Why did Typhoon Terry arrive so late？ Expert interpretation

　　The Central Meteorological Observatory issued a typhoon blue warning at 10: 00 on July 15th, and the China Meteorological Bureau launched a typhoon level 4 emergency response at 11: 00. The tropical depression in the South China Sea was strengthened into the No.4 typhoon "Taili" this year on the morning of 15th. According to the latest forecast of the Central Meteorological Observatory, the typhoon "Taili" will increase rapidly in the future, and it is expected to land in the eastern part of Hainan Island to the coastal area of western Guangdong on the night of 17th, or it will become the first typhoon to land in China this year.

　　Is Typhoon Terry, who has been "late" for a long time, secretly preparing for an "enlarged move"? Is there really a tendency to "debut" late in recent years? What is the changing trend of typhoon’s overall impact on China? Nie Gaozhen, a senior engineer of Typhoon and Marine Meteorological Forecasting Center of China Meteorological Bureau, made a detailed interpretation of the "initial stage" and related issues.

　　How late was Typhoon Terry?

　　According to the forecast of the Central Meteorological Observatory, the fourth typhoon "Taili" this year is expected to land in the coastal area from the east of Hainan Island to the west of Guangdong on the night of July 17, which is more than 20 days later than the average landing date of "Initial Taiwan" from 1991 to 2020.

　　In the fact that the typhoon was late, let’s take a look at the degree of the typhoon "Taili": the date of the first typhoon landing in China from 1991 to 2020 is ranked in four time periods, and the typhoon "Taili" is in the last 25%. It can also be seen that in 75% of the years, the "first stage" time is earlier than this year. The late arrival of Typhoon Taili is significant.

　　Do typhoons tend to land later and later in recent years?

　　Simply judging from the expected landing time of "Initial Taiwan" in the past three years and this year, the expected landing date of Typhoon Taili No.4 in 2023 is the night of July 17th, that of Typhoon Siam in 2022 is July 2nd, that of Typhoon Bear in 2021 is June 12th, and that of Typhoon Parrot in 2020 is June 14th. The intuitive feeling is that, compared with the average landing date of "Initial Taiwan" from 1991 to 2020 on June 27th, 2023 and 2022 are indeed late, but 2021 and 2020 are obviously early.

　　In fact, if we put the above-mentioned "late" records for two consecutive years into a longer time series, it is even more impossible to draw obvious simple rules. It’s too early to draw any conclusions.

　　Judging from the overall impact of typhoon season on China, has there been any obvious change in recent years?

　　Analyzing the destructive power of typhoons that landed or affected China in recent years, we will find the fact that if there was a typhoon with a particularly strong intensity that year, its impact would probably account for more than half of the overall impact of the typhoon season that year.

　　Generally speaking, a particularly strong typhoon will bring strong precipitation, which may lead to geological disasters. Or the wind is particularly strong, which will bring great losses to cities along the way.

　　From this point of view, the public feels that the overall impact of the typhoon season seems to be changing in recent years, which may be partly due to the fact that there was no typhoon with particularly strong intensity that year. On the other hand, timely and accurate forecasting, active and favorable disaster prevention response, and strong winds and heavy precipitation brought by typhoons did not appear in disaster-prone areas are all important reasons.

　　However, the impact of typhoons is a double-edged sword. Many typhoons with low intensity are also doing some "good things" silently, such as "sprinklers" and "air conditioners", which are the expectation and praise for their functions of relieving drought and cooling down summer heat.

　　How is the typhoon generated and landed this year?

　　Before Typhoon Taili was generated, three typhoons were generated in the western Pacific, and the average number for many years was 5.2. Together with Typhoon No.4, the number of typhoons generated was one less. If Typhoon Taili finally lands in China, as of its expected landing date, the average number of typhoons landing in China for many years is 1.5, which is 0.5 less this year.

　　On the whole, the "progress bar" is slow, but not very significant.

　　What is the cause of typhoon "Taili"?

　　First of all, the time is long enough. From the generation of Typhoon No.3 this year to now, there has been a typhoon "empty window period" in the northwest Pacific for more than one month. From the climate point of view, energy has been accumulated in place.

　　Secondly, the subtropical high moves northward, leaving room for the development of typhoon activities. Last week, the subtropical high was still very strong, and covered the mid-low latitude ocean surface generated by typhoons. Under the suppression of the strong subtropical high, typhoon activities in the northwest Pacific have been suppressed. However, the situation has changed in the past two days. With the subtropical high moving northward, it has brought growth space for typhoons. As long as the external conditions that inhibit the development of typhoon are eliminated, tropical disturbance may develop into tropical depression.

　　Can Typhoon Taili bring relief to the high temperature in South China?

　　According to the forecast of the Central Meteorological Observatory, Typhoon Taili will move to the north-north direction at a speed of 10 to 15 kilometers per hour, and land in the coastal area from the eastern part of Hainan Island to the western part of Guangdong on the night of the 17th. It is expected that the intensity of landing will be typhoon or strong typhoon (35 to 42 meters per second, 12 to 14).

　　It is estimated that from 16th to 19th, due to the influence of typhoon and peripheral circulation, the accumulated rainfall in southern Guangdong, Hainan Island and southwestern Guangxi will be more than 100mm, and the local area will reach 250-450mm. The strongest rainfall period is from 17th to 18th. During the rainfall, there are strong convective weather such as lightning, short-term heavy rainfall and local thunderstorms and strong winds. From the 16th, the high temperature in the south will obviously weaken under the influence of typhoon.

　　Although typhoon "Taili" can bring coolness, due to its strong intensity when landing, the risk of disaster is high. It is suggested that Guangdong, Hainan and Guangxi should make preparations for typhoon defense in advance, strengthen the prevention of strong wind disasters, urban and rural waterlogging, flash floods and geological disasters, and do a good job in grabbing early rice for harvesting and drying. It is at the peak of summer vacation tourism, so it is suggested that tourism projects involving islands and seas in coastal areas of South China should be well managed. Ships sailing in the relevant waters of the South China Sea should pay attention to avoid the typhoon-affected areas in advance, and the offshore operators should evacuate in time.

　　Will Typhoon Taili strengthen rapidly near the shore?

　　After Typhoon Taili entered the South China Sea this year, several internal and external conditions contributed to the development of its intensity.

　　One is the warm sea temperature. Compared with the western Pacific, the South China Sea is shallow and the sea temperature is relatively high, about 30-31℃, which is beneficial to the strengthening of Typhoon Taili. Second, the wind shear is not particularly large, which is more appropriate. Third, as the typhoon approaches the land, the high-altitude divergence conditions will also become better. Fourth, the typhoon "Taili" belongs to a small typhoon. Compared with a large typhoon, the intensity of a small head is easier to increase rapidly. At present, judging from the shape of typhoon, the center of typhoon is flat and long, and the convection around it is loose.

　　In the future, if the typhoon center can shrink and become round, the convection will be more dense and symmetrical, and the typhoon intensity will be greatly strengthened.