NCAR

UCAR Congressional Briefing: Subseasonal to seasonal forecasts

WASHINGTON — Federal investments in atmospheric and oceanic research are ushering in major advances in longer-term weather prediction, enabling private companies to provide their clients with valuable forecasts of weather patterns weeks to months in advance, experts said today at a congressional briefing.A panel of scientists representing universities and the private sector agreed that continued government investment in advanced computer modeling, observing tools, and supercomputers is critical for progress in forecasting on longer time scales.The nonprofit University Corporation for Atmospheric Research (UCAR) sponsored the briefing.Subseasonal to seasonal forecasts are predictions of regional weather patterns from two weeks to two years in advance, such as the likelihood of unusually dry or stormy conditions. Improving such forecasts is a national priority, emphasized in the Weather Research and Forecasting Innovation Act that Congress passed last year The panelists said the key to long-term forecasts is increased understanding of the role of the oceans and ocean-atmosphere patterns such as El Niño."Ocean conditions change more slowly than the atmosphere, and that longer memory allows us to predict weather patterns on longer time scales," said Ben Kirtman, a professor of atmospheric sciences at the University of Miami Rosenstiel School of Marine and Atmospheric Science. "How temperatures evolve below the ocean surface, and how the atmosphere and the ocean exchange heat and moisture and momentum — these processes are particularly important when you want to make subseasonal to seasonal forecasts."Gokhan Danabasoglu, chief scientist of the Community Earth System Model at the National Center for Atmospheric Research, said advanced computer models that incorporate observations of ocean conditions are being increasingly used for longer-term prediction. For example, forecasters are able to produce increasingly accurate month-ahead outlooks of temperatures over North America, and researchers have even been able to generate a 10-year forecast of Arctic sea ice conditions, which is important to shipping companies."Better models and more detailed information about ocean conditions lead to better predictions," Danabasoglu said. "We are seeing promising results in longer-term predictions that can be highly beneficial to society."Such forecasts are providing critical intelligence to the $100 billion livestock industry, said Chad McNutt, principal and co-founder of Livestock Wx, which provides livestock producers with advanced weather and climate information. He explained that cattle producers need advance information about temperature and precipitation patterns that affect winter wheat and other crops that cattle graze on. His clients also want to know the timing of insect infestations that affect cattle health."Agriculture sectors like the cattle industry need sustained support for research into improved subseasonal to seasonal forecasting," McNutt said. "The forecasts have real economic implications for producers."Alicia Karspeck, climate scientist and associate director of research partnerships with Jupiter Technology Systems, Inc., said private companies need high-quality, accessible, and continuous data from federal agencies to create long-term prediction products. Jupiter relies on federally funded observations and computer modeling to provide its clients with customized climate and weather risk analytics on timescales of weeks to decades."Federal funding for climate research, observations, and computing creates real value for the private sector, helping us deliver high-quality forecast products to our customers," Karspeck said. "Our company understands that the pipeline from scientific discovery to useful and marketable products relies on a vibrant, well-resourced research sector."Antonio Busalacchi, president of UCAR, emphasized the close partnerships among government agencies, universities, and private companies as they work to improve long-range forecasts."These collaborations are enabling us to better understand the entire Earth system in ways that will allow society to prepare for weather patterns weeks to months in advance," Busalacchi said. "Accurate subseasonal to seasonal forecasts will help to safeguard lives and property as well as benefit every economic sector."The briefing was the latest in a series of UCAR Congressional Briefings that draw on expertise from UCAR's university consortium and public-private partnerships to provide insights into critical topics in the Earth system sciences. Past briefings have focused on moving advances in Earth science research to industry, predicting wildfires, forecasting space weather, tools that improve aviation weather safety, the state of the Arctic, hurricane prediction, potential impacts of El Niño, and new advances in water resources forecasting.

