Eyewall replacement cycle

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 with a diameter of 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. Radar from reconnaissance aircraft showed an inner eye that varied from in diameter 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. 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." 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. 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 undergoing an eyewall replacement cycle while approaching the northeastern coast of Taiwan 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. 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. == Secondary eyewall formation ==

shows the beginning of an eyewall replacement cycle in Hurricane Frances. 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. 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. β-skirt axisymmetrization hypothesis In a fluid system, β (beta) is the spatial, usually horizontal, change in the environmental vertical vorticity. β is maximized in the eyewall of a tropical cyclone. The β-skirt axisymmetrization (BSA) assumes that a tropical cyclone about to develop a secondary eye will have a decreasing, but non-negative β that extends from the eyewall to approximately to from the eyewall. In this region, there is a small, but important β. This area is called the β-skirt. Outward of the skirt, β is effectively zero. == Death of the inner eyewall ==

When the secondary eyewall totally surrounds the inner eyewall, it begins to affect the tropical cyclone dynamics. Tropical cyclones are fuelled by the high ocean temperature. Sea surface temperatures immediately underneath a tropical cyclone can be several degrees cooler than those at the periphery of a storm; cyclones depend on receiving energy from the ocean transported by the inward spiralling winds. When an outer eyewall is formed, the moisture and angular momentum necessary for the maintenance of the inner eyewall is now being used to sustain the outer eyewall, causing the inner eye to weaken and dissipate, leaving the tropical cyclone with one eye that is larger in diameter than the previous eye. revealing the moat between the inner and outer eyewalls. In the moat region between the inner and outer eyewall, observations by dropsondes have shown high temperatures and dewpoint depressions. The eyewall contracts because of inertial instability. Compared to the processes involved with the formation of the secondary eyewall, the death of the inner eyewall is fairly well understood. Some tropical cyclones with extremely large outer eyewalls do not experience the contraction of the outer eye and subsequent dissipation of the inner eye. Typhoon Winnie (1997) developed an outer eyewall with a diameter of that did not dissipate until it reached the shoreline. The time required for the eyewall to collapse is inversely related to the diameter of the eyewall which is mostly because inward directed wind decreases asymptotically to zero with distance from the radius of maximum winds, but also due to the distance required to collapse the eyewall. Throughout the entire vertical layer of the moat, there is dry descending air. The dynamics of the moat region are similar to the eye, while the outer eyewall takes on the dynamics of the primary eyewall. The vertical structure of the eye has two layers. The largest layer is that from the top of the tropopause to a capping layer around 700 hPa which is described by descending warm air. Below the capping layer, the air is moist and has convection with the presence of stratocumulus clouds. The moat gradually takes on the characteristics of the eye, upon which the inner eyewall can only dissipate in strength as the majority of the inflow is now being used to maintain the outer eyewall. The inner eye is eventually evaporated as it is warmed by the surrounding dry air in the moat and eye. Models and observations show that once the outer eyewall completely surrounds the inner eye, it takes less than 12 hours for the complete dissipation of the inner eyewall. The inner eyewall feeds mostly upon the moist air in the lower portion of the eye before evaporating. ==Evolution into an annular tropical cyclone==

Annular tropical cyclones have a single eyewall that is larger and circularly symmetric. Observations show that an eyewall replacement cycle can lead to the development of an annular hurricane. While some tropical cyclones become annular without undergoing an eyewall replacement cycle, it has been hypothesized that the dynamics leading to the formation of a secondary eyewall may be similar to those needed for development of an annular eye. Annular tropical cyclones have been simulated that have gone through the life cycle of an eyewall replacement. The simulations show that the major rainbands will grow such that the arms will overlap, and then it spirals into itself to form a concentric eyewall. The inner eyewall dissipates, leaving a tropical cyclone with a singular large eye with no rainbands. == References ==