What is the system of naming hurricanes

Tropical cyclones: structure, formation, distribution, classification

Tropical cyclones are among the weather extremes that are most destructive to natural and social systems. They occur in the tropical regions of all three oceans. They are called hurricanes in the Atlantic, typhoons in the Pacific, and cyclones in the northern Indian Ocean. For the sake of simplicity, the term “hurricane” is often used universally. The focus of attention in the western world is on the tropical cyclones in the Atlantic, because of their hazard potential in the western hemisphere, but also because they have been best researched and the longest data series are available about them.


Fig. 1: Location of Hurricane Katrina on August 28, 20051

Damage

In 1998, Hurricane Mitch killed 11,000 people in Honduras and Nicaragua and caused property damage of US $ 5 billion. In 2005, Hurricane Katrina submerged the American city of New Orleans by 80%, making it largely uninhabitable, killing around 1,800 people and causing property damage of 160 billion US dollars, adjusted for inflation, making it the most expensive hurricane in the United States to date. Overall, the year 2005 went down in history as the year of by far the strongest hurricane activity in the North Atlantic, with 14 hurricanes in total, seven of which belonged to category 3-5 and three to category 5, and with two category 4 hurricanes as early as July Start of the hurricane season.2

After more than a decade of relatively little activity, the 2017 hurricane season almost reached the level of 2015 with 10 hurricanes and six strong category 3-5 hurricanes.2a The damage caused by prolonged heavy rainfall from Category 4 Storm Harvey in the Houston area was almost as large as Katrina, at $ 125 billion.2 B Another specialty was Hurricane Ophelia, which as a Category 3 hurricane did not move westwards as usual, but northwards and hit the coast of Ireland as a strong hurricane.2c

The economic damage caused by hurricanes in the USA has increased many times over since the beginning of the 20th century. The five most expensive hurricanes that have hit the mainland of the United States since 1900 are all in the period after 2005.2d However, this is primarily due to population growth and the increasing accumulation of values ​​in the affected coastal zones.3 If the damage from previous hurricanes is extrapolated to the values ​​and population figures present in the region today, no trend can be discerned. Hurricane Katrina would come in second only to the Great Miami Storm of 1926, which would have caused $ 157 billion in damage under 2005 conditions. And the loss-making decade 1996-2005 would only be in second place after 1926-1935.

distribution


Fig. 2: Mean tracks of Atlantic hurricanes in September5a

Most of the world's tropical cyclones (around 87%) occur between the 20th and 20th centuriesON and 20OS; two thirds of all tropical cyclones form in the northern hemisphere. The preferred areas are the Western Atlantic, Eastern Pacific, Western North Pacific, Northern and Southern Indian Oceans and the Southwest Pacific off Australia. Storms of the higher categories occur mainly in the northern western Pacific (Fig. 3). The hurricanes in the North Atlantic move on train tracks from the central Atlantic or the eastern Caribbean to the west and north towards Central America and the south coast of the United States (Fig. 2). With hurricane "Katrina" wind speeds of over 300 km / h were measured in August 2005 and with "Wilma" in October 2005, the strongest hurricane since records began, peak gusts of up to 340 km / h were measured.


Fig. 3: Spread of tropical cyclones. TD: Tropical low pressure area, TS: Tropical storm, 1-5: Hurricane category according to the Saffir-Simpson scale (see Table 1)6

construction

A tropical cyclone has a cylindrical structure. In the center, the so-called eye, which has a diameter of 15 to 30 km, there is almost no wind and no clouds. Around the eye there is a wall of clouds with high wind speeds and heavy rainfall. In the eye there is a very low pressure of less than 900 hPa directly above the sea surface, which sucks in air from all sides. The Coriolis force directs this air into a vortex around the eye, which moves counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

The evaporation of the warm ocean water supplies the system with latent energy, which is released again through condensation when it rises and causes cloud formation and precipitation. The energy supplied from below leads to further evaporation and condensation even at higher altitudes - and to a further rise of air masses up to the tropopause, where there is a lateral mass flow. As a result, the air pressure above the sea surface decreases further and further, more warm, humid air flows into the deep, etc. The lower the air pressure in the center, the stronger the storms circling around the center and the heavier the precipitation.