Past tornado experiences shape perception of risk

The following is a news release from the Society of Risk Analysis about a study led by NCAR scientist Julie Demuth.With much of the central plains and Midwest now entering peak tornado season, the impact of these potentially devastating weather events will be shaped in large part by how individuals think about and prepare for them. A new study published in Risk Analysis: An International Journal shows that people's past experiences with tornadoes inform how they approach this type of extreme weather in the future, including their perception of the risk.Led by Julie Demuth, a scientist from the National Center for Atmospheric Research, the study, "Explicating experience: Development of a valid scale of past hazard experience for tornadoes," characterized and measured people's past tornado experiences to determine their impact on the perceived risks of future tornadoes. Better understanding of these factors can help mitigate future societal harm, for instance, by improving risk communication campaigns that encourage preparation for hazardous weather events.The results indicate that people's risk perceptions are highly influenced by a memorable past tornado experience that contributes to unwelcome thoughts, feelings and disruption, which ultimately increase one's fear, dread, worry and depression. Also, the more experiences people have with tornadoes, and the more personalized those experiences, the more likely they are to believe their homes (versus the larger geographic area of their city/town) will be damaged by a tornado within the next 10 years.A tornado in Oklahoma. People's past experiences with tornadoes shapes how they perceive risk when new storms threaten. (Image courtesy NOAA.)In the context of this study, Demuth defines 'past tornado experience' as "the perceptions one acquires about the conditions associated with or impacts of a prior tornado event. Such perceptions are gained by the occurrence of a tornado threat and/or event; directly by oneself or indirectly through others; and at different points throughout the duration of the threat and event."The study was conducted through two surveys distributed to a random sample of residents in tornado prone areas of the U.S. during the spring and fall of 2014. The first survey evaluated an initial set of items measuring experiences, and the second was used to re-evaluate the experience items and to measure tornado risk perceptions. The sample sizes for the two surveys were 144 and 184, respectively.Since tornado experiences can occur at any time throughout one's life, and in multiplicity, the survey items measured both one's most memorable tornado experience and his or her multiple experiences. A factor analysis of the survey items yielded four factors which make up the memorable experience dimensions. Risk awareness: information pertaining to the possibility of a specific tornado hazard occurring, as well as threat-related social cues from both people and the media.Risk personalization: one's protective behavioral and emotional responses as well as visual, auditory and tactile sensations experienced during the tornado.Personal intrusive impacts: ways that one is personally affected by an experience, including intangible, unpleasant thoughts and feelings from the experience.Vicarious troubling impacts: others' tangible impacts and verbal accounts of their experiences and intangible intrusive impacts. The "others" are people known personally by the responding individual. Although all the items in this factor reflect others' accounts of a tornado experience, the respondent experiences these aspects by hearing about or witnessing them.The factor analysis revealed two factors contributing to the multiple experience dimensions: common threat and impact communication, and negative emotional responses. The first factor captures one's personal experience with receiving common types of information (e.g., sirens) about tornado threats and tornado-related news. The second factor captures the amount of experience a respondent has with fearing for their own life, a loved one's life and worrying about their property due to a tornado.Individual's past tornado experiences are multi-faceted and nuanced with each of the above six dimensions exerting a different influence on tornado risk perceptions. These dimensions have not been previously analyzed, particularly the intangible aspects - feelings, thoughts and emotions."This research can help meteorologists who provide many essential, skillful risk messages in the form of forecasts, watches, and warnings when tornadoes (and other hazardous weather) threaten. This research can help meteorologists recognize the many ways that people's past tornado experiences shape what they think and do, in addition to the weather forecasts they receive," states Demuth.The Society for Risk Analysis is a multidisciplinary, interdisciplinary, scholarly, international society that provides an open forum for all those interested in risk analysis. SRA was established in 1980 and has published Risk Analysis: An International Journal, the leading scholarly journal in the field, continuously since 1981. For more information, visit http://www.sra.org.

Reconciling Paris Agreement goals for temperature, emissions

