Sizing up cyclones

UPDATE: 2016 SEASONAL FORECASTThe NCAR-based Engineering for Climate Extremes Partnership (ECEP) has issued its first seasonal forecast using the Cyclone Damage Potential index. The forecast is for a hurricane season with higher-than-average potential to cause damage. This year's forecasted seasonal CDP is 5.7, compared to an average seasonal CDP of 3.7 for the years 1981 - 2010. For more details, visit the ECEP website.May 18, 2016 |In early July 2005, Hurricane Dennis, a Category 3 storm on the Saffir-Simpson Hurricane Wind Scale, was bearing down on the Gulf Coast. Anyone paying attention would have been forgiven for having a foreboding sense of déjà vu. Just 10 months earlier, another Category 3 storm, Hurricane Ivan, had followed a strikingly similar track, making landfall just west of Gulf Shores, Alabama. Ivan ravaged the region, ultimately causing an estimated $18.8 billion in damages. But Dennis, despite roaring ashore in practically the same neighborhood, caused only $2.5 billion in damages—less than one-seventh that caused by Ivan.The fact that two Category 3 hurricanes making similar landfall less than one year apart had such different impacts illustrates a weakness in the Saffir-Simpson scale, the system most commonly used by weather forecasters to categorize hurricane risk.Scientists at the National Center for Atmospheric Research (NCAR)—in collaboration with global insurance broker Willis—have developed a new index that they suspect can do a better job of quantifying a hurricane's ability to cause destruction. The Cyclone Damage Potential index (CDP) rates storms on a scale of 1 to 10, with 10 being the greatest potential for destruction.A prototype for an app that will allow risk managers to easily use the CDP to identify local damage potential is already available and will be among the first tools included in the beta version of the NCAR-based Global Risk, Resilience, and Impacts Toolbox when it is released later this year.Infrared satellite imagery of Hurricane Ivan (left) and Hurricane Dennis (right). Both storms were rated Category 3, both made landfall in almost the same area, and yet they caused vastly different amounts of damage. Click to enlarge. (Images courtesy NOAA.)Moving beyond wind speedOn the frequently used Saffir-Simpson Hurricane Wind Scale, hurricanes are placed in one of five categories, based on their sustained wind speeds. On the low end, Category 1 storms have sustained winds between 74–95 mph and are expected to cause "some damage." On the high end, Category 5 storms have sustained winds of 157 mph or higher and are expected to cause "catastrophic damage."Because the Saffir-Simpson scale relies solely on sustained wind speeds, it does not take into account all the characteristics of a storm linked to its destructive power."Hurricane wind damage is driven by more than simply wind speed," said James Done, one of three NCAR scientists who worked on the CDP. "The hurricane's size and forward speed also are important. A large, slowly moving hurricane that repeatedly batters an area with high winds can cause greater total damage than a smaller, faster hurricane that blows quickly through a region."Damage caused to a marina in New Orleans by Hurricane Katrina. Katrina would have received a CDP rating of 6.6, compared to a 5.0 for Hurricane Ivan and a 2.4 for Hurricane Dennis. (Image courtesy NOAA. Click here for high resolution.)For example, the critical difference between Ivan and Dennis turned out to be hurricane size, according to a study of the storms by Done and Jeffrey Czajkowski at the University of Pennsylvania's Wharton Risk Management and Decision Processes Center.To create the CDP, the scientists incorporated hurricane size and forward speed into their index, along with sustained winds. To determine the relative importance of each, the team used hurricane damage statistics from the hundreds of offshore oil and gas facilities that pepper the northern Gulf of Mexico. Because facilities are spread more-or-less evenly across the region, their exposure to hurricanes is approximately the same. Damage differences from storm to storm can therefore be attributed to differences in the storms themselves. The CDP does not predict actual damage – which could vary markedly, depending on where (or if) a hurricane makes landfall – but instead predicts the storm's potential.When applying the CDP to past hurricanes, the index was able to discern a difference between Ivan, which would have scored 5.0 on the CDP prior to landfall, and the much smaller Dennis, which would have earned a 2.4. The CDP rating for Hurricane Katrina, which ultimately caused more than $80 billion in damages in 2005, would have been 6.6.“The value of the index is in comparing current storms with storms from the past," Done said. "For example, if a hurricane is approaching New Orleans, you can compare its CDP with Hurricane Katrina's CDP and get a fuller picture of how much damage the storm is likely to cause."The CDP project was led by NCAR scientist Greg Holland, along with NCAR colleagues Done, Ming Ge, and Willis collaborator Rowan Douglas.Dive deeperFrom today's storm to tomorrow's climateIn its original form, the CDP can be easily applied in real time to existing hurricanes. But Done also wanted to find a way to examine how hurricane damage might change in the future, especially as the climate warms. The question of how climate change may influence hurricanes has been difficult to answer, in part because global climate models are typically not able to "see" the small-scale details of individual storms. Though some scientists have run climate models at a resolution that is fine enough to study hurricane formation, the effort requires so much computing power that it hasn't been practical to replicate on a large scale.To skirt this problem, hurricane researchers have looked for links between hurricane activity and phenomena that climate models can see—for example, the sea surface temperatures of ocean basins."People have used large-scale variables to infer tropical cyclone activity for decades," Done said. "I've done a similar thing, but instead of predicting how many hurricanes will form, I’m predicting hurricane damage potential."To make this "climate" version of the CDP, Done – together with NCAR colleagues Debasish PaiMazumder and Erin Towler, and Indian Space Research Organization collaborator Chandra Kishtawal – looked for variables in the large-scale environment that could be correlated to the three variables used for the original CDP: sustained winds, size, and storm speed.The team found that "steering flow," the winds that would blow along a hurricane, is correlated with forward speed. They also found that relative sea surface temperature – the difference between temperatures in the Atlantic and Pacific ocean basins – is linked to seasonal average hurricane intensity and size. This is because relative sea surface temperatures affect wind speeds higher up in the atmosphere, which in turn affect hurricane formation. The result is an index that can spit out a damage potential rating for a season, a year, or even longer, without needing to predict how many individual storms might form. Such forecasts are of interest to large reinsurance companies, like Willis Re and others."This technique enables us to translate our climate model simulations into information about extreme events that’s critical for businesses and policy makers,” Done said.Writer/ContactLaura Snider, Senior Science Writer and Public information OfficerFundersResearch Partnership to Secure Energy for AmericaWillis Re CollaboratorsEngineering for Climate Extremes PartnershipWillis Re

