What is the reason for the weakening of typhoons?

First, let me introduce to you the typhoon activities from January to June this year and their causes. The multi-year average (1971-2000 average) first typhoon time in my country (the time when a typhoon first lands in my country each year) is June 29. In the past 56 years, the time of the first station has shown obvious interdecadal changes. From the mid-1980s to the mid-1990s, the landing season occurred relatively early, but since the mid-1990s, the time of the first station has been relatively late. But 2006 was the earliest year that the sixth tropical cyclone made landfall since the founding of the People's Republic of China.

As of July 2, 2007, there were only two numbered typhoons in the northwest Pacific and South China Sea, and no typhoon landed in my country. Compared with the average time of the first typhoon in many years, the first typhoon time this year is later. Since June, the intensity of the subtropical high in the northwest Pacific has been abnormally stronger than the same period of normal years, and its location is further south than the same period of normal years. Tropical convection activities have been suppressed. The above-mentioned circulation factors are not conducive to the formation and development of typhoons. From a statistical relationship point of view, the initial typhoon time is relatively late, and the total number of typhoons that land in my country is often less. But this relationship is not absolute. For example, in some years, the first typhoon was late (1995), but the number of typhoons that landed in my country was relatively high. Generally speaking, July to September is the period of active typhoon activity. In the context of global warming, the intensity of typhoons may become stronger, and the disasters such as strong winds, heavy rains, and storm surges will intensify. Special attention must be paid to preventing disasters caused by strong typhoons to my country's coastal areas. According to statistics from the World Meteorological Organization, the 10 natural disasters that threaten human survival include typhoons, earthquakes, floods, thunderstorms and tornadoes, snowstorms, avalanches, volcanic eruptions, heat waves, landslides (mudslides), ocean tides (tsunamis), etc., and Among these 10 natural disasters, typhoons cause the highest death toll. Especially in Asia and China, typhoons and their storm surges are more serious. The economic losses caused by typhoons in our country since 1990 have averaged nearly 26 billion yuan per year, and the number of deaths caused by typhoons in our country since 1988 has averaged 453 people per year. Typhoon No. 7503 It penetrated deep into Henan and caused tens of thousands of deaths. China is not the country with the highest number of typhoon deaths in Asia. The average annual death toll in the Philippines is as high as 813. The most severe typhoon disaster in the world in the 1990s was not in China and the Philippines, but occurred in the low-lying and vulnerable zone along the coast of Bangladesh on April 29, 1991. A storm in the Bay of Bengal killed 139,000 people. Between day and night. The more serious typhoon disaster in this region occurred from November 11 to 13, 1970. The violent storm surge suddenly claimed 300,000 lives.

Host: Typhoon is the deadliest cause of human death! Now please tell us about the typhoon situation in our country.

Zhang Ling: Typhoons are a type of tropical cyclones, and typhoons are not a unique weather phenomenon in our country’s waters. There are also typhoons in tropical oceans in other regions, but they are called differently. They occur in We call those in the northwest Pacific and South China Sea typhoons, those that occur in the North Atlantic, Caribbean, Gulf of Mexico, and eastern North Pacific are called hurricanes, and those that occur in the Indian Ocean, Bay of Bengal, and Arabian Sea are called hurricanes. Cyclone or cyclonic storm; in the Philippines, people call typhoon Baguio. However, the northwest Pacific is the region with the most tropical cyclones in the world, with 36% of global tropical storms concentrated here. On average, there are about 27-28 tropical cyclones with tropical storm intensity or above in the northwest Pacific and South China Sea every year, accounting for about 30% of the total 87.7 tropical cyclones in the world. The eastern part of my country is located in eastern Asia and the west coast of the northwest Pacific. After tropical cyclones are generated, they tend to move northwest or westward. Therefore, my country is one of the few countries in the world that is most severely affected by tropical cyclones. On average, 7 tropical cyclones land in my country every year, with the most reaching 12 in a year. There are at least 3. Although some tropical cyclones do not make landfall, they can still have a major impact on the coast. Landfalling tropical cyclones are mainly concentrated in the three months of July, August and September, with an average of 1.9-2.1 per month, followed by June and October, with 0.7 and 0.6 respectively. Coastal provinces from south to north, including Hainan, Guangxi, Guangdong, Taiwan, Fujian, Zhejiang, Shanghai, Jiangsu, Shandong, Tianjin and Liaoning, may be affected by tropical cyclone activities. Tropical cyclones may land in all but Guangdong, Hainan, Taiwan, Fujian, and Zhejiang provinces are most frequently affected by tropical cyclones.