As society faces the challenge of limiting warming to no more than 2 degrees Celsius, new research finds an apparent contradiction: Achieving that goal doesn't necessarily require cutting greenhouse gas emissions to zero, as called for in the Paris Agreement. But under certain conditions, even zero emissions might not be enough.The Paris Agreement, a global effort to respond to the threats of human-caused climate change, stipulates that warming be limited to between 1.5 degrees C (2.7 degrees Fahrenheit) and 2 degrees C (3.6 degrees F). It also stipulates that countries achieve net-zero greenhouse gas emissions in the second half of this century. But the relationship between the two — is the emissions goal sufficient or even necessary to meet the temperature goal? — has not been well understood.In a new study published in the journal Nature Climate Change, scientists used a computer model to analyze a variety of possible future scenarios to better understand how emissions reductions and temperature targets are connected. The study, published March 26, was led by Katsumasa Tanaka at the National Institute for Environmental Studies in Japan and co-authored by Brian O'Neill at the U.S. National Center for Atmospheric Research."What we found is that the two goals do not always go hand in hand," Tanaka said. "If we meet temperature targets without first overshooting them, we don't have to reduce greenhouse gas emissions to zero. But if we do reduce emissions to zero, we still might not meet the temperature targets if we don't reduce emissions quickly enough."The team also found that whether temperatures overshoot the target temporarily has a critical impact on the scale of emissions reductions needed."If we overshoot the temperature target, we do have to reduce emissions to zero. But that won’t be enough," Tanaka said. "We'll have to go further and make emissions significantly negative to bring temperatures back down to the target by the end of the century."The research was supported by the Environment Research and Technology Development Fund (2-1702) of the Environmental Restoration and Conservation Agency in Japan and by the U.S. National Science Foundation, NCAR's sponsor.Drafted in 2015, the Paris Agreement has been ratified by more than 170 countries. President Donald Trump announced last year the intention to withdraw the United States from the agreement.Modeling the problem from both sidesFor the study, the researchers used a simplified integrated assessment model that takes into account the physical connections between greenhouse gases and global mean temperature in the climate as well as the economic costs of emissions reductions."We investigated the consistency between the Paris targets in two ways. First we asked, what happens if you just meet the temperature target in a least-cost way? What would emissions look like?" said O'Neill, an NCAR senior scientist. "Then we said, let's just meet the emissions goal and see what kind of temperatures you get."The team generated 10 different scenarios. They found that Earth's warming could be stabilized at 1.5 or 2 degrees C — without overshooting the goal — by drastically cutting emissions in the short term. For example, total greenhouse gas emissions would need to be slashed by about 80 percent by 2033 to hit the 1.5-degree target or by about two-thirds by 2060 to meet the 2-degree target. In both these cases, emissions could then flatten out without ever falling to zero.Due to the difficulty of making such steep cuts, the scientists also looked at scenarios in which the temperature was allowed to temporarily overshoot the targets, returning to 1.5 or 2 degrees by the end of the century. In the 1.5-degree overshoot scenario, emissions fall to zero by 2070 and then stay negative for the rest of the century. (Negative emissions require activities that draw down carbon dioxide from the atmosphere.) For the 2-degree temporary overshoot scenario, emissions fall to zero in 2085 and also become negative, but for a shorter period of time.On the flip side, the scientists also looked at scenarios where they set the emissions levels instead of the temperature. In those cases, they analyzed what would happen if emissions were reduced to zero around mid-century (2060) or at the end of the century (2100). In the first case, the global temperature peaked around the 2-degree target and then declined. But in the second case, the temperature rose above 2 degrees around 2043 and stayed there for a century or more."The timing of when emissions are reduced really matters," O'Neill said. "We could meet the goal set out in the Paris Agreement of reducing emissions to zero in the second half of the century and still wildly miss the temperature targets in the same agreement if we wait to take action."The new study is part of a growing body of research that seeks to better understand and define what it will take to comply with the Paris Agreement. For example, another recent study — led by Tom Wigley, a climate scientist at the University of Adelaide who holds an honorary appointment at NCAR — also looks at the quantity and timing of emissions cuts needed to stabilize global temperature rise at 1.5 or 2 degrees above preindustrial levels. This work focuses in particular on implications for emissions of carbon dioxide, the main component of the broader greenhouse gas emissions category that makes up the Paris emissions target.O'Neill and Tanaka believe their work might be useful as countries begin to report the progress they've made reducing their emissions and adjust their goals. These periods of reporting and readjusting, known as global stocktakes, are formalized as part of the Paris Agreement and occur every five years."Our study and others may help provide countries with a clearer understanding of what work needs to be done to meet the goals laid out in the agreement. We believe that the Paris Agreement needs this level of scientific interpretation," Tanaka said.