A meeting for storm-driven science

October 22, 2014 | Among the world’s varied climates, two populous midlatitude areas get an especially big helping of large-scale extreme rainfall: eastern Asia and central-to-eastern North America. Experts from both continents met at NCAR on September 15–18 to discuss promising avenues of research that could lead to improvements in predicting hurricanes, floods, and other phenomena affecting billions of people. The Tenth International Conference on Mesoscale Convective Systems (ICMCS) arrived in the Western Hemisphere for only the second time in the meeting’s 15-year history. Sponsored by a nongovernmental organization called the East Asia Weather Research Association, the ICMCS rotates among five member nations: China, Japan, Korea, Taiwan, and the United States. Hurricanes, typhoons, and cyclones The ICMCS’s mission is broader than its title might suggest. Originally, the meetings focused on mesoscale convective systems, huge clusters of thunderstorms that often dump torrential rain. Over the last decade, the meeting has broadened to incorporate related phenomena and emerging technologies that can help observe and model such systems. A growing priority: the tropical cyclones known as hurricanes in North America, typhoons in east Asia, and cyclonic storms in India.  Fumie Murata (University of Kochi) confers with Satoshi Okawara (Japan Radio Company) at the ICMCS meeting in Boulder. (©UCAR. Photo by Bob Henson.) “Tropical cyclones are big issues for east Asia countries, so they’ve gotten more attention from ICMCS in recent years,” says NCAR’s Wen-chau Lee. He shared organizational duties for the Boulder meeting with fellow NCAR scientist Ying-Hwa “Bill” Kuo and David Jorgensen (NOAA National Severe Storms Laboratory). Although the United States has a well-deserved reputation for wild weather—the world’s heaviest one-hour rainfall occurred in Missouri, and 43 inches of rain once fell in 24 hours near Houston—Asia is no slouch when it comes to tropical cyclones, heavy rainfall, and flooding. NASA astronaut Reid Wiseman captured this image of Super Typhoon Vongfong from the International Space Station as the storm churned across the northwest Pacific Ocean on October 9, 2014. Vongfong was the world's strongest tropical cyclone of the year thus far, with estimated winds reaching 180 mph. (NASA image courtesy Reid Wiseman.) “In fact, these events are often more frequent and severe in Asia than their U.S. counterparts,” says Lee. One of the highlights of this year’s meeting in Boulder was the presentation of results from several field experiments over the last few years that focused on processes influencing Asian and North American weather. These studies, including TiMREX, T-PARC, ITOP, and DYNAMO, are helping scientists form a deeper understanding of how processes such as the Madden-Julian Oscillation transfer energy and moisture northward from the tropics and eastward across the Northern Hemisphere. The meeting drew nearly 100 participants from the five nations behind ICMCS, including about 20 graduate students. “These students represent the next generation of our scientific work force,” notes Lee. Writer: Bob Henson, NCAR/UCAR Communications Collaborating institutions:National Center for Atmospheric ResearchUniversity Corporation for Atmospheric Research Funders:National Science Foundation

A hurricane by any name

June 11, 2014 | According to the results of a high-profile study last week, people may take a hurricane named Glen a bit more seriously than one named Glenda. But the perceived gender of a hurricane’s name is just one of many factors potentially shaping how someone reacts to a given storm, according to several scientists at NCAR who take a multifaceted approach to studying hurricane response. While hurricane forecasts have improved substantially over the years, NCAR experts stress that the forecast and warning information on a given storm must be communicated as clearly and quickly as possible in order to save lives in coastal and inland communities. This is an increasingly complex undertaking as Facebook, Twitter, and other outlets are transforming the ways that people get information. “People get a variety of information over time about the risks they face as a hurricane approaches. We need to understand how that information evolves, and how that influences people’s risk perceptions and responses,” said Julie Demuth, an NCAR researcher who studies societal aspects of hazardous weather. As Hurricane Sandy approached on October 27, 2012, emergency officials in Old Saybrook, Connecticut, combined highway signs with a dedicated website to notify the public of mandatory evacuation plans. (Screen capture from video by Robert Rose/FEMA.) Demuth and her colleagues Rebecca Morss and Jeff Lazo found themselves peppered with media inquiries after a study titled "Female hurricanes are deadlier than male hurricanes" appeared in the Proceedings of the National Academy of Sciences. The study concluded that landfalling U.S. hurricanes with feminine names took more lives in recent decades than those with masculine names. The study also included experiments to find out how subjects respond to hurricane names and their perceived gender. Critics have lobbed a volley of counterarguments at the PNAS paper. The expertise of NCAR group members allowed them to put the study in context, as they’ve spent years looking closely at a number of variables influencing how people respond to hurricane threats. Along with colleague Betty Morrow (SocResearch), the group published a comprehensive analysis last fall in the Bulletin of the American Meteorological Society examining how hurricane risk information is created and communicated. In another study, examining the response to Hurricane Ike in 2008, Morss and colleague Mary Hayden found that a National Weather Service statement warning that Ike’s storm surge in some areas would lead to “certain death” produced counterintuitive responses among some residents. “A lot of factors influence decisions during actual hurricanes,” said Lazo, who thinks any effect from the perceived gender of a hurricane’s name is likely dwarfed in this mix. “How someone responds is affected by demographics, culture, and prior experience, as well as the quality and sources of information they receive, the time of day of landfall, and other variables.” Morss notes that the experimental subjects in the PNAS study weren't in circumstances similar to those faced by coastal residents when a hurricane threatens. “In a real hurricane situation, people are receiving many different pieces of information over a period of hours or days. It can be a very high-pressure situation, where you’re dealing with family interactions, monetary issues, and other constraints,” said Morss. Such real-world complications are all the more reason why in-depth research is needed, she added. “Hurricane Sandy and other recent events have clearly illustrated that there are major gaps in weather risk communication that contribute to loss of life and significant harm,” Morss said. “This is a very important area for further work.” Demuth voiced concerns about several of the methods and conclusions in the PNAS paper, but she told the Washington Post that she hopes the study leads to continuing dialogue and research on the very real question of what factors most influence people’s response to hurricane threats. “Research in risk perception and risk communication is a vital complement to research on how hurricanes behave and how they're predicted. We need all of these in order to protect lives and reduce harm.” Writer/contactBob Henson, NCAR & UCAR Communications ResearchersJulie Demuth, NCARMary Hayden, NCARJeff Lazo, NCARRebecca Morss, NCARBetty Morrow, SocResearch Funders National Science Foundation National Oceanic and Atmospheric Administration Soldiers from the Florida Army National Guard's 3rd Battalion, 20th Special Forces Group, talk to a resident swimming in the flooded section of the White Street Fishing Pier in Key West, Florida, on September 9, 2008. People braved the dangerous conditions at the end of the pier as storm surge from Hurricane Ike crashed over the barriers and flooded the pier. Guardsmen and Key West police officers warned the swimmers of the unsafe conditions, and soon the pier was closed to visitors by city officials. In the background a man holds a child in the dangerous surf. Ike went on to produce catastrophic flooding along the upper Texas coast, resulting in the largest search-and-rescue operation in U.S. history. The hurricane resulted in more than 100 deaths. (Wikimedia Commons photo by Tech. Sgt. Thomas Kielbasa.)