Host: So, why are typhoons so lethal?

Zhang Ling: A tropical cyclone is a huge vortex that forms on the surface of the tropical ocean and rotates counterclockwise in the northern hemisphere, with a range of hundreds to thousands of kilometers. It mostly occurs in tropical oceans, where the temperature is high and water vapor is abundant. When the external wind field experiences shear, a rotating airflow field will be formed. If suitable conditions are met, this rotating airflow field will continue to develop and eventually form a tropical cyclone. The generation and development of tropical cyclones require huge amounts of energy, so they form on the tropical ocean surface where high temperature, high humidity and other suitable meteorological conditions exist. The formation of a typhoon requires the following conditions: 1) A vast warm ocean surface with a seawater temperature above 26.6°C, providing the high-temperature and high-humidity air of a tropical cyclone. 2) The vertical shear of tropospheric wind speed is small, which is conducive to heat accumulation.

3) The geostrophic parameter f is greater than a certain value (in areas with a latitude greater than 5°), which is conducive to the formation of a strong low-pressure vortex. 4) There are low-level disturbances in the tropics, providing continuous input of mass, momentum and water vapor.

Host: The area affected by a typhoon ranges from hundreds to thousands of kilometers. It’s really overwhelming!

Zhang Ling: Yes, the radius of a typhoon is generally three to four hundred kilometers, and some are even larger. Therefore, weather radars often cannot see the entire typhoon, although now weather satellites can observe the entire typhoon from high altitudes. The shape of the middle and upper layers, but its internal details cannot be observed. From the photos taken by meteorological satellites, we can see that the upper and middle layers of the typhoon are roughly circular and contain spiral rotating clouds. They rotate counterclockwise in the northern hemisphere, clockwise in the southern hemisphere, and in the center of the typhoon Sometimes the cloudless eye of a typhoon can be clearly seen in this location. Inside the typhoon, weather reconnaissance aircraft conduct observations from various heights and directions. It can be known that the typhoon is roughly a cloud column with a large radius, and its height can reach up to more than 18,000 meters. Surrounding the center of the huge cloud column is the eye of the typhoon with no clouds or very thin clouds and no wind or rain. Going outward from the eye of the typhoon is the eyewall of the typhoon, which is where the clouds are thickest and the wind and rain are heaviest. Then outward, the clouds get higher and thinner, and the wind and rain become weaker. On the ground, when a typhoon gradually approaches and the eye of the typhoon happens to pass by, and then when the typhoon gradually moves away, we can feel the above situation. The eye of a typhoon occurs because the wind in the typhoon rotates counterclockwise along the center of the typhoon. The centrifugal force generated during the rotation and the wind blowing in from the center balance and offset each other, resulting in a phenomenon of no wind within tens of kilometers of the typhoon center. At the same time, the sinking and warming effect of the air in this area causes the clouds to dissipate and the rain to disperse, forming the eye of the typhoon.

Host: How does a typhoon develop and move?

