Tropical cyclone grade

Tropical cyclone is a low-pressure vortex that occurs on the surface of tropical or subtropical ocean, and it is a powerful and profound tropical weather system. [1] is a mesoscale or synoptic warm cyclone generated on the tropical ocean surface [2]. It can be found in the western Pacific Ocean and its adjacent waters (typhoons), the Atlantic Ocean and the Northeast Pacific Ocean (hurricanes), the Indian Ocean and the South Pacific Ocean [3].

Tropical cyclones are common in summer and autumn, and their life cycle can be roughly divided into four stages: generation, development, maturity and extinction. Their intensity is divided into several grades according to the central wind speed, and they are observed as a huge spiral direct spread cloud system [2][4]. Mature tropical cyclones have macro structures such as storm eye, eye wall and spiral rain belt, with a diameter of100 ~ 2,000 km, the maximum wind speed in the center exceeds 30m/s, the central pressure can be reduced to about 960 hPa, and it can extend to the tropopause in the vertical direction [2][3]. Unlisted tropical cyclones may last for 2 to 4 weeks until they leave the tropical waters, while landing tropical cyclones usually die quickly within 48 hours after landing [5].

The formation mechanism of tropical cyclones has not been completely confirmed. According to historical statistics, warm ocean surface, initial disturbance, weak vertical wind shear and a certain intensity of beta effect are the necessary conditions for the formation of tropical cyclones [2]. Dynamically, the second kind of conditional instability (CISK) theory can well explain the generation and maintenance of tropical cyclones [4][6][7][8] Global warming is also considered to be related to the generation frequency of tropical cyclones [9].

Tropical cyclone is a low-pressure vortex that occurs on the surface of tropical or subtropical ocean, and it is a powerful and profound tropical weather system. Typhoon is a kind of tropical cyclone. In China, tropical cyclones in the northwest Pacific and South China Sea are classified into six grades according to the maximum average wind speed near the bottom center, in which the wind force is 12 or above, which is collectively called typhoon. [1] Tropical cyclone is a warm cyclone system generated and developed in tropical waters. Tropical cyclones generated in the western Pacific Ocean, the northwest Pacific Ocean and their adjacent waters are called "typhoons"; Tropical cyclones produced in the Atlantic Ocean and the Eastern Pacific Ocean are called "hurricanes"; Tropical cyclones generated in the Indian Ocean and the South Pacific can be called "cyclone storm" or "cyclone" for short [2] [10].

Tropical hurricane

The broad definition of tropical cyclone does not require the intensity of cyclone, that is, tropical cyclone can be called "tropical cyclone" in a broad sense at any stage of its life history; Narrow tropical cyclones only include strong cyclones in the mature stage, and their intensity must exceed the minimum standard of maximum wind speed according to the center of cyclone classification system [1 1]. For example, in the Atlantic Ocean, only cyclone systems with maximum wind speed exceeding 120 km/h 1 min will be classified as "severe" tropical cyclones, that is, hurricanes, and heating cyclones below this standard will be classified as "tropical depression" and "tropical storm" [16544].

Similar concepts to tropical cyclones include subtropical cyclones and temperate cyclones. As a difference, extratropical cyclone is a cold air cyclone existing in the middle and high latitudes, which can be generated in the ocean or land, and in most cases it is formed by the unstable development of baroclinic, accompanied by the appearance of front [12]. Subtropical cyclone is a weather system between tropical cyclone and temperate cyclone. Its mature form is close to tropical cyclone, but it has a cold core structure similar to temperate cyclone in dynamics [13].

Horizontal (upper) and vertical (lower) structural differences between tropical cyclones and temperate cyclones

As a connection, tropical cyclones may turn into temperate cyclones after entering the temperate ocean surface, and temperate cyclones may also turn into tropical cyclones in a few cases [12]. A subtropical cyclone will be recognized as a tropical cyclone after it enters the tropical ocean surface and transforms into a warm core structure, but when it passes through the subtropical ocean surface, it will not be recognized as a subtropical cyclone as long as its warm core structure remains unchanged.

