JAPAN - Tornadoes are relatively uncommon in Japan, but they have hit the nation with increasing frequency amid global warming.
A tornado ripped through Saitama and Chiba prefectures on Monday, blowing roofs off of buildings and injuring dozens of people. The tornado was spawned by huge cumulonimbus clouds that rapidly developed in the Kanto region on Monday afternoon.
It went on a 10-kilometer rampage, affecting an area from Koshigaya, Saitama Prefecture, and moving northeast to Noda, Chiba Prefecture. According to reports by local governments including those of the two prefectures, more than 600 buildings were damaged, and 64 people had been reported injured as of Tuesday morning.
In Saitama Prefecture, 63 people were injured, seven seriously, while one person was injured slightly in Noda.
In Koshigaya and Matsubushi in Saitama Prefecture, 458 houses and other buildings were damaged, with eight completely destroyed. In Noda, 153 buildings were damaged.
Tsumoru Matsumoto, a forecaster of the Japan Meteorological Agency, said the tornado was triggered by a so-called supercell, a massive column of cumulonimbus clouds with a diameter of more than 10 kilometers.
The agency's Doppler radar, which observes movements of winds in clouds, located in Kashiwa, Chiba Prefecture, about 18 kilometers southeast of the devastated site in Koshigaya, detected a swirl of rising updraft with a diameter of several kilometers-a feature specific to the formation of a supercell.
Cold air at minus 6 C moved over the Kanto region at an altitude of about 6,000 meters Monday afternoon. Meanwhile, near the ground, warm, moist air drifted from the south, causing atmospheric instability and rapidly developing a cumulonimbus cloud.
Shortly after 2 p.m., when the tornado is believed to have been generated, temperatures in eastern areas of Saitama Prefecture topped 30 C. The temperature difference between the high-altitude cold air mass and warm air near the ground was significant.
Furthermore, a rain front also developed over a limited area, making the atmosphere even more unstable, Matsumoto said.
Hot spell linked to tornado?
According to the agency, the average annual number of tornadoes formed over land in Japan is about 20, although the figures vary from year to year. In 2010, when the nation saw a scorching summer, the number was 37, the largest since 2007, when the agency beefed up its monitoring capability for tornadoes.
At a press conference Monday, an agency official said this summer was "abnormal" with more heat waves and localized downpours of heavy rain. The official partly blamed the abnormality on global warming.
"In general, the progress of global warming increases water vapor. As a result, we see more local heavy rains and thunderstorms as well as tornadoes, like this summer," the official said.
As of Friday, the nation had seen 17 tornadoes so far this year. As tornadoes are most likely to be generated in September, the number could rise further. The annual number of tornado warnings and advisories issued by the agency totaled 424 as of Sunday, up 87 from a year earlier.
Kanto prone to tornadoes
The Kanto region's geographical features make it easier for a tornado to form. According to Hiroshi Niino, director of the University of Tokyo's Atmosphere and Ocean Research Institute, the yearly number of tornadoes formed per area of 10,000 square kilometers was 9.1 in Okinawa Prefecture on the average from 1961 to 1993. Tokyo, excluding remote islands, came second at 2.7, followed by Chiba at 1.9, Ibaraki at 1.5 and Saitama at 1.4. Each of these Kanto prefectures far exceeded the national average of 0.5.
"As warm air tends to move into the Kanto region from the Pacific Ocean, and as the region is generally flat, an air current is unlikely to break up. The region has conditions in which cumulonimbus clouds develop easily," Niino said.
Meanwhile, forecasting tornadoes is difficult.
The agency has increased efforts to monitor tornadoes by installing 20 Doppler radars across the nation. However, it has been able to predict only 3 to 4 percent of tornadoes.
Masayuki Maki, a specially appointed professor of Kagoshima University who specializes in meteorology, said: "Forecasting [a tornado] accurately is very difficult with the current observation technology. It may be 10 years until a high-accuracy radar is developed and put into practice."