Thunderstorm: Thunderstorm is a local storm caused by strong cumulonimbus clouds, accompanied by lightning, thunder and heavy showers. Lightning and thunder without precipitation are called dry thunderstorms. When a thunderstorm crosses the border, meteorological elements and weather phenomena will undergo drastic changes, such as a sharp rise in air pressure, a sharp change in wind direction, a sharp increase in wind speed, a sudden drop in temperature, and then a downpour. Strong thunderstorms can even bring serious disasters such as hail and tornado.

Usually, thunderstorms accompanied by showers are called general thunderstorms, and those accompanied by severe weather phenomena such as heavy rain, strong winds, hail and tornadoes are called strong thunderstorms. Both are formed by strong cumulonimbus clouds called thunderstorm clouds. Thunderstorm process is not just a thunderstorm cloud, but is composed of several or several thunderstorm cell in different development stages. Although these thunderstorm cell are in the same thunderstorm cloud, each cell has an independent circulation in the cloud, but they have all gone through the development stage (updraft running through the cloud), maturity stage (downdraft with precipitation and precipitation drag in the cloud) and dissipation stage (downdraft in the cloud), and they are all in the process of continuous regeneration and disappearance.

Thunderstorm activity has certain regional and seasonal characteristics. According to statistics, the frequency of thunderstorms in low latitudes is more than that in middle latitudes, and the middle latitudes are more than that in high latitudes. This is because the low latitude is always hot and rainy, and the air is in a warm and humid unstable state, which is easy to form thunderstorms. In the mid-latitude summer half year, the temperature and humidity of the near-surface atmosphere increase, and the instability of atmospheric stratification increases. At the same time, the weather system is active and there are many thunderstorms. At high latitudes, the temperature is low, the humidity is low, the atmosphere is relatively stable, and thunderstorms rarely occur. At the same latitude, the frequency of thunderstorms is generally more in mountainous areas than in plains, and more in inland areas than in coastal areas. Thunderstorms occur most frequently in summer, followed by spring and autumn, and rarely occur in winter except in warm and humid areas.

The movement of thunderstorms is greatly influenced by geographical conditions. In mountainous areas, thunderstorms often move along the mountains. If the mountain is not high, strong thunderstorms can cross the mountain. In coastal areas, rivers and lakes, due to the low water temperature during the day, local downdraft often occurs, which weakens or even disappears the intensity of thunderstorms. However, some weak thunderstorms often cannot cross the water surface and move along the coast, but at night, thunderstorms may intensify.

The second question:

Tropical weather system: Meteorological tropics are the zone between the northern hemisphere guide and the subtropical high ridge. Because the ridge line of subtropical high moves north and south with the season, the position and range of tropical edge also change seasonally. Usually, the area within 30 degrees north and south latitude is called tropics, accounting for about half of the global area, and most of it is the ocean, which is the net gain area of heat on the earth. The lower atmosphere is often in a state of high temperature, high humidity and instability. At the same time, the tropical area is also the convergence and rising area of airflow. Such thermal and dynamic conditions are conducive to the vigorous development of convective cloud systems and the aggregation of convective cloud systems into huge clouds. It is the background and condition for the occurrence and activity of disastrous weather system.

(1) intertropical convergence zone

Intertropical convergence zone is a narrow convergence zone formed by the confluence of trade winds in the northern and southern hemispheres, also known as equatorial convergence zone. Because the pressure value of convergence zone is lower than that of nearby areas, it was once called equatorial trough. Intertropical convergence zone is one of the important large-scale weather systems in tropical areas, and it is distributed in discontinuous bands around the earth. Its fluctuation, intensity, movement and change have great influence on the long, medium and short-term weather changes in tropical areas.

Intertropical convergence zone can be divided into two types according to the characteristics of its airflow convergence: one is the airflow convergence zone formed by the intersection of the northeast trade wind and the equatorial westerly wind in the northern hemisphere in summer, which is called the monsoon convergence zone because it is active in the monsoon region; The other is the convergence zone formed by the direct intersection of the trade winds in the northern and southern hemispheres, which is called the trade wind convergence zone, as shown in Figure 5.2 1.

