This report is about tropical revolving storms. Topics covered will include the formation of tropical revolving storms, the areas mainly affected by these storms, aviation hazards with relevance to tropical revolving storms, and a personal assessment of current and future techniques used for the prediction of tropical revolving storms.
A tropical revolving storm originates in the tropics and is a cyclonic disturbance which involves strong convection that extends from the surface to the tropopause. They are generally of a smaller size than temperate depressions. The isobars around the area are generally circular, and there are no fronts involved. There is a very steep pressure gradient which gives rise to the storms great intensity. Generally, tropical storms can have the power and energy to sink any ship that sails through them, and should definitely be avoided at all times. There exists the ‘eye’, which is situated within high walls of thick cloud. The eye is where there exists the greatest danger, due to the very high and unpredictable sea waves. Temperatures in the eye are generally higher than in the surrounding atmosphere. For the most part, tropical storms are accompanied by heavy, and possibly torrential precipitation, and powerful winds. Rainfall can reach 95mm/hour in the most extreme cases, and wind speed can get up to 185kts. Sometimes, internal tornadic winds can exist. Tropical storms can last as long as thirty days, or even just for a few hours. The tropical storms can also approximately reach a maximum of 600nm in size. The most popular locations for tropical storms to form are the North Atlantic and Eastern Pacific oceans (which are coincidentally both sides of the North American continent), Western Pacific oceans, South Pacific oceans and Indian oceans between 5°N and 30°N to 35°N latitudes. Tropical storms do not occur in the European continent.
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In the North Atlantic, tropical storms are born from an easterly wave condition. A trough in the upper atmosphere of low pressure travels west, and this disturbs the tropical air situated over the water. The pressure can get to approximately 870hPa, as an extremity. Due to this extremely low pressure, the storm can cause a surge of up to 13 metres, which is the amount the sea level can raise due to the low pressure, shallow water and wind. Wave heights can reach 34 metres high. For a tropical storm to form, the water temperature must be at least 27° Celsius. The result of this is instability in the atmosphere due to the high environmental lapse rate (ELR), which is then heightened by an increasing saturated adiabatic lapse rate (SALR), and this therefore causes cloud formation, rising convective currents, and thunderstorm activity. The rising columns of air cause an atmospheric pressure drop over an expanding area.
For a tropical revolving storm to form, the latitude must be greater than 5°, so that the Coriolis force can have an effect on the cyclonic circulation and vorticity. The Coriolis force (which is caused by the rotation of the Earth) diverts the airflow into a circular, anti-clockwise motion. Another condition for a tropical storm to form is low wind shear in the troposphere. Wind shear is a change of wind speed and/or direction with altitude, and the vertical development of the storm is aided by wind shear. Conditions which permit the divergence of the airflow at altitude are also a condition for the formation of tropical storms, as this reduces atmospheric pressure by removing air from the area. There also needs to be a tropical disturbance, and this disturbance initiates the process of the formation of tropical revolving storms. The disturbance is very likely to be convection connected to an easterly wave from the Intertropical Convergence Zone (ITCZ).
Spiral cloud bands surround the centre of the storm, and within these cloud bands water vapour condenses due to the convective updraughts, resulting in precipitation, and therefore more latent heat is released into the storm system. Due to the air movement towards the low-pressure centre, the rotating winds accelerate in a process called ‘the conservation of angular momentum’. If the area of rotating winds is diminished, the wind speed must increase. The ‘eye wall’ is formed by the innermost cloud bands, which form a ring of clouds which extend from the surface of the sea to high altitudes, and this surrounds the calmer central eye.
Areas Affected By Tropical Revolving Storms
The areas affected by tropical revolving storms generally tend to be between 5° and 35° latitude. However, tropical storms usually form at around 10° latitude where the oceans are the warmest. The North Pacific West area has the highest occurrence and average annual frequency of tropical storms of anywhere else in the world, averaging at approximately 26 per annum. The North Pacific East has approximately half of that number, on average 13 per year. In the North Atlantic and North Indian oceans, tropical storms average 9 and 6 per year, respectively. Generally, in the Northern Hemisphere, tropical revolving storms occur between August and October; however it is not unfamiliar that they can form as early as May, or as late as November.
In the Southern Hemisphere, however, tropical storms usually form between January and March. The ocean in the southern hemisphere with the most frequent annual tropical storms is the South Indian Ocean in the East, where it gets on average ten tropical storms form. In the West of the South Indian Ocean, there exists on average 8 tropical storms per year. In the south of the Pacific Ocean, particularly on the western side, approximately 6 tropical storms are recorded between January and March. In the Southern Atlantic, however, only one tropical revolving storm has been recorded, which occurred in March 2004. Therefore, there are a global average total of 79 tropical revolving storms.