New views of Sandy

Bob Henson • October 1, 2013 | It’s been almost a year since the storm named Sandy pummeled the northeast United States. But the fearsome cyclone lives on within the circuits of supercomputers that are replicating its unusual evolution and track. As the hurricane threatened the Eastern Seaboard last autumn, NCAR scientists watched it unfold through high-resolution forecasts issued every six hours by Advanced Hurricane WRF. (AHW is the NCAR-based, hurricane-oriented, advanced version of the multiagency Weather Research and Forecasting model, or WRF.) Sandy’s unique track culminated in a bend to the west-northwest that brought the storm into the New Jersey coast. Brighter colors indicate higher surface winds; click on map to see original version with legend. (Image courtesy Wikimedia Commons.) Like many of its peers, AHW initially predicted an offshore track for Sandy. But several days before landfall, the model shifted to a more accurate forecast of Sandy’s nearly-unprecedented sharp left hook into New Jersey. Dramatic AHW animations of Sandy were prominently featured at a town hall in January at the annual meeting of the American Meteorological Society. Since then, several NCAR scientists have dug more deeply into the storm’s behavior for a paper now online at Monthly Weather Review. Meanwhile, the AHW forecasts have been replicated in new and stunning detail, resulting in one of the finest-scale animations to date of any hurricane. Both projects were discussed in August at the AMS Conference on Mesoscale Meteorology. (See the first two recorded presentations in the meeting program.) The forces that threw Sandy’s curve ball One of the many intriguing aspects of Sandy was its multiple phases. After striking Cuba as a Category 3 hurricane, the storm dipped below hurricane strength, but then it slowly gained power and size as it moved northeast, paralleling the Gulf Stream. Typically, such a system would continue moving toward the northeast and east, away from North America. But a huge mass of high pressure centered in Greenland partially blocked that path. At the same time, the polar jet stream was dipping sharply into the eastern United States. These two features steered Sandy on a path that swung to the west-northwest into southern New Jersey (see map at left). As the storm made its leftward bend, it reintensified to Category 2 status and then began transforming into a post-tropical cyclone. This prompted the National Hurricane Center to reclassify Sandy as post-tropical at 7:00 p.m. EDT on October 29, only an hour before landfall. Since the January AMS meeting, Tom Galarneau, along with NCAR colleagues Chris Davis and Mel Shapiro, have analyzed additional AHW simulations, which they describe in the just-released paper. They explain that Sandy’s life cycle—unique in Atlantic hurricane annals—was actually a blend of several well-studied phenomena that hadn’t been previously shown to come together in such a way near a major coastline.  In this 3-D map of potential temperature, relatively cool air wraps around Sandy's core near the surface (purple and blue colors), while air parcels gain heat from moisture condensing into clouds and precipitation as they ascend through the storm’s core. For more details on this simulation, see the YouTube videos below. (©UCAR. Image courtesy Mel Shapiro, NCAR. This image is freely available for media & nonprofit use.) As Sandy moved northeast, contrasting air masses created a pseudo-frontal system along the edge of the Gulf Stream’s warm water. The vorticity, or circulation, along this frontal zone (picture an atmospheric rolling pin oriented along the Gulf Stream) was gradually ingested by Sandy and tilted into vertical vorticity (now picture the rolling pin standing on one end). This helped the storm’s core to intensify, tighten, and regain its Category 2 status. It’s roughly similar to the smaller-scale process by which a supercell thunderstorm can ingest, tilt, and concentrate spinning air to produce storm-scale circulations (some of which can generate tornadoes). Sandy’s turn to the northwest was aided by the jet stream, which angled southeast into the mid-Atlantic in a “wave breaking” pattern. Just as a surfer caught in a breaking wave might be briefly pulled downward and seaward instead of shoreward, the breaking atmospheric wave in the jet stream pulled Sandy toward the northwest and away from the northeast track more typical for hurricanes at that location. Even while Sandy was still a full-fledged hurricane, its circulation was so large that its northern edges were more akin to an extratropical (nontropical) cyclone.  About 24 hours before Sandy made landfall, cool air began to wrap around its warm core, eventually surrounding it. Although this process meant Sandy was doomed as a tropical storm, it also may have intensified Sandy’s low-level winds. Sandy’s catastrophic storm surge was abetted not only by its vast size and rare track, but also by east-to-west near-surface winds that extended all the way across the North Atlantic, from Europe to the United States. Going into seclusion Strong winter storms at sea sometimes develop pockets of warm air within their cold cores—a process known as warm seclusion, first characterized by Shapiro and Daniel Keyser. However, in this case, the warm air being secluded was already present in Sandy’s inner core. This is the first time such a dramatic warm seclusion has been documented in a landfalling U.S. hurricane. Galarneau and colleagues did find examples documented in the scientific literature that occurred elsewhere, including hurricanes Iris (1995) and Lili (1996)—both far out in the North Atlantic—as well as eight North Pacific typhoons in 2001 and 2002 documented in a paper by Naoko Kitabatake (Japan Meteorological Agency). However, none of these other examples exhibited a dramatic northwest turn similar to Sandy's. Tom Galarneau (©UCAR. Photo by Carlye Calvin. This image is freely available for media & nonprofit use.) Although the NHC reclassification of Sandy just before landfall was a controversial move in some quarters, Galarneau thinks it was justified from a scientific point of view: the AHW runs show that the mechanisms driving Sandy’s reintensification were nontropical, and the storm’s warm core was completely secluded by cold continental air at that point. Starting this year, NHC will allow hurricane warnings to be retained even if a storm is reclassified as post-tropical before striking land (see PDF). Galarneau is now investigating Sandy’s track more closely. It’s the strongest storm known to strike as far north as New Jersey at a west-northwest angle in U.S. records going back to the 1850s. Galarneau has found several other tropical systems off the mid-Atlantic that moved toward the north or northwest during the