Eyewall replacement cycle
In meteorology, 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 are already small, some of the outer rainbands may strengthen and organize into a ring of thunderstorms—a new, outer eyewall—that slowly moves inward and robs the original, inner eyewall of its needed moisture and angular momentum. Since the strongest winds are in a tropical cyclone's eyewall, the storm 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.[1]
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.[2]
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.[3] Hurricane Allen in 1980 went through repeated eyewall replacement cycles, fluctuating between Category 5 and Category 4 status on the Saffir-Simpson Hurricane Scale several times. Typhoon June (1975) was the first reported case of triple eyewalls,[4] and Hurricane Juliette and Iris (2001) were documented cases of such.[5][6]
Secondary eyewalls[edit]
Identification[edit]
Qualitatively identifying secondary eyewalls is easy for a hurricane analyst to do. It involves looking at satellite or radar imagery and seeing if there are two concentric rings of enhanced convection. The outer eyewall is generally almost circular and concentric with the inner eyewall. Quantitative analysis is more difficult since there exists no objective definition of what a secondary eyewall is. Kossin et al. specified that the outer ring had to be visibly separated from the inner eye with at least 75% closed with a moat region clear of clouds.[12]
While secondary eyewalls have been seen as a tropical cyclone is nearing land, none have been observed while the eye is not over the ocean. Changes in the intensity of strong hurricanes such as Katrina, Ophelia, and Rita occurred simultaneously with eyewall replacement cycles and comprised interactions between the eyewalls, rainbands and outside environments.[12][13] Eyewall replacement cycles, such as occurred in Rita as it approached the Gulf Coast of the United States, can greatly increase the size of tropical cyclones while simultaneously decreasing in strength.[14]
During the period from 1997–2006, 45 eyewall replacement cycles were observed in the tropical North Atlantic Ocean, 12 in the Eastern North Pacific and 2 in the Western North Pacific. 12% of all Atlantic storms and 5% of storms in the Pacific underwent eyewall replacement during this time period. In the North Atlantic, 70% of major hurricanes had at least one eyewall replacement, compared to 33% of all storms. In the Pacific, 33% of major hurricanes and 16% of all hurricanes had an eyewall replacement cycle. Stronger storms have a higher probability of forming a secondary eyewall, with 60% of category 5 hurricanes undergoing an eyewall replacement cycle within 12 hours.[12]
During the years 1969-1971, 93 storms reached tropical storm strength or greater in the Pacific Ocean. 8 of the 15 that reached super typhoon strength (65 m/s), 11 of the 49 storms that reached typhoon strength (33 m/s), and none of the 29 tropical storms (<33 m/s) developed concentric eyewalls. The authors note that because the reconnaissance aircraft were not specifically looking for double eyewall features, these numbers are likely underestimates.[3]
During the years 1949-1983, 1268 typhoons were observed in the Western Pacific. 76 of these had concentric eyewalls. Of all the typhoons that underwent eyewall replacement, around 60% did so only once; 40% had more than one eyewall replacement cycle, with two of the typhoons each experiencing five eyewall replacements. The number of storms with eyewall replacement cycles was strongly correlated with the strength of the storm. Stronger typhoons were much more likely to have concentric eyewalls. There were no cases of double eyewalls where the maximum sustained wind was less than 45 m/s or the minimum pressure was higher than 970 hPa. More than three-quarters of the typhoons that had pressures lower than 970 hPa developed the double eyewall feature. The majority of Western and Central Pacific typhoons that experience double eyewalls do so in the vicinity of Guam.[4]