The Yomiuri Shimbun
The deadly tornado that hit a residential area in Tsukuba, Ibaraki Prefecture, on Sunday is believed to have been triggered by a temperature difference of more than 40 C between a high-altitude cold air mass and warm moist air near the ground, which developed a cumulonimbus cloud and a rotating ascending air current.
Japan Meteorological Agency radars detected a cumulonimbus cloud 10 to 20 kilometers in diameter, suggesting that a so-called supercell thundercloud developed around the time of the incident. Supercell clouds, cumulonimbus clouds several times bigger than usual, tend to trigger powerful tornadoes.
According to the agency, a cold air mass with temperatures lower than minus 21 C moved over Japan on Sunday at an altitude of about 5,500 meters, while warm humid air moved from the south toward a low-pressure area over the Sea of Japan.
At about 1 p.m., when the tornado hit Tsukuba, the surface temperature of the city was 25.6 C, higher than the average high of 21.3 C in the area.
Since about a month ago, an upper polar front jet stream has been meandering around the Japanese archipelago. As a result, unstable conditions that invite cold air are repeatedly created every seven to 10 days. The “bomb low pressure” that brought heavy rains and winds in early April also was caused by these unstable conditions.
The intensity of tornadoes is rated by the internationally adapted Fujita-Pearson Tornado Scale, or F-Scale. The six-level scale ranges from F0 tornadoes of 61 kph to 115 kph to F5 tornadoes of 421 kph to 511 kph.
The most powerful tornado recorded in Japan was an F3 tornado, in which houses were blown down and train cars were turned over.
Prof. Fumiaki Kobayashi of the National Defense Academy of Japan’s Earth and Ocean Sciences Department said: “In general, Sunday’s tornado can be categorized as an F2 twister with winds of more than 50 meters per second [180 kph]. But in places, winds may have been as powerful as an F3 [252 to 331 kph].”
(May. 8, 2012)
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