September-through-November period. However, of the 18 systems identified, only five interacted with a dipping jet stream over the eastern United States and a blocking high near Greenland, a la Sandy. Each of these five occurred much earlier than Sandy in the hurricane season and interacted with much weaker dipping jet streams. "Comparing Sandy to these other tropical systems underscores the unusual nature of Sandy,” says Galarneau. For now, Galarneau is agnostic on the controversial theory that sea ice loss in the Arctic may have increased the likelihood of blocking highs such as the one that helped steer Sandy northwestward. He and colleagues agree there’s much more science yet to be done on the dynamics of this most unusual storm. Stunning visuals tell Sandy’s story Mel Shapiro (©UCAR. Photo by Bob Henson. This image is freely available for media & nonprofit use.) NCAR’s Shapiro has led the charge in another research direction: simulating Sandy at ultra-fine-scale resolution. This project emerged after Peter Johnsen (Cray Inc.) saw Shapiro’s AMS presentation in January and suggested a deeper dive. The two joined forces with Mark Straka at the National Center for Supercomputing Applications (NCSA), an NSF-funded facility located at the University of Illinois at Urbana-Champaign. Their work was also supported by the U.S. Office of Naval Research. Over the last several months, this team has used the Advanced Research WRF (ARW) model to replicate Sandy and its environment in unprecedented detail. In these model runs, each horizontal grid point is separated by just 500 meters (about 1600 feet). Since the model includes 150 vertical layers, this means that weather conditions were calculated at more than four billion points for each second in a 96-hour simulation. Simulation 1: Cloud Tops (All videos @UCAR. All are courtesy Mel Shapiro, NCAR, et al. and are freely available for media & nonprofit use.) Simulation 2: Radar Reflectivity Simulation 3: Potential Temperature Simulation 4: Wind Speeds Simulation 5: Trajectories of Air Parcels Carrying out this Herculean task required 58 hours of time using 140,000 cores (CPUs) on Blue Waters, a Cray XE6 supercomputer at NCSA. The raw data—49 terabytes’ worth—were processed by Alan Norton and Perry Domingo at NCAR’s Computational and Information Systems Laboratory, using the lab’s VAPOR software (Visualization and Analysis Platform for Ocean, Atmosphere, and Solar Researchers). At left are links to five of the animations produced thus far, which are available on the AtmosNews YouTube channel. They’re aesthetically as well as scientifically gripping—a point not lost on Shapiro, an art and design aficionado. Shapiro and colleagues are just beginning to analyze the features brought to vivid life by the animations. These include the wrapping of cold air around Sandy’s warm core, the evolution of showers and thunderstorms that helped fuel Sandy, and bands of heavy snow that fell over the Appalachians. Finer-scale guidance for forecasters At the National Hurricane Center (NHC), forecasters don’t have the luxury of such detailed model results when a hurricane approaches, since the computational demands are so enormous. But the team behind the ultra-fine AHW simulations is pursuing a possible demonstration project that would see if such projections are actually feasible in real time and whether they could bring value to the warning process. This could occur through the ten-year, NOAA-based Hurricane Forecast Improvement Project (HFIP), which is evaluating a variety of modeling approaches. Right now, the NOAA’s Hurricane WRF model (HWRF) is being run in real time through HFIP at a more modest resolution—27 km—in an experimental basin-scale domain covering the entire region from Hawaii to West Africa, between 10°S and 60°N. This is a broadened, next-generation version of the current operational version of HWRF, a collaborative effort involving NOAA’s Hurricane Research Division and Environmental Modeling Center. Both versions of the model allow individual storms to be tracked through moving nests, each with a resolution as fine as 3 km.  “What we’ve proposed is to run the entire basin-scale domain at a uniform 3-km resolution, to learn what we are losing by the nesting and to demonstrate the benefit of uniform grids,” says NOAA’s Frank Marks. Cray has the latest version of the operational HWRF installed, says Marks, “so the next step is to see whether the proposed configuration is reasonable to run in near real time.” There’s also been preliminary discussion among NCAR, Cray, and NOAA of possibly bringing 500-meter resolution into HWRF for testing.  HFIP runs through 2019, with forecast demonstration experiments planned each year until then. Storm surge—which kills most Americans who die in hurricanes—is a priority in HFIP. Starting this year, the project is experimenting with an operational storm-surge model tied to the operational version of HWRF, a collaboration involving several parts of NOAA and its National Weather Service. As with all HFIP demonstration model runs, these storm surge forecasts will be made available to hurricane forecasters at NHC for use and evaluation, though not released directly to the public (to avoid confusion with official hurricane and storm surge forecasts). Meanwhile, the team behind the 500-meter ARW runs is looking into linking their ultra-fine-scale modeling with a storm surge model such as ADCIRC (based at the University of North Carolina), which is now being adapted by NCAR’s Carl Drews for higher-resolution use. Such a pairing might eventually allow river channels, seawalls, and other fine-scale local features to be incorporated into storm surge forecasts. Though not involved in the ultra-fine-scale modeling efforts himself, NCAR's Tom Galarneau is watching with interest. “Sandy would be a good example to use in testing the value of ultra-high resolution, because of the wide range of coastline shape and infrastructure that it affected,” he says. Enhancing the resolution of a model doesn’t necessarily improve its accuracy, of course, so more work is needed to comprehensively determine how well the 500-meter resolution performs. Thus far, Shapiro and colleagues are encouraged by how well the results match up with satellite imagery (see graphic below). As such simulations become more common, Shapiro thinks they could open major new avenues for research and prediction. As he noted in his  AMS conference talk in August, “You will discover things about our atmosphere that you never even dreamt existed.” Ultra-fine-scale simulations of Sandy’s near-surface winds (upper right) and cloud-top temperatures (lower right) closely resemble the observations derived from satellite data (at left). (©UCAR. Images courtesy Mel Shapiro, NCAR. This image is freely available for media & nonprofit use.)

