Tornadoes, microbursts, and silver linings

Bob Henson • April 1, 2014 | It takes a sharp eye to find something positive in the wreckage of the worst swarm of U.S. tornadoes on record. Ted Fujita had just such an eye, and millions of Americans are safer in the air because of it. Theodore "Ted" Fujita was renowned for his meticulous work in observing and analyzing meteorological phenomena, including tornadoes and microbursts, through photographs and damage surveys as well as weather data. (Photo courtesy University of Chicago.) Fujita, who died in 1998, is known around the world for devising the Fujita Tornado Damage Scale, or F-scale. Now reworked as the Enhanced Fujita Tornado Damage Scale, it’s the system most commonly used to rank tornado strength based on observed damage. A prolific researcher, Fujita made many other contributions to meteorology that are less well known to the public. One standout is his conceptualization of microbursts, the small yet dangerous pockets of descending wind that even weak showers can produce. Fujita and colleagues at the University of Chicago joined with NCAR researchers in the late 1970s and 1980s in definitive work that clarified the danger posed to aviation by microbursts. The effort was a spectacular success—and it all started in the wake of a devastating round of tornadoes that struck 40 years ago this week. Insight from tragedy On April 3, 1974, the U.S. South and Midwest were raked by more than 140 tornadoes that killed more than 300 people. The sheer scope of this disaster, which Fujita dubbed the Jumbo Outbreak (it’s also referred to as the Super Outbreak), went beyond anything previously measured. Since then, the only comparable event was the rash of more than 300 tornadoes across much of the eastern United States on April 25–28, 2011, again killing more than 300 people. While conducting an aerial survey of damage from the Jumbo Outbreak, Fujita—already known among peers as a meticulous observer—noticed something strange. He recounted the experience in a 1985 monograph, The Microburst: The deadliest single tornado of the Jumbo Outbreak on April 3, 1974, plowed through Xenia, Ohio, killing 36 people. (Wikimedia Commons image.) “Unlike the swirling patterns of fallen trees, commonly seen from the air in the wake of tornadoes, hundreds of trees were blown outward in a starburst pattern. Trees near the starburst center were flattened or uprooted, spattered by a brownish topsoil.” Fujita reasoned that such damage was produced not by the inflowing, rising air in a tornado, but rather from a focused surge of descending air that suddenly diverges when it hits the ground. He likened this to flow from a garden hose. If the hose is pointed directly downward, the water will spray outward in all directions, but if the hose is angled just slightly, the water fans out toward one side, much like the damage observed by Fujita. Fujita concluded that about 15 percent of the jaw-dropping 2,598 linear miles of damage he mapped from the Super Outbreak was caused by these “outburst” winds, rather than by tornadoes. The next critical event occurred on the stormy day of June 24, 1975, when an Eastern Airlines plane crashed on approach at New York’s Kennedy International Airport, killing 113 people. Analyzing eyewitness reports and black-box data, and recalling the damage patterns he’d observed the year before, Fujita concluded that a “downburst” was likely responsible for the crash. (He later coined the term “microburst” to describe a downburst that was no more than 2.5 miles wide at ground level. See this comparison.) Quelling the critics with field data As with many transformative scientific ideas, Fujita’s concept came up against prominent detractors. Most researchers at the time believed that even strong downdrafts from thunderstorms ought to weaken before reaching the surface. Fujita also bucked convention in how he carried out his research. “The large majority of his downburst work was not published in peer-reviewed journals,” noted Jim Wilson (NCAR) and Roger Wakimoto (now NSF assistant director for the Directorate for Geosciences) in a 2001 retrospective for the Bulletin of the American Meteorological Society (see PDF). They wrote: “It is likely that [the publication process] would have been an irritating, time-consuming activity for Fujita. He probably realized reviewers would have questioned his unorthodox analysis procedures and heavy use of unstated assumptions.” As Fujita continued to gather data through aerial damage surveys in the late 1970s, he also enlisted the help of NCAR and its pair of portable Doppler radars, which were then among only a handful available in the world available for atmospheric research. With encouragement from Bob Serafin, who managed NCAR’s observing facilities at the time (and who later became NCAR director), NSF agreed to fund a field campaign. “At a time when many in the scientific community had serious doubts about Fujita’s downburst hypothesis, these two [entities (NSF and NCAR)] fully supported his efforts,” noted Wilson and Wakimoto. Several landmark field projects over the next decade made it clear that Fujita was on the right track. Descending air curls outward and upward as it slams into the ground in a microburst near Denver's former Stapleton International Airport on July 6, 1984, during the CLAWS project (Classify, Locate, Avoid Wind Shear). (Photo by Wendy Schreiber-Abshire, ©UCAR. This image is freely available for media & nonprofit use.) NIMROD, the Northern Illinois Meteorological Research on Downbursts, combined NCAR’s two radars with the CHILL radar (University of Chicago/Illinois State Water Survey). As NIMROD detected its first microburst, on May 29, 1978, Fujita and Wilson stepped outside and were almost blown into a nearby farm pond.  