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The comet that disappeared: What happened to ISON?

On Thanksgiving Day in 2013, solar scientists, astronomers, and amateur skywatchers alike pointed their instruments at the Sun and waited. Comet ISON, a bright ball of frozen matter from the earliest days of the universe, was inbound from the Oort Cloud at the edge of the solar system and expected to pierce the Sun's corona on Nov. 28. Scientists were expecting quite a show.But instead of a brilliant cosmic display, there was … nothing.An enhanced image of Comet ISON taken by the Hubble Space Telescope in May 2013. (Image courtesy NASA.)"The first thing we did was make sure that we had definitely seen nothing," said Paul Bryans, a solar scientist at the National Center for Atmospheric Research (NCAR), who was looking for the comet using NASA's Solar Dynamics Observatory. "We did image processing just to make sure nothing was there, and it wasn't. But that's not necessarily a boring result. That can tell us something."And it has. Bryans and colleague Dean Pesnell, of the NASA Goddard Space Flight Center, recently published a study that sheds light on the mystery of Comet ISON."We think that the most likely thing that happened is that Comet ISON broke up before it got really close to the Sun," said Bryans, a researcher at NCAR's High Altitude Observatory.Solar scientists, like Bryans, are interested in comets like ISON because they can act as probes into the mysterious solar corona. How they behave on their journey past the Sun can offer insight into the corona's composition and the behavior of the Sun's magnetic field.Watching and waitingSun-grazing comets are not that unusual, but they're usually too small to live through the encounter. Larger comets, like Comet Lovejoy, which sailed through the Sun's corona in December 2011, can survive brushes with the Sun. But they burn off a large part of their masses in the process, sometimes leaving a dazzling trail of extreme ultraviolet emissions in their wake. Comet ISON, first spotted more than a year before it reached the Sun, was thought to be large enough to survive the trip. The comet was very bright, a sign that it might also be quite large.This animation shows Comet ISON, Mercury, and Earth from Nov. 20 to Nov. 25, 2013. The Sun sits right of the field of view. The images were taken by the Heliospheric Imager on NASA's STEREO mission. See the full video. (Animation courtesy of NASA/STEREO.) When NASA observatories failed to see a showy trail from Comet ISON — or any trail at all — scientists were left wondering what happened. In a study published in 2014, researchers hypothesized that Comet ISON did not emit the extreme ultraviolet radiation like Comet Lovejoy because ISON passed further away from the Sun.In the new study, published in The Astrophysical Journal, Bryans and Pesnell challenge those conclusions. Using data collected by the Solar Dynamics Observatory, the researchers compared ISON to Lovejoy, systematically evaluating how the conditions might have differed for the two comets — including the density of the solar atmosphere, the Sun's magnetic field, and the size of the comets — as well as how those differences might have affected the comets' emissions of extreme ultraviolet radiation."Using Lovejoy as a benchmark, we took each factor in turn," Bryans said. "The fact that ISON was further away from the Sun than Lovejoy would have made a difference, but it was not a large enough difference to explain why we saw nothing from ISON."Instead, the study finds that ISON's fizzle is best explained by the comet's size. They estimate that IOSN's radius was at least a factor of four smaller than Lovejoy's.'Dust and rubble' If Bryans and Pesnell are correct, it means that estimates of Comet ISON's size before it reached the Sun were too large. Comet size is correlated to brightness, but other factors can affect brightness as well. In ISON's case, scientists believe the comet was making its first trip around the Sun, which means that it was still packed with highly volatile matter that had not yet burned off. This matter could make the comet appear brighter for its size than a comet that had already traveled once past the Sun. "On a comet's first passage past the sun, it has all this cold, icy stuff on the outside of it that burns off easily and looks really bright," Bryans said.But even if the comet was bright because of its size, the scientists believe it’s likely that the comet broke into pieces before entering the Sun's corona."It's possible by the time it made its closest approach to the Sun, it was just a pile of dust and rubble," Bryans said. About the articleTitle: On the absence of EUV emission from Comet C/2012 S1 (ISON)Authors: Paul Bryans and W. Dean PesnellPublication: The Astrophysical Journal, DOI: 10.3847/0004-637X/822/2/77Funders:  NASACollaborators: NASA Goddard Space Flight CenterWriter/contact:Laura Snider, Senior Science Writer and Public Information Officer 

