|
 |
|
| Backgrounders |
Hurricanes, Typhoons, Cyclones |
 |
The cutting edge in hurricane research Studying hurricane
structure to understand behavior Visuals Anatomy of a hurricane Web links Current conditions, glossaries, research, education, and more
|
|
|||||||||||||||||||
|
Each year, hundreds of low-pressure centers spin up over warm waters with weak contrasts in temperature in the tropics and subtropics, entering the life cycle of tropical storms. When these systems organize to the point where their sustained winds top 34 knots (39 miles per hour), they're known as tropical cyclones. But various parts of the world use a variety of terms once a tropical cyclone packs winds of at least 65 knots (74 mph). Around North and Central America, they're called hurricanes. The god of evil for the Carib people was named Hurican, according to the authors of Hurricane Strike! That's the source, with a slight twist in spelling, of the name used in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and Northeast Pacific Ocean. In the Northwest Pacific, the same powerful storms are called typhoons. In the Southeastern Indian and Southwest Pacific Oceans they're called severe tropical cyclones. In the North Indian Ocean, they're called severe cyclonic storms, while in the Southwest Indian Ocean, they simply keep the name tropical cyclone. In recent years, we’ve seen several hurricanes and tropical storms strike in unfamiliar places. This could be a result of improved monitoring, regional changes in ocean temperatures and upper-air circulation (perhaps linked to global warming in some cases), or all of the above. Hurricanes that make Category 3 status on the Saffir-Simpson scale (winds of at least 96 knots or 111 mph) are labeled intense hurricanes. If a typhoon hits 132 knots (150 mph), it becomes a supertyphoon. The South Atlantic had been considered free of tropical cyclones—that is, until March 2004, when a mysterious storm later dubbed Hurricane Catarina (more below) made landfall in Brazil. In October 2005, Vince became the first tropical storm ever recorded in Spain. And in the Arabian Sea, Gonu became a Category 5 in June 2007—that region’s strongest tropical cyclone on record. After weakening, Gonu brought unprecedented damage from rain, wind, and flooding to parts of Oman and Iran. When is hurricane season? Hurricane season in the Atlantic Basin (including the Caribbean and the Gulf of Mexico) runs from June 1 through November 30, with most activity clustered from August to October. The typhoon and cyclone seasons follow their own patterns. In the Northeast Pacific, the official season runs from May 15 to November 30. In the Northwest Pacific, typhoons are most common from late June through December. The North Indian Ocean sees cyclones from April to December, with peaks in May and November. The Southwest Pacific and South Indian oceans (including the waters bordering Australia) get most of their activity from November to May. If the conditions are right, tropical cyclones can develop outside their official seasons, especially in the Northwest Pacific, where they occur year round. NOAA's Hurricane Research Division goes into greater detail about seasonal timescales. Does a quiet Atlantic indicate a stormy Pacific? On average there are about 80 to 100 named tropical cyclones per year across the world, including about 40 to 60 that reach hurricane strength. But the numbers in each basin may vary more than the overall global average. In part this is due to ocean-atmosphere cycles such as El Niño. Because they affect where showers and thunderstorms develop, these cycles can enhance hurricane activity in one basin while suppressing it in another. During El Niño years, the Atlantic tends to be less active than usual, while parts of the central and northeast Pacific are typically busier than usual.How big and how strong can hurricanes get? Whatever you call them, these monster storms are the most powerful atmospheric phenomena on Earth. Hurricanes gather energy from water vapor in the atmosphere stretched for hundreds of thousands of square miles across the warm ocean water of the tropics. The storms themselves can bring heavy rains and gale-force winds blowing 38 to 73 miles per hour (62-117 kilometers per hour) across an area the size of Louisiana. Hurricane-force winds can extend 50 miles (80 kilometers) or more from the storm center. Hurricanes derive much of their energy from the heat generated as water condenses in the rainbands that spiral into the storm. In a major 2005 field campaign, researchers collected unprecedented data to examine the connection between the rainbands and the storm's nucleus—its raging eyewall and its calm, often-clear eye (more about the RAINEX campaign, below). According to NOAA's National Hurricane Center, the average hurricane eye—the still center where pressure is lowest and air temperature aloft is highest—stretches 30 miles (48 km) across, with some growing as large as 120 miles (200 km) across. The eye typically shrinks as a