Eye (cyclone)

The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is usually circular and typically 25–40 miles (40–65 km) in diameter. It is surrounded by the eyewall, where the most severe weather of a cyclone occurs. The cyclone's lowest barometric pressure occurs in the eye, and can be as much as 15% lower than the atmospheric pressure outside of the storm.

Basic definitions

The eye is possibly the most recognizable feature of tropical cyclones. In strong tropical cyclones, the eye is a roughly-circular area of light winds and clear skies, surrounded on all sides by a towering vertical wall of thunderstorms, known as the eyewall. In weaker tropical cyclones, the eye is less defined, and can be covered by the central dense overcast, which is an area of high, thick clouds which show up brightly on satellite pictures. Rain may even fall heavily in the eye of a disorganized storm. In all storms, however, the eye is the location of the storm's minimum barometric pressure: the area where the atmospheric pressure at sea level is the lowest.

Structure

A typical tropical cyclone will have an eye approximately 25 mi (40 km) across, situated at the geometric center of the storm. The eye may be clear or have spotty low clouds (a clear eye), it may be filled with low- and mid-level clouds (a filled eye), or it may be obscured by the central dense overcast. There is, however, very little wind and rain, especially near the center. This is in stark contrast to conditions in the eyewall, which contains the storm's strongest winds, and highest seas.

While normally quite symmetric, eyes can be oblong and irregular, especially in weakening storms. A large ragged eye is a non-circular eye which appears fragmented, and is an indicator of a weak or weakening tropical cyclone. An open eye is an eye which can be circular, but the eyewall does not completely encircle the eye, also indicating a weakening, moisture-deprived cyclone.

While typical mature storms have eyes that are a few dozen miles across, rapidly intensifying storms can develop an extremely small, clear, and circular eye, referred to as a pinhole eye. Storms with pinhole eyes are prone to large fluctuations in intensity, and provide difficulties and frustrations for forecasters.

Pinhole eyes often trigger eyewall replacement cycles, where a new eyewall begins to form outside the original eyewall. This can take place anywhere from ten to a few hundred miles (fifteen to hundreds of kilometers) outside of the inner eye. This results in the storm having two concentric eyewalls, or an "eye within an eye". In most cases, the outer eyewall contracts soon after its formation, choking off the inner eye, and creating a much larger, but stable eye. While this process tends to weaken storms as it occurs, the new eyewall can contract fairly quickly after the old eyewall dissipates, causing the storm to re-strengthen and the process to repeat.

Because of cycles such as these, eyes can range in size from 200 miles (320 km) (Typhoon Carmen) to a mere two miles (3 km) (Hurricane Wilma) across. While it is very uncommon for storms with large eyes to become very intense, it does occur, especially in annular hurricanes. Hurricane Isabel was the eleventh most powerful Atlantic hurricane of all time, and sustained a large, 40–50 mile (65–80 km)-wide eye for a period of several days.

Formation

Tropical cyclones typically form from large, disorganized areas of disturbed weather in tropical regions. As more thunderstorms form and gather, the storm develops rainbands which start rotating around a common center. As the storm gains strength, for reasons yet unknown to scientists, a ring of stronger convection forms at a certain distance from the rotational center of the developing storm. Since stronger thunderstorms and heavier rain mark areas of stronger updrafts, air begins to build up in the upper levels of the cyclone. This results in the formation of an upper level anticyclone, or an area of high atmospheric pressure above the central dense overcast. Consequentially, most of this built up air flows outward anticyclonically above the tropical cyclone.

However, again for reasons unknown, a small portion of the built-up air, instead of flowing outward, flows inward towards the center of the storm. This causes air pressure to build even further, to the point where the weight of the air counteracts the strength of the updrafts in the center of the storm. Air begins to descend in the center of the storm, creating a mostly rain-free area; a newly-formed eye. Jonathan Vigh (2006). "Formation of the Hurricane Eye". Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

Detection

The formation of an eye is almost always an indicator of increasing tropical cyclone organisation and strength. Because of this, forecasters watch developing storms closely for signs of eye formation.

For storms with a "clear eye", detection of the eye is as simple as looking at pictures from a weather satellite. However, for storms with a filled eye, or an eye completely covered by the central dense overcast, other detection methods must be used. Observations from ships and Hurricane Hunters can pinpoint an eye visually, by looking for a drop in wind speed or lack of rainfall. In the United States, a network of NEXRAD Doppler radar can detect storms forming near the coast. Weather satellites also carry equipment for measuring atmospheric water vapor and cloud temperatures, which can be used to spot a forming eye. In addition, scientists have recently discovered that the amount of ozone in the eye is much higher than the amount in the eyewall, due to air sinking from the ozone-rich stratosphere.

Unknowns

As stated in previous sections, there are many aspects of the eye which remain a mystery, the most pressing of which is the reason that the eye forms at all. Hundreds of theories exist as to the exact process by which the eye forms: all that is known for sure is that the eye is necessary for tropical cyclones to achieve high wind speeds.

