Sustaining Category Five Intensity
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In the wake of the devastation wrought by Hurricane Ivan throughout much of the Caribbean, and Eastern United States, the folks at Hurricaneville have decided to go to work on a special series of articles it had planned to do at the end of last year on Category Five Hurricanes. What this series of reports will be about is research that we conducted during the Fall Semester 2003 at Rutgers University for a class taken called Atmospheric Dynamics.

Category Five Hurricanes are a rare breed. Since records have been taken, less than five percent of all hurricanes have gone on to become Cat Five storms. However, in just the past two seasons, the Atlantic Basin has had two such storms: Isabel and Ivan. Both Hurricane Isabel and Hurricane Ivan teetered between Category Four and Category Five strength three times with Isabel lasting the longest at that intensity with 36 hours.

While hurricanes that are able to reach Category Five are seen from space as nature's work of perfection, the represent a hurricane at its maximum level of intensity, and most severe destructive power. But, how do they get to this level? And, when they do, how are they able to maintain that intensity for long periods of time like Hurricane Isabel. These series of reports will look into these questions and others.

Storm Facts Category Five Hurricanes

Since records have been taken, there have only been 23 hurricanes that have been able to reach Category Five Hurricane strength. Now, according to the Saffir-Simpson Scale, that means a hurricane that has sustained winds greater than 155 mph, and a minimum central pressure of less than 920 millibars or 27.17 inches of Hg. There have only been three Category Five storms that have made landfall in the United States since records began. The Labor Day Hurricane of 1935, Hurricane Camille (1969), and Hurricane Andrew in August, 1992.

Believe it or not, Andrew was originally a Category Four Hurricane at landfall. Its minimum central pressure at that time was 922 mb, or 27.23 inches of Hg. However, recently, in August, 2002, the storm was reclassified to Category Five on its tenth anniversary. At this peak level, hurricanes are in their most perfect form. There is perfect outflow, which is the exhaust a hurricane needs in order to maintain itself. You'll notice this element right away in a perfectly symmetric hurricane, which most Category Five storms are. The perfect symmetry means that all sides of the storm are about equal distance from the center. In other words, the storm has the form of a perfectly round circle.

Then, there is the eye of the storm. In a hurricane at this maximum level of intensity, the eye is usually small and narrow much like a pinhole. This is because the storm needs to conserve momentum. A hurricane operates much like a figure skater going into a spin at the end of his/her routine. They start out slowly with their arms outward, and then build speed and rotation as they draw their arms inward. The eye, or center of lowest pressure, serves that function for the hurricane. Usually, the tighter the eye is, the higher the sustained winds are.

For hurricanes to reach the level of a Category Five storm, they must have the following conditions: First, they must have the basic ingredients all hurricanes must have: warm sea surface temperatures, light winds aloft, and they must have a rotation or spin to be a trigger mechanism for the storm to develop. In addition to those elementary requirements, a major hurricane such as a Category Five storm must undergo a process of rapid intensification, which is a situation when a minimal hurricane, or mild hurricane rapidly deepens into a major hurricane in a short period of time.

When hurricanes are a major intensity of either a Cat Three, Four, or Five, they undergo a phenomenon known as eyewall replacement, which is where an outer eyewall develops within the system and gradually chokes the inner (primary) eyewall as it produces instability and sinking air, which kills the thunderstorms in the inner eyewall. As this occurs, the hurricane goes through a reorganization phase where it may weaken now that the eyewall, or ring of most intense thunderstorms surrounding the eye has expanded outward. The reason for this is again the law of conservation of momentum. All you have to do to understand is think our example of the figure skater.

A consequence of this is that the storm develops concentric eyewalls, a phenomenon detected most notably in Hurricane Gilbert after it became a Category Five Hurricane with 180 mph sustained winds, and 215 mph wind gusts near the Yucatan Peninsula in September, 1988. The concentric eyewalls is a result of eyewall replacement, where you have two distinct eyewalls within the hurricane, the inner or original eyewall, and an outer, newer eyewall. Such internal changes can disrupt these tremendously potent and dangerous storms. For these storms to be at their optimum level, conditions must be perfect.

Researchers call this the satisfaction of the "Goldilocks Principle." Everything has to be just right for a Category Five Hurricane to thrive and sustain itself. It is these internal changes as well as external changes such as cooler sea surface temperatures, dry air intrusion, and moderate to strong upper level wind shear, that make it extremely difficult for a hurricane not only to reach Category Five strength, but also be able to maintain that level for long periods of time. However, there have been such storms that have defied the odds.

Out of the 23 hurricanes ever recorded to reach Category Five status, six of those have lasted at that intensity for 30 hours or more. They include: Hurricane Dog (1950), Hurricane David (1979), Hurricane Mitch (1998), Hurricane Isabel (2003), Unamed Hurricane of 1947, and Hurricane Camille in 1969. Of these six storms, Hurricane Dog lasted the longest at Category Five as it maintained that level for 60 hours. The storm also fluctuated between Category Four and Category Five strength four times, which is also a record. Next in terms of duration are David and Mitch, which both lasted at Cat Five for 42 hours.

Hurricane Isabel, which is one of the storms that we will look at in this first in a series of articles on these rare and powerful tropical cyclones, was a Category Five Hurricane for 36 hours, and fluctuated between Category Four and Category Five intensity three times before finally weakening to a Category Two when it made landfall along the Outer Banks of North Carolina. Another storm that we will look at is not on this list, but it has recenty made its own mark on history, and that storm is Hurricane Ivan (2004). Ivan was a rare storm in not only the sense that it was a powerful Category Five, but it was also a system that originated at a very low latitude.