Cutting greenhouse gas emissions would help spare cities worldwide from rising seas

BOULDER, Colo. — Coastal cities worldwide would face a reduced threat from sea level rise if society reduced greenhouse gas emissions, with especially significant benefits for New York and other U.S. East Coast cities, new research indicates.The study, by scientists at the National Center for Atmospheric Research (NCAR), used a powerful computer model to tease out the ways that winds and currents in a warming world push ocean water around, lifting it in some regions and lowering it in others. The scientists examined how these variations in sea level rise would change under two conditions: if emissions continue on their current trajectory, or if they are sharply reduced.The results showed that, if society can implement cuts soon on emissions of carbon dioxide and other heat-trapping gases, the projected increases in sea level around the globe would be significantly less toward the end of the century. This would help coastal cities in much of the world as they try to fend off rising waters, with the benefits most pronounced for cities on parts of the Atlantic and Indian oceans.Projected sea level rise for major cities worldwide will vary significantly later this century, depending on whether society continues to increase emissions of greenhouse gases at the current rate (a scenario known as RCP 8.5) or begins to sharply reduce them (RCP 4.5). Some cities, such as New York and London, would see particularly pronounced benefits if society cuts emissions. For more details on the range of projected sea level rise for major cities, click on the graphic or see table below. (Graphic by Simmi Sinha, ©UCAR. Click to enlarge. This graphic is freely available for media & nonprofit use.)  "Mitigating greenhouse gases will reduce sea level rise later this century, with some regions seeing especially significant benefits," said NCAR scientist Aixue Hu, the lead author of the new study. "As city officials prepare for sea level rise, they can factor in the compounding effect of local conditions, which are due to the winds and currents that cause internal variability in the oceans."Hu and his co-author, NCAR scientist Susan Bates, caution that the modeling study presents an incomplete picture, because it does not include runoff from melting ice sheets and glaciers — two factors that scientists are just now incorporating into computer models. Instead, it simulates the influence of climate change on variations in sea level worldwide to reveal which coastlines will benefit most from emission reductions associated with the additional heat absorbed by the ocean.The study, published this month in the journal Nature Communications, was funded by the U.S. Department of Energy and by the National Science Foundation, which is NCAR's sponsor.Global changes with local impactsSea level rise is one of the most consequential impacts of climate change, threatening to swamp low-lying islands and major coastal cities. Sea levels in some regions are expected to rise by several feet by the end of this century, due to a combination of melting ice sheets and glaciers (which account for about two-thirds of sea level rise) along with thermal expansion, or ocean waters expanding as they warm (which accounts for the remaining one-third).To study how changes in emissions would affect global sea level rise and local variations, Hu and Bates used two sets of computer simulations that are based on two different greenhouse gas scenarios.In the business-as-usual scenario, with emissions from human activity continuing to increase at current rates, global temperatures by late this century would rise by about 5.4 degrees Fahrenheit (3 degrees Celsius) over late 20th century levels. In the moderate mitigation scenario, with society taking steps to reduce greenhouse gases, warming would be held to about 3.2 degrees F (1.8 degrees C).The scientists found that reducing greenhouse gas emissions would not significantly restrain sea level rise for the next two decades. The reason, in part, has to do with the inertia of the climate system (once heat enters the oceans, it is retained for a period of time). In addition, winds and currents are naturally variable from year to year, pushing ocean water in different directions and making it hard to discern the full impact of planet-scale warming over the span of a decade or two.But the scientists found that later in the century, from 2061 to 2080, reduced emissions would have a significant impact across almost the entire world. The simulations showed that the extent of mean global sea level rise from thermal heat expansion (but not runoff from melting ice) was reduced by about 25 percent, from about 17.8 centimeters (7 inches) in the business-as-usual scenario to 13.2 centimeters (5.2 inches) in the moderate mitigation scenario.Locally, winds and currents make a differenceFor some cities, the benefits of the lower-emission scenario would be especially significant. New York City, where sea levels this century are expected to rise more than almost anywhere else in the world, would see a difference of 9.8 centimeters (3.9 inches). Other cities that would see a greater-than-average reduction include Boston (9.3 cm/3.7 in), London (8.3 cm/3.3 in), Dar es Salaam (6.8 cm/2.7 in), Miami (6.5 cm/2.6 in), and Mumbai (5.8 cm/2.3 in).On the other hand, some cities in South America (such as Buenos Aires), Asia (such as Bangkok and Jakarta), Australia (such as Melbourne), and the west coast of North America (such as Vancouver and San Francisco) would see lower-than-average benefits. And reducing greenhouse gases would have no statistically significant effect on sea level rise along the western coasts of Australia and the Philippines.The reason for the local differences in sea level rise has to do with the influence (or lack thereof) of a changing climate on major currents and on atmosphere-ocean interactions around the globe.In the northern Atlantic, for example, warming temperatures are expected to weaken the Gulf Stream that transports warmer water from the subtropics to the Arctic. The powerful current draws water away from much of the east coast of the United States, and scientists have warned that a weakening current would send those waters back toward the coastline and significantly raise sea levels. If actions taken by society resulted in reduced emissions, the Gulf Stream would be less affected and, therefore, sea level rise in the north Atlantic would be less substantial.In contrast, the currents in some other ocean basins appear to be less sensitive to climate change. Across much of the Pacific, for example, sea levels are influenced by the Pacific Decadal Oscillation, a phenomenon related to winds and sea surface temperatures. Although climate change is affecting winds and causing sea surface temperatures to rise in the Pacific, it is not disrupting currents there as much as it is in the northern Atlantic. As a result, climate change mitigation that reduces thermal expansion would generally have a less significant effect on Pacific sea levels.The study also found greater variations in future sea level rise in different regions, including some cities where local sea levels are influenced by the Pacific Decadal Oscillation or by an Atlantic climate pattern known as the North Atlantic Oscillation. As a result, the projected sea level rise in the model varied more for London and Tokyo than for New York."City planners in some places will be able to make decisions based on more certain sea level projections, but for other places it's going to be more difficult to know what the sea levels will be," Bates said.About the paperTitle: Internal climate variability and projected future regional steric and dynamic sea level rise
Authors: Aixue Hu and Susan BatesJournal: Nature CommunicationsNew research estimates the extent to which sea level rise would be reduced for major cities worldwide by later this century if society cuts emissions of greenhouse gas emissions. These tables incorporates projections based on the thermal expansion of ocean water as well as on the localized impacts of winds and currents, but they do not include additional sea level rise caused by the melting of ice sheets and glaciers. (Data produced by Aixue Hu and Susan Bates, NCAR. Graphic by Simmi Sinha, UCAR. Click to enlarge. This graphic is freely available for media & nonprofit use.)