Fig. 4: Formation and structure of a hurricane7

The energy turnover in tropical cyclones plays an important role in the dissipation of energy over the heated ocean surfaces in the summer half of the year. Because the tropical atmosphere has a very high water vapor content, the long-wave radiation cannot dissipate enough energy from the earth's surface into space. This task is partly taken over by the tropical cyclones. They transport energy through evaporation and condensation into the upper troposphere, from where it can be radiated into space. In addition, the sea surface is churned up by the strong wind and cooler water is carried upwards from the depths.

Emergence

A tropical cyclone differs significantly from the low pressure areas of medium latitudes, which in Europe lead to precipitation and storms, especially in winter. These are caused by the temperature difference between northern and southern latitudes and mix different warm air masses with one another. A tropical cyclone, on the other hand, is driven by the energy from the ocean below, which mainly enters the atmosphere as latent heat through evaporation and condensation and heats it up. The first prerequisite for the formation of a tropical cyclone is therefore a high sea surface temperature of at least 26 ° C, as can be found north and south of the equator. Since hurricanes get their energy from the water and are up to 500 kilometers in diameter, a sufficiently large, cohesive sea surface is a further prerequisite. The third prerequisite is the effect of the Coriolis force, the deflection of which leads to the typical vortex formation. Since the Coriolis force only becomes effective at a distance of at least 5-8 ° from the equator, there are no tropical cyclones in the immediate vicinity of the equator.

Whether a more or less strong tropical cyclone actually develops with the above-mentioned conditions also depends on the structure of the atmosphere over the ocean. A high sea surface temperature also means high evaporation and thus a high water vapor content in the atmosphere, which provides energy for the convection of tropical cyclones. On the other hand, there can only be a weak vertical wind shear. This means that the direction and strength of winds must be as similar as possible at all heights above the ground, otherwise the vertical development of tropical cyclones will be disturbed.4 Over the North Atlantic, for example, there is relatively strong wind shear in El Niño years, in which therefore relatively few hurricanes occur. The stability of the atmosphere due to the temperature stratification also plays an important role. The greater the temperature gradient with altitude, the greater the drive for the rise of warm air, which favors the development of tropical cyclones.5

Classification and naming

Hurricanes in the North Atlantic are divided into five categories according to the wind speed according to the so-called Saffir-Simpson scale. The designation of a disturbance as a tropical cyclone occurs from a wind speed of 56 km / h. From 118 km / h one speaks of a hurricane, from 178 km / h there is a category 3 hurricane, and above 249 km / h a category 5 hurricane.


Table 1: Hurricane categories on the Saffir-Simpson scale

The naming of individual hurricanes is also regulated. For each hurricane season, the World Meteorological Organization (WMO) sets 21 alphabetically arranged female and male first names six years in advance. The name of a hurricane that caused particularly severe damage will be removed from future lists, as happened for example with Katrina (2005) and Fay (2008). If there are more than 21 hurricanes in one season, as was the case in 2005, for example, the following are named after the Greek alphabet (alpha, beta, gamma, etc.).


Remarks:
1.
Source: NASA Earth Observatory
2. Holland, G. J. & Webster, P. J. (2007): Heightened tropical cyclone activity in the North Atlantic: natural variability or climate trend? Phil. Trans. R. Soc. A 365, 2695-2716
2a. NOAA (2017): Extremely active 2017 Atlantic hurricane season finally ends
2 B. NOAA NHC, National Hurricane Center (2018): Costliest U.S. tropical cyclones tables update
2c. Di Liberto, T., NOAA (2017): Former hurricane Ophelia batters Ireland
2d. NOAA NHC, National Hurricane Center (2018): Costliest U.S. tropical cyclones tables update, https://www.nhc.noaa.gov/news/UpdatedCostliest.pdf
3. Pielke, R.A. Jr, et al. (2008): Normalized hurricane damages in the United States: 1900-2005 Natural Hazards Review 9, 29-42
4. Zeng, Z., (2010): A statistical analysis of vertical shear effect on tropical cyclone intensity change in the North Atlantic, Geophysical research letters 37, doi: 10.1029 / 2009GL041788
5. IPCC (2007): Cambridge Univ. Press, Cambridge. Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate, Box 3.5.
5a. Modified from National Hurricane Center: Tropical Cyclone Climatology (http://www.nhc.noaa.gov/pastprofile.shtml)
6. Source: Global Warming Art (page no longer exists)
7. Own representation