Scientists tackle mystery of thunderstorms that strike at night

BOULDER – Thunderstorms that form at night, without a prod from the Sun's heat, are a mysterious phenomenon. This summer scientists will be staying up late in search of some answers. From June 1 through July 15, researchers from across North America will fan out each evening across the Great Plains, where storms are more common at night than during the day. The research effort, co-organized by the National Center for Atmospheric Research (NCAR) and several collaborating institutions, will use lab-equipped aircraft, ground-based instruments, and weather balloons to better understand the atmospheric conditions that lead to storm formation and evolution after sunset. Their results may ultimately help improve forecasts of these sometimes damaging storms. The Plains Elevated Convection at Night (PECAN) field campaign will involve scientists, students, and support staff from eight research laboratories and 14 universities. The $13.5 million project is largely funded by the National Science Foundation (NSF), NCAR's sponsor, which contributed $10.6 million. Additional support is provided by NASA, the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Department of Energy. Aloft in the night Thunderstorms that form during the day are less puzzling than nighttime storms. The Sun heats the Earth’s surface, which in turn, warms the air directly above the ground. When that warm air is forced to rise, it causes convection—a circulation of warm updrafts and cool downdrafts—and sometimes creates a storm. The formation of thunderstorms at night, however, when the Sun is not baking the land, is less well understood. "At night, the entire storm circulation is elevated higher off the ground," said NCAR scientist Tammy Weckwerth, a PECAN principal investigator. "This makes observations of the conditions leading to nighttime thunderstorms much more challenging because that part of the atmosphere is not well covered by the network of instruments we normally rely on. The vast array of instruments available to PECAN researchers will allow them to collect data higher in the atmosphere. This data will help scientists characterize the conditions that lead both to individual storm formation as well as to the clustering and organizing of these storms into large-scale systems, which can drop significant precipitation. "Nighttime thunderstorms are an essential source of summer rain for crops but are also a potential hazard through excessive rainfall, flash flooding, and dangerous cloud-to-ground lightning," says Ed Bensman, program director in NSF’s Division of Atmospheric and Geospace Sciences.  "Weather forecast models often struggle to accurately account for this critical element of summer rainfall on the Great Plains.  The PECAN field campaign will provide researchers and operational forecasters with valuable insights into thunderstorms at night—and improve our ability to model them more accurately." Deploying in the dark The campaign, based in Hays, Kansas, will begin each day at 8 a.m., when a crew of forecasters starts developing a nightly forecast. At 3 p.m. the scientists will use the forecast to determine where across northern Oklahoma, central Kansas, or south-central Nebraska to deploy their mobile resources. Moving dozens of people around the Great Plains each night will be a challenge for PECAN, but it's also what distinguishes it from past field projects.  "Previous severe weather campaigns have focused mostly on daytime storms, for largely practical reasons, as it is more difficult to set up instruments in the dark," said Bart Geerts, a professor of atmospheric science at the University of Wyoming and a PECAN principal investigator. "But the large thunderstorm complexes travelling across the Great Plains at night really are a different beast." NCAR's portable S-Pol radar is one of the many instruments that will be deployed during the PECAN field campaign to help scientists better understand nighttime thunderstorms. (©UCAR. This image is freely available for media & nonprofit use.) Scientists believe that several interacting factors may contribute to nocturnal storm formation and maintenance: a stable layer of air at the surface; a strong wind current above that layer, known as a low-level jet; and atmospheric waves, some of which are called "bores," that ripple out from the storms themselves.  "But we just don't really know how they interact," Geerts said. "That's what PECAN is about." A better understanding of these storms will have relevance for areas beyond the Great Plains. Clustered nighttime thunderstorms are common in various regions scattered across the globe. A fleet of instruments PECAN will use three research aircraft, two of which—a University of Wyoming King Air and a NASA DC-8—will fly in the clear air away from the storms. Only the third, a NOAA P-3, which is widely used in hurricane research and reconnaissance, will be able to fly into the trailing region of storms.   The researchers will also rely on a number of ground-based instrument suites, known as PECAN Integrated Sounding Arrays, or PISAs. Six of the PISAs will operate from fixed locations around the study area, and four will be mobile, allowing them to be repositioned each night depending on where storms are expected to form. The instruments within each PISA vary, but collectively they will give each array the ability to measure temperature, moisture, and wind profiles, as well as launch weather balloons. Among the instruments are several newly developed at NCAR's Earth Observing Laboratory (EOL), including one that uses an innovative laser-based technique to remotely measure water vapor and an advanced wind profiler. Finally, the scientists will have a fleet of mobile and fixed radars, including the NCAR S-Pol. In all, PECAN researchers will have access to more than 100 instruments brought to the effort by partner institutions from across North America. "The sheer number of instruments being coordinated is unprecedented," said Weckwerth, who has participated in more than 15 other field expeditions. The planning necessary to manage this large collection of instruments—from finding property suitable for a fixed radar to making sure the mobile instruments are out of harm's way while tracking a storm—is being taken on by EOL's Project Management Office. That team is also responsible for housing, food and other logistics for the scientists and students who are participating in the campaign.