Associated phenomena

Eyewall replacement cycles

Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 115 mph (185 km/h). As tropical cyclones approach and reach this threshold of intensity, the eyewall contracts and they can develop a pinhole eye (see above). When this occurs, some of the outer rainbands may organize into a ring of thunderstorms—an outer eyewall—that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. As the strongest winds are located in a cyclone's eyewall, the tropical cyclone usually weakens during this phase, as the inner wall is "choked" by the outer wall. Eventually the outer eyewall replaces the inner one completely, and the storm will most likely re-intensify. Hurricane Allen in 1980 went through repeated eyewall replacement cycles, fluctuating between Category 5 and Category 3 status on the Saffir-Simpson Scale several times. Hurricane Juliette (2001) was a rare documented case of triple eyewalls.

The discovery of this process was partially responsible for the end of the U.S. Governments's hurricane modification experiment Project Stormfury. This project set out to seed clouds outside of the eyewall, causing a new eyewall to form and weakening the storm. When it was discovered that this was a natural process due to hurricane dynamics, the project was quickly abandoned.

Moats

A moat in a tropical cyclone is a clear ring outside the eyewall, or between concentric eyewalls, characterized by slowly sinking air, little or no precipitation, and strain-dominated flow . The moat between eyewalls is just one example of a rapid filamentation zone; such strain-dominated regions can potentially be found near any vortex of sufficient strength.

Eyewall mesovortices

Eyewall mesovortices are small scale rotational features found in the eyewalls of intense tropical cyclones. They are very similar to small vortices often observed in tornadoes. In these vortices, wind speed can be up to 10% higher than in the typical eyewall. Eyewall mesovortices are most common during periods of intensification in tropical cyclones.

Eyewall mesovortices often exhibit unusual behavior in tropical cyclones. They usually rotate around the low pressure center, but sometimes they remain stationary. Eyewall mesovortices have even been documented to cross the eye of a storm. These phenomena have been documented observationally, experimentally, and theoretically.

Eyewall mesovortices are a significant factor in the formation of tornadoes after tropical cyclone landfall. Mesovortices can spawn rotation in thunderstorms, which leads to tornadic activity. At landfall, friction is generated between the circulation of the tropical cyclone and land. This can allow the mesovortices to descend to the surface, causing large outbreaks of tornadoes.

Stadium effect

The stadium effect is a phenomenon observed in tropical cyclones with eyes. It is a common event, where the clouds of the eyewall curve outward from the surface with height. This gives the eye an appearance resembling an open dome from the air, akin to a sports stadium. An eye is always larger at the top of the storm, and smallest at the bottom of the storm because the rising air in the eyewall follows isolines of angular momentum, which also slope outward with height.

Hazards

Though the eye is by far the calmest part of the storm, with no wind at the center and typically clear skies, over the ocean it is possibly the most hazardous area. In the eyewall, wind-driven waves are all traveling in the same direction. In the center of the eye, however, waves from all directions converge, creating erratic waves that can build on each other. The maximum height of typical hurricane waves is unknown, but new research indicates that typical hurricanes may have wave heights approaching 100 feet (33 m). Bjorn Carey(2005). "Hurricane's Waves Soared to Nearly 100 Feet". This is in addition to any storm surge which may occur, as storm surges often extend into the eye.

Other storms

Though only tropical cyclones have structures that are officially called "eyes", there are other storms which can exhibit eye-like structures:

Polar lows

Polar lows are mesoscale weather systems (typically smaller than 600 miles or 1000 km across) found near the poles. Like tropical cyclones, they form over water, and can feature deep convection, and feature winds of gale force or greater (> 31 mph, 50 km/h). Unlike storms of tropical nature, however, they thrive in much colder temperatures and at much higher latitudes. Despite these differences, they can be very similar in structure to tropical cyclones, featuring a clear eye surrounded by an eyewall and rain/snow bands.

Extratropical storms

Extratropical storms are areas of low pressure which exist at the boundary of different air masses. Almost all storms found at mid-latitudes are extratropical in nature, including classic North American nor'easters and European windstorms. The most severe of these can have a clear "eye" at the site of lowest barometric pressure, though it is usually surrounded by lower, non-convective clouds and is found near the back end of the storm.

Subtropical storms

Subtropical storms are cyclones which have some extratropical characteristics and some tropical characteristics. As such, they may have an eye but cannot be true tropical storms.

Tornadoes

Tornadoes are destructive, small-scale storms, which produce the fastest winds on earth. There are two main types—single-vortex tornadoes, which consist of a single spinning column of air, and multiple-vortex tornadoes, which consist of small suction vortices, resembling mini-tornadoes themselves, all rotating around a common center. Both of these types of tornadoes are theorized to have calm centers, referred to by some meteorologists as "eyes".

References

External links

Tropical cyclone meteorology

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