＜prologue＞
I started a blog called “The Baby Boomer Generation’s Miscellaneous Blog”（Dankai-sedai no garakutatyou:団塊世代の我楽多（がらくた）帳） in July 2018, about a year before I fully retired. More than six years have passed since then, and the number of articles has increased considerably.
So, in order to make them accessible to people who don’t understand Japanese, I decided to translate my past articles into English and publish them.
It may sound a bit exaggerated, but I would like to make this my life’s work.
It should be noted that haiku and waka （Japanese short fixed form poems） are quite difficult to translate into English, so some parts are written in Japanese.
If you are interested in haiku or waka and would like to know more, please read introductory or specialized books on haiku or waka written in English.
I also write many articles about the Japanese language. I would be happy if these inspire more people to want to learn Japanese.
my blog’s URL:団塊世代の我楽多（がらくた）帳 | 団塊世代が雑学や面白い話を発信しています
In recent years, there has been an increase in “guerilla downpours” that bring localized heavy rainfall during the rainy season, and the term “linear precipitation band” has become more common. While torrential rain during the rainy season didn’t seem to occur as often in the past, it has occurred almost every year in recent years.
What exactly is a linear precipitation band? And why have they become more frequent recently? We’d like to explain it in an easy-to-understand way.
1. What is a “linear precipitation band”?
A “linear precipitation band” is “a line-shaped rain area accompanied by heavy precipitation, approximately 50 to 300 km long and 20 to 50 km wide, created when a group of organized cumulonimbus clouds, consisting of successively developed rain clouds (cumulonimbus clouds), passes or lingers in roughly the same place for several hours.”
Linear rain clouds can be divided into “fast-moving” and “stationary.” Of these, “stationary” ones cause torrential rain that can lead to major disasters and are called “linear precipitation bands.”
“Linear precipitation bands” can sometimes continue to cause heavy rain in the same place for more than three hours, resulting in “unprecedented heavy rain.”
2. Causes of “linear precipitation bands”
One cause of stationary linear precipitation bands is the “backbuilding phenomenon,” in which cumulonimbus clouds form in rows like the buildings in Tokyo. Linear precipitation bands are created through the following process:
(1) First, cumulonimbus clouds form due to wind convergence or topographical effects. They slowly move, carried by the winds aloft, bringing heavy rain.
(2) New cumulonimbus clouds form upwind of the cumulonimbus clouds, and then slowly move downwind.
(3) Another cumulonimbus cloud forms in the same location, and as it develops, it is carried away, and another cumulonimbus cloud forms in the same location… This process repeats.
In this way, an organized linear precipitation band is created.
Heavy rain caused by linear precipitation bands will continue unless the factors that provide the water vapor that develops cumulonimbus clouds and cause updrafts are eliminated, or unless there is a change in the wind currents that move cumulonimbus clouds.
3. Heavy Rain Damage Caused by Linear Precipitation Bands
Since the 1990s, it has been noted that linear precipitation bands are often seen during torrential rains in Japan, but the term “linear precipitation band” only began to be used frequently after the landslides in Hiroshima City caused by heavy rains in August 2014.
Linear precipitation bands occur throughout Japan, but are particularly prevalent in Kyushu and Shikoku. The Northern Kyushu Heavy Rains that occurred on July 5-6, 2017, and caused extensive damage were also caused by linear precipitation bands.
The July 2018 Western Japan Heavy Rains that occurred from June 28 to July 8, 2018, and caused damage across a wide area, mainly in western Japan, including Hokkaido and the Chubu region, were also caused by linear precipitation bands.
The heavy rains that struck southern Kumamoto Prefecture and other parts of Kyushu in early July and continue to cause damage today were also caused by linear precipitation bands.
4. Research into the Mechanism and Prediction of Linear Precipitation Bands
While the mechanism behind their formation remains unclear, four conditions are believed to favor their formation:
(1） A continuous influx of warm, humid air, which is the source of cumulonimbus clouds.
(2） This warm, humid air rises when it collides with a mountain or a cold front.
(3) Unstable atmospheric conditions favor the formation of cumulonimbus clouds, such as dry, cold air aloft, which causes even stronger air to rise.
(4) Unidirectional winds that blow and create developed cumulonimbus clouds.
This year’s rainy season, which has seen prolonged torrential rainfall, mainly in Kyushu, is due to the lack of a Pacific high pressure system, resulting in an abnormally stationary rainy season front. Large amounts of warm water vapor flow in from the East China Sea to the west and from the Pacific Ocean to the southwest. These clouds collide, causing rapid rises and the formation of cumulonimbus clouds. Cold air from Siberia enters the atmosphere, and winds blow in a consistent direction at higher altitudes, creating conditions favorable for the formation of linear precipitation bands and heavy rain.
According to an analysis of radar observation data by the Japan Meteorological Agency, of the 261 cases of “heavy rain other than typhoons” that occurred between 1995 and 2009, 168 cases, or approximately two-thirds, were caused by linear precipitation bands.
In June 2019, the National Research Institute for Earth Science and Disaster Resilience and the Japan Weather Association began a “linear precipitation band forecasting demonstration experiment” in the Kyushu region, which has been hit particularly hard by the damage.
5. How to protect yourself from torrential rain caused by “linear precipitation bands”
To protect ourselves from torrential rain damage, it’s important to check the three pieces of information: “alert level,” “designated river flood forecast,” and “landslide warning information.” However, the following actions are also necessary.
(1) Regularly check the “hazard map” (disaster prediction map) to identify the risks in your area.
(2) If you live in a “high-risk flood area,” consider “early evacuation” as soon as a heavy rain forecast is issued.
(3) If heavy rain has already made evacuation impossible, “vertically evacuate” to a higher floor within your home, such as the second or third floor.
(4) If you live near a mountain, “horizontally evacuate” to a room away from the mountain slope.