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The Global Distribution of Atmospheric Oxygen

Britton Stephens

Research Aviation FacilityEarth Observing Laboratory

Orographic Convection in the Tropics: Wind Speed Control and Convective Initiation

The Dominica Experiment (DOMEX) took place in the eastern Caribbean in the spring of 2011 with 21 research flights of the University of Wyoming King Air (UWKA) aircraft. The goal was an improved understanding of the physics of convective orographic precipitation in the tropics. The UWKA measured upstream and downstream airflow properties as well as the convective clouds and precipitation over the island of Dominica.

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Development of low-cost laser remote sensor for high vertical resolution and continuous measurements of atmospheric water vapor

Scott Spuler

Remote Sensing FacilityEarth Observing Laboratory

Nor’easter on the radar

February 9, 2015 | While some folks were looking to Punxsutawney Phil on Groundhog Day, scientists were learning about the weather through a different route: flying a highly advanced cloud radar on its maiden voyage above a major northeast storm. The dual-polarization radar, developed at NCAR with funding from NSF, is a compact instrument that uses millimeter-long wavelengths of energy to provide highly detailed views of the atmosphere. It distinguishes between ice, water, and supercooled water droplets (droplets that exist as a liquid below freezing), measures thin ice and liquid clouds, and takes advanced wind measurements. Scientists flew the new HIAPER Cloud Radar, mounted in the white pod, over a Nor'easter to capture detailed data about a major winter storm. (Photo by Jonathan Emmett, NCAR. This image is freely available for media & nonprofit use.) The new instrument is designed to fly aboard the specially instrumented NSF/NCAR Gulfstream V aircraft, also known as the High-performance Instrumented Airborne Platform for Environmental Research (HIAPER). For that reason, it is called the HIAPER Cloud Radar. A scientific team led by University of Illinois atmospheric scientist Bob Rauber deployed the radar on February 2 for eight hours, gathering data from Washington, D.C., to Bangor, Maine. Flying over a storm that was pummeling the region with snow, freezing rain, and strong winds, the scientists obtained clear and detailed observations of various features within the storm, including circulation patterns and small regions near the top of the clouds called generating cells that contain up-and-down drafts and generate the snow that later reaches the ground. The one-day field project, known as Nor’easter, will help scientists better understand the conditions that lead to the distribution of precipitation within the storm. It may eventually lead to finer-grained forecasts that can project which locations within a region are likely to receive the most snow. Northeastern winter storms draw energy from warm Atlantic water, developing bands of exceptionally heavy snow. This creates a major forecasting challenge for the densely populated Northeast where the locations of such bands can mean the difference between a foot of snow falling on Manhattan or on less developed regions just 50 or 75 miles away. “We are hoping to understand how the circulation patterns we saw so clearly on the HIAPER Cloud Radar organize snowfall,” Rauber said. “The data from the Nor’easter Project will be valuable to forecasters and researchers studying Northeast snowstorms, helping them interpret the computer model results and observations by ground radar.” The research team was excited to put the new radar through its paces and said it passed its maiden voyage with flying colors. “The HIAPER Cloud Radar, together with the long range and endurance of HIAPER, will allow atmospheric scientists the flexibility to explore regions of storms that would be impossible to reach with ground-based radars,” said Jeff Stith, manager of NCAR’s Research Aviation Facility. “For example, they can use it to study the formation of large storm systems approaching the East Coast from the Atlantic Ocean. We expect it to be widely used in future missions, often in combination with other measurements that can be made from HIAPER.” The wait won't be long. The radar's next deployment will be this summer, when a field project known as CSET (Cloud-System Evolution in the Trades) gets underway. CSET researchers will head to the Pacific to examine the evolution of cloud systems in the important trade-wind regions, which can influence climate worldwide. The NSF/NCAR HIAPER Gulfstream V research jet in flight. (©UCAR. This image is freely available for media & nonprofit use.)  Writer/contactDavid Hosansky Nor’easter CollaboratorUniversity of Illinois Nor’easter & HIAPER Cloud Radar FunderNational Science Foundation    

