tornado

tornado

This entry is compiled and applied by "popular science China" Science Encyclopedia

to examine

.

Tornado is a kind of local scale severe weather phenomenon, which occurs between the bottom of the straight form clouds and the underlying surface. Tornadoes can be seen in tropical and temperate regions, including the interior of America, Western Australia, northeast of the Indian peninsula, etc. the common occurrence time is spring and summer

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Tornadoes can be divided into multi vortex tornado, landspout, waterspout and so on. Tornadoes are observed as long and narrow funnel clouds or similar dust / water columns. Tornadoes usually have a wind speed of 30 to 130 meters per second, a diameter of less than 2 kilometers, an activity range of 0 to 25 kilometers, and a duration of about 10 minutes. The intensity of tornadoes can be divided into five levels according to the enhanced Fujita scale (EF)

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The conditions of tornado include wind shear near the ground and significant vertical motion / instability energy. Thunderstorm is the ideal environment to meet the above conditions, and it is also the main cause of tornado

The tornado caused by supercell is called supercell tornado, and other cases are called non supercell tornado. The strength and scale of supercell tornado is usually larger than that of non supercell tornado

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Tornado is a kind of meteorological disaster, where it passes, it often happens to pull up trees, overturn vehicles, destroy buildings and so on. Modern operational weather forecast can early warn tornadoes through high frequency ground stations and remote sensing observations, but it has higher requirements for forecasting experience

. In addition, some areas also use manual observation of tornadoes, which is called storm tracking

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brief introduction

Tornado is a rare local, small-scale, sudden strong convective weather. It is a strong, small-scale air vortex caused by the air convection movement under the strong and unstable weather conditions.

A violently rotating funnel-shaped cloud tower protruding from a cumulonimbus cloud. Sometimes it's hidden, sometimes it's hanging in the air or touching the ground. The axis of tornado funnel cloud is generally perpendicular to the ground. In the later stage of development, when the wind speed difference between the upper and lower layers is large, it can be inclined or curved. The minimum diameter of the lower part is only a few meters, generally hundreds of meters, and the maximum diameter can reach more than 1000 meters. The diameter of the upper part is generally several kilometers, and the maximum diameter can reach 10 kilometers. The scale of tornado is very small, and the central pressure is very low, resulting in a large horizontal pressure gradient, resulting in strong wind speed, and the central wind speed can reach 100-200 M / s. Because of the strong rotating force of the air flow, it often rolls up the water, dust and silt on the ground, and its destructive power varies widely. The weak can only roll up the straw bundles and / or clothes. The strong can collapse the house according to the tree, and even raise the people and animals together. When passing through the water, it can absorb water and rise like a column (called a water tornado at this time). Therefore, although the scope of the tornado is small, the disaster caused by it is very serious. The direction and speed of a tornado are determined by the movement of its parent cloud (the cumulonimbus cloud that produces the tornado). The speed of the parent cloud is usually 40-50 km / h, and the fastest can be 90-100 km. Its moving path is mostly straight line, generally only a few kilometers, individual up to dozens of kilometers. Tornadoes are the products of severe convective weather. They often occur in the low-level atmospheric junction of 20-50 degrees north latitude with great convective instability. They are often accompanied by fronts, cyclones or non tropical thunderstorms. When the landing tropical cyclone moves to the middle latitude and tends to decline, tornadoes are also easy to appear, sometimes in tropical areas. According to the region of tornado, it can be divided into land Tornado (generated over land) and water Tornado (generated over sea or water). Tornadoes are difficult to predict, and the rain radar has never directly observed them.

Basic characteristics

Tornado is a natural phenomenon with certain destructive power. In general, a tornado is a cyclone. When it touches the ground, its diameter ranges from a few meters to several hundred meters, with an average of about 250 meters and a maximum of about 1000 meters. In the air, the diameter can be several thousand meters, with a maximum of 10 kilometers. The maximum wind speed can reach 150 km to 450 km per hour, and the duration of tornado is usually only a few minutes, but the longest is only a few hours. Everything is robbed wherever it goes. The funnel-shaped center of tornado is composed of dust and condensed water vapor, which is the visible "dragon mouth". On the ocean, especially in the tropics, a similar scene occurs in the sea called a tornado.