Taking the temperature of streamflow forecasts

February 21, 2018 | Adding temperature predictions into seasonal streamflow forecasts in the U.S. Southwest could increase the accuracy of those forecasts, according to a new study that analyzed historical conditions in the headwaters of the Colorado and Rio Grande rivers.Many rivers in the western United States are fed by melting snow in the spring and summer. Regional water managers depend on seasonal water supply forecasts that estimate the amount of runoff the snowpack will yield to determine how much water to allocate to farmers and ranchers, city residents, and other users.These forecasts, which are based on snowpack measurements taken in the winter and spring, tend to assume that the climate is stable and that the relationship between the amount of snowpack and the amount of runoff is also stable.But a recent study by scientists at the National Center for Atmospheric Research (NCAR), the National Oceanic and Atmospheric Administration (NOAA), and the Bureau of Reclamation found that warmer temperatures reduce the amount of meltwater that actually makes it into a stream, a finding that highlights the importance of accounting for changing climate conditions when forecasting streamflow.Building on this work, scientists at NCAR tested whether incorporating seasonal temperature predictions into statistical streamflow forecasting models could improve their accuracy. The temperature predictions reflect the recent warming trend as well as whether the months after the forecast date are likely to be warmer or colder than this trend. Results of the new study were published in the journal Geophysical Research Letters."Adding temperature predictions into streamflow forecasts will not only improve the information that water managers have today, but it also has the potential to mitigate some of the loss of predictability that we now expect in the future, as the climate continues to warm," said NCAR scientist Flavio Lehner, who co-led the study with NCAR scientist Andy Wood.The research contributes to an NCAR effort, in collaboration with several federal agencies, to build better tools and models for analysis and prediction of water resources. The new study was funded by the NOAA, the Bureau of Reclamation, and the U.S. Army Corps of Engineers.Other study co-authors include Angus Goodbody from the National Water and Climate Center, which issues streamflow forecasts for the western United States; Florian Pappenberger from the forecast department of the European Centre for Medium-Range Weather Forecasts; and Douglas Blatchford and Dagmar Llewellyn, both from the Bureau of Reclamation.NCAR scientist Flavio Lehner in the upper Rio Grande River watershed near Los Alamos, New Mexico. (Photo courtesy Flavio Lehner.) Comparing "hindcasts" with and without temperature predictionsTo test whether the addition of seasonal temperature predictions could improve streamflow forecasts, the scientists created "hindcasts" for the headwaters of the Colorado and Rio Grande Rivers (both located in Colorado) for the three decades between 1987 and 2016.The team used historical observations of snowpack, precipitation, and streamflow to issue and evaluate a series of forecasts — on Jan. 1, Feb. 1, March 1, April 1, and May 1 —  for each year. These hindcasts emulate the current method of issuing streamflow forecasts, including the calendar dates when those forecasts are issued. The scientists also issued a second set of hindcasts, this time with the addition of seasonal temperature predictions for the region. The seasonal predictions were drawn from the North American Multi-Model Ensemble and the European Centre for Medium-Range Weather Forecasts, which together comprise eight state-of-the-art models used for seasonal climate forecasts.The team found that incorporating temperature predictions improved the accuracy of seasonal streamflow forecasts at the majority of river gauges across the headwaters of both basins. The amount of improvement varied between about 1 percent and 10 percent, averaged across the basins."We think that model-based temperature predictions could be used to improve water supply forecasts in other watersheds that rely on runoff from snowpack — across the western United States and in other parts of the world," Lehner said "But the degree of improvement will certainly depend on the individual area."Smoothing the path to adoptionTo make it as easy as possible for existing operational forecasting centers to begin incorporating the research findings, the scientists chose to modify existing forecasting techniques to include temperature predictions instead of inventing an entirely new forecasting method."It's a well-known challenge to transition methods from a research experiment to an operational setting," Wood said. "Here we chose a baseline of a current operational water supply forecast technique so that the approach is an extension to the existing practice, and more likely to be supportable."  Fostering these kinds of applications is the overarching goal of the Postdocs Applying Climate Expertise program that supports Lehner's research. PACE is run by the Cooperative Programs for the Advancement of Earth System Science, a community program of the University Corporation for Atmospheric Research. "This project has enabled stakeholders and scientists to work together directly to tackle a concrete water-related problem arising from the variability and trends we’ve observed in our regional climate," said Llewellyn, a scientist at the Bureau of Reclamation and study co-author. About the articleTitle: "Mitigating the impacts of climate non-stationarity on seasonal streamflow predictability in the U.S. Southwest"Authors: Flavio Lehner, Andrew W. Wood, Dagmar Llewellyn, Douglas B. Blatchford, Angus G. Goodbody, Florian PappenbergerJournal: Geophysical Research Letters, DOI: 10.1002/2017GL076043