The radars and surface stations ended up detecting about 50 microbursts that spring and summer. JAWS, the Joint Airport Wind Shear project, took place in northeast Colorado in 1982, followed by CLAWS (Classify, Locate, and Avoid Wind Shear) in 1984. With three NCAR Doppler radars positioned more closely together, Fujita identified more than 180 microbursts and analyzed several in great detail through vertical cross sections. One of the stronger microbursts occurred near Denver’s former Stapleton Airport from a cloud with a radar reflectivity of just 17 dBZ—barely a detectable shower. (Fujita also saw his first-ever tornado that summer, near the town of Bennett.) MIST, the Microburst and Severe Thunderstorm project, brought the three NCAR Doppler radars to northern Alabama in 1986. This study focused on “wet” microbursts—those fed by great volumes of heavy rain, as opposed to the evaporation and cooling that produced the “dry” microbursts common in more arid regimes.   NCAR’s John McCarthy spent much of his time during the summers of 1984 and 1985 in the control tower of Denver’s Stapleton International Airport during the CLAWS project. (©UCAR. This image is freely available for media & nonprofit use.) Results from these field projects quickly convinced skeptics that microbursts were a bona fide threat to aviation. This initial awareness helped pilots and air traffic controllers stem the rate of major microburst-related accidents, which had been occurring every year or two in the 1970s and early 1980s. More intensive pilot training, much of it initiated by NCAR’s John McCarthy and sponsored by the Federal Aviation Administration, cemented the new concepts. Technology also made a huge difference. Building on research led by McCarthy, and with FAA support, NCAR teamed up with the Massachusetts Institute of Technology’s Lincoln Laboratory to develop software that generated wind-shear alerts, drawing on observations from surface weather stations near airports and a new network of airport-based Doppler radars. As the new tools and observing systems spread nationwide, microburst-related accidents became increasingly rare: the last major U.S. flight mishap attributed to wind shear occurred in 1995. RAL is still actively helping international governments and airport authorities procure wind shear systems. “The microburst/aircraft problem demonstrated how funding focused on a particular weather problem can lead to an operational solution,” says Wilson. The next challenges It’s likely that hundreds of deaths have been avoided thanks to Ted Fujita’s uncommon insight, his exhaustive documentation, and the careful field work carried out by NCAR scientists and their collaborators. Many of them remain active in research, savoring the microburst success even as they look to new areas where knowledge and technology can make transportation safer and more efficient. Much of this work now takes place within the framework of the FAA’s NextGen initiative, a comprehensive approach to aircraft navigation, weather information, and safety. Turbulence remains a vexing aviation problem. Even with no thunderstorms nearby, severe turbulence injured 11 people (two seriously) in February aboard a United Airlines flight en route from Denver to Billings, Montana. To date, around 200 United and Delta aircraft have tested an NCAR-developed system that automatically diagnoses turbulence and creates global maps of turbulence intensity. Southwest Airlines is expected to join the system later this year. Another NCAR software package uses Doppler radar data to remotely detect turbulence related to thunderstorms. Icing is another long-recognized hazard that’s being partially tamed by technology. An NCAR-developed system, now marketed by Vaisala, allows airports to monitor snowfall rate and other weather factors crucial to aircraft de-icing as they unfold. Two products developed at NCAR use current observations and short-range computer models to map out where in-flight icing conditions are expected to be most likely and most severe, both now and over the next 12 hours. NCAR is also collaborating with NOAA on a prototype system that would provide a global picture of predicted icing severity. This graphic display from the FIP-Severity software program depicts a two-hour icing severity forecast across the United States on March 15, 2011. The forecast is for a column extending from 1,000 to 30,000 feet above mean sea level. The shades of blue denote the level of severity, with dark blue indicating heavy icing. The red areas of "SLD threat" are warnings for the presence of supercooled large drops, an indicator of severe icing potential. Icing