Investigation of serious tornado damage in Saroma-cho, Hokkaido

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An investigation was carried out of a tornado and damage caused on November 7, 2006 in Hokkaido, Saroma-cho from both meteorological and a engineering viewpoints. The following points were clarified. The cumulonimbus that brought the tornado was about 10 km high and 20 to 30 km in diameter. It had typical supercell characteristics, i.e., a pair of a strong up and down draft regions and the vault and embryo curtain. The foehn phenomenon had been caused by wind from the south flowing over mountains. This phenomenon greatly affected the generation of the tornado. A complex geometrical feature around the site was another factor in the tornado's generation. A technique using Doppler radar data and numerical cloud resolving storm simulator was applied to a short-period tornado forecast. Applying horizontal wind speeds and the mixing ratio of rain obtained by dual Doppler radar analysis improved the estimation.A detailed investigation of damage to buildings showed that the damaged area was about 1 km long and about 100 to 250 m wide. Most of the debris was scattered in the northern area of Iwasa district, while some flew more than 20 km, reaching the Okhotsk-sea. A questionnaire survey was circulated to 151 families throughout the damaged area. From the primary result of the questionnaire, relations between dam age and structure of houses, meteorological phenomena under the tornado, people's behaviors and government actions were studied. There was considerable damage to houses due to the impact of flying debris, and some roofs were blown off. The standard spec and method for inspecting the exterior cladding materials damaged by flying debris were surveyed. The results should be valuable for further evaluation of countermeasures to tornados.