The location of intertropical convergence zone moves north and south with the seasons, but the range of movement in different areas is not equal. It is mainly active in the trade wind convergence area of the East Pacific, Atlantic Ocean and West Africa, with a small moving range, and is located in the northern hemisphere most of the year; The monsoon convergence in East Africa, Asia and Australia has a large seasonal shift, which is located in the southern hemisphere in winter and moved to the northern hemisphere in summer. In some years, in June+10 of 65438, a monsoon convergence zone (double intertropical convergence zone) appeared in the southern hemisphere and the northern hemisphere, which is closely related to the land and sea distribution and topographic characteristics of the active area.

Intertropical convergence zone generally exists only in the middle and lower troposphere. The axis of the monsoon convergence zone inclines to the south or southwest with height, because the equatorial westerly belt mostly appears below the 500hPa layer. On the other hand, the convergence area of the trade winds in the ocean almost coincides at different heights, because there is almost no difference in temperature and humidity between the two intersecting airflow, and the geostrophic action near the equatorial belt disappears.

Intertropical convergence zone, especially the monsoon convergence zone, is the area where water vapor and heat are most concentrated in low latitudes, with an average monthly precipitation of 300-400 mm, and a large amount of latent heat released by water vapor condensation becomes the most important heat source. After intertropical convergence zone was heated, it stimulated the formation of convective clouds, tropical cyclones and other tropical weather systems. On satellite images, the monsoon convergence zone is usually shown as a huge east-west cloud zone, which is composed of discrete clouds stretching for several Qian Qian meters.

(2) East wind wave

It is the fluctuation caused by the deep easterly airflow disturbance on the south side of subtropical high (northern hemisphere). Generally, the wavelength is1000-1500 km, the elder is 4 000—5 000km, and the extension height is generally 6-7 km, and some of them reach the tropopause. The maximum intensity occurs between 700 and 500 hectopascals. The cycle is 3-7 days. The moving speed is about 20-25 km/h.

East wind waves are generally characterized by shear between northeast wind and southeast wind. Its structure varies from region to region. In the western Atlantic and Caribbean, the easterly wave is inverted V-shaped, and the wave axis inclines eastward with height. The northeast wind blows in front of the trough and the southeast wind blows behind the trough. There is a divergent downdraft area in front of the trough, and the wet layer is thin, only some small cumulus clouds or clear skies are generated. There is a convergent updraft area behind the trough, a large amount of water vapor is transported upwards, and the wet layer is thick, forming clouds and rain. This pattern is formed because the easterly wind speed in the middle and lower troposphere decreases with the increase of height.

The easterly wave in the western Pacific Ocean is generated in the eastern part of the western Pacific Ocean, with an average wavelength of about 2 000km and a moving speed of about 25-30 km/h. Due to the easterly wind at the lower level and the westerly wind at the upper level in the eastern part of the western Pacific Ocean, the easterly wave axis is easterly, and rainy weather occurs in the rising area of air convergence behind the trough. When the easterly wave moves to the western and southern waters of the western Pacific Ocean, it can reach the middle and upper troposphere upwards because of the equatorial westerly wind in the lower level, and it is clearest between 400 and 200 hectopascals. Moreover, with the increase of height, the wave axis of the easterly wave gradually inclines to the west. Results The air flow converged and rose in front of the trough, and the wet layer was thick, cloudy and rainy, while the air flow diverged and sank behind the trough, and the wet layer was shallow and sunny. The easterly waves in the western western Pacific often affect South China, the middle and lower reaches of the Yangtze River and East Asia, bringing heavy rain and windy weather. The strong easterly wave may have a closed circulation, which reduces the air pressure, increases the central wind force and strengthens the precipitation. The easterly wave can also develop into a tropical cyclone under appropriate conditions.

