Eyewall replacement cycle
In meteorology, an eyewall replacement cycle, also called a concentric eyewall cycle, is a process whereby some of the outer rainbands of a tropical cyclone with an eye 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.
They naturally occur in intense tropical cyclones with maximum sustained winds greater than, or hurricane-force, and particularly in major hurricanes of Saffir–Simpson category 3 to 5.
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, undergo eyewall replacement cycles. Hurricane Allen in 1980 went through repeated eyewall replacement cycles, fluctuating between Category 5 and Category 4 status on the Saffir–Simpson scale several times. Typhoon June was the first reported case of triple eyewalls, and Hurricane Juliette and Iris were documented cases of such.
History
The first tropical system to be observed to have 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 and an outer eyewall at. During a subsequent flight 8 hours later, the inner eyewall had disappeared, the outer eyewall had reduced to 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 at low altitude to near the tropopause. In between the two eyewalls was an area of clear skies that extended vertically from to. The low-level clouds at around were described as stratocumulus with concentric horizontal rolls. The outer eyewall was reported to reach heights near while the inner eyewall only extended to. 12 hours after identifying concentric eyewalls, the inner eyewall had dissipated.Hurricane Beulah in 1967 was the first tropical cyclone to have its eyewall replacement cycle observed from beginning to end. Previous observations of concentric eyewalls were from aircraft-based platforms. Beulah was observed from the Puerto Rico land-based radar for 34 hours during which time a double eyewall formed and dissipated. It was noted that Beulah reached maximum intensity immediately prior to undergoing the eyewall replacement cycle, and that it was "probably more than a coincidence." Previous eyewall replacement cycles had been observed to decrease the intensity of the storm, but at this time the dynamics of why it occurred was not known.
As early as 1946 it was known that the introduction of carbon dioxide ice or silver iodide into clouds that contained supercooled water would convert some of the droplets into ice followed by the Bergeron–Findeisen process of growth of the ice particles at the expense of the droplets, the water of which would all end up in large ice particles. The increased rate of precipitation would result in dissipation of the storm. By early 1960, the working theory was that the eyewall of a hurricane was inertially unstable and that the clouds had a large amount of supercooled water. Therefore, seeding the storm outside the eyewall would release more latent heat and cause the eyewall to expand. The expansion of the eyewall would be accompanied with a decrease in the maximum wind speed through conservation of angular momentum.
Project Stormfury
Project Stormfury was an attempt to weaken tropical cyclones by flying aircraft into them and seeding with silver iodide. The project was run by the United States Government from 1962 to 1983.The hypothesis was that the silver iodide would cause supercooled water in the storm to freeze, disrupting the inner structure of the hurricane. This led to the seeding of several Atlantic hurricanes. However, it was later shown that this hypothesis was incorrect. In reality, it was determined, most hurricanes do not contain enough supercooled water for cloud seeding to be effective. Additionally, researchers found that unseeded hurricanes often undergo the eyewall replacement cycles that were expected from seeded hurricanes. This finding called Stormfury's successes into question, as the changes reported now had a natural explanation.
The last experimental flight was flown in 1971, due to a lack of candidate storms and a changeover in NOAA's fleet. More than a decade after the last modification experiment, Project Stormfury was officially canceled. Although Project Stormfury did not achieve its goal in reducing the destructiveness of hurricanes, the observational data and storm lifecycle research generated by Stormfury helped improve meteorologists' ability to forecast the movement and intensity of future hurricanes.
Secondary eyewalls
Identification
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.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. 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 their strength.
During the period from 1997 to 2006, 45 eyewall replacement cycles were observed in the tropical North Atlantic Ocean, 12 in the Eastern North Pacific and two 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.
During the years 1969–1971, 93 storms reached tropical storm strength or greater in the Pacific Ocean. Eight of the 15 that reached super typhoon strength, 11 of the 49 storms that reached typhoon strength, and none of the 29 tropical storms developed concentric eyewalls. The authors note that because the reconnaissance aircraft were not specifically looking for double eyewall features, these numbers are likely underestimates.
During the years 1949–1983, 1268 typhoons were observed in the Western Pacific. Seventy-six 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.
Multiple secondary eyewalls
The formation of more than one secondary eyewall at the same time is a rare occurrence; two secondary eyewalls and a primary eyewall are referred to as triple eyewalls. Typhoon June was the first reported case of triple eyewalls, and Hurricane Juliette and Iris were documented cases of such.Secondary eyewall formation
Secondary eyewalls were once considered a rare phenomenon. Since the advent of reconnaissance airplanes and microwave satellite data, it has been observed that over half of all major tropical cyclones develop at least one secondary eyewall. There have been many hypotheses that attempt to explain the formation of secondary eyewalls. The reason why hurricanes develop secondary eyewalls is not well understood.Early formation hypotheses
Since eyewall replacement cycles were discovered to be natural, there has been a strong interest in trying to identify what causes them. There have been many hypotheses put forth that are now abandoned. In 1980, Hurricane Allen crossed the mountainous region of Haiti and simultaneously developed a secondary eyewall. Hawkins noted this and hypothesized that the secondary eyewall may have been caused by topographic forcing. Willoughby suggested that a resonance between the inertial period and asymmetric friction may be the cause of secondary eyewalls. Later modeling studies and observations have shown that outer eyewalls may develop in areas uninfluenced by land processes.There have been many hypotheses suggesting a link between synoptic scale features and secondary eyewall replacement. It has been observed that radially inward traveling wave-like disturbances have preceded the rapid development of tropical disturbances to tropical cyclones. It has been hypothesized that this synoptic scale internal forcing could lead to a secondary eyewall. Rapid deepening of the tropical low in connection with synoptic scale forcing has been observed in multiple storms, but has been shown to not be a necessary condition for the formation of a secondary eyewall. The wind-induced surface heat exchange is a positive feedback mechanism between the ocean and atmosphere in which a stronger ocean-to-atmosphere heat flux results in a stronger atmospheric circulation, which results in a strong heat flux. WISHE has been proposed as a method of generating secondary eyewalls. Later work has shown that while WISHE is a necessary condition to amplify disturbances, it is not needed to generate them.