can create a significant hazard for some types of aircraft. (Image courtesy NOAA/NWS/ADDS.) Even though U.S. air travel is safer than ever, passengers can still encounter major inconvenience with long delays and cancelled flights. A new NCAR-based tool for air traffic awareness could help give airlines a probabilistic sense of where, when, and how many flights are likely to be hindered by thunderstorms. The system converts output from high-resolution computer forecast models into estimates of how much airspace capacity might be reduced—essentially switching the forecast product from weather itself to its impact on flights. The time frame of several hours into the future represents a sweet spot, with the probabilistic approach allowing for guidance beyond “nowcasting” of thunderstorms and other threats while giving traffic controllers and airlines a valuable planning window. The experimental project has been funded by NASA, the FAA, and most recently the National Weather Service. It’s also possible that some of the next transformative breakthroughs in transportation safety will occur on terra firma. Weather-related highway accidents kill thousands of Americans each year. NCAR’s Research Applications Laboratory—which emerged after the successful attack on the microburst problem—now includes a branch focused on surface transportation. Among their current projects is a winter-weather decision support system that uses weather and road data sent by specially equipped snowplows to help determine where plowing and sanding are most needed. When deployed on a larger scale, millions of “intelligent” vehicles might someday give drivers crucial advance notice of ice, rain, and other potential road hazards just around the bend.  As with microbursts and aircraft, there’s nothing like the power of plentiful data and thorough analyses to help keep people safe from weather’s worst.

Triggering turbulence in clear air

January 16, 2013 | Turbulence is the leading cause of injuries to passengers and crew aboard commercial aircraft, and it indirectly increases travel expenses by costing airlines tens of millions of dollars yearly. While much of the rough air occurs within clouds, planes sometimes unexpectedly encounter turbulence while cruising through regions of clear air. New research by NCAR researchers and collaborators points to gravity waves, which ripple unseen through the atmosphere, as the culprit in many cases of clear-air turbulence. If those waves can be forecast, the research suggests that planes in many cases could be rerouted around them. “Clear-air turbulence forecasting is one of the last great challenges of numerical weather prediction,” says Bob Sharman, who leads NCAR’s turbulence research team. “As we better understand what causes turbulence, we can begin developing systems to predict it.” Sharman presented the findings at December’s annual meeting of the American Geophysical Union. Gravity waves are a common atmospheric phenomenon. They are caused when air is forced upward, generally over mountains or in thunderstorms, and bumps up against the stable floor of the stratosphere. This sets off ripples that can travel hundreds of miles before breaking. (Gravity waves are unrelated to gravitational waves, which are perturbations in the gravitational field.) For their research, Sharman and colleagues from NCAR; the University of Melbourne in Australia; the University of California, Los Angeles; and the Naval Research Laboratory collected observations of turbulence from more than 100 commercial aircraft. The team then compared those reports to the locations of cloud and other possible sources of gravity waves. The observations, which measure the extent to which rough air causes up-and-down movements of an airplane, were recorded by a special onboard system devised by NCAR several years ago. Sharman and his team then used the NCAR-based version of the Weather Research and Forecasting model (WRF) to simulate atmospheric conditions associated with observed turbulence events. They found that gravity waves “break” against aircraft, much as ocean waves break on the beach. Although clear-air turbulence has traditionally been thought to be due mainly to areas of high wind shear associated with jet streams, the research indicates that gravity wave breaking events actually account for much of the observed turbulence. Gravity waves often break within a relatively shallow altitude range, so pilots might be able to avoid them if they knew where the waves were. But the waves cannot be detected by radars aboard commercial aircraft. For the next phase of their research, Sharman and his colleagues are using a computer model to better understand the initiation and evolution of gravity waves and gravity wave breaking. If they can successfully predict turbulence associated with gravity waves at least 85% of the time, the aviation industry may find it cost effective to reroute aircraft. Even if the planes did not take action to avoid the waves, pilots could often alert passengers about a bumpy ride ahead. “The goal is to make flying as safe and comfortable as possible,” Sharman says. The research is supported by NASA and the National Oceanic and Atmospheric Administration.