(3) Tropical clouds

From the satellite images, it is found that there are a large number of deep convective clouds in tropical areas with the diameter of 100 ~ 1000km, which are called cloud clusters. It is difficult to analyze the weather system corresponding to the cloud cluster on the weather map, but most weather systems such as easterly wave and tropical cyclone are developed on the basis of the cloud cluster. Strong winds and heavy rains often occur in areas where clouds pass.

Clouds can be divided into three types according to their scales and regions: ① monsoon clouds, named after southwest monsoon activities, are the largest clouds on the earth. North-south width 10 latitude, east-west length 20-40 latitude, mainly occurring in tropical Indian Ocean and Southeast Asia. In winter, the cloud cluster is located at 5-10 N, and it begins to move northward with the monsoon in mid-June, and then moves into 20-30 N in August. Monsoon depression often appears in clouds, and sometimes it can develop into a storm in the Bay of Bengal, causing heavy rain. (2) Ordinary clouds, which often appear in the intertropical convergence zone on the ocean, are the first embryos of tropical cyclones, easterly waves and other weather systems with scales above four latitudes. This kind of cloud has a great influence on coastal areas such as South China and East China, and can form rainstorm weather. ③ Small-scale clouds (popcorn clouds) are composed of some cumulonimbus clouds with a horizontal scale of 50×50km, and each cumulonimbus cloud group is composed of about 65,438+00 cumulonimbus clouds, which mostly appear in tropical South America and southern Tibet, China, with obvious daily variation.

The cloud cluster consists of a moderate convective cloud system with a scale of 10- 100 km and a small convective cloud system with a scale of 4- 10 km and a life span of 30 minutes to several hours. In the process of moving with the prevailing wind, small and medium-sized convective cloud systems often form on the upwind side, disappear on the downwind side, and constantly metabolize, but they often stay still on the sea surface with high temperature, and sometimes the cloud systems accumulate and rainstorm occurs.

(4) Tropical cyclones

Tropical cyclone is a strong cyclonic vortex with warm core structure formed in tropical ocean. When it comes, it often brings strong winds, heavy rains and stormy waves, which is extremely destructive and threatens people's lives and property. This is a disastrous weather. At the same time, tropical cyclones also bring abundant rainfall, which is beneficial to alleviate or alleviate the drought phenomenon in midsummer and is the most important weather system in tropical areas.

1. Classification

The intensity of tropical cyclones varies greatly. Accordingly, the international standard names and grades of tropical cyclones are:

(1) Typhoon (hurricane): the maximum wind speed near the ground center is ≥32.6m/s (that is, the wind force is above 12).

(2) Tropical storm: the maximum wind speed near the ground center17.2-32.6m/s (i.e. wind force 8- 1 1). The maximum wind speed near the ground center is 24.5-32.6m/s (wind 10- 1 1), which is called a severe tropical storm.

(3) Tropical depression: the maximum wind speed near the ground center10.8-17.1m/s (wind force 6-7).

China has adopted international regulations since 1989. Previously, China's meteorological department had stipulated that the maximum wind speed near the ground center in tropical cyclones was 17.2-32.6 m/s (that is, the wind force was 8- 1 1), which was called typhoon. The maximum wind speed ≥32.6m/s (above 12) is called a strong typhoon; The maximum wind speed10.8-17.1m/s (wind force 6-7) is called tropical depression.

In order to better identify and track tropical storms and typhoons with strong winds, they are often named or numbered. According to the regulations of China's meteorological department, tropical storms and typhoons that appear in the west of east longitude 150 and north of the equator every year are numbered according to the order of occurrence. For example, Tropical Storm No.9306, Severe Tropical Storm No.9304 and Typhoon No.9302 refer to Tropical Storm No.6, Severe Tropical Storm No.4 and Typhoon No.2 that appeared in the west of East longitude 1993.