Tornadoes are usually extremely fast, and the wind speed of 100 meters per second is not surprising, even reaching more than 175 meters per second, which is five or six times larger than a typhoon of category 12. The range of wind is very small, generally only 25-100 meters in diameter, only in very few cases the diameter can reach more than one kilometer; from occurrence to disappearance, there are only a few minutes, at most a few hours.

Most tornadoes rotate counter clockwise in the northern hemisphere and clockwise in the southern hemisphere, with some exceptions. The exact mechanism of tornado formation is still under study, which is generally considered to be related to the violent activities of the atmosphere.

Since the 19th century, the accuracy of weather forecast has been greatly improved. Weather radar can monitor tornadoes, hurricanes and other disasters.

Tornadoes often pull up big trees, overturn vehicles and destroy buildings where they pass. They often destroy crops and trees in an instant, interrupt traffic, collapse houses and cause loss of human and animal lives and economy.

Formation conditions

Tornado, a natural phenomenon, is the product of Thunderstorm in clouds. Specifically, tornado is a form of concentrated release of a small part of the huge energy of Thunderstorm in a small area.

The formation of tornado can be divided into four stages

(1) The instability of the atmosphere produces a strong updraft, which is further strengthened by the influence of the largest cross flow in the jet stream.

(2) Due to the interaction with the wind with shear in vertical velocity and direction, the updraft begins to rotate in the middle of the troposphere, forming a mesoscale cyclone.

(3) With the development and upward extension of mesoscale cyclone, it becomes thinner and stronger. At the same time, a small area of enhanced complementary, that is, the primary tornado formed in the cyclone, and formed the tornado core in the same process.

(4) The rotation in the core of a tornado is different from that in a cyclone, which is strong enough to make the tornado extend all the way to the ground. When the developing vortex reaches the ground height, the ground pressure drops sharply and the ground wind speed rises sharply, forming a tornado.

Detection method

Doppler weather radar detection

The time from tornado occurrence to dissipation is short, and the action area is very small, so that the existing detection instruments do not have enough sensitivity to accurately observe tornadoes. Relatively speaking, Doppler radar is an effective and commonly used observation instrument. The Doppler radar aims at the microwave beam emitted by the tornado, and the microwave signal is reflected by the debris and raindrops in the tornado and then received by the radar again. If the tornado is far away from the radar, the reflected microwave signal frequency will move to the low frequency direction; otherwise, if the tornado is closer to the radar, the reflected microwave signal frequency will move to the high frequency direction. This phenomenon is called Doppler shift. After receiving the signal, the radar operator can calculate the speed and moving direction of the tornado by analyzing the frequency shift data.

Dual polarization weather radar detection

The emergence of dual polarization technology makes a powerful supplement to the detection of cyclones and tornadoes by Doppler weather radar, and comprehensively improves the level of analysis and early warning of tornado microphysical characteristics. ① Because Doppler weather radar needs to have good spatial resolution for detecting small-scale tornado vortex, it does not need high accuracy for dual polarization detection. ② Dual polarization characteristic signal is different from Doppler characteristic signal, because it is "isotropic", so it does not depend on the change of observation angle. ③ When tornadoes occur at night or are wrapped by a large amount of precipitation, which is difficult to be detected by Doppler radar observation, dual polarization information can identify them more effectively.

Fast scan radar detection

Wurman designed and developed the first X-band mobile fast scanning radar, rapid Dow. The radar can complete 360 ° volume scanning every 7 seconds, and can detect 12 beam range data in 14 seconds. Its range resolution reaches 11 meters, which makes it easier to study the three-dimensional structure of tornado. From the current detection technology of tornado, fast scanning radar has unique advantages in observing tornado in space-time scale. The next generation weather radar network planned by the United States is also positioned as multi-functional phased array radar. So it can be seen that this technology will become a research hotspot in the future

Chinese PinYin : Long Juan Feng