The rate of sea level rise is accelerating, a new study finds

NCAR scientist John Fasullo is a co-author of a new study appearing in the Proceedings of the National Academies of Science. The study finds that the rate of sea level rise is accelerating. The following is an excerpt from a news release by the Cooperative Institute for Research in Environmental Sciences. February 13, 2018 | Global sea level rise is not cruising along at a steady 3 mm per year. It’s accelerating a little every year, according to a new study that harnessed 25 years of satellite data to calculate that the rate is increasing by about 0.08 mm/year every year — which could mean an annual rate of sea level rise of 10 mm/year, or even more, by 2100.“This acceleration, driven mainly by accelerated melting in Greenland and Antarctica, has the potential to double the total sea level rise by 2100 as compared to projections that assume a constant rate—to more than 60 cm instead of about 30.” said lead author Steve Nerem, a scientists at the Cooperative Institute for Research in Environmental Sciences. "And this is almost certainly a conservative estimate," he added. "Our extrapolation assumes that sea level continues to change in the future as it has over the last 25 years. Given the large changes we are seeing in the ice sheets today, that's not likely."If the oceans continue to change at this pace, sea level will rise 65cm (26 inches) by 2100—enough to cause significant problems for coastal cities, according to the new assessment by Nerem and several colleagues from CU Boulder, the University of South Florida, NASA Goddard Space Flight Center, Old Dominion University, and the National Center for Atmospheric Research. The team, driven to understand and better predict Earth’s response to a warming world, published their work today in the journal Proceedings of the National Academy of Sciences.Rising concentrations of greenhouse gases in Earth’s atmosphere increase the temperature of air and water, which causes sea level to rise in two ways. First, warmer water expands, and this "thermal expansion" of the oceans has contributed about half of the 7 cm of global mean sea level rise we've seen over the last 25 years, Nerem said. Second, melting land ice flows into the ocean, also increasing sea level across the globe.These increases were measured using satellite altimeter measurements since 1992, including the U.S./European TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3 satellite missions. But detecting acceleration is challenging, even in such a long record. Episodes like volcanic eruptions can create variability: the eruption of Mount Pinatubo in 1991 decreased global mean sea level just before the Topex/Poseidon satellite launch, for example. In addition, global sea level can fluctuate due to climate patterns such as El Niños and La Niñas (the opposing phases of the El Niño Southern Oscillation, or ENSO) which influence ocean temperature and global precipitation patterns.Read the full news release here. 

Drier and wetter: The future of precipitation variability

January 17, 2018 | Precipitation variability — the swing from dry to wet and back again — will continue to increase across the majority of the world's land area as the climate warms, according to a new study led by scientists at the National Center for Atmospheric Research.The researchers expect precipitation variability to become greater from day to day, year to year, and even decade to decade. The new research, published in the Nature journal Scientific Reports, provides results from sophisticated computer simulations that predict that there will be both more droughts and more floods within the same areas as the climate warms. The findings are relevant for water managers who need to make long-range plans."When it's dry, it will be drier. When it's wet, it will be wetter — in the same place," said NCAR scientist Angeline Pendergrass, lead author of the study. "There will be a broader range of conditions that will become 'normal.'"The research was funded by the National Science Foundation, which is NCAR's sponsor, and by the U.S. Department of Energy.As the climate continues to warm, the range of precipitation that is "normal" in a particular place is likely to grow, meaning a single location can become both wetter and drier. The image on the left shows a flood in Colorado. The image on the right shows a droughtin Texas. (Images courtesy the U.S. Department of Defense and U.S. Department of Agriculture.)New tools to study changes in precipitationHistorically, changes in precipitation variability have been difficult to pin down because the amount of rain or snow a particular region gets can vary a great deal naturally.But in recent years, the availability of large ensembles of climate model runs has allowed scientists to begin separating some of the more subtle impacts of climate change from the natural chaos in the climate system. These ensembles may include 30 or 40 runs of a single climate model over the same time period with slightly different, but equally plausible, initial conditions.Pendergrass and her colleagues, NCAR scientists Flavio Lehner, Clara Deser, and Benjamin Sanderson, along with ETH-Zürich professor Reto Knutti, took a closer look at precipitation variability using large ensembles of runs from the NCAR-based Community Earth System Model (CESM) and from the Geophysical Fluid Dynamics Laboratory (GFDL) climate model. They also looked at a collection of individual runs taken from many different climate models and known as the Climate Model Intercomparison Project Phase 5, or CMIP5.The team found that precipitation variability will likely increase substantially over two-thirds of the world's land areas by the end of the century if greenhouse gas emissions continue unabated. They also found that, on average, variability increases 4 to 5 percent over land per degree Celsius of warming and that variability increases across all time scales, from days to decades."This increase in variability is arising due to more moisture in the atmosphere and a weakening of global atmospheric circulation," Pendergrass said. "That's important because it means that changes in precipitation variability are not just linked to changes in El Niño and La Niña events, as some previous work implied."Helping water managers plan for the futurePendergrass hopes the study's findings will be used by water managers in their future planning. Models used today by water managers often assume that the change in precipitation variability in the future will track with the expected increase in average precipitation.But the new study finds that the increase in precipitation variability will outstrip the increase in average precipitation, which means that water managers may be miscalculating the magnitude of future swings from wet to dry or vice versa."Water managers may be underestimating how much heavy events — floods or droughts — will change," Pendergrass said.About the articleTitle: Precipitation variability increases in a warmer climateAuthors: Pendergrass, A. G., R. Knutti, F. Lehner, C. Deser, and B. M. SandersonJournal: Scientific Reports, DOI: 10.1038/s41598-017-17966-yWriter/contact:Laura Snider, Senior Science Writer