2. Typhoon

The range of typhoon is usually measured by the diameter of the outermost closed isobar. The range of most typhoons is 600- 1000 km, the largest is 2 000km, and the smallest is only about 100km. The typhoon circulation height can reach 12- 16 km, and the typhoon intensity is determined by the maximum average wind speed on the ground near the typhoon center and the minimum sea level pressure at the typhoon center. The wind speed of most typhoons is 32-50m/s, and the largest is 1 10m/s, or even more. The atmospheric pressure in the center of a typhoon is generally 950 hectopascals, with the lowest being 920 hectopascals, and some are only 870 hectopascals.

Typhoons mostly occur on the ocean surface with high seawater temperature of 5-20 north latitude, mainly in eight sea areas (Figure 5.22), namely, five sea areas of western and eastern North Pacific in the northern hemisphere, western North Atlantic, Bay of Bengal and Arabian Sea, and three sea areas of western South Pacific and western and eastern South Indian Ocean in the southern hemisphere. There are about 80 typhoons (including tropical storms) every year, of which more than half occur in the North Pacific (about 55%), 73% in the northern hemisphere and only 27% in the southern hemisphere. There are no typhoons in the South Atlantic and Southeast Pacific.

Typhoons in the northern hemisphere (except the Bay of Bengal and Arabian Overseas) mainly occur at 7- 10 with relatively high SST, and in the southern hemisphere during June-March with relatively high SST, and decrease significantly in other seasons (Table 5.7).

(1) Structure: The typhoon is a powerful and deep cyclone vortex and a mature typhoon. The lower layer is divided into three areas according to the convergent airflow speed: ① The outer ring is also called the gale area, and the radius from the typhoon edge to the outer edge of the vortex area is about 200-300 km. Its main feature is that the wind speed increases sharply to the center, and the wind power can reach more than 6 levels. ② The middle circle, also known as the vortex area, has a radius of about 100km from the edge of the windy area to the eye wall of the typhoon, which is the area with the strongest convection, wind and rain and the most destructive force. ③ The inner ring is also called typhoon eye area, with a radius of about 5-30km. Most of them are round, and the wind speed decreases rapidly or the wind is calm.

The vertical distribution of typhoon wind field can be roughly divided into three layers: ① the low-level inflow layer, from the ground to 3km, the airflow converges strongly to the center, and the strongest inflow and outflow is in the planetary boundary layer below 1km. Due to the geostrophic bias, the airflow inside the cyclone rotates, and the closer it is to the typhoon center when it flows inward, the shorter the rotation radius, the greater the curvature of the isobar and the corresponding increase in inertial centrifugal force. Results Under the action of geostrophic bias and inertial centrifugal force, the inward airflow could not reach the center of the typhoon, but strongly spiraled near the eye wall of the typhoon. ② In the updraft layer, from 3km to about 10km, the airflow mainly rises tangentially around the eye wall of the typhoon, and the rising speed reaches the maximum between 700-300 HPA. ③ The upper outer layer, from 10km to the tropopause (12- 16 km), releases a lot of latent heat during the rising process, which makes the temperature in the middle of the typhoon higher than that in the periphery, and the horizontal pressure gradient force in the typhoon gradually decreases with the height. When it reaches a certain height (about 10- 12 km). The outflow of air is roughly equal to the inflow of low-level air, otherwise the typhoon will strengthen or weaken.

The temperature field on each isobaric surface of typhoon is a warm center structure close to a circle. As can be seen from Figure 5.23, the horizontal distribution of typhoon low-level temperature gradually increases from the periphery to the eye area, but the temperature gradient is very small. This horizontal temperature field structure is gradually obvious with height, which is the result of the release of latent heat of condensation in the rain area outside the eye wall and the sinking and warming of air in the eye area.

