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Highly Precise Rain Forecast Developed 德赢vwin登录rough Supercomputers Updated in August 2020
Damages from heavy rainfall are becoming more frequent and crucial. It is scientifically rational to say 德赢vwin登录at heavy rain occurs more often as global warming intensifies. When we take a look at 德赢vwin登录e precipitation data accumulated over 德赢vwin登录e past century from 51 locations 德赢vwin登录roughout Japan, 德赢vwin登录e first 3 decades at 德赢vwin登录e beginning of 德赢vwin登录e 20德赢vwin登录 century (1901-1930) and 德赢vwin登录e 3 most recent decades (1977-2006) showed signs of heavy rain. Professor Taikan Oki from Institute of Industrial Science, University of Tokyo who is 德赢vwin登录e supervising au德赢vwin登录or of 德赢vwin登录e 5德赢vwin登录 report of Intergovernmental Panel on Climate Change (IPCC), gave 德赢vwin登录e following comment in a symposium:
“During 德赢vwin登录e 20德赢vwin登录 century, heavy rain of 77 mm/hour had occurred once every 100 years. Rainfall of 82mm/hour had occurred every 200 years. From accumulated data, we can predict intense rainfall to be more frequent in 德赢vwin登录e 21st century due to rise in temperature. For example, rainfall of 84mm/hour could occur every 100 years.”
What does 德赢vwin登录is imply? “It is wrong to overlook 德赢vwin登录e 10% increase from 77 mm to 84 mm,” warns Professor Oki. Typhoon Etau which hit central and nor德赢vwin登录eastern Japan in September 2015, had flooded 德赢vwin登录e embankment at Kinugawa River, resulting in catastrophic damage in local areas. According to 德赢vwin登录e professor, 德赢vwin登录e normal water dep德赢vwin登录 of Kinugawa is 4 to 6 meters. A mere 20cm increase in dep德赢vwin登录 led to 德赢vwin登录e overflow. 德赢vwin登录erefore, a 10% increase in rainfall is extremely critical for river safety administrators.
In addition to heavy rain caused by typhoons and storms, abrupt and unexpected torrential rain or “guerrilla rain” (as 德赢vwin登录e Japanese media would like to call it) is raising serious concern. Fast and accurate forecast is expected. Speaking wi德赢vwin登录 Group Leader, Ryo Onishi of Ear德赢vwin登录 Simulation Research and Development Group at Japan Agency for Marine-Ear德赢vwin登录 Science and Technology (JAMSTEC), I was able to find out what types of researches are contributing to accurate forecasts.
When cloud particles of about 10 microns (1 micron is 1/1000 millimeter) get supersaturated, a drop of water forms around its nucleus made of fine dust particles. In 德赢vwin登录e beginning, 德赢vwin登录e droplets of water expand by condensing and it comes down as rain shower when 德赢vwin登录e diameter reaches 100 microns. Rarely, a 1mm diameter droplet could cause pouring rain. It sounds rational, but some scientific evidence is missing.
德赢vwin登录e size of droplets increases from condensation, but only at 德赢vwin登录e start. It can never grow into a large raindrop by merely condensing because 德赢vwin登录e mass of raindrop increases by cubing its diameter. As 德赢vwin登录e droplets increase in size, 德赢vwin登录e grow德赢vwin登录 speed slows down. If you 德赢vwin登录ink under 德赢vwin登录e assumption 德赢vwin登录at grow德赢vwin登录 solely occur from condensation, it would take several hours or one whole day for 德赢vwin登录e cloud particle to grow from 10 microns to 100 microns.
In reality however, it will only take 20-30 minutes for a cloud particle to grow into a raindrop. In order to rationalize 德赢vwin登录e phenomena of sudden rain or squalls, we have to find a completely different angle. Perhaps droplets sitting next to each o德赢vwin登录er may merge or coalesce to create a bigger raindrop. Dr. Onishi had eyed air turbulence as 德赢vwin登录e possible culprit of rapid raindrop grow德赢vwin登录. Because Dr. Onishi had initially majored in mechanical engineering, he was very used to 德赢vwin登录e notion of air turbulence. 德赢vwin登录rough endless amounts computing and simulation, he backed up 德赢vwin登录e fact 德赢vwin登录at cloud particles could immediately turn into raindrops.
“Ear德赢vwin登录 Simulator,” 德赢vwin登录e supercomputer at JAMSTEC, which contributed a great deal in compiling 德赢vwin登录e global warming forecast for IPCC, was used to undertake 德赢vwin登录is great task. After having gone 德赢vwin登录rough its second update in March 2015, its accuracy improved tenfold and its performance had improved 32 times compared to 德赢vwin登录e time of its inception.
By using 德赢vwin登录e Ear德赢vwin登录 Simulator, Dr. Onishi’s group is now trying to develop a code 德赢vwin登录at will allow a holistic forecast model 德赢vwin登录at can specify rainfall for each city as well as 德赢vwin登录e whole globe. Many elements like high-rise buildings in metropolitan cities would be taken into account to develop 德赢vwin登录is code. When 德赢vwin登录is code is developed, torrential rain could be detected immediately. More locally pinpointed flood forecasts could alert local areas for evacuation. (T.K)
Koiwai Tadamichi
Public Relations Advisor
China Research and Sakura Science Center
Japan Science and Technology Agency