Tropical cyclones contain a lot of unstable energy and may become meteorological disasters. In the mature landing stage, tropical cyclones bring significant destructive strong winds, a lot of precipitation and secondary disasters such as storm surges and thunderstorms [4]. Tropical cyclones in the ocean are a great threat to the shipping industry. Modern operational weather forecast can identify and observe tropical cyclones through satellite remote sensing, and forecast and warn their development and movement in combination with numerical weather forecast [14] [15]. The main members of WMO will name the tropical cyclones in each sea area and release the information to the public [16] [17].

structure

eye of wind

Main items: eye of the wind

The eye of the wind is a relatively calm area located at the center of rotation (usually geometric center) of a tropical cyclone. The eye may not have a clear eye or an eye full of low and medium clouds, which is the lowest point of tropical cyclone near-surface pressure [10]. The wind speed in the eye is obviously lower than that in the peripheral area, usually less than 24 km/h, with little or no rain. It mainly flows downward in the eye, and the edge area near the eye wall is an upward airflow with gas vorticity. The potential temperature of the atmosphere inside and above the eyes is higher than its surrounding environment [19].

Typhoon eye diagram

The common value of eye size is about 50 km, which increases with the height. The eye diameter of tropical cyclones in the northern hemisphere is usually smaller than that of tropical cyclones in the southern hemisphere [20]. Extreme examples of eye size include 1960 Typhoon Carmen, 370 km, and Hurricane Wilma, 3.7 km in 2005 [2 1] [22]. The intensity of tropical cyclones is very sensitive to the diameter of the eye. Under the same thermal and dynamic conditions, tropical cyclones with smaller eye diameter have higher maximum potential intensity [23].

With the change of tropical cyclone life stage, the geometric characteristics of eye will also change. The rapidly increasing tropical cyclone has a small, clear and highly symmetrical eye, sometimes called "pinhole eye"; A mature tropical cyclone has a symmetrical round eye with a continuous eye wall around it, that is, "close your eyes" [24] [25]. Extinct or incompletely developed tropical cyclones have irregular eye, such as open eye, asymmetric shape or fragmented eye [25]. The dynamic change of eye can be used as a reference for tropical cyclone operational weather forecast [24].

Not all tropical cyclones have mature eyes (eyes closed). According to the climate statistics of Atlantic tropical cyclones in 1989-2008, 60% of hurricanes have clear eye. When the eye first appeared, the average maximum wind speed in the center of tropical cyclone was 29.8 m/s, that is, the intensity was slightly lower than that of 1 hurricane [25].

Eye wall

The eye wall is a tower-shaped cumulus cloud formed around the eye of tropical cyclone, and its height can extend from sea level to the top of stratosphere, which is about 15 km for tropical waters. The eye wall contains strong convective activity, which forms latent heat release in the middle troposphere. The eye wall is also the area with the largest wind speed and unit precipitation rate in tropical cyclones. The intensity of tropical cyclone [10] can be estimated by observing the maximum wind speed on the eye wall.

The eye wall of tropical cyclones with high development and maturity intensity may include two parts: primary eye wall and secondary eye wall, which is usually related to the replacement cycle of eye wall [26]. When the convective activity in the main eye wall reaches a certain intensity, strong convective activity will develop in the main rain belt near the eye wall, forming a new secondary eye wall. The secondary eye wall will gradually move towards the eye, replacing the original eye wall [26]. In the process of replacing the eye wall, the original eye wall is isolated and weakened because it is out of the area conducive to convection, while the secondary eye wall is not fully developed, so the intensity of tropical cyclone will be temporarily weakened. After replacing the eye wall, the tropical cyclone maintained by the new eye wall will be strengthened again [26].

1997 Typhoon Abby is undergoing eye wall replacement.

exterior structure

Spiral rain belt

Spiral rain belt is a fully developed and mature structure of tropical cyclone, which is essentially the sum of all convection systems except eye wall in tropical cyclone [27]. The spiral rain belt rotates in the positive vorticity direction with the cyclone center, and the tangential velocity decreases with the increase of height, and it contains discontinuous convective precipitation [27]. In the near-surface area affected by spiral rain belt, there may be weather phenomena such as formation precipitation and strong wind, so in the weather forecast, spiral rain belt defines the position of peripheral strong wind area and precipitation area [28].