The climate secrets of southern clouds

BOULDER, Colo. — This month, an international team of scientists will head to the remote Southern Ocean for six weeks to tackle one of the region's many persistent mysteries: its clouds.What they discover will be used to improve climate models, which routinely underestimate the amount of solar radiation reflected back into space by clouds in the region. Accurately simulating the amount of radiation that is absorbed or reflected on Earth is key to calculating how much the globe is warming.The field campaign, called the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study, or SOCRATES, could also help scientists understand the very nature of how clouds interact with aerosols — particles suspended in the atmosphere that can be from either natural or human-made sources. Aerosols can spur cloud formation, change cloud structure, and affect precipitation, all of which affect the amount of solar radiation that is reflected.During the mission, which will run from mid-January through February, the scientists will collect data from a bevy of advanced instruments packed onboard an aircraft and a ship, both of which are specially designed for scientific missions."SOCRATES will allow for some of the best observations of clouds, aerosols, radiation, and precipitation that have ever been collected over the Southern Ocean," said Greg McFarquhar, a principal investigator and the director of the University of Oklahoma Cooperative Institute for Mesoscale Meteorological Studies (CIMMS). "These data will provide us with critical insight into the physics of cloud formation in the region, information we can use to improve global climate models."The U.S. portion of SOCRATES is largely funded by the National Science Foundation (NSF).“The Southern Ocean is famously remote and stormy and it's hard to imagine a worse place to do a field campaign. But a vast, stormy ocean is a great laboratory for studying clouds, and it's clear from our models that we have a lot to learn about them,” said Eric DeWeaver, program director for Climate and Large-Scale Dynamics in NSF’s Geoscience directorate."I'm excited about this campaign because I think it will answer some fundamental questions about clouds and their dependence on atmospheric conditions," DeWeaver said. "We'll be able to use this information to understand cloud behavior closer to home and how clouds are likely to adjust to changing climatic conditions."Critical observing and logistical support for SOCRATES is being provided by the Earth Observing Laboratory (EOL) at the National Center for Atmospheric Research (NCAR). Other U.S. principal investigators are based at the University of Washington.The Australian portion of SOCRATES is largely funded by the country's government through the Australian Marine National Facility, which is owned and operated by CSIRO.A supercooled mysteryMcFarquhar and his colleagues think the reason that climate models are not accurately capturing the amount of radiation reflected by clouds above the Southern Ocean is because they may not be correctly predicting the composition of the clouds. In particular, the models may not be producing enough supercooled water — droplets that stay liquid even when the temperature is below freezing.One possible explanation for the problem is the way models represent how clouds interact with aerosols, a process that affects the amount of supercooled water in a cloud. These representations were developed from atmospheric observations, largely in the Northern Hemisphere, where most of the world's population lives.But the atmosphere over the Northern Hemisphere — even over the Arctic — contains many more pollutants, including aerosols, than the atmosphere over the Southern Ocean, which is relatively pristine."We don't know how appropriate the representations of these processes are for the Southern Hemisphere," McFarquhar said. "SOCRATES will give us an opportunity to observe these cloud-aerosol interactions and see how much they differ, if at all, from those in the Northern Hemisphere."Flying through hazardous cloudsThe NSF/NCAR HIAPER Gulfstream V has been modified to serve as a flying laboratory. (©UCAR. This figure is freely available for media & nonprofit use.)For the SOCRATES field campaign, observations will be taken from the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research, or HIAPER, a highly modified Gulfstream V aircraft, and the R/V Investigator, an Australian deep-ocean research vessel."Much of what we currently know about Southern Ocean cloud, aerosol, and precipitation properties comes from satellite-based estimates, which are uncertain and have undergone few comparisons against independent data," said co-investigator Roger Marchand, a scientist at the University of Washington. "The data collected during SOCRATES will also enable us to evaluate current satellite data over the Southern Ocean, as well as potentially help in the design of better satellite-based techniques."The research aircraft will be based out of Hobart, Tasmania, and will make about 16 flights over the Southern Ocean during the course of the campaign. The many high-tech instruments on board will measure the size and distribution of cloud droplets, ice crystals, and aerosols, as well as record the temperature, winds, air pressure, and other standard atmospheric variables.The instruments include NCAR's HIAPER Cloud Radar (HCR) and High Spectral Resolution Lidar (HSRL). The wing-mounted HCR is able to "see" inside clouds and characterize the droplets within, while the HSRL can measure air molecules and aerosols. Together, the two highly advanced instruments will give scientists a more complete picture of the wide range of particles in the atmosphere above the Southern Ocean.The nature of the research — flying a plane in search of supercooled water —presents some challenges with aircraft icing."Oftentimes, the cleaner the air, the more probable large drops and severe icing conditions become," said Cory Wolff, the NCAR project manager who is overseeing aircraft operations for SOCRATES. "We have a number of precautions we're taking to mitigate that risk."First, a mission coordinator whose sole job is to monitor icing conditions will join each flight. Second, the design of the flights themselves will help the crew anticipate icing conditions before they have to fly through them. On the flight south from Tasmania, the HIAPER GV will fly high above the clouds — and the icing danger. During that leg of the flight, the scientists will collect information about the clouds below, both with onboard radar and lidar as well as with dropsondes — small instrument packages released from the aircraft.With that information, the scientists can determine whether it's safe to pilot the aircraft through the clouds on the return trip, collecting detailed information about the cloud composition.Sailing the stormiest seasThe Australian R/V Investigator will take measurements of the atmosphere and ocean during its six-week voyage. (Image courtesy CSIRO.)The measurements taken from the sky will be complemented by data collected from instruments on board the Australian R/V Investigator, including the NCAR Integrated Sounding System. The ISS gathers extensive data by using a radar wind profiler, surface meteorology sensors, and a balloon-borne radiosonde sounding system. The team will launch soundings every six hours, and sometimes more often, throughout the campaign."Observations from the ship will help us understand the background state of the atmosphere — how it's behaving," said NCAR scientist Bill Brown, who traveled to Australia in late November to prepare the ISS for the voyage.The ship will be deployed for the entire six weeks and will face its own challenges, notably the notorious roughness of the Southern Ocean, sometimes called the stormiest place on Earth."There are no land masses to break up the winds down there," Brown said. "So the ocean can be quite rough."SOCRATES investigators will also draw on measurements from another Australian ship as it travels between Tasmania and Antarctica on resupply missions, the R/V Aurora Australis, as well as observations from buoys and some land-based instruments on Macquarie Island."I am excited that we will have such a comprehensive suite of observations," McFarquhar said. "If we just had the cloud observations we wouldn’t have the appropriate context. If we just had the aerosols and measurements below the clouds, we wouldn't be able to understand the complete picture."For more about the SOCRATES campaign, visit the project website.Collaborating institutions:Australian Antarctic DivisionAustralian Bureau of MeteorologyAustralian Department of Environment and EnergyColorado State UniversityCooperative Institute for Mesoscale Meteorological StudiesCSIROKarlsruhe Institute of TechnologyMonash UniversityNational Center for Atmospheric ResearchNational Science FoundationNorthWest Research AssociatesQueensland University of TechnologyUniversity of California San DiegoUniversity of Colorado BoulderUniversity of Illinois at Urbana-ChampaignUniversity of MelbourneUniversity of OklahomaUniversity of Washington