Expanding Antarctic sea ice linked to natural variability

BOULDER — The recent trend of increasing Antarctic sea ice extent — seemingly at odds with climate model projections — can largely be explained by a natural climate fluctuation, according to a new study led by the National Center for Atmospheric Research (NCAR). The study offers evidence that the negative phase of the Interdecadal Pacific Oscillation (IPO), which is characterized by cooler-than-average sea surface temperatures in the tropical eastern Pacific, has created favorable conditions for additional Antarctic sea ice growth since 2000. The findings, published in the journal Nature Geoscience, may resolve a longstanding mystery: Why is Antarctic sea ice expanding when climate change is causing the world to warm? The study's authors also suggest that sea ice may begin to shrink as the IPO switches to a positive phase. "The climate we experience during any given decade is some combination of naturally occurring variability and the planet's response to increasing greenhouse gases," said NCAR scientist Gerald Meehl, lead author of the study. "It's never all one or the other, but the combination, that is important to understand." Study co-authors include Julie Arblaster of NCAR and Monash University in Australia, Cecilia Bitz of the University of Washington, Christine Chung of the Australian Bureau of Meteorology, and NCAR scientist Haiyan Teng. The study was funded by the U.S. Department of Energy and by the National Science Foundation, which sponsors NCAR. On Sept. 19, 2014, the five-day average of Antarctic sea ice extent exceeded 20 million square kilometers (about 7.7 million square miles) for the first time since 1979, according to the National Snow and Ice Data Center. The red line shows the average maximum extent from 1979-2014. (Image courtesy NASA's Scientific Visualization Studio/Cindy Starr) Expanding ice The sea ice surrounding Antarctica has been slowly increasing in area since the satellite record began in 1979. But the rate of increase rose nearly five fold between 2000 and 2014, following the IPO transition to a negative phase in 1999. The new study finds that when the IPO changes phase, from positive to negative or vice versa, it touches off a chain reaction of climate impacts that may ultimately affect sea ice formation at the bottom of the world. When the IPO transitions to a negative phase, the sea surface temperatures in the tropical eastern Pacific become somewhat cooler than average when measured over a decade or two. These sea surface temperatures, in turn, change tropical precipitation, which drives large-scale changes to the winds that extend all the way down to Antarctica. The ultimate impact is a deepening of a low-pressure system off the coast of Antarctica known as the Amundsen Sea Low. Winds generated on the western flank of this system blow sea ice northward, away from Antarctica, helping to enlarge the extent of sea ice coverage. “Compared to the Arctic, global warming causes only weak Antarctic sea ice loss, which is why the IPO can have such a striking effect in the Antarctic," said Bitz. "There is no comparable natural variability in the Arctic that competes with global warming.” Sifting through simulations To test if these IPO-related impacts were sufficient to cause the growth in sea ice extent observed between 2000 and 2014, the scientists first examined 262 climate simulations created by different modeling groups from around the world. When all of those simulations are averaged, the natural variability cancels itself out. For example, simulations with a positive IPO offset those with a negative IPO. What remains is the expected impact of human-caused climate change: a decline in Antarctic sea ice extent. But for this study, the scientists were not interested in the average. Instead, they wanted to find individual members that correctly characterized the natural variability between 2000-2014, including the negative phase of the IPO. The team discovered 10 simulations that met the criteria, and all of them showed an increase in Antarctic sea ice extent across all seasons. "When all the models are taken together, the natural variability is averaged out, leaving only the shrinking sea ice caused by global warming," Arblaster said. "But the model simulations that happen to sync up with the observed natural variability capture the expansion of the sea ice area. And we were able to trace these changes to the equatorial eastern Pacific in our model experiments." Scientists suspect that in 2014, the IPO began to change from negative to positive. That would indicate an upcoming period of warmer eastern Pacific Ocean surface temperatures on average, though year-to-year temperatures may go up or down, depending on El Niño/La Niña conditions. Accordingly, the trend of increasing Antarctic sea ice extent may also change in response. "As the IPO transitions to positive, the increase of Antarctic sea ice extent should slow and perhaps start to show signs of retreat when averaged over the next 10 years or so," Meehl said. About the article Title: Antarctic sea-ice expansion between 2000 and 2014 driven by tropical Pacific decadal climate variability Authors: Gerald A. Meehl, Julie M. Arblaster, Cecilia M. Bitz, Christine T. Y. Chung, and Haiyan Teng Publication: Nature Geoscience, DOI: 10.1038/NGEO2751 WriterLaura Snider, Senior Science Writer and Public Information Officer

