Typhoon What Do You Know?
Roadmap • Common terms • Naming • Definitions • Common expressions
Typhoons, Hurricanes and Tropical Cyclones • Cyclone (generic term) • Tropical Cyclone • Typhoon • Hurricane • Cyclone (one of tropical cyclones) • Extratropical Cyclone • Willy-Willy • Tornado
Cyclone • Cyclone is the most general term to refer to a low-pressure system. • Typhoons and other types of low pressure systems are all cyclones. The direction of rotation is opposite in the northern hemisphere and the southern hemisphere, but other essential features of a cyclone are shared in both hemispheres.
Tropical Cyclone • Tropical cyclone is in general a cyclone formed in the tropical areas. • The word "tropical" does not refer to the place of formation, and it actually refers to the structure of a cyclone. • Typhoons, hurricanes and others are all "intense" tropical cyclones, so they are regarded as same meteorological phenomena. It is, however, still unclear whether there is subtle regional differences between tropical cyclones in the world.
Extratropical Cyclone • Extratropical cyclone literally means a cyclone outside of the tropical areas. Just like a tropical cyclone, this term also does not refer to the place of formation, but refers to the structure of a cyclone. • Formation: • tropical cyclone: warm air only • extratropical cyclone: consists of both cold air and warm air. • Finally, we often see a tropical cyclone transformed into an extratropical cyclone, but the inverse is rare.
Typhoon • Typhoon is a tropical cyclone located in the western north Pacific basin (between 100E and 180E in the northern hemisphere). The category of a typhoon is decided by the maximum sustained winds. • Finally, among tropical cyclones in the world, the typhoon is the most frequent and the strongest tropical cyclone.
Hurricane • Hurricane is a tropical cyclone located in the north Atlantic, eastern north Pacific (east of 180W in the northern hemisphere), eastern south Pacific (east of 160E in the southern hemisphere). • The category of a hurricane follows the same international standard as the typhoon based on the maximum sustained wind. • When a hurricane goes across the 180 degree date line and enters into the basin of the typhoon, it starts to be called a typhoon.
Cyclone (Contd.) • Originally a generic term for a cyclonic system. • It is also used to refer to something equivalent to a typhoon (tropical cyclone) when there is no special term available. Indian Ocean, western south Pacific, and Australia are the examples of this area.
Willy-Willy • Willy-Willy is often introduced as the name of a tropical cyclone around Australia. • But it seems that it actually means something like a dust devil, and has little relationship with a tropical cyclone. • A dust devil is a rotating updraft, 1000 meters high or more and tens of meters in diameter. They are distinct from the common dust storms of this area, because they resemble small tornados.
Tornado • The tornado and the tropical cyclone share the same feature as the vortex of atmosphere, but other features, such as formation, structure, scale and duration, are totally different. • For example, the scale of a tornado is usually on the order of 100m-1000m, while a tropical cyclone is on the order of 100km-1000km.
Naming Typhoons • Number-based conventions and a list-based convention. • The latter convention is more popular in most countries, such as human names for hurricanes.
Number-based Conventions • Number-based conventions are based on the sequential number from the beginning of a typhoon season. • For example, Typhoon No. 14 is the 14th typhoon of the typhoon season. • On the other hand, a 4-digit YEAR+NUMBER identification code such as Typhoon 0314, or T0314 for short, is a more preferred convention in technical and professional areas. • Number-bases conventions, however, directly use sequential numbers, so it is sensitive to subsequent changes of orders. • Secondly, when the order of birth is changed, names do not change to reflect the actual order.
List-based Conventions • List-based conventions are based on the list of typhoon names defined in advance by the committee of meteorological organizations worldwide. • A new name is automatically chosen from the list upon the genesis of a typhoon. The list is defined for each basin and managed by the meteorological organization responsible for the respective basin. • The majority of countries in the world seems to prefer list-based conventions to number-based conventions. • Advantages: explicit representation of the sequential order of the typhoon during the typhoon season • Disadvantage: similarity of all names which leads to the difficulty of making distinction.
Named after Women? • The most famous list with the longest history is the list of hurricane names of the Atlantic Ocean. This list contains alphabetically-ordered names from A (Q, U, X, Y, Z are skipped). • On the birth of a new hurricane, a new name is chosen in the alphabetical order, and the next list is in use when the current list is exhausted. • Only female names were used in the beginning, because hurricanes were named after girlfriends or wives of US Army Air Corp and Navy meteorologists. In 1979, however, male names are included in the list from an argument on gender equality.