Groundbreaking data set gives unprecedented look at future weather

Dec. 7, 2017 | How will weather change in the future? It's been remarkably difficult to say, but researchers are now making important headway, thanks in part to a groundbreaking new data set at the National Center for Atmospheric Research (NCAR).Scientists know that a warmer and wetter atmosphere will lead to major changes in our weather. But pinning down exactly how weather — such as thunderstorms, midwinter cold snaps, hurricanes, and mountain snowstorms — will evolve in the coming decades has proven a difficult challenge, constrained by the sophistication of models and the capacity of computers.Now, a rich, new data set is giving scientists an unprecedented look at the future of weather. Nicknamed CONUS 1 by its creators, the data set is enormous. To generate it, the researchers ran the NCAR-based Weather Research and Forecasting model (WRF) at an extremely high resolution (4 kilometers, or about 2.5 miles) across the entire contiguous United States (sometimes known as "CONUS") for a total of 26 simulated years: half in the current climate and half in the future climate expected if society continues on its current trajectory.The project took more than a year to run on the Yellowstone supercomputer at the NCAR-Wyoming Supercomputing Center. The result is a trove of data that allows scientists to explore in detail how today's weather would look in a warmer, wetter atmosphere.CONUS 1, which was completed last year and made easily accessible through NCAR's Research Data Archive this fall, has already spawned nearly a dozen papers that explore everything from changes in rainfall intensity to changes in mountain snowpack."This was a monumental effort that required a team of researchers with a broad range of expertise — from climate experts and meteorologists to social scientists and data specialists — and years of work," said NCAR senior scientist Roy Rasmussen, who led the project. "This is the kind of work that's difficult to do in a typical university setting but that NCAR can take on and make available to other researchers."Other principal project collaborators at NCAR are Changhai Liu and Kyoko Ikeda. A number of additional NCAR scientists lent expertise to the project, including Mike Barlage, Andrew Newman, Andreas Prein, Fei Chen, Martyn Clark, Jimy Dudhia, Trude Eidhammer, David Gochis, Ethan Gutmann, Gregory Thompson, and David Yates. Collaborators from the broader community include Liang Chen, Sopan Kurkute, and Yanping Li (University of Saskatchewan); and Aiguo Dai (SUNY Albany).Climate and weather research coming togetherClimate models and weather models have historically operated on different scales, both in time and space. Climate scientists are interested in large-scale changes that unfold over decades, and the models they've developed help them nail down long-term trends such as increasing surface temperatures, rising sea levels, and shrinking sea ice.Climate models are typically low resolution, with grid points often separated by 100 kilometers (about 60 miles). The advantage to such coarse resolution is that these models can be run globally for decades or centuries into the future with the available supercomputing resources. The downside is that they lack detail to capture features that influence local atmospheric events,  such as land surface topography, which drives mountain weather, or the small-scale circulation of warm air rising and cold air sinking that sparks a summertime thunderstorm.Weather models, on the other hand, have higher resolution, take into account atmospheric microphysics, such as cloud formation, and can simulate weather fairly realistically. It's not practical to run them for long periods of time or globally, however — supercomputers are not yet up to the task.As scientific understanding of climate change has deepened, the need has become more pressing to merge these disparate scales to gain better understanding of how global atmospheric warming will affect local weather patterns."The climate community and the weather community are really starting to come together," Rasmussen said. "At NCAR, we have both climate scientists and weather scientists, we have world-class models, and we have access to state-of-the-art supercomputing resources. This allowed us to create a data set that offers scientists a chance to start answering important questions about the influence of climate change on weather."Weather models are typically run with a much higher resolution than climate models, allowing them to more accurately capture precipitation. This figure compares the average annual precipitation from a 13-year run of the Weather Research and Forecasting (WRF) model (left) with the average annual precipitation from running the global Community Earth System Model (CESM) to simulate the climate between 1976 and 2005 (right). (©UCAR. This figure is courtesy Kyoko Ikeda. Ite is freely available for media & nonprofit use.)Today's weather in a warmer, wetter futureTo create the data set, the research team used WRF to simulate weather across the contiguous United States between 2000 and 2013. They initiated the model using a separate "reanalysis" data set constructed from observations. When compared with radar images of actual weather during that time period, the results were excellent."We weren't sure how good a job the model would do, but the climatology of real storms and the simulated storms was very similar," Rasmussen said.With confidence that WRF could accurately simulate today's weather, the scientists ran the model for a second 13-year period, using the same reanalysis data but with a few changes. Notably, the researchers increased the temperature of the background climate conditions by about 5 degrees Celsius (9 degrees Fahrenheit), the end-of-the-century temperature increase predicted by averaging 19 leading climate models under a business-as-usual greenhouse gas emissions scenario (2080–2100). They also increased the water vapor in the atmosphere by the corresponding amount, since physics dictates that a warmer atmosphere can hold more moisture.The result is a data set that examines how weather events from the recent past — including named hurricanes and other distinctive weather events — would look in our expected future climate.The data have already proven to be a rich resource for people interested in how individual types of weather will respond to climate change. Will the squall lines of intense thunderstorms that rake across the country's midsection get more intense, more frequent, and larger? (Yes, yes, and yes.) Will snowpack in the West get deeper, shallower, or disappear? (Depends on the location: The high-elevation Rockies are much less vulnerable to the warming climate than the coastal ranges.) Other scientists have already used the CONUS 1 data set to examine changes to rainfall-on-snow events, the speed of snowmelt, and more.Running the Weather Research and Forecasting (WRF) model at 4 kilometers resolution over the contiguous United States produced realistic simulations of precipitation. Above, average annual precipitation from WRF for the years 2000-2013 (left) compared to the PRISM dataset for the same period (right). PRISM is based on observations. (©UCAR. This figure is courtesy Kyoko Ikeda. Ite is freely available for media & nonprofit use.) Pinning down changes in storm trackWhile the new data set offers a unique opportunity to delve into changes in weather, it also has limitations. Importantly, it does not reflect how the warming climate might shift large-scale weather patterns, like the typical track most storms take across the United States. Because the same reanalysis data set is used to kick off both the current and future climate simulations, the large-scale weather patterns remain the same in both scenarios.To remedy this, the scientists are already working on a new simulation, nicknamed CONUS 2.Instead of using the reanalysis data set — which was built from actual observations — to kick off the modeling run of the present-day climate, the scientists will use weather extracted from a simulation by the NCAR-based Community Earth System Model (CESM).  For the future climate run, the scientists will again take the weather patterns from a CESM simulation — this time for the year 2100 — and feed the information into WRF.The finished data set, which will cover two 20-year periods, will likely take another year of supercomputing time to complete, this time on the newer and more powerful Cheyenne system at the NCAR-Wyoming Supercomputing Center. When complete, CONUS 2 will help scientists understand how expected future storm track changes will affect local weather across the country.Scientists are already eagerly awaiting the data from the new runs, which could start in early 2018. But even that data set will have limitations. One of the greatest may be that it will rely on a single run from CESM. Another NCAR-based project ran the model 40 times from 1920 to 2100 with only minor changes to the model's starting conditions, showing researchers how the natural chaos of the atmosphere can cause the climate to look quite different from simulation to simulation.Still, a single run of CESM will let scientists make comparisons between CONUS 1 and CONUS 2, allowing them to pinpoint possible storm track changes in local weather. And CONUS 2 can also be compared to other efforts that downscale global simulations to study how regional areas will be affected by climate change, providing insight into the pros and cons of different research approaches."This is a new way to look at climate change that allows you to examine the phenomenology of weather and answer the question, 'What will today's weather look like in a future climate?'" Rasmussen said. "This is the kind of detailed, realistic information that water managers, city planners, farmers, and others can understand and helps them plan for the future."Get the dataHigh Resolution WRF Simulations of the Current and Future Climate of North America, DOI: 10.5065/D6V40SXPStudies that relied on the CONUS data setExtreme downpours could increase fivefold across parts of the U.S.Slower snowmelt in a warming worldNorth American storm clusters could produce 80 percent more rainWriter/contact:Laura Snider, Senior Science Writer

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