NCAR weather ensemble offers glimpse at forecasting's future

July 1, 2016 | Last spring, scientists at the National Center for Atmospheric Research (NCAR) flipped the switch on a first-of-its-kind weather forecasting system. For more than a year, NCAR's high-resolution, real-time ensemble forecasting system has been ingesting 50,000 to 70,000 observations every six hours and creating a whopping 90,000 weather maps each day.The system has become a favorite among professional forecasters and casual weather wonks: Typically more than 200 people check out the site each day with more than a thousand coming during major weather events. During this experimental period, the NCAR ensemble has also become a popular source of guidance within the National Weather Service, where it has already been referenced several hundred times by forecasters at more than 50 different offices. But perhaps more important, the data accumulated from running the system daily — and there is lots of it — is being used by researchers at universities across the country to study a range of topics, from predicting hail size to anticipating power outages for utilities."We wanted to demonstrate that a real-time system of this scale was feasible," said NCAR scientist Craig Schwartz. "But it's also a research project that can help the community learn more about the predictability of different kinds of weather events."Schwartz is a member of the team that designed and operates the system, along with NCAR colleagues Glen Romine, Ryan Sobash, and Kate Fossell.This animation shows the forecast for accumulated snowfall made by each of the NCAR ensemble's 10 members for the 48-hour period beginning on Jan. 22, 2016. In the run-up to the blizzard, which ultimately dropped more than 30 inches of snow on parts of the Mid-Atlantic, more than 1,000 people visited the NCAR ensemble's website. (©UCAR. This animation is freely available for media & nonprofit use.)Testing a unique toolNCAR's high-resolution ensemble forecasting system is unique in the country for a couple of reasons, both of which are revealed in its name: It's an ensemble, and it's high resolution.Instead of producing a single forecast, the system produces an "ensemble" of 10 forecasts, each with slightly different (but equally likely) starting conditions. The degree to which the forecasts look the same or different tells scientists something about the probability that a weather event, like rain, hail, or wind, will actually occur.By comparing the actual outcomes to the forecasted probabilities, scientists can study the predictability of particular weather events under different circumstances. The forecasting system's high resolution (the grid points are just 3 kilometers apart) allows it to simulate small-scale weather phenomena, like the creation of individual storms from convection — the process of moist, warm air rising and then condensing into clouds. The combination of fine grid spacing and ensemble predictions in the NCAR system offers a sneak peek at what the future of weather forecasting might look like, and weather researchers across the country have noticed. Cliff Mass, a professor of atmospheric sciences at the University of Washington whose specialty is forecasting, said: "It's extremely important for the United States to have a convection-allowing ensemble system to push our forecasting capabilities forward. We were delighted that NCAR demonstrated that this could be done."'The cat's meow'The treasure trove of accruing weather data generated by running the NCAR ensemble is already being used by researchers both at NCAR and in the broader community. Jim Steenburgh, for instance, is a researcher at the University of Utah who is using the system to understand the predictability of mountain snowstorms."NCAR's ensemble not only permits the 'formation' of clouds, it can also capture the topography of the western United States," he said. "The mountains control the weather to some degree, so you need to be able to resolve the mountains' effects on precipitation."Steenburgh has also been using the ensemble with his students. "We’re teaching the next generation of weather forecasters," he said. "In the future, these high-resolution ensemble forecasts will be the tools they need to use, and this gives them early, hands-on experience." Like Steenburgh, Lance Bosart, an atmospheric researcher at the University of Albany, State University of New York, has used the ensemble both in his own research — studying the variability of convective events — and with his students. He said having 10 members in the ensemble forecast helps students easily see the great spread of possibilities, and the visual emphasis of the user interface makes it easy for students to absorb the information."What makes it an invaluable tool is the graphical display," he said. "It's visually compelling. You don't have to take a lot of time to explain what you're looking at; you can get right into explaining the science. I like to say it's the cat's meow." Setting an exampleThe NCAR ensemble is also enabling the researchers running it to further their own research. "We're collecting statistics on the misfit between the model predictions and observations and then we're trying to use that to improve our model physics," Romine said. The ensemble project is also teaching the team about the strengths and weaknesses of the way they've chosen to kick off, or "initialize," each of the ensemble members. "The NCAR ensemble happens to produce a pretty good forecast, but we realize there are some shortcomings," Schwartz said. "For example, if we were trying to make the best forecast in the world, we would probably not be initializing the model the way we are. But then we wouldn’t learn as much from a research perspective." The NCAR ensemble began as a yearlong trial, but the project is continuing to run for now. The team would like to keep the system online until next summer, but they don't yet have the computing resources they need to run it past September.If the system does continue to run, the researchers who are using it say there's still more that they and their students can learn from the project. And if not, there's loads of data already collected that are still waiting to be mined. In any case, Mass says NCAR's ensemble has been a valuable project. "It set a really good example for the nation," he said.Community members interested in collaborating or helping support the NCAR ensemble project are encouraged to contact the team at ensemble@ucar.edu.Writer/contact:Laura Snider, Senior Science Writer and Public Information Officer 

