Summary of typhoons

Summary of typhoons

What is typhoon?

Talking about typhoons, we should start with cyclones.A cyclone is a horizontal vortex with a central pressure lower than its surroundings at the same altitude. In the northern hemisphere, air rotates counterclockwise; in the southern hemisphere, it rotates counterclockwise.Because typhoons, the atmospheric vortex, which are generated on the tropical ocean surface, called "tropical cyclones".Typhoon means a tropical cyclone with a maximum wind force of 12 or more near the center.Therefore, typhoon is a strong tropical cyclone which originates from the tropical ocean.In fact, a typhoon is a swirl of air whirling around its own center and moving forward at the same time.

Formation and origin of typhoons

Formation of typhoons

Formation of typhoons require the following conditions:

(1)The wide warm ocean surface, with sea water temperature above 26.6°C, provides the high temperature and humid air needed by tropical cyclones.

(2)Vertical shear of tropospheric wind speed is smaller, which is beneficial to heat accumulation.

(3)The geostrophic parameters/greater than a certain value (latitude is greater than 5 °), to form a strong low pressure vortex.

(4)There are low-level disturbances in the tropics (unstable atmospheric stratification with lower heat and upper cooling),to provide continuous mass, momentum and water vapor input.

According to statistics, typhoons often occur in areas with ocean surface temperature over 26 to 27°C.In the high temperature sea area, which happened that some disturbances occurred in the atmosphere, and a large amount of air began to rise. At this time, the surrounding air of the rising sea area was continuously flowing into the rising area.Due to the rotation of the earth, the flowing air rotates like a wheel.When the rising air expands and cools, the condensation droplets emit heat, which in turn contributes to the rising of low-level air, lowers the ground pressure and makes the air spin more violently,then typhoons are formed.

Origin of Typhoons

According to the statistical results of the world meteorological organization (WMO) on the data from 1968 to 1990, the global average of about 83 tropical cyclones occurred every year.Global tropical cyclone source include eight sea areas, which mainly distribute between latitudes of 5°〜20 in the north and South hemispheres. Among them, 65% of the total number of tropical cyclones are generated between 10°〜20°latitudes.Only 13% of tropical cyclones occur in higher dimensions beyond 20, and both occur in the Northwest Pacific and the Northwest Atlantic. Few tropical cyclones occur near the equator within 5°.

About two-thirds of the tropical cyclones formed in the Northern Hemisphere, more than half of them occurred in the North Pacific, and the western part of the Ocean was about twice as many as the eastern part. That is to say, the Northwest Pacific Ocean had the largest number of tropical cyclones, and about 80% of these tropical cyclones would develop into typhoons.The Northwest Pacific Ocean is also the only area where typhoons are likely to occur every month of the year.

The main sources of typhoons are as follows:

(1)Western North Pacific: including the South China Sea, the affected areas include China, the Philippines, South Korea, Japan, Vietnam, the Pacific islands, and in the meantime, Thailand and Indonesia can also be affected. Tropical cyclones in the Northwest Pacific Ocean account for one third of the global total annually.China's coast is the largest tropical cyclone landing site in the world; 6 to 7 Tropical Cyclones Landing in the Philippines every year.

(2)Eastern North Pacific: The second most tropical cyclone-producing region, including Mexico, Hawaii, the Pacific island countries, in rare cases can affect California and the northern part of Central America.

(3)North Atlantic: including the Caribbean Sea, Gulf of Mexico. There is a wide gap in the number of generations per year, ranging from one to more than 20, with an average of about 10 generations per year.It mainly affects the eastern coast of the United States and the states along the Gulf of Mexico, Mexico and the countries of the Caribbean Sea, with the farthest impacts reaching Venezuela and Canada.

(4)Western South Pacific: mainly affecting Australia and Oceania countries.

(5)North Indian Ocean: including the Bay of Bengal and the Arabian Sea, mainly formed in the Bay of Bengal.The North Indian Ocean monsoon has two peaks: one in April and May before the onset of the monsoon;and the other in October and November after the end of the monsoon.Affecting India, Bangladesh, Sri Lanka, Thailand, Myanmar and Pakistan, and sometimes the Arabian Peninsula. Eastern South Indian Ocean: affects Indonesia and Western Australia.

(6) Western South Indian Ocean: mainly affecting Madagascar, Mozambique, Mauritius, Reunion Island, Tanzania, Comoros and Kenya.

Structure of typhoons

Typhoon is nearly circular. A mature typhoon can be divided horizontally into typhoon eye area, strong wind area near the center and rainstorm area (called typhoon eye wall), strong wind area and precipitation area around the periphery (Fig. 6-5).