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Analyzing Hurricane Isabel

Hurricane Isabel as well as our other featured storm, Hurricane Ivan, were both classic Cape Verde storms. You'll often here that term when forecasters refer to a hurricane that forms in the Far Eastern Atlantic just off the West Coast of Africa near a series of tiny islands called the Cape Verde Islands. The storms are often the most powerful storms of any given hurricane season that usually form during the peak months of August and September.

Isabel started off modestly as a tropical wave, and didn't become a depression until some five days later. .Once it became a depression, the storm started blossoming. Within 36 hours, it was a strong tropical storm with winds of 65 mph. Some twenty-four hours later, Isabel was closing in on major hurricane status with winds of 110 mph. This system had the earmarks of a rapidly deepening hurricane that was in the right environment to do such a thing. The ingredients were coming together for a Category Five Hurricane.

The next 84 hours saw the hurricane go from a Category Two to a Category Five with a 51 millibar, or 1.53 inch drop in its minimum central pressure. This rapid drop in pressure was already on top of the decrease it had from being a tropical depression, and storm to a Category Two system, which was even more rapid (that occurred in about 60 hours). Under the influence of the Bermuda High, or subtropical ridge in the Atlantic, Hurricane Isabel was able to stay at that level of intensity for a day and a half.

After that, the storm weakened as it went through a reorganization phase, probably an eyewall replacement. It then regained optimum intensity at Cat Five, and repeated the weakening phase followed by reintensification. Isabel then entered an area unfavorable for development with hostile upper level winds. It also encountered some intrusion of dry air, and that caused the powerful storm to go through a gradual weakening phase prior to making landfall along the Outer Banks of North Carolina in the middle of September, 2003.

Isabel struck land as a strong Category Two storm with winds sustained at 105 mph according to official reports. The storm has grown in size, which is what many weakening hurricanes do when they lose strength in order to obey the laws conserving momentum and energy. The storm brought widespread rain and wind to the Eastern United States as it combined with a high pressure system over the New England states to create a very strong pressure gradient, which exacerbated the winds in coastal areas. It ended up causing some $3.37 billion dollars in damage, and was the strong storm to hit the Mid-Atlantic since Hurricane Hazel in October, 1954.

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Analyzing Hurricane Ivan

As mentioned earlier, Ivan like Isabel was a classic Cape Verde system. It also marked the second time in as many years that a hurricane had reached such an optimum intensity. Prior to Isabel and Ivan, the last Category Five Hurricane was Hurricane Mitch in October, 1998, which was a catastrophe of major proportions. Mitch dumped as much as 75 inches of rain over parts of Central America including Honduras and Nicaragua, which produced devastating floods and mudslides that killed some 11,000 people, and even perhaps more.

Back to Ivan, the storm like Isabel went through fluctuations between Category Four and Category Five intensity. However, it made its own mark by becoming the sixth strongest storm on record with its lowest pressure at 910 mb, or 26.87 inches of Mercury. The only storms to have deeper lows in the Atlantic Basin were Mitch and Camille with 905 mb, Hurricane Allen (1980) with 899 mb, The Labor Day Hurricane of 1935 with 892 mb, or Hurricane Gilbert with 888 mb. Actually, Gilbert's original minimum pressure was 884, or 26.13 inches of Hg (Hg=Mercury), but there were some errors, and it was recalculated.

Another thing that was so rare about Ivan was the fact that it formed at such a low latitude. It emerged as a tropical depression when it was some 700 miles to the South-Southwest of the Cape Verde Islands. The exact latitude was lower than 10 degrees North, which is unusual because tropical systems normally formed between 10 and 30 degrees North Latitude. The reason why that is so in most cases is because of one of the factors in development: rotation or spin. The closer you get to the equator, the less likely you are going to encounter spin because the tropical wave axis need to be bent by the curvature of the earth in order for there to be rotation.

However, being far south may have also helped the system since the sun angle is higher, and that in turn, could produce warmer sea surface temperatures to flourish in. And, apparently, it did do just that. Within three days of developing, Ivan was already a Category Four Hurricane with winds of 135 mph, and a minimum central pressure of 948 mb, or 27.99 inches of Hg. However, it appeared to have underwent eyewall replacement, and reorganization. As a result, the storm weakened to a strong Category Two with winds still sustained at 105 mph, and a minimum central pressure of 968 mb, or 28.59 inches of Hg.

The stage was set though for a new phase of rapid intensification. Sea surface temperatures were more than adequate, and upper level conditions were conducive for restrengthening. Ivan did exactly that. Within 56 hours, Ivan's pressure dropped some 43 mb, or 1.29 inches of Hg to 925 mb, or 27.31 inches of Hg. Ivan then underwent some weakening in another reorgnaization, or eyewall replacement phase as its winds decreased to 150 mph, and its pressure increased to 921 mb, or 27.20 inches of Hg on Thursday afternoon, September 9th.

It stayed at Category Four strength for about 48 hours, but toward the end of that phase, it began another deepening trend. This time, the pressure would drop another 11 mb in about nine hours time, and that brought it to its lowest point at 910 mb. From there, Ivan fluctuated in intensity between Category Four and Category Five as it ransacked Jamaica, the Caymans, and Cuba after demolishing Grenada and other neighboring Windward Islands earlier in the week. After bypassing the Western Tip of Cuba, Ivan encountered more hostile upper level wind conditions, and weakened to a strong Category Three Hurricane at landfall.

Nevertheless, the effects of Ivan's 130 mph winds, and tremendous surge along a large area of coastline were devastating for residents of the Gulf Coast from Mississippi to the Florida Panhandle. While Category Five storms may not last at such a high level of intensity for long periods of time, they still contain a tremendous amount of energy that, when released, can have devastating and even deadly results on those affected by them.

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