Scientists observe first signs of healing in the Antarctic ozone layer

NCAR scientists Doug Kinnison and Michael Mills are co-authors on a new study published today in the journal Science. This is an excerpt from a news release by the Massachusetts Institute of Technology, a UCAR member institution, about the study.This animation shows the opening and closing of the Antarctic ozone hole (dark blue) in 2015. (Animation courtesy of NASA.)June 30, 2016 | Scientists at MIT and elsewhere have identified the “first fingerprints of healing” of the Antarctic ozone layer, published today in the journal Science.The team found that the September ozone hole has shrunk by more than 4 million square kilometers — about half the area of the contiguous United States — since 2000, when ozone depletion was at its peak. The team also showed for the first time that this recovery has slowed somewhat at times, due to the effects of volcanic eruptions from year to year. Overall, however, the ozone hole appears to be on a healing path.The authors used “fingerprints” of the ozone changes with season and altitude to attribute the ozone’s recovery to the continuing decline of atmospheric chlorine originating from chlorofluorocarbons (CFCs). These chemical compounds were once emitted by dry cleaning processes, old refrigerators, and aerosols such as hairspray. In 1987, virtually every country in the world signed on to the Montreal Protocol in a concerted effort to ban the use of CFCs and repair the ozone hole.“We can now be confident that the things we’ve done have put the planet on a path to heal,” says lead author Susan Solomon, the Ellen Swallow Richards Professor of Atmospheric Chemistry and Climate Science at MIT. “Which is pretty good for us, isn’t it? Aren’t we amazing humans, that we did something that created a situation that we decided collectively, as a world, ‘Let’s get rid of these molecules’? We got rid of them, and now we’re seeing the planet respond.”Solomon’s co-authors include Diane Ivy, research scientist in the Department of Earth, Atmospheric and Planetary Sciences, along with researchers at the National Center for Atmospheric Research in Boulder, Colorado, and the University of Leeds in the U.K.Read the full release at MIT News.About the articleTitle: Emergence of Healing in the Antarctic Ozone Layer Authors: Susan Solomon, Diane J. Ivy, Doug Kinnison, Michael J. Mills, Ryan R. Neely, and Anja SchmidtJournal: Science, DOI: 10.1126/science.aae0061

Pioneering NCAR photochemist passes

June 29, 2016 | Jack Calvert, a preeminent researcher in photochemistry, atmospheric chemistry, and air pollution, died on June 1 at the age of 93 in Tennessee.

Calvert joined NCAR as a senior scientist in 1981, after serving for more than three decades as a professor of chemistry at Ohio State University. His chosen specialty was the investigation of chemical reactions initiated by light.

While at NCAR, Calvert led the Atmospheric Kinetics and Photochemistry Group until he retired in 1993 and was appointed emeritus senior scientist.

Trailblazing HAO research recognized by National Academy of Sciences

June 28, 2016 | Former HAO Senior Scientist Tim Brown has received a prestigious award from the National Academy of Sciences for landmark research that he conducted while at NCAR in the 1990s and early 2000s.

The James Craig Watson Medal is presented every two years for outstanding contributions to the science of astronomy. It recognizes Brown for pioneering instrument developments and observations, and for formulating a method to make extremely sensitive images of the Sun, which became key to the field of helioseismology.

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