Zhang Ling: The movement path of typhoons is related to many factors, including large-scale flow fields (such as subtropical high pressure, westerly troughs, etc.), sea surface and atmospheric temperatures, topography, and the structure and intensity of the typhoon itself. Among them, the environmental flow field of typhoons often plays a major role. Due to sudden changes in the environmental flow field or the conditions of the typhoon itself, as well as being affected by other complex factors, the typhoon path will undergo sharp deflections, jumps, stagnation, rotations, and swings. Forecasting these abnormal paths is difficult. There are many reasons for changes in typhoon intensity, and the conditions are similar to those for the occurrence and development of typhoons. There are three important aspects: 1) The influence of sea temperature, which determines the source of water vapor and the maintenance of thermal instability. 2) Whether the large-scale environmental flow field is favorable. These include low-altitude convergence, high-level divergence outflow, the magnitude of vertical wind shear, the role of cold air, etc. 3) The influence of terrain. This is mainly the impact of land and large-area islands on typhoons. In addition, the paths of typhoons in the northwest Pacific can be divided into three categories according to their movement directions: westward, northwestward and turning typhoons.

Host: So what conditions are needed for a typhoon to weaken automatically?

Zhang Ling: There are two situations when a typhoon weakens: one is when it lands on the mainland or the Philippines or Taiwan Island, mainly due to the increase in friction on the underlying surface, which leads to changes in the typhoon's internal structure and weakens; the other is when the typhoon weakens. One situation is that the typhoon weakens in the open ocean or sea surface. Possible factors include cold air, increased vertical shear at upper and lower levels, and low sea temperatures in the sea areas it passes through. The mechanism of typhoon intensity change is very complex and is currently unknown. Under further research.

Host: Typhoon is one of the most serious natural disasters that harms mankind. But we have also heard that typhoons have both harm and benefits. So can typhoons bring us benefits?

Zhang Ling: Typhoons will inevitably bring disasters such as violent winds, heavy rains, and storm surges. It is estimated that a moderate-intensity typhoon contains energy equivalent to twenty million-ton atomic bombs. The energy released by an explosion. The huge energy of typhoons is mainly released through strong winds, heavy rains and storm surges, which can easily lead to huge disasters. Wherever the typhoon's center passed, the damage was the most severe. Huge waves caused by strong winds often cause offshore work ships to capsize and kill people. Once they land, they can blow down buildings, destroy telecommunications and power facilities, uproot trees, and cause casualties to humans and animals. Typhoons can also cause sea tide levels to surge, causing Storm surges and even tsunamis can cause seawall breaches, seawater intrusion, and salinization of farmland; heavy rains associated with typhoons may cause local flooding and trigger geological disasters such as mudslides or landslides. For example, strong typhoon No. 7503 penetrated into Henan and caused heavy rains, causing the Banqiao Reservoir dam to collapse, killing tens of thousands of people; Typhoon No. 0513 Taili penetrated into the inland after landing in Fujian, causing local flash floods in Jiangxi, Anhui and other places, and waterlogging and mudslides in some areas. , landslides and other geological disasters. For example, Typhoon Sinlaku made landfall in Cangnan County, Wenzhou City, Zhejiang Province on September 216, 2002. Affected by it, storm surges of 1-2m generally occurred along the northeastern coast of Taiwan Province, Fujian, Zhejiang, and Shanghai. The largest storm surge The tide occurred at Aojiang Station in southern Zhejiang, reaching 3.21 meters, with a maximum tide level of 6.9 meters, breaking the highest tide level record in the history of the station, and exceeding the local warning water level by 1.3 meters (Figures 2 to 7).