Figure 6-5 Typhoon remote sensing images taken by satellite (source: website of China Meteorological Administration)

Typhoon eye is an important sign of the development and maturity of typhoon, which is an area surrounded by typhoon eye wall.In the eyes of the typhoon, there is neither strong wind nor heavy rain, and there are only thin clouds in the sky.Typhoon eye area will suddenly become calm, rainstorm stops, and may last 20 minutes to 1 hour, people often mistake that the typhoon has passed, but it is just in the eye of the typhoon.The calmer the storm, the stronger the typhoon.

Typhoon eyewall (near the center of the strong wind and rainstorm area) has a strong convective ring around the eyewall of the typhoon, known as the eyewall or typhoon cloud wall.This is the most important part of the typhoon. The strongest gale above grade 12 and the raging waves above 10m are mainly located in the eye wall area.At the same time, the eye wall of typhoon is also one of the main areas causing typhoon rainstorm. Tornadoes, lightning, thunderstorms and sometimes hailstorms also occur in the eye wall area.

Typhoon spiral cloud-rain belt (outer gale and precipitation area) is close to the eye wall of typhoon with wide range and various forms,which is a very important feature of typhoon structure. It is also an important sign to judge whether the intensity of tropical cyclone can develop and increase, which can also be used to judge the distribution of typhoon affecting the outer cloud-rain belt and the characteristics of its burst.Where the spiral rain belt affects, there are often showers and gusts, the intensity and time of rainfall vary with the range and intensity of the cloud rain belt.

Classification of tropical cyclones

According to the World Meteorological Organization (WMO), tropical cyclones are divided into six stages, as shown in Table 6-4.

Number of typhoons

The number of typhoons are also the number of tropical cyclone.The reason why tropical cyclones are numbered is that a tropical cyclone usually lasts for more than a week, several tropical cyclones may occur in the ocean at the same time, there will be no confusion with the numbering.In China, tropical cyclones with maximum wind force greater than 8 degrees near the center, west of 150 degrees longitude to the east, north of 10 degrees latitude to the north, which are numbered in the order of annual occurrence.China's typhoon number is according to the requirements of the World Meteorological Organization (WMO), the number is composed of four digits,the first two digits represent the ten digits and the individual digits of the Gregorian calendar year, and the second two are the sequence of the annual occurrence of typhoons, such as Typhoon No. 7 of 1997, which is No. 9707.

Naming of typhoons

At the 30th meeting of the World Meteorological Organization (WMO) Typhoon Committee, held in Hong Kong from November 25 to December 1, 1997, which was decided that the names of tropical cyclones in the Northwest Pacific and South China Sea should be named with Asian style.The new nomenclature is a predetermined list of names that are then recycled year after year in a sequential fashion.There are 140 names in the naming table, which are provided by 14 countries and regions in the Asia-Pacific region of WMO, including Cambodia, China, Korea, Hong Kong, Japan, Laos, Macao, Malaysia, Micronesia, Philippines, Korea, Thailand, the United States and Vietnam.Each country or region provides 10 names. The 140 names are divided into 10 groups, each of which has 14 names arranged in alphabetical order according to the English names of each member country or region, and recycled in sequence.

Disaster of typhoons

Typhoon is a kind of disastrous weather system with strong destructive force, when typhoon moves on the sea, it will set off huge waves, storms and rainstorms will follow, which will pose a serious threat to ships.When typhoon landed, heavy storms would cause huge losses to people's lives and property, especially to agriculture and buildings.Typhoon mainly causes harm through three ways: strong wind, heavy rain and storm surge.

(1)Strong wind.Typhoon is a huge energy pool, the wind speed is above 17.2m/s (wind power level 8), even above 60m/s.It is estimated that the wind pressure per square metre on the plane perpendicular to the wind direction can reach 230kg when the wind reaches 12 grades.

(2)Rainstorm.Typhoon is a very strong rainfall system, when a typhoon landfall, rainfall center in a day can fall 100 to 300mm heavy rain, and even up to 500 to 800mm.The flood disaster caused by typhoon and rainstorm is the most dangerous one.The intensity of typhoons and rainstorms is large, which is high frequency and wide scope with fierce and destructive force.

(3) Storm surge.Storm surge is a serious natural disaster occurring along the coast of the ocean, caused by strong winds and high tide water level, which makes local areas increase water drastically and causes major disasters.When typhoon moves to land, due to the strong wind and low pressure of typhoon, the sea water accumulates strongly toward the coast, the tide level rises sharply, and the wave head of sea water presses toward the coast like mountains and seas. The storm surge of a strong typhoon can raise the coastal water level by 5 to 6m. The storm surge meets the astronomical high tide level, which produces high-frequency tidal level, causes overflow of tide, breach of seawall, destruction of houses and various construction facilities, flooding towns and farmlands, causing a large number of casualties and property losses.Storm surges can also cause coastal erosion, seawater irrigation, land salinization and other disasters.