Eye (cyclone)
The eye is a region of mostly calm weather at the center of a tropical cyclone. The eye of a storm is a roughly circular area, typically in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather and highest winds of the cyclone occur. The cyclone's lowest barometric pressure occurs in the eye and can be as much as 15 percent lower than the pressure outside the storm.
In strong tropical cyclones, the eye is characterized by light winds and clear skies, surrounded on all sides by a towering, symmetric eyewall. In weaker tropical cyclones, the eye is less well defined and can be covered by the central dense overcast, an area of high, thick clouds that show up brightly on satellite imagery. Weaker or disorganized storms may also feature an eyewall that does not completely encircle the eye or have an eye that features heavy rain. In all storms, however, the eye is where the barometer reading is lowest.
Structure
A typical tropical cyclone has an eye approximately 30–65km across at the geometric center of the storm. The eye may be clear or have spotty low clouds, it may be filled with low- and mid-level clouds, 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. Due to the mechanics of a tropical cyclone, the eye and the air directly [|above] it are warmer than their surroundings.File:Winnie 1997-08-17 0600Z.png|thumb|Typhoon Winnie is tied with Typhoon Carmen for the largest eye on record at.
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 or a weak but strengthening one. Both of these observations are used to estimate the intensity of tropical cyclones via Dvorak analysis. Eyewalls are typically circular; however, distinctly polygonal shapes ranging from triangles to hexagons occasionally occur.
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, sometimes referred to as a pinhole eye. Storms with pinhole eyes are prone to large fluctuations in intensity, and provide difficulties and frustrations for forecasters.
Small/minuscule eyesthose less than ten nautical miles acrossoften trigger [|eyewall replacement cycles], where a new eyewall begins to form outside the original eyewall. This can take place anywhere from fifteen to hundreds of kilometers outside the inner eye. The storm then develops two concentric eyewalls, or an "eye within an eye". In most cases, the outer eyewall begins to contract soon after its formation, which chokes off the inner eye and leaves a much larger but more stable eye. While the replacement cycle tends to weaken storms as it occurs, the new eyewall can contract fairly quickly after the old eyewall dissipates, allowing the storm to re-strengthen. This may be followed by another cycle of eyewall replacement.
Eyes can range in size from to a mere across. While it is uncommon for storms with large eyes to become very intense, it does occur, especially in annular hurricanes. Hurricane Isabel was the eleventh most powerful North Atlantic hurricane in recorded history, and sustained a wide65–80km eye for a period of several days.
Formation and detection
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, 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, the barometric pressure at the surface begins to drop, and 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. Consequently, most of this built up air flows outward anticyclonically above the tropical cyclone. Outside the forming eye, the anticyclone at the upper levels of the atmosphere enhances the flow towards the center of the cyclone, pushing air towards the eyewall and causing a positive feedback loop.However, 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 areaa newly formed eye.
Many aspects of this process remain a mystery. Scientists do not know why a ring of convection forms around the center of circulation instead of on top of it, or why the upper-level anticyclone ejects only a portion of the excess air above the storm. Many 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.
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 storm's center. In the United States, South Korea, and a few other countries, a network of NEXRAD Doppler weather radar stations can detect eyes 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. Instruments sensitive to ozone perform measurements, which are used to observe rising and sinking columns of air, and provide indication of the formation of an eye, even before satellite imagery can determine its formation.
Approximately 60% of Atlantic tropical cyclones form eyes. One satellite study found eyes detected on average for 30 hours per storm.
Associated phenomena
Eyewall replacement cycles
Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185km/h, or major hurricanes. When tropical cyclones reach this intensity, and the eyewall contracts or is already sufficiently small, some of the outer rainbands may strengthen and organize into a ring of thunderstormsan outer eyewallthat slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. Since 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 can re-intensify.The discovery of this process was partially responsible for the end of the U.S. government's hurricane modification experiment Project Stormfury. This project set out to seed clouds outside 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.
Research shows that 53 percent of intense hurricanes undergo at least one of these cycles during its existence. Hurricane Allen in 1980 went through repeated eyewall replacement cycles, fluctuating between Category5 and Category4 status on the Saffir–Simpson scale several times, while Hurricane Juliette is a documented case of triple eyewalls.
Moats
A moat in a tropical cyclone is a clear ring outside the eyewall, or between concentric eyewalls, characterized by subsidence and little or no precipitation. The air flow in the moat is dominated by the cumulative effects of stretching and shearing. The moat between eyewalls is an area in the storm where the rotational speed of the air changes greatly in proportion to the distance from the storm's center; these areas are also known as rapid filamentation zones. Such areas can potentially be found near any vortex of sufficient strength, but are most pronounced in strong tropical cyclones.Eyewall mesovortices
Eyewall mesovortices are small scale rotational features found in the eyewalls of intense tropical cyclones. They are similar, in principle, to small "suction vortices" often observed in multiple-vortex tornadoes. In these vortices, wind speeds may be greater than anywhere else in the eyewall. Eyewall mesovortices are most common during periods of intensification in tropical cyclones.Eyewall mesovortices often exhibit unusual behavior in tropical cyclones. They usually revolve 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 individual convective cells or updrafts, 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 tornadoes. These tornadic circulations in the boundary layer may be prevalent in the inner eyewalls of intense tropical cyclones but with short duration and small size they are not frequently observed.