In recent years, heavy rains have occurred frequently in various regions, significantly impacting our lives. However, opportunities to correctly understand their mechanisms and dangers are limited.

Therefore, in this project, five faculty members from different specialized fields will convey the reality, risks, and countermeasures of heavy rains and disasters through our university's initiatives. "To know is to prepare." We hope this will empower individuals in their judgment and actions.

This time, we spoke with Professor Norihiro Nishi of the Department of Earth and Planetary Sciences, Faculty of Science, Fukuoka University (Specialty: Tropical Meteorology).

<How do cumulonimbus clouds and linear rainbands that cause heavy rain form?>

Warm, moist air containing water vapor is light, so it rises and cools in the upper atmosphere, forming clouds. When water droplets in the clouds grow larger, they fall to the ground as rain. This is how rain falls.

There are two types of rain-producing clouds: "nimbostratus" and "cumulonimbus." Cumulonimbus clouds are responsible for the intense rain that leads to disasters. These are the "thunderheads" often seen in summer.

Cumulonimbus clouds are about 5 km wide, so even if heavy rain falls, it will last for about an hour at most. However, if cumulonimbus clouds form one after another in a line and continue to rain in the same location, it results in prolonged heavy rainfall. This is a "linear rainband."

<The rainy season is a season where conditions for the formation of linear rainbands tend to align>

There are several factors that cause cumulonimbus clouds to form linear rainbands:

Large amounts of water vapor

Wind carrying warm, moist air

Complex wind patterns

Topography where air tends to converge

Islands and mountainous areas, etc.

Not all of these conditions are necessary. On the other hand, the conditions considered indispensable are a large amount of water vapor and the wind that continuously carries it.

In the Kyushu region, warm, moist air flows in from the south during the rainy season. This is precisely a situation where the conditions essential for the formation of linear rainbands – "large amounts of water vapor" and "wind that continuously carries warm, moist air" – are likely to be met.

Typhoons also develop by taking in warm, moist air from their surroundings, thus possessing the conditions for cumulonimbus cloud formation. Typhoons that visit during the rainy season interact with the rainy season front, so even small typhoons cannot be ignored.

<Situations unique to Kyushu>

We who live in Kyushu experience prolonged rainfall during the rainy season, but it turns out this is a characteristic unique to Kyushu.

"For example, in the Kanto region, rainfall in autumn tends to be greater than during the rainy season. Kyushu is the 'heartland of the rainy season,'" says Professor Nishi.

Reference: Ministry of Land, Infrastructure, Transport and Tourism, Normal Precipitation Amounts for Prefectural Capitals, etc.

The warm, moist air that causes rain during the rainy season flows into Japan from the seas south of Taiwan and mainland China. The sea serves as a source of water vapor.

Because moist air is carried in large quantities continuously, Kyushu tends to have higher precipitation than other regions.

There are many islands in the sea west of Kyushu, and mountainous areas extend through Nagasaki and Saga on the way to Fukuoka. When warm, moist air collides with such terrain, it is lifted, making it easier for cumulonimbus clouds to form.

By the way, many people who watch the news in Fukuoka might feel, "Heavy rains often occur in Ogori and Kiyama."

"While I cannot say for sure as there are no observation points, people who watch the Fukuoka weather forecast closely will have that impression. I believe the influence of the mountains is significant," says Professor Nishi, showing a map.

So, is it safe outside of mountainous areas? Not necessarily. Linear rainbands can form and cause heavy rain even in flat areas.

Professor Nishi explaining with a map. An overview of water vapor observation in which our university participates, and the vertical distribution of water vapor observed in Koshikijima, Kagoshima Prefecture. (Provided by Assistant Professor Koichi Shiraishi, Department of Earth and Planetary Sciences)

<To what extent can heavy rains and linear rainbands be predicted in weather forecasts?>

Weather and typhoon predictions are made with high accuracy using weather satellites, radar, and computers.

Nevertheless, predicting linear rainbands remains difficult. While it is possible to grasp the conditions that make them likely to form based on water vapor inflow and wind patterns, pinpointing the exact location and time is not easy.

There are three reasons for this:

Linear rainbands form in narrow areas.

There are multiple formation conditions, and it cannot be said that "these conditions will always lead to formation."

