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 4 status on the Saffir-Simpson Hurricane Scale several times. Typhoon June (1975) was the first reported case of triple eyewalls, and Hurricane Juliette (2001) was a documented case of such.
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". 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 outer 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 concentric eyewalls, the inner eyewall had dissipated.The storm was observed by a reconnaissance aircraft to have an inner eyewall at
Hurricane Beulah in 1967 was the first tropical cyclone to have its eyewall replacement cycle observed from beginning to end. [ citation needed ]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 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 a failure in its goal of reducing the destructiveness of hurricanes, Project Stormfury was not without merit. 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 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.
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. July offers the best background environmental conditions for development of a secondary eyewall.[ citation needed ] 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 in strength.
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.
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.
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.
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 (WISHE) 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.
In the vortex Rossby wave hypothesis, the waves travel radially outward from the inner vortex. The waves amplify angular momentum at a radius that is dependent on the radial velocity matching that of the outside flow. At this point, the two are phase-locked and allow the coalescence of the waves to form a secondary eyewall.
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 50 kilometres (30 mi) to 100 kilometres (60 mi) 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.
Convective available potential energy (CAPE) is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. The higher the CAPE, the more likely there will be convection. If areas of high CAPE exist in the β-skirt, the deep convection that forms would act as a source of vorticity and turbulence kinetic energy. This small-scale energy will upscale into a jet around the storm. The low-level jet focuses the stochastic energy a nearly axisymmetric ring around the eye. Once this low-level jet forms, a positive feedback cycle such as WISHE can amplify the initial perturbations into a secondary eyewall.
After the secondary eyewall totally surrounds the inner eyewall, it begins to affect the tropical cyclone dynamics. Hurricanes are fueled 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, and therefore cyclones are dependent upon receiving the energy from the ocean from the inward spiraling 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.
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.Contraction of the eyewall occurs if the area of convection occurs outside the radius of maximum winds. After the outer eyewall forms, subsidence increases rapidly in the moat region.
Once the inner eyewall dissipates, the storm weakens; the central pressure increases and the maximum sustained windspeed decreases. Rapid changes in the intensity of tropical cyclones is a typical characteristic of eyewall replacement cycles.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 200 nautical miles (370 km) 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.
Annular hurricanes 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 hurricanes develop into annular hurricanes without an eyewall replacement, 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.Hurricane Daniel (2006) and Typhoon Winnie (1997) were examples where a storm had an eyewall replacement cycle and then turned into an annular hurricane. Annular hurricanes 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 spiral into itself to form a concentric eyewall. The inner eyewall dissipates, leaving a hurricane with a singular large eye with no rainbands.
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, and this led to seeding several Atlantic hurricanes. However, it was later shown that this hypothesis was incorrect. It was determined that most hurricanes do not contain enough supercooled water for cloud seeding to be effective. Additionally, researchers found that unseeded hurricanes often undergo the same structural changes that were expected from seeded hurricanes. This finding called Stormfury's successes into question, as the changes reported now had a natural explanation.
The Fujiwhara effect, sometimes referred to as the Fujiwara effect, Fujiw(h)ara interaction or binary interaction, is a phenomenon that occurs when two nearby cyclonic vortices orbit each other and close the distance between the circulations of their corresponding low-pressure areas. The effect is named after Sakuhei Fujiwhara, the Japanese meteorologist who initially described the effect. Binary interaction of smaller circulations can cause the development of a larger cyclone, or cause two cyclones to merge into one. Extratropical cyclones typically engage in binary interaction when within 2,000 kilometres (1,200 mi) of one another, while tropical cyclones typically interact within 1,400 kilometres (870 mi) of each other.
An annular tropical cyclone is a tropical cyclone that features a normal to large, symmetric eye surrounded by a thick and uniform ring of intense convection, often having a relative lack of discrete rainbands, and bearing a symmetric appearance in general. As a result, the appearance of an annular tropical cyclone can be referred to as akin to a tire or doughnut. Annular characteristics can be attained as tropical cyclones intensify; however, outside the processes that drive the transition from asymmetric systems to annular systems and the abnormal resistance to negative environmental factors found in storms with annular features, annular tropical cyclones behave similarly to asymmetric storms. Most research related to annular tropical cyclones is limited to satellite imagery and aircraft reconnaissance as the conditions thought to give rise to annular characteristics normally occur over water well removed from landmasses where surface observations are possible.
A tropical cyclone forecast model is a computer program that uses meteorological data to forecast aspects of the future state of tropical cyclones. There are three types of models: statistical, dynamical, or combined statistical-dynamic. Dynamical models utilize powerful supercomputers with sophisticated mathematical modeling software and meteorological data to calculate future weather conditions. Statistical models forecast the evolution of a tropical cyclone in a simpler manner, by extrapolating from historical datasets, and thus can be run quickly on platforms such as personal computers. Statistical-dynamical models use aspects of both types of forecasting. Four primary types of forecasts exist for tropical cyclones: track, intensity, storm surge, and rainfall. Dynamical models were not developed until the 1970s and the 1980s, with earlier efforts focused on the storm surge problem.
The eye is a region of mostly calm weather at the center of strong tropical cyclones. The eye of a storm is a roughly circular area, typically 30–65 kilometers in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather and highest winds 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.
