by Bob Henson and David Hosansky
Along with harried seaside dwellers, forecasters and researchers have been racing to keep up with this year's string of Atlantic hurricanes, particularly the unprecedented trio of Category 5 storms: Katrina, Rita, and Wilma. Below is a summary of recent NCAR work pertaining to tropical cyclones. See also the article on global-regional coupled modeling in the tropics.
The Gulf delivers for RAINEX
If somebody had told Wen-Chau Lee that he'd be sampling two Category 5 hurricanes within a month of each other, "I wouldn't have believed it," he says.
The NCAR scientist flew into Katrina and Rita during the Hurricane Rainband and Intensity Change Experiment. RAINEX is the first study to collect airborne Doppler radar data simultaneously from two critical parts of a hurricane: the rainbands that spiral toward a hurricane's eyewall and the fierce winds and updrafts in the eyewall itself. The goal of RAINEX is to analyze the role of rainbands in hurricane dynamics, including the poorly understood process of rapid intensification and the eyewall replacements that lead to cyclic weakening and strengthening.
This year's record crop of Atlantic hurricanes gave Lee and his RAINEX colleagues more data than they could have imagined. Both Katrina and Rita hit peak strength within easy flying range of MacDill Air Force Base in Tampa, where RAINEX aircraft operations were based. Two of NOAA's hurricane-hunter P-3s sliced through the eyewalls of Katrina and Rita, gathering data for public forecasts as well as for RAINEX. Meanwhile, Lee flew into and around rainbands aboard a third P-3, operated by the Naval Research Laboratory (NRL), that carries the Electra Doppler Radar (ELDORA).
Even a first glimpse of the RAINEX data was eye-opening, according to Lee. 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. Flown 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 (below). Strong couplets of vorticity reveal that the cells had impressive circulations of their own within the larger-scale flow along rainbands and into the eyewall. The cells were oriented at an angle toward the rainbands, which also intrigues the RAINEX group.
"I never imagined that rainbands were this complex in structure," says Lee. "This data set will help us understand the role mesoscale vorticity plays in storm-scale intensity changes." Robert Houze (University of Washington), one of the two principal investigators for RAINEX, concurs. "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 says.
||This horizontal cross-section from ELDORA shows two of the primary rainbands about 100 km (60 mi) east of the eye of Katrina on 28 August as the NRL P-3 flew between them (the thick dashed line shows the flight track). The tightly packed convective cells along the inner band feature strong couplets of vorticity (circulation), shown in solid and lightly dashed contours. (Image courtesy Wen-Chau Lee and Michael Bell.)
Shuyi Chen (University of Miami), the other RAINEX PI, notes the importance of RAINEX for mesoscale modeling of tropical cyclones. "The data we collected in Katrina and Rita provided a unique opportunity for evaluating and validating high-resolution computer models that are capable of resolving the eyewall and rainbands for future hurricane predictions," says Chen.
The logistics for RAINEX also paved new ground with the precision of its aircraft deployment. Flight track data for all three planes, as well as radar data from the two NOAA P-3 aircraft, were transmitted to the RAINEX Operations Center in real time. Combined with satellite data, they were then transmitted to the NRL P-3 within minutes to help the aircraft crew survey the most scientifically critical parts of the storm. Chris Burghart (NCAR) was among those responsible for the critical links between the aircraft and communications center. "This type of communication really sets a standard for future multi-aircraft missions," says Lee.
Is global change fueling intense hurricanes?
On 16 September, not long before Hurricane Rita broke Katrina's short-lived record for the lowest surface pressure observed in the Gulf of Mexico, Science published an intriguing study of tropical cyclones. The research team—led by Peter Webster (Georgia Institute of Technology)—found that the proportion of tropical cyclones that reach Category 4 or 5 status on the Saffir-Simpson intensity scale has nearly doubled in the last 35 years (see graphic below).
The graph at left shows the increase in Category 4 and 5 tropical cyclones (red line), summed over five-year periods since 1970. Between 2000 and 2004, there were more of these intense systems than there were Category 1 storms (blue) or Category 2 and 3 storms combined (green). The maximum wind speed observed in any tropical cyclone globally (black) has changed little since 1970. (Image by Peter Webster et al., courtesy Science.)