(2) Weather: According to the satellite cloud picture and radar echo of typhoon, a mature typhoon cloud system has been developed (Figure 5.24). From the outside to the inside, it is as follows: ① Peripheral spiral cloud belt, which consists of stratocumulus or cumulus, spirals into the typhoon at a small angle. Cloud belts are often blown away by high-altitude winds and become "flying clouds". ② The inner spiral cloud belt consisting of several cumulonimbus clouds or cumulonimbus clouds directly participates in typhoons and forms precipitation. (3) The cloud wall is a concentric cloud belt around the center of the typhoon, consisting of towering cumulonimbus clouds. The height of the cloud top can reach more than 12km, which is like a towering cloud wall, causing severe weather such as gale and rainstorm. (4) The airflow in the eye area sinks, and the weather is clear and cloudless. If the water vapor in the lower layer is abundant, some stratocumulus and cumulus may be generated below the inversion layer, but the vertical development is not strong, there are many cloud gaps, and there is generally no precipitation.

(3) Formation and extinction: The formation and development mechanism of typhoon has not yet reached a perfect conclusion. Most scholars believe that typhoons are developed from tropical weak disturbances. When the weak tropical cyclone system is generated on the high-temperature ocean surface or removed from the peripheral area, the airflow will generate a component that flows into the weak cyclone due to friction, which will gather the high-temperature and high-humidity air on the ocean surface to the cyclone center, and transport it to the middle and upper parts for condensation with the ascending motion, releasing latent heat, heating the air column above the cyclone center, and forming a warm core. The feedback from the warm heart makes the air lighter, the ground pressure drops, and the cyclone circulation strengthens. The strengthening of circulation further increases friction convergence, increases upward water vapor, continues to heat the middle and upper troposphere, and the ground pressure continues to drop, and so on until it becomes a typhoon. From the above, it can be seen that the important mechanism of typhoon formation and development is the formation of typhoon warm heart, and the formation, maintenance and development of warm heart need suitable environmental conditions and the flow field that produces tropical disturbance, which are interrelated and indispensable. It is generally believed that the environmental conditions and flow fields suitable for typhoon formation are:

① Broad high-temperature ocean surface: Typhoon is a very violent weather system with considerable energy, which is mainly converted from the latent heat released by a large amount of water vapor condensation, and the release of latent heat is the result of the unstable development of atmospheric stratification. Therefore, the instability of atmospheric stratification has become an important prerequisite for the formation and development of typhoons. The instability of the lower troposphere mainly depends on the vertical distribution of temperature and humidity in the atmosphere. The higher the temperature and humidity of the lower atmosphere, the stronger the instability of atmospheric stratification. Therefore, the vast high-temperature ocean surface has become a necessary condition for the formation and development of typhoons. According to statistics, typhoons generally do not occur on the ocean surface where the sea surface temperature is lower than 26.5℃, while typhoons are prone to occur on the ocean surface where the sea surface temperature is higher than 29-30℃. In the warm season (July-65438+1October) in the western part of the North Pacific Ocean, the sea surface temperature can reach above 30℃ and the water vapor is abundant, making it the area with the most typhoons in the world.

② Appropriate geostrophic parameter value: The development and expansion of tropical initial disturbance depends on the action of certain geostrophic bias, which makes the convergent airflow gradually turn into a cyclone rotating horizontal vortex, and the cyclone circulation is strengthened. Otherwise, if there is no geostrophic deflection force or the geostrophic deflection force is too small to reach a certain value, the horizontal convergent airflow can directly reach the low-pressure center, resulting in air accumulation and central stuffing, leading to the weakening or failure of cyclone vortex formation. According to the calculation, F can reach a certain value only in areas five latitudes away from the equator, which is beneficial to the formation of typhoons. In fact, most typhoons occur between 5 and 20 degrees latitude.

