Atmospheric dynamics have been intense in developing susceptible disturbances in both extratropical and tropical areas. As we have stated earlier, extra tropical cyclones as moving cyclones in the mid-latitude zone. As the mid-latitudes represent a point of convergence for contrasting air masses, it is there that the cyclones and anticyclones traverse with varying regularity with the prevailing westerly winds. But in the case of tropical cyclones, the tropical atmosphere acts as a heat sink of the radiant energy obtained through short-wave solar radiation. With the high rate of exchange of energy between oceans and atmosphere, it brings about innumerable atmospheric disturbances, which further get triggered with the varied dynamics.
Hence, in this article, we will put emphasis on tropical cyclones and their dynamics. Firstly, we will discuss the genesis of these cyclones; how the radiant energy from the sun supplies atmospheric heat that leads to the development of the disturbances. Simultaneously, we will discuss the structure of the tropical cyclones, which are warm-cored, powerful cyclonic atmospheric vortices, and form over warm tropical oceans and have a horizontal scale of hundreds to 1000 km, extending throughout the depth of the troposphere. Also, how intense mature tropical cyclones produce an eye with modest subsidence in its centre, surrounded by fast swirling flow and a deep convective ring that slopes radially outward with height. The presence of active spiral rain bands outside the convective eye wall is another distinguishing feature of powerful tropical cyclones. These rain bands are divided into inner and outer rain bands based on their fundamental distinctions in dynamics and thermodynamics. These rain bands’ activity has the potential to create major changes in the structure and severity of tropical cyclones, which will be discussed as well. In addition, the regional distribution of these tropical cyclones will be studied, taking into consideration factors such as latitude, moisture supply, and Coriolis force. Finally, the evolution of tornadoes and how they differ from cyclones will be discussed.
Tropical Cyclones: Origin
A hurricane or a tropical cyclone is similar to a heat engine that is powered by the latent heat of condensation. A hurricane’s overall energy output is believed to be equal to the total amount of electricity utilised in the United States over a six-month period. The power equivalent of the energy liberated within a storm in one day, according to William L. Donn, is around ten thousand times the daily power consumption in the entire United States. According to him, a hurricane’s daily energy discharge is equivalent to nearly a hundred thousand megaton bombs. The specific mechanism that causes tropical cyclones to arise is not well known at this time. However, there are some basic characteristics that must be met in order for a hurricane to form, as listed below.
A plentiful supply of warm, moist air is available at all times. Tropical cyclones of hurricane strength develop over warm tropical waters where the surface temperature is around 27°C, allowing water vapour to fill the lower layers of air next to the ocean surface. In the process of cloud and rain production, latent heat is transferred into storms and released. Tropical cyclones form in the western hemisphere of the oceans, where temperatures are higher than in the eastern hemisphere. The cold currents lower the surface temperatures of the tropical oceans in the eastern parts, rendering them unsuitable for storm spawning. Furthermore, the warmer months of the year provide optimal circumstances for tropical storm development.
The fact that tropical storms rarely form within a 50–80 wide belt on both sides of the equator suggests that Coriolis force is important in cyclonic air circulation. The majority of these cyclones are restricted to a belt spanning 50 to 200 latitudes, with the highest frequency occurring between latitudes 100 and 150.
Also, there is the existence of weak tropical disturbances. Pre-existing mild tropical disturbances increase and evolve into hurricanes or strong tropical storms under favourable conditions. Minor differences in water and air temperature produce a number of small low-pressure centers. Weak cyclonic circulation emerges around these locations. Then, due to the warm, humid air and the air column’s latent instability, a true hurricane vortex might form quickly. It should be noted, however, that only a small percentage of these disturbances turn into hurricanes.
Furthermore, there must be an anticyclonic circulation at the height of 9000 to 15000 meters above the surface disturbance in order for ascending air currents within the cyclone to continue to be pumped onto it in order to maintain the low pressure at the cyclone’s centre. Because of these unique dynamic conditions in the airflow near 200 millibars, a tropical disturbance intensifies. As a result, the upper-level outflow of air circulation emerges. In addition, hurricane development processes are limited to latitudes equatorward of the subtropical jet stream due to weak vertical wind shear in the basic current.
In the intertropical convergence zone, there are small atmospheric vortices that get formed. Hurricane formation is triggered by these atmospheric vortices. It’s worth noting that the trade winds from both hemispheres meet along a line known as the intertropical front. When the inter-tropical convergence zone is farthest from the equator, temperature disparities between these air masses must occur. As a result, the convergences of these different-temperature winds, as well as the resulting instability, are necessary for the emergence and growth of powerful tropical storms.
