the eastward propagation of Kelvin-wave signals and an intraseasonal variability (ISV) along the equator. During a few days of pre-depression stage, convective activity within a 250-km radius from the vortex center showed a significant diurnal variation with a peak in the midnight (12—18 UTC). The relative vorticity was enhanced in the lower troposphere during the nocturnal convective phase, while the enhancement of vorticity in the middle troposphere took place under a less convective environment in the daytime. The diurnal changes in the horizontal vorticity distribution and the moving direction were also seen. These results may provide information necessary to understand the mechanisms associated with tropical cyclogenesis over a monsoon trough in the western North Pacific.