Annular tropical cyclone

An annular tropical cyclone is a tropical cyclone that features a normal to large, symmetric eye surrounded by a thick and uniform ring of intense convection, often having a relative lack of discrete rainbands, and bearing a symmetric appearance in general. As a result, the appearance of an annular tropical cyclone can be referred to as akin to a tire or doughnut.[1] Annular characteristics can be attained as tropical cyclones intensify; however, outside the processes that drive the transition from asymmetric systems to annular systems and the abnormal resistance to negative environmental factors found in storms with annular features, annular tropical cyclones behave similarly to asymmetric storms. Most research related to annular tropical cyclones is limited to satellite imagery and aircraft reconnaissance as the conditions thought to give rise to annular characteristics normally occur over water well removed from landmasses where surface observations are possible.

Satellite image of Hurricane Isabel of 2003, displaying a large, circular, and symmetric eye, and symmetrical storm shape, which are characteristics of annular tropical cyclones. Isabel is also exhibiting a pinwheel eye, a rare feature found in some annular tropical cyclones.

Characteristics and identification

The annular hurricane was first defined as a subset of tropical cyclones by John Knaff of Colorado State University and James Kossin of the University of Wisconsin–Madison in 2002 by use of infrared satellite imagery, which serves as the visual means of ascertaining annular characteristics within tropical cyclone. Knaff and Kossin defined an annular tropical cyclone as a tropical cyclone that maintains either an average or larger-than-average eye surrounded by deep convection containing the storm's inner core and a lack of convection occurring outside the central dense overcast for at least three hours. As a result, annular storms lack the rainbands characteristic of typical tropical cyclone. These features lend the storm an axisymmetric appearance common to annular tropical cyclones. However, this definition is only applicable while a storm maintains these characteristics—when and while a storm does not feature annular characteristics, the tropical cyclone is considered asymmetric. In addition to the primary defining characteristics, the diurnal pulsation of the cirrus cloud canopy associated with outflow is subdued once storms become annular.[1] Some annular tropical cyclones may also display a "pinwheel eye", a feature in which conditions in the storm causes its eye to take the appearance of a spoked wheel.[2] An algorithm for identification of annular tropical cyclones in real-time by objective criteria has been developed, and shows some power, but is not yet operational.[3]

Although tropical cyclones can achieve annular characteristics across a wide spectrum of intensities, annular storms are typically strong tropical cyclones, with average maximum sustained windspeeds of 108 kn (200 km, 124 mph). In addition, storms attaining annular characteristics are less prone to weakening as a result of negative environmental factors. Annular cyclones can maintain their respective peak intensities for extended periods of time unlike their asymmetric counterparts. Following peak intensity, such systems will tend to gradually taper off. This unusual intensity persistence makes their future intensities difficult to forecast and often results in large forecast errors. In an analysis of hurricanes in the East Pacific and North Atlantic between 1995 and 1999, Knaff and Kossin observed that the National Hurricane Center underestimated the intensity of annular hurricanes 72 hours out by 18.9 kn (35.0 km/h, 21.7 mph).[1]

A survey of Pacific typhoons between 1990 and 2009 found only 12 with annular characteristics, representing an occurrence rate of 4 percent.[4]

Transition from asymmetric cyclones and necessary conditions

Typhoon Noru of 2017, displaying all annular characteristics (minus the pinwheel eye)

Tropical cyclones can become annular as a result of eyewall mesovortices mixing the strong winds found in the eyewalls of storms with the weak winds of the eye, which helps to expand the eye. In addition, this process helps to make the equivalent potential temperature (often referred to as theta-e or ${\displaystyle \theta _{e}}$) within the eye relatively uniform. This transition takes roughly 24 hours to complete and can be considered a type of eyewall replacement cycle. Winds have also been found to decrease in a stairstep like fashion within the radius of maximum wind, which may indicate that more wind is mixed between the eye and eyewall as cyclones strengthen, which helps to explain why annular characteristics are generally exclusive to storms of higher intensities.[1]