Tropical cyclones form over tropical oceans where there is a severe paucity of meteorological observation stations. As a result, the needed data on atmospheric conditions is missing, making the task of determining the components that cause tropical storm development all the more difficult. If more surface observing stations are made available, the unresolved questions about tropical cyclones may be answered.
Tropical Cyclone: Structure
In this section, we will analyse the recent progress over the tropical cyclone structure, rain bands, eye, and eyewall are the key components of a tropical cyclone. In the northern hemisphere, air spirals in counter-clockwise toward the centre (clockwise in the southern hemisphere) and out the top in the opposite way. Air sinks in the storm’s core, generating an “eye” that is mainly cloud-free.
The Eye
The hurricane’s centre is a relatively calm, often clear patch of sinking air with low winds that rarely exceed 15 mph (24 km/h) and measures 20–40 miles (32–64 km) in diameter. When maximum sustained wind speeds exceed 74 mph (119 km/h), an eye forms, which is the calmest part of the storm.
But what causes an eye to form? The exact cause of eye development is still unknown. It’s most likely due to the interaction of “angular momentum conservation” and centrifugal force. The conservation of angular momentum states that when objects travel closer to the centre of circulation, they will spin faster. As a result, air speeds up as it approaches the core of the tropical cyclone. Observing figure skaters spin is one way of looking at this. The quicker they spin, the closer their hands are to their bodies. The further apart the hands are from the body, the slower they spin. As the air flows into the core of a tropical storm, the speed must increase.
The centrifugal force, which happens as the wind speed increases, is an outward-directed force caused by the wind’s momentum wanting to move the wind in a straight line. There is a pull outward when the wind rotates around the tropical cyclones centre. The centrifugal force is stronger when the curvature is steeper and/or the rotation is faster. The intense rotation of air around the cyclone, which occurs at roughly 74 mph (119 km/h), balances inflow to the centre, causing air to ascend about 10-20 miles (16-32 km) from the centre, generating the eye wall. This rapid rotation also causes an air vacuum in the middle, causing some of the air flowing out the top of the eye wall to turn inward and sink to compensate for the loss of air mass. Because of the sinking air, cloud development is suppressed, resulting in a pocket of relatively clear air in the middle.
The Eye Wall
The eye wall is where the strong wind gets as close as it can. The eye wall is made up of a ring of tall thunderstorms that provide heavy rain and the fiercest winds. Wind speed, which is an indicator of the storm’s strength, can alter due to changes in the eye and eyewall structure. Double (concentric) eye walls can form as the eye grows or shrinks in size.
Some of the outer rain bands may assemble into an outer ring of thunderstorms in powerful tropical cyclones, slowly moving inside and robbing the inner eyewall of moisture and momentum. At this stage, the tropical cyclone has weakened. When the outside eye wall entirely replaces the inner one, the storm can be the same intensity as before or even stronger in rare situations.
Rain Bands
The curved bands of clouds and thunderstorms that trail away from the eye wall in a spiral fashion. These bands have the potential to produce tornadoes as well as significant rain and wind bursts. There are some instances when there are gaps between spiral rain bands where there is no rain or wind.
In fact, if one were to travel from the hurricane’s outer edge to its centre, one would normally progress from light rain and wind to a dry and weak breeze, then back to increasingly heavier rainfall and stronger wind, over and over, with each period of rainfall and wind becoming more intense and lasting longer.
Tropical Cyclone: Tracks and Regional Distribution
Hurricanes normally head westward after forming, away from the equator. The airflow above guides them, and subsequently, the intensity of moving hurricanes increases at first. Their average speed is between 15 and 30 km/h. Tropical cyclones, according to some weather scientists, migrate from east to west following the basic currents of trade winds and equatorial westerlies. These storms steadily gain speed as they migrate away from the equator. Tropical storms always follow certain courses because warm ocean currents most likely influence the cyclones’ paths to a considerable extent. Tropical cyclones curve towards the poles after reaching the western sections of tropical oceans. Under the influence of upper westerlies, some storms curve strongly poleward between 20 and 30 degrees north latitude, then recurve towards the east. In rare cases, they remain motionless for an extended period of time. These cyclones usually bend about the western edges of summer oceanic high-pressure regions (in the northern hemisphere) and deflect into the westerlies. The hurricanes are moving at a speed of 100 kilometers per hour at this point. Tropical cyclones in the tropics do not have any fronts. They are dragged into eastward or north-eastward moving extratropical cyclones after leaving the tropics and develop different fronts and air masses.