Proper alignment may be key to hurricanes

February 25, 2013 | Rita, Katrina, Sandy—these are familiar as the names of three of the most destructive hurricanes in recent history. Early warnings from reliable forecasts can allow thousands of people to evacuate in the path of such frightening and potentially deadly weather events, and the sooner such a warning can be issued, the better.  But if a storm fizzles after a warning goes out, that also creates risks, as people may be less likely to take precautions the next time a storm threatens—a syndrome known as warning fatigue. As a step toward meeting the goal of providing earlier warnings, NCAR scientists and their colleagues examined what enables poorly organized clusters of thunderstorms to develop into tropical storms and hurricanes. A study by NCAR scientists Chris Davis and David Ahijevych, published in the Journal of Atmospheric Sciences, details that the development of a tropical storm depends on certain critical features. These include a pre-existing tropical weather system that has a counter-clockwise swirl of air in the Northern Hemisphere (a clockwise one in the Southern Hemisphere) and is also aligned through a vertical column at least three miles high. However, this swirling air may not align at different altitudes.  For instance, the center of the circling air at the ocean’s surface may be at a different location from the center of the circling air several miles above. When that happens, the misalignment seems to delay or even break down the formation of a tropical storm. Davis and his team observed eight storm systems during one hurricane season to explore what conditions allowed storms to congeal and what made them fall apart. A field experiment funded by the National Science Foundation and NCAR known as PREDICT—the Pre-Depression Investigation of Cloud Systems in the Tropics—provided the means to collect data. The researchers relied on the NSF/NCAR Gulfstream V aircraft that flew multiple missions over the Atlantic basin during the heart of hurricane season, from August 15 to September 30, 2010. Dropsondes—parachute-outfitted devices that gather data as they fall to Earth—were released on each occasion at different altitudes. Sensors on the dropsondes measured air pressure, temperature, humidity, location, height, dewpoint, windspeed, and wind direction. Digging into the data, Davis and team confirmed that a high moisture content through at least the lowest three miles is important for tropical storm formation because it greatly increases the efficiency of rainfall. But an offset of circulating air contributes to delaying storm formation or makes things fizzle out altogether. “One likely reason that misalignment is important,” Davis says, “is that it allows the intrusion of air from outside the circulation, and this air tends to be drier than the air inside the circulation.” The structure of conditions that may or may not develop into a tropical storm are difficult to see in satellite data, Davis explains, unless the misalignment of the circulating air is extreme. However, these conditions can sometimes be depicted by computer models used in weather prediction. The new findings bring the science a bit closer to distinguishing the real threats, vital to protecting populations and saving them from warning fatigue. Christopher A. Davis and David A. Ahijevych, Thermodynamic Environments of Deep Convection in Atlantic Tropical Disturbances, Journal of the Atmospheric Sciences (January 1, 2013); DOI: 10.1175/JAS-D-12-0278.1

The eyes of winter

Bob Henson • February 22, 2013 | It’s been quite a month for cloud appreciators. Satellite images have revealed at least three dramatic eye-like features not far off the U.S. Atlantic and Pacific coasts over the last several weeks. While these can look startlingly like the eyes of hurricanes, they’re not quite the same thing. Two of these pseudo-eyes formed within major winter storms (extratropical cyclones) centered east of New England. One of these systems—dubbed Nemo by the Weather Channel—pummeled the Northeast with up to 40 inches of snow on February 8–9. A small eye-like feature appears amid the intense circulation around this low-pressure center. The storm brought extremely heavy snowfall to the northeast U.S. on February 8–9. (Image courtesy SSEC/CIMSS.) As the blizzard wound down in New England on the 9th, the still-powerful center of low pressure, well offshore, developed a tiny eye-like feature (see photo). This isn’t uncommon in the strongest nor’easters. Although extratropical cyclones typically have cold cores, a small pocket of warm air can be pulled from the warm side of the circulation and pinched off at the center of the cyclone. It’s one aspect of the Shapiro-Keyser process, outlined by researchers Mel Shapiro (NCAR) and Daniel Keyser (University of Albany, State University of New York). This process also factored into Superstorm Sandy as it neared landfall; see my writeup from November 2. Eye-like features have been spotted in nor’easters for decades. In a 1981 paper for Monthly Weather Review (PDF), Lance Bosart (University at Albany) discussed the presence of such a feature in the President’s Day snowstorm of 1979. And sharp-eyed Syracuse meteorologist/blogger Dave Eichorn noted the visual similarity between this month’s northeast blizzard and its colossal predecessor, the Blizzard of 1978, which sported its own eye-like center. What makes a pseudo-eye? In a well-developed hurricane, air rises rapidly through a tight ring of convection (showers and thunderstorms). But these furious updrafts are counterbalanced by air that’s being forced downward, both outside the storm and at its center. Air must rise to generate clouds and precipitation, so the subsidence in the middle of the storm tends to produce an eye with clear skies and relatively calm conditions—all surrounded by powerful thunderstorms that can tower up to 8 miles (13 km) or more. Some of the same circulation patterns may be at work in extratropical cyclones that produce eye-like features. But these cyclones aren’t as symmetric as hurricanes, and much of their rain and snow is produced by stratus clouds that are significantly more shallow than thunderstorms. Thus, the eye-like components in extratropical cyclones aren’t usually as clear-cut as in a hurricane, and they seldom last long. “I would call these ‘false eyes,’ ” says David Nolan (University of Miami). “A real hurricane eye has deep convection and also dynamically forced sinking down the middle.” Poleward and Pacificward Other types of cold-season weather can produce even more spectacular “eyes.” Polar lows—often informally called Arctic hurricanes—are a common feature of high-latitude winter, and they’re indeed like cousins of hurricanes in some ways. Despite the chilly environment overall, the warmest air in polar lows is at the center, in contrast to the cold-core nature of most extratropical cyclones. Winds in a polar low can exceed gale force, and their eye-like features can be quite prominent. The Polar Lows website (spotlighting “the coolest weather on the planet”) has plenty of examples. Despite its eerie visual similarity to a hurricane, this coastal eddy brought winds of little more than 10 mph as it approached the California coast near San Diego on February 17. (Aqua/MODIS image courtesy SSEC/CIMSS.) Atmospheric eddies off the coast of southern California can also produce distinctly clear centers that resemble hurricane eyes. One dramatic example occurred last weekend west of San Diego (see photo). The islands off southern California, such as Catalina and San Clemente, help produce many of these coastal eddies. Often they occur within vast fields of marine stratocumulus clouds. Sometimes these eddies are individual, but when strong winds strike an island, the flow can create pairs of oppositely rotating vortices that move downstream. The result is called a von Karman vortex street. These patterns are entrancing enough that an entire website is devoted to images, explanations, and predictions of von Karman vortex streets off Guadalupe Island. How could a modest coastal eddy produce a feature as cloud-free and symmetric as the eye of a Category 5 hurricane?  “I must admit the clearness is remarkable,” says Nolan. “It may be possible that a portion of the updraft is recirculating back to the center and descending, like in a hurricane. Perhaps it only takes a tiny amount of descent to clear out the shallow stratocumulus.” As Scott Bachmeier (CIMSS Satellite Blog) notes, eddy features off the coast of Southern California are not uncommon. However, the case pictured at right caught Bachmeier’s eye, as it were. “I’ve never seen an eddy there take on such a well-defined eye-like feature,” he told me. We can be grateful that this eddy brought only light breezes to southern California. If nothing else, that spared us from the headline, “False Eye Lashes Coast.” (Hat tip to climate scientist and fellow pun lover Joe Barsugli.)