The Naming Process of Tropical Revolving Storms
The classification of tropical storms is based on wind speed and is set out by the World Meteorological Organisation. A ‘tropical depression’ has a wind speed of less than, or equal to 33kts, a ‘moderate tropical storms’ has a wind speed of 34 to 47kts, a ‘severe tropical storm’ has a wind speed of 48 to 63kts, and finally a ‘hurricane’ (or equivalent synonym) has a wind speed of greater than 64kts.
Tropical revolving storms are almost always named with a human name. The reason for this is that it is much easier to identify storms with a name for weather warnings; it also means that the storms are much more memorable. It is also widely believed that naming storms makes it easier for the media to report, and it also means that there is a heightened interest in the storm and its movement and intensity, and for this reason it keeps the public more prepared for its occurrence. It also means that it is much easier to transfer news and weather forecasts to various stations such as ships at sea, scattered weather stations and coastal bases, rather than using longer and more cumbersome latitude-longitude identifications.
Female names started being used for naming tropical storms in the mid-1900’s. However, meteorologists soon decided to discuss how to make naming a tropical storm more efficiently, and decided to identify tropical storms from a pre-prepared alphabetical list of names. A list would be drawn-up for each year, and each tropical storm would be names in alphabetical order. An international committee of the World Meteorological Organisation maintains and updates the list of names, and originally only used female names; however since 1979 male names began being used, and now alternate with female names. There are currently six different lists, and rotate with each other, so for example a list used in 2010 would then be used again in 2016. Different regions of the world use different lists, usually with names more appropriate with that region of the world. If a tropical storm occurs which causes a significant amount of damage, or a high death toll, a name can be ‘retired’, meaning that the name will be taken from that list and replaced with another one of the same letter after a meeting of the World Meteorological Organisation Tropical Cyclone Committees. An example of a retired name is Katrina, after Hurricane Katrina caused billions of American dollars’ worth of damage in Southern USA in 2005.
Caribbean Sea, Gulf of Mexico and the North Atlantic Names
The Size and Structure of a Fully Developed Tropical Revolving Storm
A fully developed tropical revolving storm can reach a vertical height of up to 9 miles, and can reach a radius of up to 600 nautical miles, which is the radius of the gale force winds. A tropical storm has a thermally direct, strong circulation, where warm air rises near the centre of the storm, and cooler air around the outside sinks. The warmer centre of the storm is a reservoir of potential energy, and this energy is constantly being converted into kinetic energy by the thermally direct circulation. Cloud bands in the tropical storm are formed due to the weak uplift of the air and low precipitation regions.
A tropical revolving storm has a wind speed ranging from 33kts to greater than 64kts, and the wind speed determines the nomenclature of the storm:
Moderate Tropical Storm
34 – 47kts
Severe Tropical Storm
Hurricane (or synonym)
The tropical revolving storm consists of many identifiable elements that make up the structure. Moving from the outside, there is firstly an outer band of convective cumulus cloud, which is due to warm air rising. Then there is an annular zone of sinking air, which is usually clear of cloud but there can exist some shallow cumulus clouds. Moving further into the tropical storm, there is an inner band of deep, convective cumulus clouds and cumulonimbus, which extend towards the tropopause in spiral bands which move towards the centre. The eye wall is an area of high velocity wind which move parallel to the isobars and rise rapidly. The eye wall surrounds at least half of the eye of the storm, and winds here can gust up to approximately 173kts (200mph). The eye wall consists of a band of very tall thunderstorms which create torrential precipitation and strong winds. On the side of the eye wall where the wind direction is the same as the direction of forward movement of the tropical storm is where the most destructive part of the storm exists.
DIAGRAM 1: Tropical Revolving Storm Structure
The centre of the storm, also known as the ‘eye’ of the storm, is an area of descending air which is warming adiabatically, and there tends to be an absence of clouds in the eye. Additionally, the wind speed in the eye tends to be very light, not exceeding 13kts (15mph). The eye, on average, has a diameter of approximately 17 to 26 nautical miles (20 to 30 miles). An eye usually forms when the maximum sustained wind speeds go above 68kts (78mph). The eye is the calmest part of the storm, and tends to become smaller as the storm strengthens.
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Usually, a canopy of cirrus clouds exist which form at the tropopause in the divergent outflow, and some of it descends further down into the annular zone. These high cirrus clouds exist due to the low temperatures at altitude. The cloud walls of a tropical storm can extend up to more than 6 nautical miles, which consequently spread out to become anvil-shaped at the base of the stratosphere. These clouds then extend downwind for many miles. The direction of the tropical revolving storm tends to be in the same direction as the winds aloft.
