Keeping an eye on weather's biggest bullies
Hurricane season in the Atlantic Basin runs from June 1 through November 30, with most activity clustered from August to October. In the Pacific Basin, typhoons and tropical cyclones are most common from late June through December, but storms can occur in either basin at almost any time of year.
The first months of the 2005 Atlantic season have unfolded at the fastest pace on record, with sea-surface temperatures among the warmest ever observed. NOAA's Climate Prediction Center foresees continued above-normal activity for the rest of the season.
What's the longer-term outlook? Recent studies indicate that, on a global scale, hurricanes are already packing more punch and may continue to intensify on average as the planet warms.
Whatever you call them, these monster storms are the most powerful physical 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. This year, researchers are examining the connection between the rainbands and the storm's nucleus—its raging eyewall and its calm, often-clear eye.
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.
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).
One of the best ways to figure out how hurricanes operate is to fly right into them. This summer, skilled pilots are steering research aircraft as close as safety permits to storms forming in the Atlantic and Pacific. The planes are part of NOAA's hurricane hunter fleet and other research operations supported by the U.S. Navy, Air Force, universities, and research labs.
From mid-August to the end of September, researchers are gathering 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) is studying 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 are few if any studies of their interaction.
"To study this relationship, we need a detailed map of the whole storm, including both the eyewall and rainbands," explains NCAR scientist Wen-Chau Lee.
Lee is the lead scientist for the Naval Research Laboratory's P-3 aircraft as it profiles rainbands using dropsondes and ELDORA , a sophisticated Doppler radar built by NCAR and the French government. When dropped from a plane, the GPS Dropsonde sensors measure temperature and wind as they descend through the storm.
Two P-3 aircraft operated by NOAA are also carrying Doppler radars, and extra dropsondes are being released from NOAA and Air Force aircraft. By combining the data from the aircraft and assimilating them into computer models, researchers will gain a better sense of whether the storm's circulation speeds up or slows down as rainbands wrap around the hurricane.Charting lifecycles
During July 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).
Once again, the goal is better understanding of what makes hurricanes strengthen or weaken.
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.
Amid the raindrops and ice particles, parachute-borne weather instruments fell at a faster clip. NOAA's Shirley Murillo was the GPS dropsonde scientist aboard the NOAA P-3s. Murillo, who spent several summers at NCAR during undergraduate and graduate school as part of the SOARS program, processed and transmitted data from the dropsondes in real time so they could be fed into weather forecasting models during the experiment.
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, says Murillo. Through such work, researchers will add greatly to their understanding and prediction of intensity changes.
Back on the ground, Wen-Chau Lee has developed a single-Doppler wind retrieval technique to pull more information out of ground-based radar detection of sizeable swirls of wind, or vortices. The technique has been tried on both hurricanes and tornadoes.
Lee tested his mathematical method with coastal radar data from Hurricane Charley gathered in August 2004. He found the method provided early notice of Charley's rapid intensification just before landfall. This technique could someday be a valuable complement to hurricane hunter aircraft as storms approach landfall; the method is being considered by the National Hurricane Center as a way to extract quick estimates of a hurricane's peak winds and central pressure as it approaches land.
NCAR's Advanced Research version of the Weather Research and Forecasting model (WRF) has been configured to high resolution to reveal tropical storm activity in detail since Hurricane Isabel in 2003. A multiagency effort, WRF is a next-generation computer model for weather prediction that can be used by both researchers and operational forecasters.
For an experiment during 2004's Hurricane Ivan, NCAR's WRF team used a fine-grained 4-kilometer (2.5-mile) grid to bring the hurricane's inner features into sharp focus.
Next-generation research and forecasting
For this NCAR experimental forecast, the Advanced Research WRF model (left) started with observed conditions at 8:00 p.m. Eastern Daylight Time on September 15, 2004, and projected Ivan's track and precipitation amounts up to 48 hours ahead with remarkable precision. A composite of radar observations of the hurricane's behavior as the storm later developed is on the right. Click here or on the image to watch the animation. (Animation courtesy Christopher Davis and Kristin Conrad, NCAR.)
WRF joined the suite of official operational models used by the National Weather Service in 2004. Researchers hope to see WRF coupled with ocean and/or wave models to produce even better forecasts of hurricanes in the future.
Are hurricanes changing as the average global temperature increases?
NCAR scientist Kevin Trenberth points out that, because the number of hurricanes varies so much in any region from year to year and decade to decade, it is difficult to use statistical techniques to reveal longer-term trends in the number of hurricanes that form and where they move.
"There is no sound theoretical basis for drawing any conclusions about how anthropogenic climate change affects hurricane numbers or tracks, and thus how many hit land," Trenberth says.
However, he notes, 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.
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.