New system for aircraft forecasts potential storm hazards over oceans

BOULDER—The National Center for Atmospheric Research (NCAR) has developed a prototype system to help flights avoid major storms as they travel over remote ocean regions. The 8-hour forecasts of potentially dangerous atmospheric conditions are designed for pilots, air traffic controllers, and others involved in transoceanic flights. A new online system, developed by an NCAR-led research team, provides eight-hour forecasts of major storms over much of the world's oceans. The prototype system can help transoceanic flights avoid potentially dangerous atmospheric conditions.  (©UCAR. Screen capture courtesy Cathy Kessinger, NCAR. This image is freely available for media & nonprofit use.) The NCAR-based system, developed with funding from NASA’s Applied Sciences Program, combines satellite data and computer weather models to produce maps of storms over much of the world’s oceans. The system is based on products that NCAR has developed to alert pilots and air traffic controllers about storms and related hazards, such as turbulence and lightning, over the continental United States. Development of the forecasts was spurred in part by the 2009 crash of Air France Flight 447, which encountered a complex of thunderstorms over the Atlantic Ocean. NCAR worked with the Massachusetts Institute of Technology's Lincoln Laboratory, the Naval Research Laboratory, and the University of Wisconsin-Madison to create the system. “These new forecasts can help fill an important gap in our aviation system,” says NCAR’s Cathy Kessinger, the lead researcher on the project. “Pilots have had limited information about atmospheric conditions as they fly over the ocean, where conditions can be severe. By providing them with a picture of where significant storms will be during an eight-hour period, the system can contribute to both the safety and comfort of passengers on flights over the ocean.” The forecasts, which continue to be tested and modified, can be viewed here. They cover most of the Atlantic and Pacific oceans, where NCAR has real-time access to geostationary satellite data. The forecasts are updated every three hours. Flying with limited information Pilots of transoceanic flights currently get preflight briefings and, in certain cases involving especially intense storms, in-flight weather updates every four hours. They also have onboard radar. Cathy Kessinger (©UCAR. Photo by Carlye Calvin.) The information, however, is of limited value for strategic flight planning while en route. Pinpointing turbulence associated with storms over the oceans is far more challenging than over land because geostationary satellites, unlike ground-based radar, cannot see within the clouds. Thunderstorms may develop quickly and move rapidly, rendering the briefings and weather updates obsolete. Onboard radars lack the power to see long distances or through dense clouds. As a result, pilots often must choose between detouring hundreds of miles around potentially stormy areas or flying through a region that may or may not contain intense weather. Storms may be associated with hazardous windshear and icing conditions, in addition to lightning, hail, and potentially severe turbulence. “Turbulence is the leading cause of injuries in commercial aviation,” says John Haynes, Applied Sciences program manager at NASA Headquarters in Washington. “This prototype system is of crucial importance to pilots and is another demonstration of the practical benefit of NASA's Earth observations.”

