Extended Abstract (1.5M)4D.3
The Double Eyewall and Cycloid-like Track in the Typhoon Dujuan (2003)
PAPER WITHDRAWN
Jing-Shan Hong, Central Weather Bureau, Taipei, Taiwan; and P. L. Chang
This paper is to document the prominent double eyewall of typhoon Dujuan (2003) in radar view over south of Taiwan. As Dujuan approaching Taiwan, the radius of maximum wind (RMW) associated with the outer ring was continuously contracted from about 120 km to 45 km, however, the inner eye was rather inactive and the associated RMW was about 20 km and almost kept constant. The intense convection in outer ring was rapidly cyclonic stretched and resulted in a banded structure with thin strip wrapped around the vortex. The double eyewall in Dujuan was not concentric. Thus the inner eye tended to orbit the outer ring by the steering of the rotational flow in the moat with period of 3.5~4-h and eventually exhibited as a cycloid-like track. Orbited inner eye contributed to the oscillated speed of the inner eye motion, which was deviated from about 5 to 15 m s-1. The estimated orbit speed of the inner eye (Ve) was about 5 m s-1, whereas the motion of the entire typhoon (Vt) was about 8 m s-1. By the way, the motion of the entire typhoon and the orbited inner eye projected to the radial component and resulted in the oscillation of the observed maximum inbound/outbound radial velocity. The impact of such the oscillation appeared in two folds. First, the projection of Ve and Vt could produce a wind burst up to 15 m s-1 on the inbound/outbound velocity in certain condition depending on the relative relation of typhoon motion and the orbit of the inner eye. It is very important for the local forecasting/nowcasting of the strong wind, in particular in the landfall case. Second, the oscillation of the inbound/outbound velocity associated with the inner eye can enhance the wind shear across the moat. The large differences could enhance the differential rotation in the moat region and result in the inactive inner eye and the spiral band wrapped around the outer ring as the type 1 instability proposed by Kossin et al. (2002). The further analysis showed that the orbit of the inner eye obviously shifted to the south portion of the outer ring. The hodograph of the vertical wind shear derived from the composite dropsonde displayed a significant northerly vertical shear from 700 to 300 hPa. Since the vertical development of the inner eyewall was up to 10~12 km whereas only 7 km for the outer ring, thus, the inner eye was expected to experience more forcing of the vertical wind shear than the outer ring and resulted in a drift toward the south portion of the outer ring.
Session 4D, tropical cyclone observations and structure II
Monday, 3 May 2004, 3:45 PM-5:15 PM, Napoleon III Room
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