The spiral rain belt of tropical cyclone is usually divided into "main rain belt", "secondary rain belt" and "far rain belt" [29] [28]. Among them, the main rain belt, also known as the "inner rain belt", is the main part of the spiral rain belt, and it is almost motionless with the eye wall during the cyclone movement [27], which is considered as the dividing line between tropical cyclone body and environment in some studies [30]. The secondary rain belt is a group of convective cells rotating around the main rain belt. The outer rain belt may be gradually involved in the main rain belt along the cyclone radius, or it may be loosely organized around the tropical cyclone [27]. The peripheral rain belt is the sum of the outermost sporadic convective systems of tropical cyclones, and it is also called "outer mesoscale convective system (OMCS)" in some studies [31].

Spiral rain belt has a complex mesoscale structure. According to the observation conclusion of radar echo, in the middle and lower layers, the airflow inside the spiral rain belt deviates from the cyclone center and blows to the rain belt, and the intensity increases with height [32]. The lower layer of the spiral rain belt is a strong convergence area, accompanied by the inflow of air near the ground. The position of convergence zone deviates from the cyclone center with height, and the convergence intensity weakens with height, and turns to divergence in the upper troposphere. On the section along the cyclone center, the spiral rain belt contains secondary vertical circulation, in which the updraft is located inside the rain belt (convergence zone), inclining outward with height, and may contain convective heavy precipitation. Downdraft is located outside the updraft, and its intensity is lower than that of updraft [32].

The formation of secondary rain belt in spiral rain belt is considered to be related to the outward propagation of vortex Rossby wave in tropical cyclone [33]. The dynamic mechanism of the main rain belt in the spiral rain belt is not completely clear. The numerical simulation results show that the main rain belt will change the dynamic structure of tropical cyclones, and it is related to the formation and replacement of eye walls [34].

Surrounding windy area

Strong winds can be observed outside tropical cyclones, including the area where the peripheral rain belt is located, and its coverage is generally called "peripheral gale area", which can be defined by "gale radius" according to diagnostic parameters [35]. Strong wind radius is the direct weather influence range of tropical cyclone, which is usually symmetrical with tropical cyclone itself [36]. When tropical cyclones land, the wind speed and its range within the strong wind radius will change due to the influence of the underlying surface [35].

trait

cloud system

The cloud system of tropical cyclone is the internal expression of its dynamic characteristics. Generally speaking, the direct cloud system generated by convection plays a central role in the generation and development stage of tropical cyclones [37]; After entering the mature period, tropical cyclones have always been dominated by direct expansion cloud systems, which contain strong rotating updraft and relatively weak downdraft. Stratocumulus clouds can be seen in the eye of tropical cyclone, and its vertical height is near the top of the boundary layer, and the top downward airflow is above the boundary layer. Eye wall clouds are mainly cumulus clouds, and their vertical height can reach the tropopause. On satellite images, they appear as central dense clouds (CDO) [29].

Morphologically, the eye wall cloud is affected by the gradient wind balance in the cyclone and the oblique transport of neutral stratified airflow in the boundary layer, and usually extends outward at a certain slope, and is dynamically changed by the buoyancy uplift driven by wet static energy below it [29]. The main rain belt and the secondary rain belt are composed of convective cells, and the cloud system is also dominated by tower cumulus, including overturning updraft and stable downdraft [29]. When the eye wall is replaced, the cloud system of the main rain belt and the eye wall will merge. The cloud system in the peripheral rain belt has the same characteristics as that in the general mesoscale convective system, and its morphology is dominated by vortex dynamics, including the heavy precipitation cloud system represented by cumulonimbus clouds [29].

Characteristic scale

The spatial scale of tropical cyclones has obvious dynamic changes. Generally speaking, a tropical cyclone may be a mesoscale system with a scale of 100 km or a synoptic scale system with a scale of 1000km [38] [39], and its movement is considered to be influenced by planetary scale fluctuations and turning airflow above 5000km [40] According to some special circumstances, the diameter of the tip of super typhoon reaches 2200 km[4 1], while the diameter of tropical storm Kyle is only 18.5 km[42]. In some studies, tropical cyclones are considered as a "scale-up" process, that is, the pre-existing cyclone disturbance on the weather scale is superimposed with the energy and vorticity characteristics on the convection scale [29].

On the time scale, tropical cyclones usually exist for several weeks before landing, which belongs to the type of mesoscale system that can last for a long time, but there are also a few cases, such as Hurricane John [43], which lasts for 365,438+0 days, which may exceed the weather scale systems such as frontal cyclones on the time scale. In addition, considering the situation that tropical cyclones become temperate cyclones, the time scale of weather impact estimation (rather than the system itself) may be longer.