hurricane intensifies, sometimes narrowing to less than 10 miles (16 km) in width. Eventually, a new eye may form around the old one; hurricanes often weaken during this transition but can intensify again as the new eye contracts. Size does not determine intensity. Some of the most destructive hurricanes to hit the U.S. coastline, including Hurricane Andrew in 1992, have been relatively small. Long-lived hurricanes can "spin down," with their strong winds spreading over a large area as they weaken. Hurricane winds can be fierce, and some storms even spawn tornadoes. But the worst hurricane damage is often the result of a storm surge that causes coastal flooding. Investigators are now working to determine the size of the storm surge and the extent of damage caused by Hurricane Katrina. Storms that move inland often bring much-needed rain that farmers and water managers count on. But if too much falls at once it can quickly overwhelm stream and river beds, producing serious riverine flooding. Can we control hurricanes? Even smaller hurricanes pack a mind-boggling amount of power. The heat energy released by a hurricane equals 50 to 200 trillion watts—or about the same amount of energy released by exploding a 10-megaton nuclear bomb every 20 minutes. We are unlikely to come up with methods to control such overwhelming natural power for the foreseeable future—though that has not kept people from speculating about what it would take. Researchers instead focus on understanding every aspect of hurricane structure and behavior, with the hope their work will lead to better predictions of storm tracks and intensity so warnings can be issued to protect life and property. The societal side of the equation includes communication between forecasters and emergency managers who make decisions about when and where to call for evacuation from threatened areas and then get the word out with help from local public safety offices and the mass media. Researchers at NCAR are collaborating with colleagues elsewhere to address the human side of hurricane forecasts and warnings through the Collaborative Program on the Societal Impacts and Economic Benefits of Weather Information (SIP). Is global warming affecting hurricanes? Are hurricanes changing as the average global temperature increases? The physical interactions among warmer oceans, more moisture in the atmosphere, and other effects of present and future global warming all point to an increase in hurricane intensity and rainfall on a warmed planet. Studies with computer models suggest that, with warmer sea-surface temperatures and moister air, more energy goes into the showers and thunderstorms that feed hurricanes, pushing more of them into the extreme category. The modeling work, based on known physical laws, was supported by a study in 2005 by researchers at the Georgia Institute of Technology and NCAR. The team found that the number of Category 4 and 5 hurricanes worldwide has nearly doubled over the past 35 years, even though the total number of hurricanes has dropped since the 1990s. The shift occurred as global sea surface temperatures have increased over the same period. The research, described in an NCAR news release, appeared in the September 16, 2005 issue of Science. Also in 2005, longtime hurricane researcher Kerry Emanuel of the Massachusetts Institute of Technology reported that "the amount of energy released in [tropical cyclones] in both the North Atlantic and the North Pacific oceans has increased markedly since the mid-1970s. Both the duration of the cyclones and the largest wind speeds they produce have increased by about 50 percent over the past 50 years," according to an MIT news release on Emanuel's findings. These studies generated intense interest, especially after the catastrophic events following Hurricane Katrina's landfall. At a hurricane conference in April 2006, scientists from NOAA's National Hurricane Center and elsewhere pointed out limitations in the observational record of tropical cyclones, especially outside the Atlantic Ocean, due to changes in observing tools and techniques. Debate also continues on the role of multidecadal cycles in the the Atlantic and how much they affect hurricane frequency and intensity. Ongoing research on this topic includes several recent papers by NCAR scientists. Studying hurricane structure to understand behavior One of the best ways to figure out how hurricanes operate is to fly right into them. During the 2005 season, skilled pilots steered research aircraft as close as safety allowed to storms forming in the North Atlantic and Pacific, including Hurricanes Katrina and Rita. The planes were part of NOAA's hurricane hunter fleet and other research operations supported by the U.S. Navy, Air Force, universities, and research labs. The unprecedented data collected in 2005's busy season is still being analyzed. Improving forecasts close to shore In the summer of 2007, forecasters tested a new technique that provides a detailed 3-D view of an approaching hurricane