The intensity of annular systems is typically greater than 83.5% of the maximum potential intensity, suggesting that the conditions in which storms gain annular characteristics are generally conducive for tropical cyclone persistence and intensification. Annular tropical cyclones also require low wind shear, and of the storms in the East Pacific and North Atlantic studied by Knaff and Kossin, all exhibited easterly winds and cold air in the upper troposphere. In addition to strong outflow, suggesting that the conditions that give rise to annular tropical cyclones are most optimal towards the equatorward side of a subtropical ridge and within the tropics. However, warmer sea surface temperatures (SSTs) are not required for annular tropical cyclones, with annular characteristics developing only within a narrow range of modest SSTs, ranging from 25.4–28.5 °C (77.7–83.3 °F).[1]

Conditions favorable for annular typhoon development in the Western North Pacific are localized within two areas within a zonal belt between 20°N−30°N; one of these areas lies over the central part of the basin, while the other is located east of Taiwan.[4] Within the Eastern North Pacific, such conditions were present only 3 percent of the time between 1998 and 1999. In the same timeframe, the North Atlantic basin only exhibited conducive conditions for annular development 0.8 percent of the time.[1]

References

1. Knaff, John A.; Kossin, James P. (April 2003). "Annular Hurricanes". Weather and Forecasting. 18 (2): 204–223. Bibcode:2003WtFor..18..204K. doi:10.1175/1520-0434(2003)018<0204:AH>2.0.CO;2.
2. ^ Montgomery, Michael T. (2014). Advances in Tropical Cyclone Research: Chapter 21: Introduction to Hurricane Dynamics: Tropical Cyclone Intensification (PDF). Naval Postgraduate School. Retrieved 18 May 2019.
3. ^ Knaff, John A.; Cram, T.A.; Schumacher, A.B.; Kossin, J.P.; DeMaria, M. (February 2008). "Objective Identification of Annular Hurricanes". Weather and Forecasting. 23 (1): 17–28. Bibcode:2008WtFor..23...17K. CiteSeerX 10.1.1.533.5293. doi:10.1175/2007WAF2007031.1.
4. ^ a b Chu, Kekuan; Tan, Zhe-Min (April 2014). "Annular Typhoons in the Western North Pacific" (PDF). Weather and Forecasting. Boston, Massachusetts: American Meteorological Society. 29 (2): 241–251. doi:10.1175/WAF-D-13-00060.1. Retrieved 19 August 2019.
Anticyclonic storm

An anticyclonic storm is a weather storm where winds around the storm flow in the direction opposite to that of the flow above a region of low pressure.

An anticyclonic tornado is a tornado which rotates in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. The term is a naming convention denoting the anomaly from normal rotation which is cyclonic in upwards of 98 percent of tornadoes. Many anticyclonic tornadoes are smaller and weaker than cyclonic tornadoes, forming from a different process, as either companion/satellite tornadoes or nonmesocyclonic tornadoes.

Azores High

The Azores High (Portuguese: Anticiclone dos Açores) also known as North Atlantic (Subtropical) High/Anticyclone or the Bermuda-Azores High, is a large subtropical semi-permanent centre of high atmospheric pressure typically found south of the Azores in the Atlantic Ocean, at the Horse latitudes. It forms one pole of the North Atlantic oscillation, the other being the Icelandic Low. The system influences the weather and climatic patterns of vast areas of North Africa and southern Europe, and to a lesser extent, eastern North America. The aridity of the Sahara Desert and the summer drought of the Mediterranean Basin is due to the large-scale subsidence and sinking motion of air in the system. In its summer position (the Bermuda High), the high is centered near Bermuda, and creates a southwest flow of warm tropical air toward the East Coast of the United States. In summer, the Azores-Bermuda High is strongest. The central pressure hovers around 1024 mbar (hPa).

This high-pressure block exhibits anticyclonic nature, circulating the air clockwise. Due to this direction of movement, African eastern waves are impelled along the southern periphery of the Azores High away from coastal West Africa towards the Caribbean, Central America, or the Bahamas, favouring tropical cyclogenesis, especially during the hurricane season.

