“Guerrilla Rainstorms” Assault Tokyo

Summer Roads Covered with Ice

On June 24, 2014, a torrent of large hailstones lashed down in a small area west of central Tokyo, startling residents in the areas of Mitaka and Chōfu. More than 20 centimeters rapidly fell in one location and around 40 households were flooded in Mitaka. According to the Japan Meterological Agency, this is the first time since 2007 that there has been major hail in the capital, and television viewers were shocked by scenes of summer roads covered with ice.

On the following day, June 25, more than 110 millimeters of rain fell in an hour in northern Tokyo, submerging local roads. Then on June 29, severe rain struck central Tokyo, resulting in fountains of rainwater shooting out of sewer manholes and the halting of some train services.

Tokyo Area Most Commonly Hit

The phrase “guerrilla rainstorm” first became widely used in Japan in 2008 when a number of fatalities were caused around the country by localized downpours. On July 28 of that year, the Togagawa River in Kobe rose 1.3 meters in just 10 minutes, sweeping five people to their deaths, including three children. Later that summer, on August 5, five municipal workers were killed in sewers in Tokyo’s Toshima Ward after a sudden deluge; rain and flooding warnings were not issued until after the accident.

Guerrilla rainstorms differ from ordinary showers in the strength and volume of the rainfall. Showers often last half an hour or less, whereas guerrilla rainstorms might bucket down 100 millimeter-per-hour rain for over an hour. As urban sewers are generally constructed to handle a maximum of around 50 or 60 millimeters of rain per hour, this quickly leads to flooding.

According to the private weather forecasting service Weathernews, 2,923 of these hard-to-predict localized storms took place around Japan in the summer of 2013 (from July 23 until September 30). Due in part to its massive size, Hokkaidō was the prefecture struck most often by guerrilla rainstorms during that period, with around 200 storms. After Hokkaidō, and more than 10 times smaller in area, were Ibaraki with 173, Saitama with 163, and Chiba with 158, all three located near metropolitan Tokyo, which itself recorded 116 storms.

Meanwhile, a JMA research team has found from observation data for the past 118 years that short-duration rainfall on summer evenings in Tokyo (between 5 and 11 pm from June until August) has increased by around 50% over the past century. This trend is not apparent for any other season or time of day. Analysis of data for the last 30 years reveals that there was 30% more summer evening rain in central Tokyo than in surrounding areas.

The “Heat Island” Connection

Based on this sort of research and data, some believe that the “heat island” phenomenon is the main cause of guerrilla rainstorms in the Tokyo area.

The term “heat island” describes the phenomenon of higher temperatures in major cities than in the surrounding areas. It is caused by numerous factors including the increase in waste heat from air conditioning and cars; lower surface water evaporation due to the increase in paved and other artificial surfaces for roads and buildings; and blocking of cooling sea winds by tightly clustered skyscrapers. The rise in the number of “tropical nights” on which the temperature does not fall below 25°C and the increased hospitalization of people due to heatstroke have become major issues for Tokyo.

As air heats up in the center of Tokyo, an updraft forms, drawing in moist air from over the surrounding ocean water—namely, Kashimanada to the east, Tokyo Bay to the south, and Sagami Bay to the southwest. This leads to the development of the cumulonimbus clouds that produce guerrilla rainstorms. Some researchers believe that this mechanism combines with the collision over western Tokyo between sea winds that separate to go around skyscrapers in the city center, making localized downpours more frequent in the west of the city.

The JMA reports that there were 237 occurrences of more than 50 millimeters of rainfall within one hour in 2013 (at 1,000 locations observed). This was based on observations by the Automatic Meteorological Data Acquisition System, commonly known as AMeDAS, consisting of 1,300 unmanned observation facilities across Japan. This figure rose by an average of 21.5 occurrences per decade between 1976 and 2013. However, given the relatively short statistical period, the JMA has stated that at present “it is not clear” whether there is an influence from global warming.

(Originally written in Japanese. Banner photo shows fallen hailstones on a residential street in Chōfu, Tokyo on June 24, 2014. © Jiji)