Cause of typhoon

A strong warm-hearted cyclone vortex that occurs in the tropical ocean is a tropical cyclone.

The horizontal scale of typhoon is about several hundred kilometers to several thousand kilometers, and the vertical scale can reach the lower stratosphere from the ground. This is a profound weather system. The air pressure in the typhoon center is very low, generally between 990 and 870 hectopascals, and the maximum wind speed on the ground near the center is generally 30-50 m/s, sometimes exceeding 80 m/s. The huge waves caused by strong winds can cause great damage to ships at sea. When a typhoon approaches the coast, strong winds will cause a large range of huge tides (see storm surge) and affect coastal towns. There are often heavy rains or extremely heavy rains in the area where the typhoon center passes, and the daily rainfall can be as high as 1600 mm, causing large-scale floods.

The record of typhoons in China has a long history. In the Chronicle of South Vietnam written by Liu Song and Shen Huaiyuan, there is such a description: "There are many hurricanes between Angola. Hurricanes also blow in all directions. When it comes to being afraid of the wind, you are often afraid in June and July. "The hurricane referred to here is a typhoon. In the 24th year of Qing Emperor Kangxi (A.D. 1685), Lingji of Zhuluo County, Taiwan Province Province wrote The Wind? At some point, "there will be a north wind from the summer solstice, there will be? Letter ",and said that" the wind and rain will last for three or four days? That is, suddenly. From one day and one night to three days. " "In May 678? . "It's not fair"? "The earliest written records also describe the typhoon season, the length of the invasion time, and the phenomenon that rain comes with the wind.

The name and intensity of typhoons vary from place to place. It is used to be called a typhoon that occurs in the northwest Pacific Ocean and the South China Sea. According to the regulations of China's meteorological department, the maximum wind speed in the center is greater than or equal to 32.7m/s (wind power is above 12), and the wind speed is between17.2 ~ 32.6 m/s (8 ~11), which is called typhoon, and the wind speed is less than or equal to/kloc. Occurring in the southern hemisphere are called tropical cyclones (above magnitude 8) and tropical disturbances (below magnitude 7), sometimes collectively referred to as Willie.

In typhoon season, several typhoons can appear in the same ocean at the same time. In order to better identify and track typhoons, each typhoon is often named and numbered. 195 1 year, the United States meteorological department first named hurricanes, and then in the Atlantic Ocean, the Caribbean Sea and the Northeast Pacific Ocean, they also formulated their own named hurricane tables. In China and Japan, typhoons in the western Pacific or the South China Sea are identified by numbers. According to China's regulations, typhoons that appear west of east longitude 150 and north of the equator are numbered according to the order of appearance. For example, typhoon 1 in 1974 is numbered as 740 1. Japan numbered the typhoons that entered the west of east longitude 180 and north of the equator.

The horizontal distribution of various meteorological elements and weather phenomena in structural typhoons can be divided into three areas: outer cloud area (including outer cloud area and inner cloud area), cloud wall area and typhoon eye area. The vertical direction can be divided into three levels: low-altitude inflow layer (below 1km), high-altitude outflow layer (above 10km) and middle-level updraft layer (from 1km to 10km).

. In the lower layer of the periphery of the typhoon, several spiral air streams with an angle of10 ~ 20 with the isobar of the typhoon area are involved in the typhoon area, and they converge and rise, which promotes the development of the convective cloud system (see cloud) and forms the outer and inner cloud bands in the outer layer of the typhoon. There are several spiral rain belts in the corresponding cloud system. According to the conservation principle of angular momentum, the tangential wind speed becomes larger and larger as the involved airflow spirals into the typhoon. At a certain distance from the typhoon center, the pressure gradient force is balanced with centrifugal force and Coriolis force (see the equilibrium state of atmospheric motion), and the airflow is no longer spiral, so a lot of wet air is forced to rise strongly, forming a towering cloud wall around the center, and the top of the cumulonimbus cloud wall can reach 19 km, which is the cloud wall area. In typhoon, the maximum wind speed occurs inside the cloud wall, and the maximum rainstorm occurs in the cloud wall area, so the cloud wall area is the storm area most prone to disasters. Because a large amount of water vapor condenses in the cloud wall area, the latent heat released by condensation warms the air column, which makes the ground pressure drop sharply and increases the radial pressure gradient in the lower layer. However, when the updraft in the cloud wall area reaches high altitude, due to the weakening of the pressure gradient, the pressure gradient force is no longer balanced with centrifugal force and Coriolis force, and a large amount of air is forcibly thrown out, forming an outer layer, and only a small amount of air flows inward into the typhoon center and sinks, forming a clear typhoon center, which is the typhoon eye area. The radius of typhoon eye is about 10 ~ 70km, with an average of about 25km. The release and warming of latent heat in the cloud wall area and the sinking and warming of the typhoon eye area make the typhoon a warm-hearted low-pressure system.