Webster, joined by NCAR's Greg Holland and Georgia Tech's Judith Curry and Hai-Ru Chang, drew on the best available data on tropical cyclones from around the globe, focusing on the post-1970 satellite era. In contrast to the sustained rise in Category 4 and 5 storms, which increased in number in all areas studied, they found that the overall frequency of tropical cyclones showed no sustained trend, increasing early in the study period but dropping in the last decade.
The 16 September article, widely discussed in scientific circles and quoted in the media, added fuel to an already contentious debate over the cause of this year's unusually active Atlantic hurricane season. The study, along with a Science paper earlier this year by Kerry Emanuel (Massachusetts Institute of Technology), lends support to climate models that show tropical cyclones will become more powerful as the world warms during the next century.
Scientists remain cautious, however, because other factors affect the intensity of a hurricane, such as atmospheric moisture and variations in winds. In addition, long-term cycles, such as the El Niño–Southern Oscillation, also induce multiyear fluctuations in hurricane intensity. "There could be problems with the data, and we cannot rule out natural variability," Holland says. But, he adds, "The balance of probability is that this is associated with the changing climate."
NCAR's Kevin Trenberth is confident that greenhouse gases are at least partly to blame for the recent increase in hurricane intensity. After Katrina, Trenberth began examining the extent to which warming temperatures are affecting moisture in the atmosphere. His conclusion: since water vapor amounts over the ocean have increased by an estimated 4% since 1970, which has enhanced both storm winds and moisture inflow to the storms, a preliminary estimate of the net global enhancement of hurricane intensity and rainfall is 3.6% to 11.8% since 1970.
Webster and colleagues did not calculate such a percentage for the influence of climate change on their findings. Nevertheless, Holland is struck by the extent to which cyclones have increased in intensity across the tropics. "There used to be a one-in-five chance that a hurricane would reach Category 4 or 5 status," he notes. "Now, there's a one-in-three chance. If you live in the tropics, that's a pretty big difference."
Pegging a hurricane's damage potential
Well before this year's biggest hurricanes charged ashore, it was clear that billions of dollars in destruction were quite possible across the oil-rich Gulf of Mexico and the heavily populated Gulf states. With so much at risk, some sectors of the economy needed more than general warnings. Identifying the most vulnerable power plants or pipelines, for example, could help utilities get the jump on an oncoming storm.
This year scientists parlayed output from the advanced research version of the Weather Research and Forecasting model (ARW-WRF) into groundbreaking portraits of where hurricane damage is most likely to be focused. Based at NCAR's Developmental Testbed Center, the ARW-WRF depicted 2005's terrible trio of hurricanes with resolutions as fine as 4 kilometers (2.5 miles) up to 48 hours in advance.
Swiss Re, one of the world's leading reinsurance companies, is collaborating with NCAR on an experimental simulation of Hurricane Katrina's impact on the Gulf Coast. By coupling Swiss Re's economic models of property in a given city with WRF's simulations of hurricane winds and rainbands, insurers hope to eventually better anticipate property losses.
WRF's prowess was also leveraged in real time this season to help utilities deal with Katrina and other hurricanes. As part of a broad, multiyear collaboration between NCAR and Los Alamos National Laboratory, LANL's Brian Bush is spending two years at NCAR. He's working to link WRF with hurricane damage models developed at LANL for the U.S. Department of Energy (DOE).
Over the past several years, LANL has used forecasts from the National Hurricane Center to drive its own damage models. That approach has its limits, says Bush. "NHC aggregates a lot of data, and you don't have visibility into all those underlying data."
This year Bush's colleagues at LANL combined high-resolution WRF output for winds and precipitation with storm surge information from the U.S. Army Corps of Engineers. In one experimental product derived for DOE, the North American Model (NAM) output for wind speed and direction was used to generate damage probabilities for generation stations, substations, and power lines across the central Gulf Coast. This aided LANL modelers in providing more accurate estimates of damage to electric power infrastructure. "It's been such a busy season that we haven't done the full validation yet, but subjectively the models did pretty well," says Bush.