Their source of warm, moist air is cut off after they leave the tropical oceans. In the absence of the essential energy in the form of latent heat of condensation, tropical cyclones lose all of their initial characteristics and resemble regular extratropical cyclones. Tropical cyclones that are travelling over open seas tend to maintain their intensity at low latitudes. Cyclones that travel away from the equator and follow erratic routes lose a lot of their intensity.
When a tropical cyclone hits land, it loses a lot of its strength because its source of moisture is cut off. In other words, there isn’t enough energy on land to keep the cyclone going. Increased friction is also contributing to hurricanes weakening over land. As a result, it is apparent that tropical cyclones form over the waters and then dissipate over land.
The path of tropical cyclones across the northern Indian Ocean differs from what has been documented about the general movement of such storms in other regions. These storms are superimposed on the monsoon circulation of the summer months, and they migrate northward with the monsoon currents.
Storms that originate in the Caribbean Sea region migrate westward through Texas and Mexico, disintegrating once they reach land. However, a few storms recurve to the north and continue onward, with some of them merging into middle-latitude depressions heading eastward. Storms that form in the south-westerly section of the North Pacific migrate northward through the Philippines, China, and Japan. Tropical storms that form near the west coast of the United States of America migrate north-west to California. Storms from the northwestern Pacific are moving westward towards the Coral Sea and New Zealand in the southern hemisphere.
Tropical cyclones are completely absent east of 140o W longitude in the tropical zones of the South Atlantic and eastern South Pacific. Tropical cyclones do not form over land, despite the fact that they frequently wander to the edges of continents. The southernmost part of the North Pacific Ocean is where the most tropical storms form. Furthermore, hurricanes of the most destructive type frequent the western half of the tropical North Atlantic and the Caribbean Sea region. The activity of tropical cyclones peaks in late summer and early autumn. It’s worth noting that the maximum frequency of tropical storms is observed when the ocean surface temperatures are at their highest. This phase also coincides with the intertropical convergence zone’s maximum poleward displacement.
Tornadoes
In contrast with cyclones, a tornado is a violently rotating column of air accompanied by a funnel-shaped or tubular cloud that extends downward from a cumulonimbus cloud. Although tornadoes are localised in nature, they are the most dangerous storms on the planet. They are modest in size and have a brief lifespan. These atmospheric disturbances may be among the most destructive forces in nature. They are low-pressure areas with a whirlpool-like structure of winds whirling around a centre cavity where centrifugal forces produce a partial vacuum. This small-scale severe storm has a diameter of 150 to 600 metres. Storms are expected to travel at a speed of 30 to 45 kilometres per hour across land. The path of a tornado might be more than 26 kilometres long. A tornado’s pressure and velocity cannot be measured directly because of the storm’s intensity. As a result, the majority of the knowledge is derived indirectly, i.e., from studying the storm’s aftermath. Excessive atmospheric instability and a steep lapse rate are required for the emergence and growth of a tornado. It is inextricably linked to violent thunderstorms.
Tornado pressures maybe 100 mb lower than pressures immediately outside the storm due to the significant pressure gradient, and speeds of around 650 km per hour have been estimated. The air caught in the storm’s vortex rises quickly and cools adiabatically. The cloud funnel takes on a dark colour as a result of the condensed moisture. Because the outside pressure drops quickly during a tornado, the explosive action of air under normal pressure within a building explodes violently, devastating lives and property.
Tornadoes can occur anywhere on the planet, with the exception of the severely cold northern areas of continents during the winter and the Polar Regions. In the United States of America and Australia, they are quite common. Because of their small size and complex development processes, forecasting and issuing warnings for these storms is extremely challenging. Forecasting tornadoes, detecting them, and disseminating warnings to the public have all improved dramatically in recent years. It is now possible to predict the timing and location of their onset. Once these storms have formed, radar storm detection technologies can track them closely.
However, there are conflicting viewpoints on the genesis of these storms. According to some meteorologists, the cold and dry air aloft, which is often polar continental air, prevails over tropical maritime air. As a result, a lid forms in the atmosphere. When the tropical maritime air penetrates this lid, it creates a powerful updraft, similar to a chimney, resulting in a cyclonic or spinning motion. According to some meteorologists, polar continental air penetrates the lid, forcing tropical maritime air fiercely upward.
Reference
- Y, Wu C.C, (26 February 2004) Current understanding of tropical cyclone structure and intensity changes – a review. Meteorology and atmospheric physics. Scholars
- Wang Yuqing (4 January 2019) Recent Research Progress on Tropical Cyclone Structure and Intensity. ScienceDirect
- National Weather Service. NOAA
- S Lal. Climatology.
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