What is a typhoon? How is it formed?

Typhoon is a kind of tropical cyclone. In meteorology, according to the definition of the World Meteorological Organization, a tropical cyclone with a sustained wind speed of 12 (that is, more than 32.7 meters per second) is called a hurricane or a local synonym, and the name of a hurricane is used in the North Atlantic and the East Pacific; Typhoon is a synonym for the western North Pacific (north of the equator, west of international date line, east of east longitude 100).

Typhoon formation

Conditions: Direct sunlight on the tropical sea surface raises the temperature of seawater, and seawater evaporation provides sufficient water vapor. The water vapor condenses in the process of uplift, releasing a lot of latent heat, promoting the further development of convection, reducing the sea level pressure, making the surrounding warm and humid air flow in and replenish, and then rising again. This cycle forms a positive feedback, that is, the second conditional instability (CISK) mechanism. On the vast sea surface with suitable conditions, the influence range of circulation will continue to expand, reaching hundreds to thousands of kilometers.

Due to the high-speed rotation of the earth from west to east, there is friction between the air column and the surface. The closer to the equator, the greater the friction, which causes the steering column to rotate counterclockwise (clockwise in the southern hemisphere). Because of the fast rotation of the earth, the air column can't keep up with the speed of the earth's rotation, forming a westward sense and forming what we now call typhoons and typhoon paths.

In tropical or subtropical oceans where the sea surface temperature exceeds 26℃, due to the high temperature near the ocean surface, a large amount of air expands and rises, which reduces the air pressure near the ocean surface and continuously supplements the inflow and rise of peripheral air. Affected by the geostrophic force, the inflowing air rotates. The rising air expands and cools, and when the water vapor in it cools and condenses into water droplets, it will release heat and promote the rising of the lower air. In this way, the air pressure near the ocean surface drops lower, and the air rotates more violently, eventually forming a typhoon.

Judging from the typhoon structure, such a huge behemoth must have unique conditions: a vast high-temperature ocean. The seawater temperature should be higher than 26.5℃ and the depth should be 60m. Typhoon is a very violent weather system, with an average daily consumption of 3100 ~ 4,000 calories/cm? Energy, this huge energy can only be supplied by the latent heat released by the vast tropical ocean. In addition, the strong winds around the typhoon will cause the sea water 60 meters deep near the center to churn. In order to ensure that the sea surface temperature is always higher than 26.5℃ during tumbling, the thickness of warm water layer must reach 60 meters. There should be a suitable flow field. The formation of a typhoon requires a strong upward movement. Suitable flow fields (such as East Storm and equatorial convergence zone) are easy to produce tropical weak cyclones, and the pressure of tropical weak cyclones is low in the middle and high in the periphery, which makes the airflow converge to the cyclone center and move upward. In the process of rising, water vapor condensation releases huge latent heat, forming a warm heart to supply typhoon energy, making the rising movement stronger and stronger. There should be enough geostrophic deflection. If the converging airflow reaches the cyclone center and the air accumulation block, a typhoon cannot be formed. The large enough geostrophic deflection force makes it difficult for the convergent airflow to flow directly into the low-pressure center, but rotates along the center, which strengthens the cyclonic circulation. The geostrophic skewness of the equator is zero, and gradually increases towards the poles, so typhoons occur at places more than 5 latitudes from the equator, ranging from 5 degrees to 20 degrees. The vertical shear of airflow should be small. That is, there is little difference between high and low wind directions and wind speeds. If the air speed difference is too large, the latent heat will flow out quickly, which is not conducive to the formation and maintenance of typhoon warm heart. In areas with latitude greater than 20 degrees, there is strong wind at the top, which is not conducive to warming up and typhoons are not easy to appear.