Formation conditions and causes It is generally believed that typhoons can only be formed if the following conditions are met: ① There is a vast high-temperature ocean surface. The sea surface temperature must be higher than 26 ~ 27℃ to keep the air on the ocean surface in a high temperature and high humidity state, which will lead to stratification instability in the middle and lower layers (see atmospheric static stability). ② The local Coriolis parameters (f = 2 ω sin, ω is the angular velocity of the earth rotation and ω is the latitude) should be greater than a certain value to ensure that the primary cyclone circulation will not weaken. Therefore, the equator (Zi = 0) will not form a typhoon, and the formation area is generally more than 5 latitudes away from the equator. ③ The vertical shear of basic airflow should be small. In this way, the latent heat of condensation will not be blown far by the high-rise strong wind, but concentrated in the same vertical column, which is beneficial to the formation of warm heart. According to statistics, typhoons are mostly formed in the area where the wind speed difference between 200 hectopascals and 850 hectopascals is less than 10 m/s ④ There is a stable convergent flow field at low altitude or a strong divergent flow field at high altitude. For example, the intersection of strong southwest airflow in central and southern intertropical convergence zone and strong northeast airflow in the north, easterly waves, high-altitude anticyclone fronts and other areas meet this condition, which are the most prone to typhoon formation. In the warm season, the western Pacific can often meet the above four conditions, and it is the region with the most typhoons, accounting for 36% of the total number of typhoons in the world every year. The main typhoon sources in the world are: the eastern North Pacific (16%), the North Atlantic (1 1%), the South Pacific (1%), the western South Indian Ocean (10%) and the South Indian Ocean. According to the standard statistics of China Meteorological Department, there are about 28.8 typhoons in the western North Pacific and the South China Sea every year. There are roughly three sources of typhoons affecting China: ① the ocean surface east of the Philippines and near the Ryukyu Islands. (2) the waters near the Mariana Islands and Marshall Islands. ③ South China Sea. As far as seasons are concerned, typhoons mostly occur in warm seasons. For example, in the western North Pacific, August is the most, February is the least, and June ~ 10 accounts for 75% of the whole year; The Bay of Bengal is concentrated in June, September and 10.

The formation and development mechanism of typhoon is the formation of typhoon warm heart. At present, the consistent view is that the warm core is formed by the convergence and rising of a large number of wet air flows at low altitude, and the latent heat of condensation released by the middle and high levels heats the air. Some scholars put forward the theory of "the second conditional instability" mechanism. This theory holds that when the conditions in the middle and lower tropical troposphere are often unstable and cyclone circulation occurs at low altitude, friction can cause friction convergence in the atmospheric boundary layer, so that water vapor can be transported upward through the top of the boundary layer and condensed in the middle and upper parts, and the released latent heat will warm the center; At this time, the ground pressure drops and the cyclone circulation strengthens, which further promotes friction convergence and water vapor upward, and then continues to heat the upper troposphere, and the ground pressure continues to drop. This cycle forms a typhoon. Some scholars have put forward some different views on the above theory, and think that the energy of typhoon warming mainly comes from the adiabatic warming effect when the cloud sinks, not from the heating of latent heat of water vapor condensation in convective clouds. One of the facts is that in the typhoon stage, the typhoon center is often not in the convective cloud area, but in the clear sky area outside the cloud.

The path and forecast typhoon path are very complicated, and parabolic path is the most common wind. There are roughly three typhoon paths in the northwest Pacific: ① westward. After the typhoon enters the South China Sea, it will continue to move westward or land in China, Guangdong and Fujian. ② North-west. Landing in China, Fujian and Zhejiang; (3) It first goes west, and then turns north and east in a parabolic shape at sea.

There are four main factors that affect the typhoon path: ① the internal force of the typhoon (which tends to move the typhoon to the west of high latitude) and the internal eccentricity (which tends to move the typhoon to the direction with the strongest wind force). (2) The guiding function of large-scale basic airflow (generally referring to the airflow on the isobaric surface of 500 hectopascals). ③ Interaction between typhoon and surrounding weather system. For example, when the typhoon approaches the long wave trough, the trough absorbs; Another example is the rotation of two typhoons. ④ The influence of ocean surface temperature. For example, typhoons tend to move around the supercooled ocean surface to the warmer ocean surface.

The forecasting methods of typhoon path and intensity can be basically divided into three categories: ① Weather chart forecasting methods. Including the use of satellite images and weather radar echoes. ② Statistical prediction method. Using mathematical statistics method and reasonable meteorological factors in the early stage, the statistical prediction equation is made. There is also a similar statistical prediction method, which uses similarity factors to find out similar paths that have appeared in history, and uses these similar paths to make the most possible prediction path. This similar method has relatively stable prediction accuracy and has been adopted by many countries. ③ Hydrodynamic numerical weather forecast method. At present, a layer of barotropic primitive equation model is mostly adopted, and typhoon is regarded as point vortex. In addition, there are multi-layer baroclinic models and models considering the internal structure of typhoon.