③ The vertical shear of airflow should be small: in order to make latent heat accumulate in the same vertical column and not spread out, the vertical shear of basic airflow should be small. Otherwise, the difference between high and low wind speeds is too large or the wind direction is opposite, and the latent heat will flow out quickly, which is not conducive to the formation and maintenance of the warm heart, which is not conducive to the development of a typhoon. According to statistics, typhoons are mostly formed between isobars of 200hPa and 850hPa, and the wind speed difference is less than10 m/s. The vertical shear of wind speed in the western Pacific is very small in one year and even smaller in summer, so typhoons occur frequently. In the Bay of Bengal and Arabian Sea in the northern Indian Ocean, the southwest monsoon in midsummer is in the lower layer, and the strong easterly jet on the south side of the Qinghai-Tibet high is in the upper layer. The vertical wind shear is very large, and the possibility of typhoon is very small. In spring and autumn, the vertical wind shear is small, and typhoons occur frequently.

(4) Suitable flow field: Whether a large amount of unstable energy accumulated in the atmosphere can release kinetic energy that has not been converted into typhoon is closely related to the start and induction of favorable flow field. Satellite cloud pictures show that there was a disturbance system before the typhoon, which developed from disturbance to typhoon. This is because there is a strong convergence field in the lower atmosphere disturbance and a divergence field in the upper atmosphere, which is beneficial to the release of latent heat. Especially when the intensity of divergent airflow in the upper atmosphere is higher than that in the lower atmosphere, the lower atmosphere disturbance will strengthen and gradually develop into a typhoon. Intertropical convergence zone and easterly wave are both airflow convergence systems, which are prone to generate weak vortices and become favorable flow fields for the formation and development of typhoons.

Globally, typhoon generation has certain regional and seasonal characteristics.

The main conditions of typhoon extinction are that high temperature and high humidity air can not be continuously supplied, low-altitude convergence and high-altitude divergence flow fields can not be maintained, and the vertical shear of wind speed increases. There are generally two ways to cause these situations: first, after the typhoon landed, the high temperature and high humidity air could not be continuously replenished, and the energy needed to maintain strong convection was lost. At the same time, the friction in the lower layer is strengthened, the internal airflow is strengthened, and the typhoon center is gradually stuffed, weakened or even disappeared. Second, after the typhoon moved to the temperate zone, cold air invaded, which destroyed the warm core structure of the typhoon and turned it into an temperate cyclone.

(4) Motion and path

The direction and speed of typhoon movement depend on the force acting on the typhoon. Power is divided into internal force and external force. The internal force is the resultant force from north to west caused by the different geostrophic skewness caused by the latitude difference between north and south within the typhoon range. The larger the typhoon range, the stronger the wind speed and the greater the internal force. The external force is the force of the external environment flow field on the typhoon vortex, that is, the guiding force of the easterly belt of the basic airflow on the south side of the subtropical high in the northern hemisphere. Internal force mainly plays a role in the initial generation of typhoon, while external force is the leading force to manipulate the movement of typhoon, so typhoon basically moves from east to west. Due to the influence of the shape, position and intensity of subtropical high, the typhoon's moving path is not uniform and becomes diverse. Take the typhoon moving path in the western North Pacific as an example, there are generally three moving paths (see Figure 5.25).

① Westward movement path: When the North Pacific ridge is east-west and strong and stable, or the North Pacific subtropical high continues to extend westward, the typhoon moves from the west-east direction of the Philippines and lands on Hainan Island or Vietnam through the South China Sea.

② Northwest path: When the north Pacific ridge line is northwest-southeast, the typhoon moves from the east of the Philippines to the northwest, passes through Iwo Jima, and lands in Jiangsu and Zhejiang provinces or crosses the Taiwan Province Strait in Zhejiang and Fujian provinces. This road has a great influence on China, especially in East China.

③ Turning path: When the subtropical high in the North Pacific recedes eastward, the typhoon moves from the sea area east of the Philippines to the northwest, and then turns to the northeast, with a parabolic path. It has a great influence on the eastern coastal areas of China and Japan.

In addition, some typhoons have special paths such as swinging or rotating from side to side during the movement. Obviously, this is related to the circulation situation at that time.

The average speed of typhoon movement is 20-30km/h, and when turning, the speed first slows down and then accelerates.