The affected population in Zhejiang Province was 7.922 million, of which more than 500,000 were evacuated, and 29 people died. The affected area was 210,000 hectares, and 105,000 hectares were affected by disasters. Among them, 28,000 hectares of marine aquaculture were affected, 9,100 houses collapsed, and 659 dams were damaged, with a total length of 231.6 kilometers, 1,397 bank protections were damaged, 443 dike breaches, a total length of 25.3 kilometers, 89 sluices were damaged, 314 ponds and dams were damaged, 320 ships were sunk, and the direct economic losses totaled 2.96 billion yuan; the affected population in Fujian Province was 2.210 million, and 1 person died. People were injured, 39 people were injured, 125,000 hectares of farmland were affected, 17,000 hectares of aquaculture were affected, 35,000 houses collapsed, 1,666 ships were damaged, 358 embankments were damaged, with a total length of 123.6 kilometers, 127 breaches, a total length of 11.5 kilometers, direct economic losses Nearly 3.26 billion yuan; tide levels in Shanghai are generally high, with the highest tide level at Huangpu Park Station reaching 5.33 meters, which is the third highest tide level in the history of tide monitoring records. Due to effective measures, only some moisture-proof facilities were damaged, resulting in a direct economic loss of about 210 million yuan. Ten thousand yuan. Typhoon Rananim made landfall on the coast of Shitang Town, Wenling City, Zhejiang Province on August 0414, 2004. Affected by the typhoon and its storm surge, Zhejiang, Fujian, Shanghai, Jiangsu, Jiangxi, Anhui, Hubei, Henan and other provinces and cities ***18.18 million people were affected by the disaster, 169 people died (another 25 people were missing), more than 2,100 people were injured, 740,000 hectares of crops were affected, 70,000 collapsed houses, 210,000 damaged houses, and direct economic losses of 20.1 billion yuan . Among them, Zhejiang Province suffered the most severe losses. A total of 639 towns and villages in 50 counties (cities) in Zhejiang Province were affected, with a population of 8.59 million. 42,400 houses collapsed due to the typhoon, resulting in 164 deaths and 24 missing. More than 1,800 people were injured, the crop damage area reached 2.7137 thousand hectares, of which 144.22 thousand hectares were affected, 31,000 livestock died, 284.07 thousand hectares of aquaculture area was lost, 141,500 tons of aquatic products were lost, 502 roads were interrupted, and the road bed was destroyed (area) 505.4 kilometers, 2,101,000 transmission lines were damaged, and 1,104.9 kilometers of communication lines were damaged. The entire urban area of ??Taizhou suffered a power outage, damaging 2,184 embankments covering a distance of 584.9 kilometers, 706 embankment breaches covering a distance of 50.7 kilometers, 706 sluices, 2,164 irrigation facilities, and 42 hydrological measuring stations. The province's direct economic losses were estimated at 15.33 billion. Yuan. In other parts of the world, countries such as Japan, the Philippines, Bangladesh, India in Asia, Caribbean coastal countries in the Americas and the United States are also often hit by tropical cyclones and the violent storms and storm surges they bring. In November 1970, Bangladesh was hit by a severe storm and the torrential rain and tsunami it brought, killing 300,000 people in three days. In December 1988, the southeastern coast of Bangladesh was hit by another severe storm and the tsunami it produced. At least 15,000 sika deer, 2,000 wild boars, baboons and monkeys, and 9 precious Bengal tigers were killed in the forest. In April 1991, the coastal areas of Bangladesh were once again hit by strong storms and their accompanying heavy rains and tsunamis. After the attack, 16 of the 64 counties in the country became disaster areas, with 10 million residents affected, 140,000 deaths, and economic losses of 3 billion US dollars. Affected by tropical storms in 2006, more than 71 million people were affected in Zhejiang, Fujian, Guangdong, Hunan, Jiangxi, Guangxi, Hainan and other provinces and regions, and 1,261 people died due to disasters; direct economic losses reached 76.4 billion yuan, accounting for 10% of all Natural disasters cause as much as 38% of losses.

Although typhoons mostly bring disasters to people due to violent storms, they are not "all bad". For example, the abundant rainfall that typhoons bring to the areas they pass through will have certain benefits in alleviating drought, moistening the climate, and improving the environment. Typhoon precipitation is the main source of summer rainfall in areas such as Jiangnan and South China in my country. It is precisely the typhoon that relieves the drought in the Pearl River Delta, the Lianghu Basin and the Northeast Plain, ensuring a bumper agricultural harvest. It is also precisely because the typhoon brings a large amount of Precipitation has caused many dry reservoirs to refill with water. Typhoons also bring cooler weather to the areas they pass through.