EOL Seminar Series

Manned and Unmanned Aircraft in Air-Sea Interaction Research

Ken MelvilleScripps Institution of OceanographyUC San Diego

Cold facts of air pollution

February 2, 2015 | The difference between a breath of cold air and a breath of warm air isn’t just the temperature. It’s also the pollutants they might contain. Until now, wintertime air pollution hasn’t been studied in much detail. Scientists have focused more on warm air, partly because summertime's stagnant atmospheric conditions and intense sunshine tend to worsen ozone pollution. But that's about to change as researchers turn their attention to winter air quality in the eastern United States. The WINTER field project will focus on the Northeast urban corridor, Ohio River Valley, and Southeast Mid-Atlantic. (©UCAR. Image by Alison Rockwell, NCAR, based on NASA satellite map. This image is freely available for media & nonprofit use.) This month, a major air quality project known as WINTER (Wintertime Investigation of Transport, Emissions, and Reactivity) takes to the air to examine pollutants across the Northeast urban corridor, Ohio River Valley, and Southeast Mid-Atlantic. Scientists will home in on wintertime emissions from urban areas, power plants, and farmland, and seek to better understand the chemical processes that take place as pollutants move through an atmosphere that is not only colder but also darker than in summer. The field campaign, which runs from February 1 to March 15, is being led by scientists at the University of Washington, NOAA's Earth System Research Laboratory, University of California Berkeley, Georgia Institute of Technology, University of Colorado Boulder, and the University of New Hampshire. The research team will use the NSF/NCAR C-130, a flying laboratory equipped with more than 20 instruments to measure gases and particles. The aircraft is owned by the National Science Foundation and operated by NCAR. NCAR is also managing the project, including coordinating research flights and providing data services. Flight operations will be based at the NASA Langley Research Center in Hampton, Virginia. "Aircraft missions will occur at different times during the campaign so that the pollutant gases and reactions can be observed during the day, at night, from night into day, and day into night,” said NCAR project manager Cory Wolff. WINTER's regional differences NCAR scientist Alan Hills (right) and University of California, Irvine, graduate student Jason Schroeder operate instruments for the WINTER field project aboard the NSF/NCAR C-130. (©UCAR. Photo by Alison Rockwell, NCAR. This image is freely available for media & nonprofit use.) A number of factors affect wintertime air: colder temperatures, snow cover, lower absolute humidity, and fewer hours of sunlight. Plants tend to emit fewer chemicals, while people may emit more as they burn heating oil and other fuels to heat their homes. In addition, pollutants may travel farther because chemical reactions take place more slowly in cold air. By flying over several regions, the WINTER research team will better understand the atmospheric impacts created by different types of emissions from major cities in the Northeast and coal-fired power plants in the Ohio River Valley. The scientists will compare those emissions with data they gather in the Southeast, where winters are milder, plants have a more pronounced influence on the atmosphere, and emissions come from agricultural burning. The project’s findings will be used to provide more detailed information to decision makers and improve computer models of the atmosphere. “Wintertime pollution has not been the focus of many campaigns—most are during the spring and summer months when the Sun has maximum impact,” said Wolff. “By sampling the air in the cold and darkness of winter, the science team can get a better sense of the atmospheric chemistry of the eastern United States and compare that to other times of year. " Writer/contactDavid Hosansky CollaboratorsUniversity of WashingtonNOAA's Earth System Research LaboratoryUniversity of California BerkeleyGeorgia Institute of TechnologyUniversity of Colorado BoulderUniversity of New Hampshire FundersNational Science Foundation (NSF)National Oceanic and Atmospheric Administration (NOAA)        

