Eyewall replacement cycle

Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 km/h (115 mph), or major hurricanes (Category 3 or above). 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 thunderstorms—an outer eyewall—that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. Since the strongest winds are 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 may 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, apparently 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.

Almost every intense hurricane undergoes at least one of these cycles during its existence. Recent studies have shown that nearly half of all tropical cyclones, and nearly all cyclones with sustained winds over 204 kilometres per hour (127 mph; 110 kn), undergo eyewall replacement cycles.Hurricane Allen in 1980 went through repeated eyewall replacement cycles, fluctuating between Category 5 and Category 3 status on the Saffir-Simpson Hurricane Scale several times. Hurricane Juliette (2001) was a rare documented case of triple eyewalls.Typhoon June (1975) was the first reported case of triple eyewalls.

The first tropical system to be observed with concentric eyewalls was Typhoon Sarah by Fortner in 1956, which he described as "an eye within an eye". The storm was observed by a reconnaissance aircraft to have an inner eyewall at 6 kilometres (3.7 mi) and an outer eyewall at 28 kilometres (17 mi). During a subsequent flight 8 hours later, the inner eyewall had disappeared, the outer eyewall had reduced to 16 kilometres (9.9 mi) and the maximum sustained winds and hurricane intensity had decreased. The next hurricane observed to have concentric eyewalls was Hurricane Donna in 1960. Radar from reconnaissance aircraft showed an inner eye that varied from 10 miles (16 km) at low altitude to 13 miles (21 km) near the tropopause. In between the two eyewalls was an area of clear skies that extended vertically from 3,000 feet (910 m) to 25,000 feet (7,600 m). The low-level clouds at around 3,000 feet (910 m) were described as stratocumulus with concentric horizontal rolls. The inner eyewall was reported to reach heights near 45,000 feet (14,000 m) while the inner eyewall only extended to 30,000 feet (9,100 m). 12 hours after identifying a concentric eyewalls, the inner eyewall had dissipated.

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