Asian Names of the Typhoon • The Asian names are list of words submitted to the typhoon committee, consisting of meteorological organizations of 14 countries and regions in the Asia / Pacific, that belongs to Tropical Cyclone Programme, World Meteorological Organization (WMO). • This list contains words of so diverse meanings -- such as animals, plants, and natural phenomena -- that preference to human names do not remain any more. • Asian names are sorted according to the alphabetical name of countries, so typhoon names themselves are not necessarily arranged in an alphabetical order. • When a hurricane moves across the 180 degree line and enters into the basin of the typhoon, it starts to be called a typhoon. A new Asian name, however, is not assigned in this case, and the hurricane name is kept as the name of the tropical cyclone.
Common Problems • One problem with typhoon names is the ambiguity of typhoon names. That is, we sometimes have multiple typhoons with the same name in history, and that causes the problem of resolving ambiguities. This is in particular a big problem with the natural language processing of typhoon-related text, or named entity recognition, because the computer should disambiguate the typhoon name automatically by the contents of the text.
Retiring Typhoon Names • To cope with the ambiguity problem, the typhoon name is sometimes "retired," and removed from the list. • This case happens when the typhoon has a severe impact on lives or the economy to the level that is remembered for generations after the devastation. Whenever a typhoon has had this level of impact, any country affected by the storm can make a request to the World Meteorological Organization (WMO) that the name of the typhoon be retired to facilitate historic references, legal actions, insurance claim activities, etc.
The Nameless Cyclone and No-Name Typhoons • An article from NASA on April 2, 2004 says that we finally had "the nameless cyclone (hurricane)." The story goes that they observed a cyclone in the South Atlantic basin where tropical cyclones have never been observed before. The list of cyclone names for this basin was therefore not prepared so this cyclone could not be named. • (Note) It seems that this hurricane is now called Catarina after the name of the place it made landfall.
The Definition of Typhoon Season • Generally speaking, the term "typhoon season" is a period of time during which we have frequent typhoon effects. • Within the recording of typhoons this term means the borders of one year in terms of the formation of typhoons. The starting point of counting for the number-based conventions of typhoon names is based on the typhoon season. • Northern hemisphere: Western North Pacific Basin From January to December • Southern hemisphere: Western South Pacific Basin From July to June (next year)
Unit of Pressure and Wind • Hecto Pascal (hPa) is a unit for pressure, and, used mainly for representing the central pressure of a typhoon. • Knot (kt) is a unit for speed, used mainly for representing the maximum wind speed at the center of a typhoon. Conversion of units is 1kt = 1.852 km/h = 0.514 m/s, so roughly speaking, 2 kt = 1 m/s and 1 kt =2 km/h.
The Life Period of Typhoons • "Typhoon" is a name given to a tropical cyclone with the maximum wind of more than 34 knots. • The difference between a tropical depression and a typhoon is merely the difference of the strength of the wind. • The ground truth measurement of the maximum wind speed around the center is not always possible. So whether a tropical cyclone should be called a typhoon cannot be decided automatically and this decision making requires the skills of human experts. • Human experts declare their birth and death, so we can explicitly define the life period of typhoons. This is one of the unique characteristics of the typhoon. • Some typhoons come to life again after weaken into tropical depressions. We even have a typhoon with two rebirths. Hence there are two possibilities as to the definitions of the life period of the typhoon in those cases. • The period between the first birth and the last death of the typhoon. • The period as above except for the period of its being a tropical depression, or not being a typhoon.
Understanding some common expressions • The typhoon is changed to the extratropical cyclone: • change of structure, not the change of intensity. • More specifically, this expression is used when the structure of the typhoon is changed from a tropical cyclone to an extratropical cyclone. • In most cases this change does not go backward. • The typhoon is changed to the tropical depression: • the change of intensity, not the change of structure. • More specifically, this expression is used when the maximum wind around the center happens to be smaller than the threshold to be a typhoon. • It only means that the maximum wind is below the threshold, so we sometimes see a tropical depression revives as a typhoon. • In short, change from a typhoon to an extratropical cyclone means structural change, while change from a typhoon to a tropical depression means intensity change. • We should be aware that these expressions do not imply the typhoon to be weakened in terms of rainfall.