台風9918号外縁部で発生した1999年9月24日の東海地方の竜巻とメソサイクロン

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Three severe tornadoes occurred in Tokai District, the central part of Japan, on 24 September 1999 when Typhoon 9918 moved northeastward over the westernmost part of Japan. Photograph and video images showed that their width was several hundred meters and their rotation was cyclonic. The Doppler radar of Nagoya University observed the parent mesoscale convective systems of the tornadoes. PPI display of Doppler velocity showed five meso-cyclones passed over the district during the period from 1100 to 1230 JST, 24 September 1999. Three meso-cyclones of the five were accompanied by the tornadoes. The Doppler radar observation found characteristics of supercell in the convective systems : a hook-shaped echo and a bounded weak-echo region. Vorticity of the meso-cyclones estimated from the Doppler velocity was an order of 10^<-2>s^<-1>. The sounding at 0900 JST, 24 September 1999 at Shionomisaki showed that the lower atmosphere was significantly unstable and the vertical shear was strong. CAPE of the profile was 2140 J kg^<-1>. This condition was favorable for formation of a supercell. In order to examine whether the profile had a potential to produce a supercell, we performed a numerical simulation experiment using a cloud-resolving model (ARPS). The result showed that a quasi-steady supercell was formed with a significant vorticity at the central part of the intense upward motion. The result suggests that the tornadoes were produced by intense stretching of the vorticity by the intense upward motion.
2000-11-30

台風9918号外縁部で発生した1999年9月24日の東海地方の竜巻とメソサイクロン

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