Tropical forests have large appetite for carbon dioxide

BOULDER — A new study led by NASA and the National Center for Atmospheric Research (NCAR) shows that tropical forests may be absorbing far more human-emitted carbon dioxide than many scientists thought. The study estimates that tropical forests absorb 1.4 billion metric tons of carbon dioxide out of a total global absorption of 2.5 billion, in response to rising atmospheric levels of the greenhouse gas. This means, if left undisturbed, the tropical trees should be able to continue reducing the rate of global warming. “This is good news, because uptake in northern forests may already be slowing, while tropical forests may continue to take up carbon for many years,” said David Schimel of NASA’s Jet Propulsion Laboratory in Pasadena, California. Schimel is lead author of a paper on the new research, appearing this week in the Proceedings of National Academy of Sciences. The question of which type of forest is the bigger carbon absorber “is not just an accounting curiosity,” said NCAR scientist Britton Stephens, a co-author on the paper. “It has big implications for our understanding of whether global terrestrial ecosystems might continue to offset our carbon dioxide emissions or might begin to exacerbate climate change.” Forests and other land vegetation currently remove up to 30 percent of human carbon dioxide emissions from the atmosphere by absorbing carbon dioxide during photosynthesis. If the rate of absorption were to slow down, the rate of global warming would speed up in return. Tropical forests like this one in the Serra do Mar Paranaense in Brazil may be absorbing far more human-emitted carbon dioxide than many scientists thought. (Photo by Deyvid Setti e Eloy Olindo Setti via Wikimedia Commons.) The new study is the first to devise a way to make apples-to-apples comparisons of carbon dioxide uptake estimates from many sources at different scales: computer models of ecosystem processes, atmospheric models used to deduce the sources of today’s concentrations (called atmospheric inverse models), satellite images, data from routine and experimental forest plots, and more. The researchers reconciled these analyses and assessed the accuracy of the inverse models based on how well they reproduced independent, airborne and ground-based measurements. They obtained their new estimate of the tropical carbon absorption from the weighted average of atmospheric, ecosystem model, and ground-based data. “Until our analysis, no one had successfully completed a global reconciliation of information about carbon dioxide effects from the atmospheric, forestry, and modeling communities,” said coauthor Joshua Fisher of the Jet Propulsion Laboratory. “It is incredible that all these different types of independent data sources start to converge on an answer.” The research was funded by NASA and by the National Science Foundation, which sponsors NCAR. Growing forests, more fires As human-caused emissions add more carbon dioxide to the atmosphere, forests worldwide are using it to grow faster, reducing the amount that stays airborne. This effect is called carbon dioxide fertilization. But climate change also decreases water availability in some regions and makes Earth warmer, leading to more frequent droughts and larger wildfires. In the tropics, humans compound the problem by burning wood during deforestation. Fires don’t just stop carbon absorption by killing trees, they also spew huge amounts of previously-stored carbon into the atmosphere as the wood burns. For about 25 years, most atmospheric inverse models have been showing that mid-latitude forests in the Northern Hemisphere absorb more carbon than tropical forests. That result was initially based on the then-current understanding of global air flows and limited data suggesting that deforestation was causing tropical forests to release more carbon dioxide than they were absorbing. In the mid-2000s, Stephens used measurements of carbon dioxide made from aircraft to show that many atmospheric inverse models were not correctly representing flows of carbon dioxide in the air above ground level. Models that matched the aircraft measurements better showed more carbon absorption in the tropical forests. However, there were still not enough global data sets to validate the idea of large tropical-forest absorption. Schimel said that their new study took advantage of a great deal of work other scientists have done since Stephens’ paper to pull together national and regional data of various kinds into robust, global data sets. He noted that the new paper reconciles results at every scale from the pores of a single leaf, where photosynthesis takes place, to the whole Earth, as air moves carbon dioxide around the globe. “What we’ve had up till this paper was a theory of carbon dioxide fertilization based on phenomena at the microscopic scale and observations at the global scale that appeared to contradict those phenomena,” he said. “Here, at least, is a hypothesis that provides of a consistent explanation that includes both how we know photosynthesis works and what’s happening at the planetary scale.” Atmospheric models have improved over the past decade, but there is still considerable disagreement among atmospheric inverse estimates of the distribution of carbon uptake, owing to remaining differences in modeling global air flows. “It is critical that we rigorously test these models against observations so that we can further reduce uncertainty on the terrestrial feedback to climate change,” Stephens said. About the article Title: The effect of increasing CO2 on the terrestrial carbon cycle Authors: David Schimel, Britton Stephens, and Joshua B. Fisher Journal: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1407302112


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