Katrina brought a new twist to the forecasts of power restoration. Because Katrina's damage was so intense and widespread, the outlook had to consider not only the time required to restore systems but also the time needed for workers to get to the damaged equipment in the first place. Moreover, many of the electric substations were completely submerged in the New Orleans area, and some of these substations were critical to bringing the electric system back on line. This further increased the time required for complete restoration. "In some cases, that was three weeks," Bush says.
Beyond hurricane season, Bush and his colleagues at LANL and NCAR will continue refining their models while preparing for a conference in the fall of 2006 on climate and weather resilience, focusing on the energy sector.
A stormy game with a clear purpose
Video games and mayhem go hand in hand. But NCAR's Seth McGinnis and colleagues have built a set of online games that employ cyber-calamity to prove a point: what people do after a natural disaster can be just as important as what they do before.
NCAR's Disaster Dynamics project, led by Robert Harriss, is an ambitious effort to help train decision makers in how to build disaster-resilient communities, not just before but after disaster strikes. Hurricane Landfall, the first game in the Disaster Dynamics series, is aimed at undergraduates taking an emergency management course. The game involves four players, each at a different computer (either locally or at a distance). As they get acquainted with their virtual home town, which is located on a vulnerable coastline, each player takes a role: year-round resident, small-business manager, hotel manager, or vacationer. The group then sorts through a variety of proposals for town improvement.
Several of the Disaster Dynamics prototypes have been tested at the Boulder and Denver campuses of the University of Colorado and at the University of California, Irvine, where social ecologist Raul Lejano says the gaming approach piqued students' interest. "The Web-based game is a great tool for simulating the negotiation process, especially as applied to land use and hazard planning. The students really got into the process and deliberated, online, in earnest."
Texas A&M University at Galveston is interested in creating a customized version of Hurricane Landfall for the Houston-Galveston area. Future games will look at other natural hazards. A beta version of the hurricane game can be downloaded from the Internet (see "On the Web").
"Emergency managers spend a lot of time planning how they're going to react to a disaster," says McGinnis. Often ignored is the recovery phase, where a community's resiliency can be bolstered. "There's a window of opportunity after a major event when you can get a lot done," says McGinnis, pointing to Katrina as a classic example: "What happens now in New Orleans is going to have consequences decades down the road."
Like their real-world counterparts, the players of Hurricane Dynamics work with limited budgets and resources, says McGinnis, "so they have to propose things that improve the town with some reasonable compromise. Of course, all of these decisions have an impact on hazard exposure." The hurricane actually hits early in the game, and players then review a new set of proposals in the context of rebuilding and planning for future risk. "We want people to come away from the game with an appreciation for the big picture, realizing that issues like land use will have a big impact on their jobs as emergency managers."
Atmospheric gamers interested in moving on to a global setting might enjoy Keep Cool—Gambling with the Climate. This bilingual German-English game was recently issued by the Potsdam Institute for Climate Impact Research (PIK). The game allows three to six players to represent groups of countries, choosing modes of growth while dealing with lobby groups and adapting to inevitable climate impacts like droughts or floods. The winner is the player who most effectively reconciles climate protection with special interests. "Feedback has revealed surprising insights and exciting discussions about conflicts of interest in climate politics," says Klaus Eisenack, a PIK mathematician and one of the game's two authors.
One for the record books
Below are a handful of the records established as of 7 November by this year's astounding Atlantic hurricane season.
Most major U.S. landfalls (four, tied with 2004)
Strongest hurricane on record in Atlantic basin (Wilma, 882 millibars)
Most named storms in a season (23)
Most hurricanes in a season (13)
Earliest date for fourth storm (5 July) and for all storms thereafter except one
First-ever storms to begin with "V" and "W"; first time storms have required the use of Greek letters (Alpha and Beta)
Strongest Gulf hurricane in month of July (Dennis)
Most tropical cyclones to develop in July (5)
Most major hurricanes to develop in July (2, tied with 1916)
Latest major U.S. landfall (Wilma, 24 October)