New airport system facilitates smoother take-offs and landings

BOULDER—For airline passengers who dread bumpy rides to mountainous destinations, help may be on the way. A new turbulence avoidance system has for the first time been approved for use at a U.S. airport and can be adapted for additional airports in rugged settings across the United States and overseas. The new turbulence avoidance system enables pilots to view areas of moderate (yellow) and severe (red) turbulence. (©UCAR. Screen capture from NCAR's Juneau Airport Warning System. This image is freely available for media & nonprofit use.) The system, developed by the National Center for Atmospheric Research (NCAR), provides information pilots can use to route aircraft away from patches of potentially dangerous turbulence. It uses a network of wind measuring instruments and computational formulas to interpret rapidly changing atmospheric conditions. The Federal Aviation Administration formally commissioned the system in July for Alaska’s Juneau International Airport. NCAR researchers can now turn their attention to adapting the system to other airports that often have notoriously severe turbulence, in areas ranging from southern California and the Mountain West to Norway and New Zealand. The Juneau system was patterned after a similar system, also designed by NCAR, that has guided aircraft for several years at Hong Kong’s heavily trafficked Chek Lap Kok Airport. “By alerting pilots to areas of moderate and severe turbulence, this system enables them to fly more frequently and safely in and out of the Juneau airport in poor weather,” says Alan Yates, an NCAR program manager who helped oversee the system’s development. “It allows pilots to plan better routes, helping to reduce the bumpy rides that passengers have come to associate with airports in these mountainous settings.” The system offers the potential to substantially reduce flight delays. In Alaska’s capital city, where it is known as the Juneau Airport Wind System or JAWS, it enables the airport to continue operations even during times of turbulence by highlighting corridors of smooth air for safe take-offs and landings. Al Yates (©UCAR. Photo by Carlye Calvin. This image is freely available for media & nonprofit use.) “The JAWS system has nearly eliminated all the risk of flying in and out of Juneau,” says Ken Williams, a Boeing 737 captain and instructor pilot with Alaska Airlines. “I wish the system would be deployed in other airports where there are frequent encounters with significant turbulence, so pilots can get a true understanding of what the actual winds are doing on the surrounding mountainous terrain as you approach or depart.” The project was funded by the Federal Aviation Administration. NCAR is sponsored by the National Science Foundation. Steep terrain, rough rides Turbulence has long been a serious concern for pilots approaching and departing airports in steep terrain. Rugged peaks can break up air masses and cause complex and rapidly changing patterns of updrafts and downdrafts, buffeting an aircraft or even causing it to unexpectedly leave its planned flight path. In Juneau, after several turbulence-related incidents in the early 1990s—including one in which a jet was flipped on its side during flight and narrowly avoided an accident—the FAA imposed strict rules of operation that effectively shut down the airport during times of atmospheric disturbance. The agency then asked NCAR to develop a system that would allow pilots to avoid regions of turbulence. Otherwise, Alaska’s capital would be isolated at many times from the rest of the state, since the only way to travel in and out of Juneau is by airplane or boat. The NCAR team used research aircraft and computer simulations to determine how different wind patterns—such as winds that come from the north over mountains and glaciers and winds that come from the southeast over water—correlated with specific areas of turbulence near the airport. To do this they installed anemometers and wind profilers at key sites along the coast and on mountain ridges. The team has installed ruggedized, heated instruments that can keep functioning even when exposed to extreme cold, wind, and heavy icing conditions. The Federal Aviation Administration accepted JAWS for operational use this year.    The five anemometer sites and three wind profiler sites around the airport transmit data multiple times every minute. Pilots can get near-real-time information about wind speed and direction, and a visual readout showing regions of moderate and severe turbulence in the airport’s approach and departure corridors, from the FAA’s Flight Service Station or online at a National Weather Service website, where an integrated display "app" is also available for download. “Juneau was an extremely challenging case, and we’re pleased that the new system met the FAA’s high standards,” Yates says. “We look forward to exploring opportunities to support development of turbulence avoidance systems at additional airports. Our goal is to improve flying safety and comfort for millions of passengers.”
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