A more perfect storm

Bob Henson • October 25, 2012 | It’s a scenario that seemed outlandish just days ago. But it now looks as if Hurricane Sandy will likely pummel the mid-Atlantic coast early next week. Technically, Sandy may not be classified as a hurricane by that point (I'll ponder that question in a future post; here, let's call it Sandy for convenience). Nevertheless, this storm is in a position to carve out multiple niches in U.S. weather history while producing what could easily be billions of dollars in damage. What makes Sandy so unusual?  And how might it rewrite weather history? The big hook The National Hurricane Center's official forecast issued at 2100 UTC (5:00 p.m. EDT) on Thursday, October 25, shows Hurricane Sandy snaking its way across the western Atlantic before striking New Jersey early on Tuesday, October 29. (Image courtesy NHC.) As I write, Sandy is in the Bahamas as a Category 2 hurricane, stronger than expected, and projected to move northeast from the Bahamas, as hurricanes so often do. But then—thanks to a strong upper-level storm dipping into the eastern U.S., and an extremely strong center of high pressure toward Greenland—the storm is expected to take a grand counterclockwise loop and arc northwest, which would bring it to the U.S. coast by Tuesday and drive it well inland. The official National Hurricane Center forecast issued at 5:00 p.m. EDT Thursday (see map at right) has the center of Sandy making landfall early Tuesday in New Jersey while moving toward the northwest. To say this path would be unusual is a major understatement. Few if any hurricanes in the last 100 years have struck the mid-Atlantic or New England with quite such a dramatically hooking path (which resembles a big question mark, as several forecasters have noted). Unfortunately, this curving path would also maximize Sandy’s time close to the warm waters of the Gulf Stream. As shown in NHC's weekly sea-surface map, temperatures are running 1-2°C above average off the mid-Atlantic coast. Update | 30 October:  Sandy's path indeed hooked—in fact, even a bit more sharply than indicated in this forecast graphic. The storm moved west-northwest into Atlantic City, New Jersey, and took a due-west course not long afterward. Extreme low pressure Leading forecast models are producing spectacularly low pressures at the center of Sandy.  Here are several results from this morning’s 1200 UTC runs for approximate projected intensity in hectopascals at or near landfall. Note that 945 hPa is close to 28.00 inches of mercury on a home barometer.           GFDL GHM ECMWF NOAA GFS             928 hPa 936 hPa 944 hPa             central NJ, Tuesday AM Delmarva peninsula, Monday PM Long Island, Tuesday PM This forecast panel from the NOAA Geophysical Fluid Dynamics Laboratory hurricane model (GFDL GHM), issued at 1200 UTC on Thursday, 25 October, shows Hurricane Sandy producing record-low barometric pressure near Philadelphia on Tuesday, 30 October. (Image courtesy NOAA.) While a couple of hurricane landfalls in Florida have produced pressures in this range, most cities in the Northeast have never reached such values, as is evident in this state-by-state roundup. The region’s lowest pressure on record occurred with the 1938 hurricane at Bellport, Long Island (946 hPa). As noted by Weather Underground’s Jeff Masters, the landfall pressures with Sandy may be less extreme than predicted, because of known model difficulties in handling tropical systems moving into midlatitudes. However, all-time records could still be well within reach at some locations. Update | 30 October:  Sandy's landfall air pressure at Atlantic City—946 mb—tied the 1938 record mentioned above. The storm set all-time records for low pressure at Philadelphia, Baltimore, Harrisburg, and a number of other locations. Big water Sandy’s hooking path and low pressure will help funnel mammoth amounts of seawater toward the Atlantic coast north of wherever the center makes landfall. Thanks to the massive high pressure to the north, and a separate low pressure center far to the east, models project a ribbon of easterly winds all the way from Europe to the mid-Atlantic as Sandy approaches. In addition, there’s a full moon on Monday afternoon, which will exacerbate any storm tides. All of these factors should help strengthen a rare and powerful wave- and surge-generating machine. Of the hurricanes ranked Category 2 or greater that have moved within 200 nautical miles of New York City since the 1850s, none are known to have taken a westward hook of the type indicated for Sandy. The hook raises the odds that water will be pushed perpendicular to the coastline, as opposed to more common paths that often parallel the coast. (Image courtesy NOAA Historical Hurricane Tracks.) The closest recent analog in this realm may be the late-October system of 1991 dubbed "the perfect storm" and made famous in the book and movie of the same name. This was a nontropical winter-type storm southeast of the Canadian Maritimes that absorbed the remnants of Hurricane Grace and arced west. Fortunately for North America, it veered back eastward before making landfall. However, it still produced waves of 10 to 30 feet from North Carolina to Nova Scotia, with tides as high as 14 feet in Boston and a record 7.8 feet in Ocean City, Maryland. It's worth noting that the surface pressures with Sandy are predicted to be substantially lower than for the 1991 storm (972 mb). Moreover, Sandy is expected to actually make landfall. New York City is at particular risk for serious impacts from storm surge. If Sandy moves inland on the New Jersey coast, huge amounts of water will flow toward New York’s harbor, so predictions of storm surge will be critical. NOAA is now issuing surge predictions for landfalling hurricanes using a probabilistic, ensemble-based approach. And at the Urban Ocean Observatory of the Stevens Institute of Technology, Philip Orton and colleagues are using the New York Harbor and Ocean Observing System (NYHOPS) to produce surge forecasts based on the NAM model. “I am personally very concerned about storm surges in New York City,” says Orton, who’s blogged on the topic. He’s working to quantify the longer-term threat with the Consortium for Climate Risk in the Urban Northeast. “City managers and scientists agree that we’re not ready for a 100-year flood event, in major part because we haven’t had one in well over 100 years," Orton says. Inland flooding is also a serious threat with Sandy. NOAA guidance and recent model runs suggest that 5–10” of rain could fall along and near the storm's path. Heavy snow is even possible along some of the highest terrain of the Appalachians. Update | 30 October: Needless to say, the concerns about Sandy's storm surge were well founded. With the surge peaking near high tide, New York City experienced water levels of 13.81 feet at the Battery, not only breaking the official 1960 record of 10.5 feet from Hurricane Donna but also topping the 11.2 feet observed in an 1821 hurricane. University of Wyoming researcher Eric Debenham launches a radiosonde during a 2003 field study. Hundreds of extra radiosondes will be launched across the nation over the next several days to help nail down Sandy's path. (©UCAR. Photo by Carlye Calvin. This image is freely available for media & nonprofit use.*) Trees and power lines Perhaps the biggest potential threat from Sandy for millions of people inland is the risk of power outages and tree loss. Winds may not be sustained above hurricane force, especially inland, but strong gales of 50–60 mph with higher gusts could easily stretch across a vast swath of the most heavily populated part of the country. These winds may be enough to bring down many trees and power lines, especially when accompanied by soil-loosening rains. If enough utility customers are affected, it could take days if not weeks to restore power to some areas—potentially affecting the November 6 elections. All eyes on the storm As this threat builds, NOAA has planned an unprecedented array of extra observations. Radiosondes (weather balloons) are normally launched twice a day, at 00Z and 12Z UTC, from nearly 100 points across the United States. Starting today, the National Weather Service will launch extra balloons at 06Z and 18Z, nationwide at first (to capture an evolving storm now over the western U.S. that will steer Sandy), and then focusing on the eastern U.S. into early next week. “While we have run extra soundings in the past, we have never done this for the entire continental U.S.,” Louis Uccellini told me. Uccellini is director of NOAA’s National Centers for Environmental Prediction. “Doing this on such short notice is an indication of how seriously the NWS views the potential threat from this system, especially as it relates to coastal inundation, flooding from heavy rains, and the like.” In addition, the GOES-14 satellite will be collecting special imagery above Sandy at 1-minute intervals until the storm dissipates or makes landfall. Refresh these maps (Colorado State University) to see loops of the 1-minute data.    

