Tropical Cyclones

Other-Science

Tropical cyclones are one of the biggest threats to life and property even in the formative stages of their development. They include a number of different hazards that can individually cause significant impacts on life and property, such as storm surge, flooding, extreme winds, tornadoes and lighting. Combined, these hazards interact with one another and substantially increase the potential for loss of life and material damage.

Over the past 50 years, 1 942 disasters have been attributed to tropical cyclones, which killed 779 324 people and caused US$ 1 407.6 billion in economic losses – an average of 43 deaths and US$ 78 million damages every day.

Characteristics of tropical cyclones

A tropical cyclone is a rapid rotating storm originating over tropical oceans from where it draws the energy to develop. It has a low pressure center and clouds spiraling towards the eyewall surrounding the “eye”, the central part of the system where the weather is normally calm and free of clouds. Its diameter is typically around 200 to 500 km, but can reach 1000 km. A tropical cyclone brings very violent winds, torrential rain, high waves and, in some cases, very destructive storm surges and coastal flooding. The winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Tropical cyclones above a certain strength are given names in the interests of public safety.

This weather phenomenon is named with different terms depending on the location.

  • In the Caribbean Sea, the Gulf of Mexico, the North Atlantic Ocean and the eastern and central North Pacific Ocean, such a weather phenomenon is called “hurricane”
  • In the western North Pacific, it is called “typhoon”
  • In the Bay of Bengal and Arabian Sea, it is called “cyclone”
  • In western South Pacific and southeast Indian Ocean, it is called “severe tropical cyclone”
  • In the southwest Indian Ocean, it is called “tropical cyclone”

They are associated with extremely heavy rain which can result in widespread flooding. Cyclones are also associated with damaging or destructive winds and in the most intense systems, surface winds may reach speeds in excess of 300 km/h. The combination of wind-driven waves and the low-pressure of a tropical cyclone can produce a coastal storm surge – a huge volume of water driven ashore at high speed and with immense force that can wash away structures in its path and cause significant damage to the coastal environment.

Depending on the maximum sustained wind speed, tropical cyclones will be designated as follows:

  • Tropical depression is when the maximum sustained wind speed is less than 63 km/h.
  • Tropical storm is when the maximum sustained wind speed is more than 63 km/h. It is then also given a name.
  • Hurricane, typhoon, tropical cyclone, very severe cyclonic storm – depending on the basin, when the maximum sustained wind speed exceeds 116 km/h or 63 knots.

According to the Saffir-Simpson Hurricane Wind Scale in use in the Caribbean Sea, the Gulf of Mexico, the North Atlantic Ocean and the eastern and central North Pacific Ocean, the hurricane strength varies from Category 1 to 5:

  • Category 1 hurricane is referring to the hurricane with maximum sustained wind speeds of 119-153 km/h.
  • Category 2 hurricane is referring to the hurricane with maximum sustained wind speeds of 154-177 km/h.
  • Category 3 hurricane is referring to the hurricane with maximum sustained wind speeds of 178-209 km/h.
  • Category 4 hurricane is referring to the hurricane with maximum sustained wind speeds of 210-249 km/h.
  • Category 5 hurricane is referring to the hurricane with maximum sustained wind speeds exceeding 249 km/h.

The impact of a tropical cyclone and the expected damage depend not just on wind speed, but also on factors such as the moving speed, duration of strong wind and accumulated rainfall during and after landfall, sudden change of moving direction and intensity, the structure (e.g. size and intensity) of the tropical cyclone, as well as human response to tropical cyclone disasters.

Average annual cycle of tropical cyclone occurrence for each ocean basin. The abscissa spans the 13 months, December through January of the following year; the ordinate is the number of storms per hundred years. For each day, the graph shows the number ofAverage annual cycle of tropical cyclone occurrence for each ocean basin. The abscissa spans the 13 months, December through January of the following year; the ordinate is the number of storms per hundred years. For each day, the graph shows the number of years that a cyclone was present (normalized per 100 years). The blue line represents all tropical cyclones (surface winds greater than 17 m s-1 or 34 knots); shading represents tropical cyclones of hurricane strength (surface winds greater than 33 m s-1 or 64 knots).

Tropical Cyclone Forecasting

Meteorologists around the world use modern technology, such as satellites, weather radars and computers, to track tropical cyclones as they develop. Tropical cyclones may be difficult to forecast, as they can suddenly weaken or change their course. However, meteorologists use state-of-art technologies and develop modern techniques such as numerical weather prediction models to forecast how a tropical cyclone evolves, including its movement and change of intensity; when and where one will hit land and at what speed. Official warnings are then issued by the National Meteorological Services of the countries concerned.

About 80 tropical cyclones form every year. The WMO Tropical Cyclone Programme provides information on these hazards and the WMO Severe Weather Information Centre provides real-time tropical cyclone advisories.

The WMO framework allows the timely and widespread dissemination of information about tropical cyclones. As a result of international cooperation and coordination, tropical cyclones are increasingly being monitored from their early stages of formation. The activities are coordinated at the global and regional levels by WMO through its Tropical Cyclone Programme. The Regional Specialized Meteorological Centres with the activity specialization in tropical cyclones, and Tropical Cyclone Warning Centres, all designated by WMO, are functioning within the Organization’s Tropical Cyclone Programme. Their role is to detect, monitor, track and forecast all tropical cyclones in their respective regions. The Centres provide, in real-time, advisory information and guidance to the National Meteorological and Hydrological Services.

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