The Speed and Direction of Movement of Tropical Revolving Storms
A tropical revolving storm, on average, travels at approximately 10kts nearer to the equator, and about 25kts in the higher latitudes. Tropical storms also tend to move with the flow of air in the troposphere. Most tropical storms move about the oceanic anticyclone towards higher latitudes, whilst others move in a westerly direction, indefinitely towards the poles. However, within the general path of the storm, its movement can be fairly unpredictable.
In the Atlantic, larger tropical storms are formed from atmospheric disturbances, also known as ‘easterly waves’. These easterly waves move off the western African coast, and are consequently carried westwards across the Atlantic by the prevailing atmospheric flow. If the ICTZ (Intertropical Convergence Zone) moves more towards the north, the Coriolis force will affect the south-eastern trade wind, and the airflow will become cyclonic. If there is a further vertical airflow pattern, this will allow cumulonimbus clouds to grow to high altitudes, and a tropical revolving storm can develop. There exists a latent heat release, convergence, and a tendency to cyclonic curvature.
This developing storm will habitually move along the Southern edge of the ‘Azores-Bermuda High’, which is a high pressure zone in the mid-Atlantic, and is usually found between 30° and 35° north in the summer season. If this high pressure zone is strong enough and in its normal position, the easterly wave will continue more westwards past the West Indies and into the Caribbean Sea, or the Gulf of Mexico. On the other hand, if a trough of low pressure travels in a southerly direction from milder latitudes, the high pressure zone will weaken, and therefore permit the tropical storm to travel in a north, north-westerly direction through the trough. The centre of the storm will tend to then head towards the North American mainland or the offshore waters of the North Atlantic seaboard. If, then, the storm enters an area of westerly winds north of 35°N, it will then quickly move north-easterly over the cooler North Atlantic waters, and consequently lose its tropical structure.
Wind Speeds, Weather, Cloud, and Aviation Hazards In and Around a Tropical Revolving Storm
From a pilot’s perspective, the characteristics of tropical revolving storms to look out for are strong gusts, spiral cloud patterns, circular isobars, and a central eye. The weather conditions in and around a tropical revolving storm vary considerably depending where one is situated within the storm.
Moving from the annular zone of sinking air towards the eye wall, pressure decreases gradually whilst the wind speed increases. As the wind speeds start to increase, this will mean that the sea wave heights increase too. The swell waves, which are waves of long wavelengths created by the tropical storms, also increase in height, and their direction is from the wind field near the eye. The cloud cover from the annular zone to the eye wall is always eight oktas; this means that the clouds cover the whole of the sky. Precipitation in this area of the storm also increases in intensity towards the eye wall.
In the eye of a tropical revolving storm, the pressure tends to steady, and the wind speed drops significantly to around 15kts. Due to this much lower wind speed, the sea waves created by wind decrease in size, however the swell waves are extremely high, and tend to move in all directions. In the eye, there is usually no cloud cover at all, though usually one or two oktas of cloud cover. Due to the general absence of clouds, there is no precipitation in the eye.
Now moving from the eye wall to the annular zone of sinking air, the pressure increases gradually, and the wind speeds immediately increase to their maximum speed, and then gradually decrease with distance from the eye wall. On this side of the eye wall, the wind direction is in the opposite direction to the other side. The sea waves are at maximum height, and then decrease in size from the eye wall. The swell wave heights also decrease from the eye wall. There is total cloud coverage in this area of the tropical storm, and the precipitation levels increase to maximum then decrease gradually.
DIAGRAM 2: Wind and Pressure Variations within a Tropical Revolving Storm
In aviation, tropical revolving storms are treated as an area of weather to avoid under any circumstances. In and around the tropical storm, there can be severe turbulence around the spiral bands of cumulonimbus, especially within the boundary around the eye. It is possible for airports to be closed due to low-level strong winds and turbulence around the airport. Travelling away from a tropical revolving storm, the winds become calmer in the outflow above approximately 30,000ft. It is important to know whereabouts the aircraft is situated within or around a tropical revolving storm due to the different wind velocities experienced around a tropical storm due to the cyclonic circulation. If the eye is situated on the left side of the aircraft, there will be an increased tailwind component (in the Northern Hemisphere). The advance front quadrant of a tropical storm is the most dangerous area to be in, due to the interaction between the storm system’s movement and wind speeds. The weakest part of the storm is generally believed to be in the rear left quadrant, as it is most unlikely that the storm will move in that direction.
DIAGRAM 3: Various Quadrants of a Tropical Revolving Storm
The path is the route the storm is forecasted to follow, the vortex is the eye of the storm, and the vertex is the most westerly point of the forecasted path curve. As the diagram shows, the dangerous quadrant is in the advance right section (in the Northern Hemisphere). This is because not only is the storm most likely to move in that direction, but also the winds in that part of the storm generally drive any aircraft into the path of the storm.