After the storm: Interviews with coastal residents following Hurricane Ike

February 25, 2011 | A case study from NCAR looks at how coastal residents assessed their risks and made decisions leading up to Hurricane Ike, along with how they perceived a statement issued by the National Weather Service that people in some areas would face “certain death” if they didn’t evacuate. Hurricane Ike made landfall as a category 2 hurricane near Galveston, Texas, on September 13, 2008. It generated a substantial storm surge of 10–20 feet (3–6 meters) in coastal southwestern Louisiana and eastern Texas, with the worst surge near Galveston. The surge inundated parts of Galveston Island and other areas, destroying structures and causing fatalities. In a series of interviews with 49 coastal Texas residents affected by Ike conducted five weeks after the storm, NCAR scientists Rebecca Morss and Mary Hayden gathered data on people’s perceptions of the hurricane’s risk, their preparation and evacuation decisions, and their opinions of the hurricane forecasts and warnings. They found that while most interviewees paid close attention to Ike as it approached and were aware that the storm was potentially dangerous, the extent of flooding surprised interviewees, as many had prepared primarily for strong winds. Although some interviewees reported that evacuation orders were very important to their decisions to stay or go, the majority also took other factors such as weather forecasts into account and used their own personal judgment. Of the interviewees who heard the NWS’s “certain death” statement, reactions were mixed. The statement helped convince several residents to evacuate, but others had strong negative opinions—describing it as “overblown” and “ridiculous,” for example—and indicated that it might decrease their sensitivity to similar warnings in the future. Morss, Rebecca E., and Mary H. Hayden, 2010: “Storm Surge and ‘Certain Death’: Interviews with Texas Coastal Residents following Hurricane Ike,” Weather, Climate and Society, doi:10.1175/2010WCAS1041.1