every six minutes, helping to determine whether the storm is gathering strength as it nears land. The technique is known as VORTRAC, which stands for Vortex Objective Radar Tracking and Circulation. Developed by researchers at NCAR and the Naval Research Laboratory (NRL), VORTRAC relies on the existing NOAA network of Doppler radars along the Southeast coast to closely monitor hurricane winds. About 20 of these radars are scattered along the Gulf and Atlantic coastlines from Texas to Maine. Each radar can measure winds blowing toward or away from it, but no single radar could provide a 3-D picture of hurricane winds before now. NCAR scientist Wen-Chau Lee and his collaborators developed a series of mathematical formulas that combine data from a single radar with general knowledge of Atlantic hurricane structure in order to map the approaching system's winds in three dimensions. The technique also infers the barometric pressure in the eye of the hurricane, a very reliable index of its strength. Forecasters using VORTRAC can update information about a hurricane each time a NOAA Doppler radar scans the storm, which can be as often as about every six minutes. That could enable forecasters to monitor it for the critical 10-15 hours before landfall. "With this technique, meteorologists for the first time will be able to monitor the strength of a hurricane every few minutes as it approaches landfall and quickly alert coastal communities if it suddenly intensifies or weakens," says NCAR scientist Wen-Chau Lee.Examining rainbandsFrom mid-August to the end of September 2005, researchers gathered in Miami to focus an unprecedented array of airborne radars on two distinct hurricane structures to tease out how they work together to make a storm stronger or weaker. The Hurricane Rainband and Intensity Change Experiment (RAINEX) studied how the outer rainbands and inner eye of a hurricane interact to influence the storm's intensity. While the two structures have been studied separately, there were few if any studies of their interaction. Wen-Chau Lee is the lead scientist for ELDORA, a sophisticated Doppler radar built by NCAR and the French government. During RAINEX, ELDORA's 500-meter (1600-foot) resolution, four times sharper than that of hurricane-hunter radars, offered the crispest picture of rainbands obtained to date. Lee worked with pilots aboard the Naval Research Laboratory's P-3 aircraft as they brought the tail-mounted ELDORA into some of the season's strongest hurricanes. Later, he combined the ELDORA data with information gathered by two other radar-equipped P-3 aircraft. All three planes also released GPS dropsonde sensors, which measured temperature and wind as the sensors descended through the storm.
As it flew between a pair of Katrina's rainbands, ELDORA detected winds of more than 70 meters per second (156 mph), as well as a string-of-pearls pattern of individual cells nestled within the rainbands. The cells had impressive circulations of their own within the larger-scale flow along rainbands and into the eyewall. Lee was impressed with the structural complexity they discovered in the rainbands. Robert Houze (University of Washington), one of the two principal investigators for RAINEX, concurred. "Intensity is driven in part by internal dynamics between the rainbands and the eyewall—something that is very hard to get to—so this is landmark information," he said. More details and a graphic illustrating the string-of-pearls pattern may be found in a fall 2005 UCAR Quarterly report, Busy times in the tropics. Charting lifecyclesDuring July 2005, scientists converged in Costa Rica to measure the full lifecycle of storms, from birth until they make landfall or fizzle out over the ocean. They got close looks at Hurricanes Dennis and Emily and Tropical Storm Gert and checked out convection (showers and thunderstorms) in the Eastern Pacific off the Costa Rican coast. NOAA and NASA joined forces to lead the experiments (see Learn More, below). NCAR's Aaron Bansemer went to Costa Rica to run microphysics experiments on one of NOAA's P-3 aircraft. He used three probes to measure the size and shapes of rain and ice particles in clouds as well as their concentrations. Those measurements will be compared with Doppler radar data showing the particles' upward and downward motions. Ice is trickier to study than rain, according to Bansemer. "Ice particles take such a variety of shapes," he explains. "So when they fall, their motions are more difficult to describe." The goal for Bansemer and NCAR colleague Andrew Heymsfield is better representation of how fast these tiny bits of rain and ice fall, which is important for understanding the strength of hurricanes. "The hurricane models are very sensitive to particle fall speeds," says Heymsfield. It's a Goldilocks type of problem. If the fall speeds in a computer model are too fast, the modeled storms dissipate too soon. If they're too slow, the updrafts are weighed down with precipitation, producing a less-intense storm.