Bar (tropical cyclone)

The bar of a mature tropical cyclone is a very dark gray-black layer of cloud appearing near the horizon as seen from an observer preceding the approach of the storm, and is composed of dense stratocumulus clouds. Cumulus and cumulonimbus clouds bearing precipitation follow immediately after the passage of the wall-like bar. Altostratus, cirrostratus and cirrus clouds are usually visible in ascending order above the top of the bar, while the wind direction for an observer facing toward the bar is typically from the left and slightly behind the observer.

Cyclone Jasmine

Severe Tropical Cyclone Jasmine (RSMC Nadi designation: 12F, JTWC designation: 10P) was a powerful and long-lived annular tropical cyclone that affected several countries, particularly Vanuatu and Tonga, over a 16-day span in February 2012. The system was the second cyclone and the only severe tropical cyclone of the relatively quiet 2011–12 South Pacific cyclone season. Cyclone Jasmine developed from an area of disturbed weather on 1 February in the Gulf of Carpentaria. Initially, the storm moved towards the east and across the Cape York Peninsula. As it moved across the South Pacific, earlier existing wind shear conditions lessened, and Jasmine began to strengthen at a faster rate. Steadily intensifying, Jasmine reached peak intensity on 8 February as a Category 4 equivalent on the Saffir–Simpson Hurricane Scale, while beginning to show annular characteristics.

The next day Jasmine entered an area of vertical wind shear, which consequently weakened the cyclone and caused its eye to expand. A high-pressure area south of Jasmine later steered the weakening cyclone to the northeast on 12 February. Although it entered an area of warmer sea surface temperatures, Jasmine subsequently entered extratropical transition and later degenerated into an extratropical cyclone on 16 February, and later dissipated completely on 19 February.

Cyclone Jasmine affected five countries during its existence. The predecessor to Jasmine brought heavy rainfall to areas of extreme northern Queensland. Jasmine also brought rainfall to areas of the Solomon Islands. As a result, pest infestations occurred across the region. In Vanuatu, heavy rains and wind from Jasmine destroy numerous crops. Banana trees in particular are affected by the cyclone. Jasmine inundated areas of Tonga that had already been affected by Cyclone Cyril just a week prior. Nuku'alofa recorded half of its average monthly rainfall in a 24‑hour span due to rains associated with the cyclone. After the season, the name Jasmine was retired from the Australian list of tropical cyclone names.

Hypercane

A hypercane is a hypothetical class of extreme tropical cyclone that could form if ocean temperatures reached approximately 50 °C (122 °F), which is 15 °C (27 °F) warmer than the warmest ocean temperature ever recorded. Such an increase could be caused by a large asteroid or comet impact, a large supervolcanic eruption, a large submarine flood basalt, or extensive global warming. There is some speculation that a series of hypercanes resulting from an impact by a large asteroid or comet contributed to the demise of the non-avian dinosaurs. The hypothesis was created by Kerry Emanuel of MIT, who also coined the term.

Kalahari High

The Kalahari High is an anticyclone that forms in winter over the interior of southern Africa, replacing a summer trough. It is part of the subtropical ridge system and the reason the Kalahari is a desert. It is the descending limb of a Hadley cell.

Kona storm

Kona storms (also called Kona lows) are a type of seasonal cyclone in the Hawaiian Islands, usually formed in the winter from winds coming from the westerly "kona" (normally leeward) direction. They are mainly cold core cyclones, which places them in the extratropical cyclone rather than the subtropical cyclone category. Hawaii typically experiences two to three annually, which can affect the state for a week or more. Among their hazards are heavy rain, hailstorms, flash floods and their associated landslides, high elevation snow, high winds which result in large surf and swells, and waterspouts.

Landspout

Landspout is a term created by atmospheric scientist Howard B. Bluestein in 1985 for a kind of tornado not associated with a mesocyclone. The Glossary of Meteorology defines a landspout as

"Colloquial expression describing tornadoes occurring with a parent cloud in its growth stage and with its vorticity originating in the boundary layer.

The parent cloud does not contain a preexisting mid-level mesocyclone. The landspout was so named because it looks like "a weak Florida Keys waterspout over land."