Hurricane study to tackle long-standing mystery

  News Release Multimedia Gallery   BOULDER—Scientists are launching a major field project next month in the tropical Atlantic Ocean to solve a central mystery of hurricanes: Why do certain clusters of tropical thunderstorms grow into the often-deadly storms while many others dissipate? The results should eventually help forecasters provide more advance warning to those in harm’s way. “One of the great longstanding mysteries about hurricanes is how they form,” says Christopher Davis, a scientist with the National Center for Atmospheric Research (NCAR) and a principal investigator on the project. “There are clusters of thunderstorms every day in the tropics, but we don’t know why some of them develop into hurricanes while others don’t. We need to anticipate hurricane formation to prepare for hazards that could develop several days later.” PREDICT, the Pre-Depression Investigation of Cloud Systems in the Tropics, will run from August 15 to September 30, the height of hurricane season. The project is funded primarily by the National Science Foundation (NSF), NCAR’s sponsor. In addition to NCAR, collaborators include the Naval Postgraduate School; University at Albany-SUNY; University of Illinois at Urbana-Champaign; University of Miami; NorthWest Research Associates, Redmond, Washington; New Mexico Tech; Purdue University; and University of Wisconsin–Madison. Based on St. Croix in the U.S. Virgin Islands, PREDICT will deploy the NSF/NCAR Gulfstream V research aircraft. The G-V jet, also known as HIAPER, has a range of up to 7,000 miles and will reach an altitude of about 43,000 feet, enabling scientists to take observations near the tops of storms that form thousands of miles from the coast. (View the G-V's external instrumentation for PREDICT.) By better understanding the formation of tropical storms that may become hurricanes, scientists can help the National Hurricane Center attain the goal of seven-day hurricane forecasts, rather than the current limit of five days. Long-term predictions are needed by shippers, offshore oil operators, emergency managers, and others involved in public safety to better prepare for incoming storms. Currently, many storms develop too quickly for society to make sufficient preparations. In 2007, for example, scattered thunderstorms in the Atlantic Ocean organized into a larger-scale storm system that quickly grew into Hurricane Felix, a category 5 storm that caused widespread loss of life and destruction in Nicaragua and Honduras. Three projects with a common purpose The PREDICT flights will be coordinated each day with flights for two other hurricane studies taking place this summer. NASA is leading a project known as GRIP (Genesis and Rapid Intensification Processes), while the National Oceanic and Atmospheric Administration (NOAA) is leading IFEX (Intensity Forecasting Experiment). A cluster of thunderstorms took just a few days to build into Hurricane Felix in 2007. By the time of this satellite image—1810 UTC (2:10 p.m. EDT) on August 2—Hurricane Felix had winds exceeding 130 mph. (Image courtesy NOAA Satellite and Information Service, via Wikipedia.) More images are available in the PREDICT Multimedia Gallery. Although the three projects are independent, their observations have the potential to capture the complete evolution of one or more hurricanes from formation until landfall, as well as capture non-developing storms that are equally important for understanding why some disturbances develop beyond the wave stage while many others do not. “We hope the information we gather this summer will unlock some of the secrets of how hurricanes form and evolve,” Davis says. “This is key information we all need to better protect lives and property from major storms.” Rotating updrafts and marsupial pouches One of the central goals of PREDICT is to pinpoint the differences between a tropical thunderstorm cluster that is capable of growing in power and one that is likely to weaken. Scientists theorize that part of the secret may lie in the 3-D air motions within a larger system, such as a tropical easterly wave or a subtropical disturbance, that can serve as a safe haven for rotating thunderstorms. As these thunderstorms draw in rotating air from their sides, they can develop increasingly powerful, tightly wound circulations, analogous to figure skaters who spin faster and faster by drawing their arms inward. In contrast, strong downdrafts that reach the surface and spread out can slow the spin of the storm. When a thunderstorm cluster is surrounded by a deep layer of moist air, the likelihood of downdrafts is significantly reduced. But the problem of hurricane formation is not as simple as the formation of one or several rotating thunderstoms, which persist typically for a few hours at most. To unlock the mystery one must look also at the larger-scale environment, including the structure and composition of the large-scale disturbances and tropical easterly waves that encompass clusters of thunderstorms. PREDICT will focus on regions where tropical easterly waves and embedded thunderstorm complexes are most likely to form tropical storms. The project will gather data across areas as large as 500 by 500 miles, using remote sensing instruments on NOAA and NASA aircraft to probe for details within these preferred regions of development. Some scientists have advanced a concept, known as the “marsupial pouch,” that they believe is key to tropical cyclone development. According to this hypothesis, if a storm cluster moves at a similar speed to the surrounding flow in the lower to middle troposphere and is not adversely deformed by horizontal wind shear, then it is largely protected from being torn apart. This protective environment, known informally as the “marsupial pouch,” can also help insulate storms from dust and dry air that might impede their growth. Within such a protective pouch, the system could draw energy from warm ocean waters, develop a closed circulation of winds, and form a tropical depression, perhaps eventually becoming a tropical storm or hurricane. “We think the marsupial pouch provides a focal point or ‘sweet spot’ where favorable conditions could persist for several days and where rotating thunderstorms are most likely to aggregate into a larger-scale storm,” says Michael Montgomery, a PREDICT principal investigator and lead scientist, as well as a professor at the Naval Postgraduate School. “This would dramatically increase the chances of a tropical depression or larger storm forming.” The findings from PREDICT will be particularly useful for forecasts in the North Atlantic. However, the results will also help forecasters in parts of Asia and Australia where coastlines are vulnerable to typhoons and cyclones (as hurricanes are known there). The findings may also help provide important insights into the equally difficult question of whether climate change will significantly increase the frequency or intensity of these powerful storms. “If we can better understand the processes that lead to hurricanes, we can apply that knowledge to our changing climate and how it is likely to influence future tropical storms and hurricanes,” Davis says. Measurements in a remote environment Part of the reason that hurricane formation has remained such a mystery is that scientists have comparatively little information in general about storms that develop over the ocean. Observations from ships and aircraft are few and far between, while satellites have difficulty providing wind and temperature information beneath cloud tops within a storm. The PREDICT research team will fly near thunderstorm complexes and probe the surrounding environment once or twice per day when tropical systems of interest come within about 1,500 miles of St. Croix. Using dropsondes (parachute-borne instrument packages) and a variety of other instruments, they will take measurements of temperature, humidity, wind speed and direction, and water vapor. They will also gather fine-scale details of clouds, including ice particles and water droplets. One of the main goals is to take measurements of airborne Saharan dust and associated dry air that can interfere with hurricane formation. “We’ll be scrutinizing developing storms on many scales, from the invisible to the enormous,” says Lance Bosart, a professor at the University at Albany-SUNY. The airborne observations will be compared with data gathered from satellites as well as from ground-based radars in the Caribbean.

Impact of hurricanes on global climate

January 23, 2009 | While there has been much attention focused on the question of whether climate change influences hurricanes, scientists are also interested in whether the reverse holds true: do hurricanes significantly impact global climate? NCAR scientists Aixue Hu and Jerry Meehl are analyzing whether changes in hurricane seasons over time can alter the meridional overturning circulation (MOC) in the Atlantic Ocean. This circulation, driven by variations in salt and temperatures in ocean waters, transports heat north from the tropics and moderates the climate of northern Europe. Paleoclimate studies have indicated that past shutdowns of the MOC were associated with abrupt climate change. Aixue and Jerry used the Community Climate System Model to simulate ocean conditions in three scenarios: no hurricanes, hurricane winds only, and hurricane winds with torrential rains. They found that the winds tend to strengthen the MOC, increasing heat transport to upper latitudes, and warming the subpolar North Atlantic by several tenths of a degree Celsius. The reason has to do with hurricane winds evaporating water from the surface, thereby increasing the salinity of the remaining water while also warming subsurface temperatures—all factors that would increase the northward movement of warmer water. Hurricane rains, however, have a different impact. The rains reduce salinity, thereby weakening the MOC. This cools sea-surface temperatures in the subpolar North Atlantic by up to 0.5 degrees Celsius. A key direction for future research is to determine which mechanism is more powerful: the hurricane winds that strengthen the MOC or the associated rains that weaken it. Aixue and Jerry note that the answer may be complex, depending on the dynamics of individual hurricanes. Their analysis indicates that, in cases where water vapor is drawn into a hurricane from a large region relative to the size of the storm, there may be comparatively more weakening of the MOC. The research will be published in Geophysical Research Letters.


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