It is difficult to collect data necessary for predicting heavy rainfall in the Kyushu region.

Linear rainbands occur in a narrow range. Typhoons affect a wide area, but linear rainbands occur in such a narrow range that they might not be noticed even if they happen in the next town. Predicting the pinpoint location is difficult.

There are multiple formation conditions, and it cannot be said that "these conditions will always lead to formation." Linear rainbands can form over a wide area if the conditions for rain are met. Topography is just one trigger, and sometimes it is not involved.

Clouds that form over islands in western Kyushu can develop and be carried by the wind, causing heavy rain in distant areas like Fukuoka. Predicting the location and time is very difficult.

Kyushu's geography makes it difficult to collect the necessary data for prediction. To predict the formation of linear rainbands, it is necessary to grasp the amount of water vapor and where it is flowing from.

For example, the Kinki region has many observation points on the surrounding land, and since it is already raining in Kyushu and the Chugoku region, it is an environment where prediction is easier.

On the other hand, in Kyushu, the "upstream" area where moist air enters is over the sea, and there are no observation points. The Japan Meteorological Agency observes by ship to grasp the situation, but prediction is said to be more difficult compared to other regions.

<Linear rainbands have existed for a long time but have become clearly recognized in recent years>

Q: In recent years, we have heard the term "linear rainbands" frequently in the news. Why have they become so common?

The meteorological phenomenon of cumulonimbus clouds forming continuously and causing heavy rain, the so-called "linear rainbands," has existed since ancient times. For example, the "Isahaya Heavy Rain" of 1957 and the "Nagasaki Great Flood" of 1982 were likely linear rainbands or similar phenomena.

Perhaps many people feel they have suddenly increased because the term "linear rainbands" has been used and spread on television.

Q: Many people also say that heavy rains have increased...

While it is true that the number of heavy rain events and the amount of rainfall are said to have increased, I don't think they have increased to the extent that the general public can clearly feel it.

Now, weather conditions can be monitored moment by moment with satellites and radar. In the past, it took time to grasp the extent of disasters. While predicting linear rainbands is difficult, we can still tell if they are likely to occur in a certain area. I believe that the increased accuracy of weather forecasts makes them more noticeable...

Reference: Ministry of Land, Infrastructure, Transport and Tourism, Japan Meteorological Agency, Changes in Heavy Rainfall and Extremely Hot Days (Extreme Phenomena)

<How to deal with warnings that require evacuation?>

Thirty to forty years ago, when I was a student, there were no computer forecasts, and they were completely inaccurate. When I was a child, small typhoons would make landfall in Kyushu without anyone knowing. Now, weather forecasting technology is almost established, and it's a completely different world compared to the past.

A warning is information that a disaster may occur over a wide area. While it is more common for nothing to happen when a warning is issued, a disaster does occur about once every few times.

The Japan Meteorological Agency stated that rather than warnings being perceived as "nothing will happen even if a warning is issued," they should be conveyed as accurately as possible, trusting the public. However, they also said that if the expression is "the possibility of a disaster is not high, but there is a possibility of a severe disaster," no one will evacuate.

Even with the development of prediction technology, how to communicate effectively to ensure that many people evacuate appropriately remains a difficult problem.

It has only been about 100 to 200 years since humans began systematically observing the weather. Changes in weather and the environment fluctuate over long periods. While much is scientifically understood, there are still uncertainties.

In meteorology, many studies are ongoing, but it remains a field with many difficult challenges.

For example, typhoons of the scale of the Ise Bay Typhoon have not occurred in Japan for decades, but they have not disappeared from the world. We don't know if they will continue to decrease.

Therefore, I want to emphasize that it is dangerous to judge that a major disaster will not occur based solely on the experience that "warnings have been issued before, but flooding and landslides have not reached here."

In reality, disasters of a scale that occur only once over many years do happen. Even if long-term residents of an area feel that "this place is safe," it does not guarantee future safety. I hope you will utilize forecasts with this understanding.

◎Mini Information◎

It is extremely rare for rain of 100 mm per hour to continue for a full hour and be recorded, but it is not uncommon for rain of the same intensity to fall for a short period, and we may have experienced it.

【Related Links】 Faculty of Science, Department of Earth and Planetary Sciences Website

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  • Source: PR TIMES
  • Category: 解説