Shulou(Shulou.com)11/24 Report--

This year, the northwest Pacific is not peaceful, and strong typhoons frequently approach or land in China. A week ago, No. 5 Typhoon "du Surui" made landfall in Fujian, China with the upper limit of a strong typhoon (wind force 15). Now, No. 6 Typhoon "Kanu" lingers in the East China Sea with the intensity of a strong typhoon. Meteorological agencies once predicted that it would make direct landfall in Zhejiang, China. However, it is worth noting that the tracks of these two typhoons have some characteristics, and there are many variables in their development, which often lead to major differences among the major meteorological agencies. And their actual path is even more peculiar. "du Suri" has successively walked out of several "right angles" in the Bashi Strait. "Kanu" even turned around in place and rushed to our country, but after entering the East China Sea, it slowly changed its direction and began to head for Japan. Can't help but wonder, does Typhoon also have to take a driver's license? Why did you take such a strange path?

The Magic right Angle of du Suri (typhoon network of the Central Meteorological Observatory) first of all, we need to understand how typhoons can continue to move. Typhoon itself is a large weather system, but its motion path is affected by many factors, the greatest of which is the "subtropical high" that we are familiar with. In the hot summer, the "subtropical high" is very strong and can extend from the depths of the Pacific Ocean to the inland areas of China and even to the northern grasslands. As a tropical low pressure system, typhoon can only move along the edge of the subtropical high and it is difficult to "cross". When the typhoon passes through the westernmost position of the subtropical high, it will continue to "turn" along the edge of the subtropical high. If the westernmost end of the subtropical high is "sharp", there may be a typhoon "U-turn". For example, Typhoon Terry made a sharp turn to Japan after Typhoon Terry entered the East China Sea in September 2017. Another possibility is that the structure of the subtropical high changes during the approach of the typhoon, retreating rapidly to the east or even "breaking" (there is a trough of low pressure on the north side), and the typhoon will take this opportunity to turn to Japan and South Korea. For example, typhoons "Bawei" and "Maisak" in 2020 took advantage of the opportunity of the trough to cut off the subtropical high and continuously moved northward to hit North and South Korea.

Chih-wen Hung et al. (2010). The arrows show several common tracks of typhoons. "H" refers to the subtropical high. The situation encountered by the "Kanu" typhoon is another situation. As can be seen from the pressure chart, on August 3, the subtropical high seems to "open its mouth to contain" Typhoon "Kanu", making "Kanu" begin to turn in place. This situation is called "saddle field" in meteorology, and the typhoon in the saddle field can not continue to move until the outfield circulation situation changes and a new guiding airflow appears. According to forecasts, "Kanu" will gradually move slowly towards Japan in the next few days, but the possibility of turning again after that cannot be ruled out.

Situation chart of 500 HPA pressure field on August 3. Within the thick blue line is the scope of the subtropical high. You can see that Typhoon "Kanu" is almost completely surrounded by the subtropical high. In addition to being guided by the subtropical high, the typhoon will also be affected by the monsoon and westerly belts. It will also be affected by other typhoons, that is, the "Fujiwara effect". In history, there are many typhoons with strange tracks, many of which are due to the emergence of other typhoons, sometimes even three or four typhoons appear at the same time, which influence each other, resulting in their path prediction is extremely difficult, the situation is complex and changeable. For example, Typhoon Wayne in 1986, Typhoon Lion Rock in 2016, Typhoon Olu in 2017, and so on.

In 1986, the 21-day performance of Typhoon Wayne, which lasted for 21 days, made many U-turns and sharp turns, which can be described as "shrimp catchers in the South China Sea" (Picture Source Wikipedia)

In the typhoon version of the Romance of the three Kingdoms encountered by "Lion Rock" in 2016, "Lion Rock" had the last laugh and walked out of a strange path, while the "right-angle turn" of "du Suri" actually had something to do with the time interval of the meteorological agency. All kinds of typhoon tracks we can see are not completely continuous tracks, but update the position of the center of the typhoon every few hours, and then connect the two adjacent positions to get the path of the typhoon. Sometimes the typhoon itself moves very slowly and may spin in place for a few hours, and the track happens to be at a "right angle", not because the typhoon can really make a perfect right-angle turn. Even if driving a "right-angle turn", we have to understand that the actual trajectory of the car can not be a perfect right angle, but an arc. The same is true of the trajectory of typhoons. Another possibility is that the typhoon itself is so loose that it is difficult to determine the center of its circulation, leading to a coincidence of "right-angle trajectories". When "du Suri" appeared, it was just after it passed through the Philippine island of Luzon and its core structure was destroyed by the mountains of Luzon Island in the Philippines. At that time, its wind eye structure was loose and the circulation center was difficult to determine, so it had a "right angle" on the real-time path.

The actual path of "du Suri" given by American institutions has a long time interval, so there is no obvious right-angle reference.

1. Wikipedia related entries

2.Zoom Earth

3. Official website of the Central Meteorological Observatory

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), by Sikong.

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