Line echo wave pattern

A line echo wave pattern (LEWP) is a weather radar formation in which a single line of thunderstorms presenting multiple bow echoes forms south (or equatorward) of a mesoscale low-pressure area with a rotating "head". LEWP often are associated with a multiple-bow serial derecho and often produce tornadoes, some of which can be strong. The existence of a LEWP on radar means that a serial derecho has developed or is likely to develop soon, much as a hook echo indicates the same for a tornado.

Mesohigh

A mesohigh (sometimes called a "bubble high") is a mesoscale high-pressure area that forms beneath thunderstorms. While not always the case, it is usually associated with a mesoscale convective system. In the early stages of research on the subject, the mesohigh was often referred to as a "thunderstorm high".

Mesoscale meteorology

Mesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers. Examples of mesoscale weather systems are sea breezes, squall lines, and mesoscale convective complexes.

Vertical velocity often equals or exceeds horizontal velocities in mesoscale meteorological systems due to nonhydrostatic processes such as buoyant acceleration of a rising thermal or acceleration through a narrow mountain pass.

A multiple-vortex tornado is a tornado that contains several vortices (called subvortices or suction vortices) rotating around, inside of, and as part of the main vortex. The only times multiple vortices may be visible are when the tornado is first forming or when condensation and debris are balanced such that subvortices are apparent without being obscured. They can add over 100 mph to the ground-relative wind in a tornado circulation, and are responsible for most (if not all) cases where narrow arcs of extreme destruction lie right next to weak damage within tornado paths.

North Pacific High

The North Pacific High is a semi-permanent, subtropical anticyclone located in the northeastern portion of the Pacific Ocean, located northeast of Hawaii and west of California. It is strongest during the northern hemisphere summer and shifts towards the equator during the winter, when the Aleutian Low becomes more active. It is responsible for California's typically dry summer and fall and typically wet winter and spring, as well as Hawaii's year-round trade winds.During the 2011–2017 California drought, the North Pacific High persisted longer than usual, due to a mass of warm water in the Pacific Ocean, resulting in the Ridiculously Resilient Ridge. This significantly limited the number of powerful winter storms that were able to reach California, resulting in historic drought conditions in that state for several years.

Polar High

The polar highs are areas of high atmospheric pressure around the north and south poles; the north polar high being the stronger one because land gains and loses heat more effectively than sea. The cold temperatures in the polar regions cause air to descend to create the high pressure (a process called subsidence), just as the warm temperatures around the equator cause air to rise to create the low pressure intertropical convergence zone. Rising air also occurs along bands of low pressure situated just below the polar highs around the 50th parallels of latitude. These extratropical convergence zones are occupied by the polar fronts where air masses of polar origin meet and clash with those of tropical or subtropical origin. This convergence of rising air completes the vertical cycle around the polar cell in each latitudinal hemisphere. Closely related to this concept is the polar vortex.

Surface temperatures under the polar highs are the coldest on Earth, with no month having an average temperature above freezing. Regions under the polar high also experience very low levels of precipitation, which leads them to be known as "polar deserts".

Air flows outwards from the poles to create the polar easterlies in the arctic and antarctic areas.

Rapid intensification

Rapid intensification is a meteorological condition that occurs when a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period.

South Pacific High

The South Pacific High is a semi-permanent subtropical anticyclone located in the southeast Pacific Ocean. The area of high atmospheric pressure and the presence of the Humboldt Current in the underlying ocean make the west coast of Peru and northern Chile extremely arid. The Sechura and Atacama deserts, as the whole climate of Chile, are heavily influenced by this semi-permanent high-pressure area. This high-pressure system plays a major role in the El Niño–Southern Oscillation (ENSO), and it is also a major source of trade winds across the equatorial Pacific.

Superstorm

A superstorm is a large, unusually-occurring, destructive storm without another distinct meteorological classification, such as hurricane or blizzard. As the term is of recent coinage and lacks a formal definition, there is some debate as to its usefulness.

Whirlwind

A whirlwind is a weather phenomenon in which a vortex of wind (a vertically oriented rotating column of air) forms due to instabilities and turbulence created by heating and flow (current) gradients. Whirlwinds occur all over the world and in any season.

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