The navigable semi-circle is the semi-circle on the left of the storm (in the Northern Hemisphere). The reasons for this being the navigable area of the storm is that it is more unlikely that the storm will travel in this direction and the winds generally move any aircraft away from the path of the storm in the advance quadrant.
If there is no information about the storm, or whereabouts in the storm one is currently situated, simple assumptions can be made about the distance the centre of the storm is. If the pressure has dropped approximately 5hPa and the wind speed is approximately 28kts, then assume the centre is about 200nm away. If the wind speed is approximately 34kts, then assume the centre is about 100nm away.
The Type and Extent of Damage That Can Occur on the Ground Due to Tropical Revolving Storms
The impact a tropical storm can have on people depends on an array of factors, such as the intensity of the cyclone, the distance the storm is from the sea, the speed of movement of the storm, and the topography of the coastal area. Also, the amount of preparation that communities have made before a tropical storm reduces the vulnerability of those communities.
On the oceans, tropical revolving storms can cause large swells due to the strong winds. There are many problems associated with these large swells and waves, such as the disruption of international shipping, and they can sometimes cause shipwrecks. The storm can cause ‘storm surges’ to develop, which means that the sea level rises. Storm surges account for approximately 90% of deaths due to cyclonic activity.
A ‘landfalling’ storm (a storm that is beginning to move over the land) can give rise to tornadoes due to the general rotation of the storm and its wind shear. A tropical storm can also produce ‘eye wall mesovortices’, rotational features found in the eye wall of a tropical storm on a smaller scale, and these mesovortices can cause subsequent tornadoes until landfall occurs. On the land, tropical storms can cause heavy damage to any buildings, depending on the strength of the winds, and the strength of the buildings. Tropical storms can also damage, or even destroy vehicles, bridges, and the storm can cause smaller objects to become deadly debris projectiles through the air. Rainfall often exceeds 100mm per day in a tropical storm, and this can cause severe flooding, and even landslides. Landslides are very common in Hong Kong due to cyclones. Also, as a tropical storm moves over an area of higher relief, such as a mountain range, the storm rapidly weakens, and torrential precipitation can occur, sometimes up to 700mm per day.
On a demographic side, due to the heavy precipitation and possible flooding caused by a tropical storm, there may be prolonged periods of time where the water becomes stagnant on the surface, and this can cause infections, and give rise to mosquito-borne illnesses such as malaria. Because evacuees of a tropical storm are usually placed into shelters for long periods of time, this can cause infection propagation. There has been over 1.9million deaths over the past 200 years due to tropical storms worldwide.
Although tropical storms cause much damage to infrastructure, as well as loss of communication, there can be good impacts of tropical storms. For example, in dryer regions of the world, the heavy precipitation brought on by tropical storms may be well-needed for agriculture. Another example is the fact that tropical storms assist in keeping the heat in the atmosphere, as tropical storms transport warmer air from the tropics towards the cooler mid-latitudes and Polar Regions.
The Tropical Revolving Storm Dissipation Process, and the Reasons for It
As long as the necessary conditions to keep a tropical revolving storm ‘alive’ are met, the tropical storm will sustain itself for as long as possible. The tropical storm will start to dissipate if the storm cannot sustain itself any longer due to the lack of energy which is required. The reason that the storm loses its energy is because it moves over an area of reduced temperature and humidity. This area could be either when the storm moves over the land (landfall) because of the cooler temperature as land takes a longer time to heat up and cool down, or the area could be a sea surface in the tropics or over higher latitude areas where the water temperatures are cooler. When a tropical storm landfalls, it reduces in intensity relatively quickly and becomes an ‘extratropical cyclone’. If a tropical storm moves over a mountainous area, the storm will weaken much more rapidly, and there may be torrential precipitation, which has the potential to cause many fatalities due to floods and mudslides. Also, if the storm stays in the same area of ocean for a prolonged period of time, it can cause the water beneath it to cool down due to mixing, and the storm will dissipate. Even if a tropical storm has lost its tropical characteristics and become extratropical or dissipated, it can still retain its wind speeds and produce several inches of precipitation. This phenomenon can be experienced on the west coast of North America, or even on the European continent.
A tropical revolving storm can regenerate after crossing a landmass. As the tropical storm moves over the landmass, it decreases in intensity, and then if the storm meets another area of warm seas, it can regenerate. This regeneration is commonplace in areas such as the Central American Isthmus, Taiwan, the Malaysian Peninsula, and Australia. Usually, if the tropical storm moves into the mid-latitudes of around 35° to 45°, it will decrease in strength, lose its tropical physiognomies (such as thunderstorms near the centre), and become a ‘mid-latitude depression’.
A Personal Assessment of Current and Future Techniques Used For the Prediction of Tropical Revolving Storms
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