Gathering measurements is critical at all stages in the tropical cyclone lifecycle, from genesis through development into mature hurricanes and then landfall or decay over open water, according to project participant Shirley Murillo of NOAA. Through such work, researchers will add greatly to their understanding and prediction of intensity changes. High-resolution forecasting from an advanced weather model NCAR's Advanced Research version of the Weather Research and Forecasting model has been configured to reveal tropical storm activity in great detail. A multiagency effort, WRF is a next-generation computer model for weather prediction that can be used by both researchers and operational forecasters. To accurately depict the small but intense features within hurricanes, Advanced Research WRF (or ARW) sharpens the detail over targeted regions to 12 kilometers (7.5 miles) for forecasts out to 120 hours, with a resolution as fine as 1.33 km (0.8 mi) near hurricanes. Since 2005, NCAR's experimental ARW forecasts of track and intensity ranked among the most accurate of the computer models used by researchers and forecasters to predict the season's hurricanes.
WRF joined the suite of official operational models used by the National Weather Service in 2004; the NWS dubbed this version the North American Mesoscale model (NAM). Researchers hope to see WRF coupled with ocean and/or wave models to produce even better forecasts of hurricanes in the future. A South American hurricane? The case of Catarina No hurricanes had ever been reported in the South Atlantic. So Brazilian meteorologists didn't know what to make of the unusual storm packing hurricane-force winds as it swept toward their shores in March 2004. Warnings were issued in time to prevent all but one death, but hundreds of homes were destroyed.
The Brazilian forecasters called it the Catarina phenomenon, for the region where it came ashore. But satellite imagery looked so familiar, with rainbands swirling around a central eye, that reports in the media around the world started calling it Hurricane Catarina. Researchers including NCAR's Greg Holland, a tropical cyclone specialist, gathered with Brazilian colleagues in July to examine data collected during the storm in search of clues to how and why it formed. Meanwhile, the Brazilian weather service has decided to call all future storms of comparable strength hurricanes. And they're considering adopting the Weather Research and Forecasting model to take advantage of its hurricane-forecasting abilities.
Anatomy of a hurricane
The eyewall replacement cycle during Katrina
Animation of 72-hour forecast on August 27, 2005, about 60 hours before landfall
Hurricane Web links Current ConditionsNOAA National Hurricane Center Glossaries and FAQs NOAA NHC Glossary of Hurricane Terms NOAA/AOML Hurricane
Research Division FAQ Research and Training Our Research: Hurricanes & Typhoons COMET: Hurricane Features MetEd: Hurricane & Typhoon training modules MetEd: Community Hurricane Preparedness SIP: Hurricane
Forecast & Warning Social Science For Learners Hurricane Strike! interactive science & safety module Look Out for Dangerous Weather: Hurricanes Windows to the Universe: Hurricanes Updated June 2007 Backgrounders provide supplementary information and should not be considered comprehensive sources. Opinions, findings, conclusions, or recommendations expressed in this publication do not necessarily reflect the views of any of UCAR's sponsors. UCAR is an Equal Opportunity/Affirmative Action employer. *News media reproduction to illustrate this story and nonprofit use permitted with proper attribution. |
|||||||||||||||||||||