The Dvorak technique is a widely used system to estimate tropical cyclone intensity based solely on visible and infrared satellite images. Within the Dvorak satellite strength estimate for tropical cyclones, there are several visual patterns that a cyclone may take on which define the upper and lower bounds on its intensity. The primary patterns used are curved band pattern (T1.0-T4.5), shear pattern (T1.5–T3.5), central dense overcast (CDO) pattern (T2.5–T5.0), central cold cover (CCC) pattern, banding eye pattern (T4.0–T4.5), and eye pattern (T4.5–T8.0).
The central dense overcast, or CDO, of a tropical cyclone or strong subtropical cyclone is the large central area of thunderstorms surrounding its circulation center, caused by the formation of its eyewall. It can be round, angular, oval, or irregular in shape. This feature shows up in tropical cyclones of tropical storm or hurricane strength. How far the center is embedded within the CDO, and the temperature difference between the cloud tops within the CDO and the cyclone's eye, can help determine a tropical cyclone's intensity. Locating the center within the CDO can be a problem for strong tropical storms and with systems of minimal hurricane strength as its location can be obscured by the CDO's high cloud canopy. This center location problem can be resolved through the use of microwave satellite imagery.
Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain or squalls. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".
Tropical cyclone forecasting is the science of forecasting where a tropical cyclone's center, and its effects, are expected to be at some point in the future. There are several elements to tropical cyclone forecasting: track forecasting, intensity forecasting, rainfall forecasting, storm surge, tornado, and seasonal forecasting. While skill is increasing in regard to track forecasting, intensity forecasting skill remains nearly unchanged over the past several years. Seasonal forecasting began in the 1980s in the Atlantic basin and has spread into other basins in the years since.
Hurricane Daniel was the second strongest hurricane of the 2006 Pacific hurricane season. The fourth named storm of the season, Daniel originated on July 16 from a tropical wave off the coast of Mexico. It tracked westward, intensifying steadily to reach peak winds of 150 mph (240 km/h) on July 22. At the time, the characteristics of the cyclone resembled those of an annular hurricane. Daniel gradually weakened as it entered an area of cooler water temperatures and increased wind shear, and after crossing into the Central Pacific Ocean, it quickly degenerated into a remnant low-pressure area on July 26, before dissipating two days later.
Mesovortices are small scale rotational features found in convective storms, such as those found in bow echos, supercell thunderstorms, and the eyewall of tropical cyclones. They range in size from tens of miles in diameter to a mile or less, and can be immensely intense.
Hurricane Norbert marked the first time a core of a hurricane was fully mapped in three-dimensions. First forming on September 14, 1984 west of the Mexican coast, Norbert gradually intensified, reaching hurricane intensity two days after formation. On September 22, Norbert peaked in strength as a Category 4 hurricane on the Saffir-Simpson Hurricane Scale. While intensifying, Norbert meandered. It moved east, then north, then west, then south, then back towards the east, and finally towards the northeast. After fluctuating in intensity for two more days, Norbert rapidly weakened. It turned towards the northwest and made landfall in southern Baja California Norte as a tropical storm. The combination of Norbert and several other storms left thousands homeless throughout Mexico. The remnants of Hurricane Norbert produced moderate rain over Arizona.
Typhoon Ida, also known as the Kanogawa Typhoon, was the sixth-deadliest typhoon to hit Japan, as well as one of the strongest tropical cyclones on record. On September 20, Ida formed in the Western Pacific near Guam. It moved to the west and rapidly intensified into a 115 mph (185 km/h) typhoon by the next day. On September 22, Ida turned to the north and continued its quick rate of intensification. Two days later, the Hurricane Hunters observed a minimum barometric pressure of 877 mb (25.9 inHg), as well as estimated peak winds of 325 km/h (200 mph). This made Ida the strongest tropical cyclone on record at the time, although it was surpassed by Typhoon June 17 years later. Ida weakened as it continued to the north-northeast, and made landfall in Japan on southeastern Honshū with winds of 80 mph on September 26. It became extratropical the next day, and dissipated on the 28th to the east of the country. Ida caused torrential flooding to southeastern Japan, resulting in over 1,900 mudslides. Damage was estimated at $50 million, and there were 1,269 fatalities.
Typhoon Nora, known in the Philippines as Typhoon Luming, was the fourth-most intense tropical cyclone on record. Originating from an area of low pressure over the western Pacific, Nora was first identified as a tropical depression on October 2, 1973. Tracking generally westward, the system gradually intensified, attaining typhoon status the following evening. After turning northwestward, the typhoon underwent a period of rapid intensification, during which its central pressure decreased by 77 mb in 24 hours. At the end of this phase, Nora peaked with winds of 295 km/h (185 mph) and a pressure of 877 mb, making it the most-intense tropical cyclone on record at the time; however, this pressure has since been surpassed by three other typhoons and one hurricane. The typhoon subsequently weakened and turned northwestward as it approached the Philippines. After brushing Luzon on October 7, the system passed south of Taiwan and ultimately made landfall in China on October 10. Once onshore, Nora quickly weakened and dissipated the following day.
The Hurricane Rainband and Intensity Change Experiment (RAINEX) is a project to improve hurricane intensity forecasting via measuring interactions between rainbands and the eyewalls of tropical cyclones. The experiment was planned for the 2005 Atlantic hurricane season. This coincidence of RAINEX with the 2005 Atlantic hurricane season led to the study and exploration of infamous hurricanes Katrina, Ophelia, and Rita. Where Hurricane Katrina and Hurricane Rita would go on to cause major damage to the US Gulf coast, Hurricane Ophelia provided an interesting contrast to these powerful cyclones as it never developed greater than a category 1.