S&TR December 2017 Lawrence Livermore National Laboratory 4

S&TR December 2017 Smoke from the 2016 Soberanes Fire in Monterey County, California—the costliest wildfire up to that time—begins to block out the Milky Way. Climate change makes droughts more likely and such fires more frequent and larger in scale. (Photo by Li Liu, M.D.) 4 Lawrence Livermore National Laboratory S&TR December 2017 Climate Modeling The Atmosphere around CLIMATE MODELS Supercomputers, the laws of physics, and Lawrence Livermore’s nuclear weapons research all interact to advance atmospheric science and climate modeling. INCE the 1960s, computer models power than we carry in our pockets now, In fact, Livermore’s climate models S have been ensuring the safe whereas today’s advanced computers trace their origins to the Laboratory’s return of astronauts from orbital and allow us to study phenomena vastly more initial development of codes to simulate lunar missions by carefully predicting complex than orbital dynamics.” That nuclear weapons. Hendrickson states, complicated spacecraft trajectories. A computational power offers unprecedented “Our primary mission is nuclear weapons slight miscalculation could cause a craft to insight into how the physical world works, design, which has required us to create zoom past the Moon or Earth and become providing details about phenomena that unique computational capabilities. These lost in space, or approach too steeply would be infeasible to study with physical capabilities have also been applied to and face an equally disastrous outcome. experiments. At Lawrence Livermore, other national needs, including modeling Bruce Hendrickson, Livermore’s associate numerical models running on high- the atmosphere and the rest of the director for Computation, points out, “In performance computers are a vital part climate system.” the 1960s, scientists and engineers put of research in many programs, including Over the years, advances in scientific people on the Moon with less computing stockpile stewardship and climate studies. understanding and increased computational Lawrence Livermore National Laboratory 5 Climate Modeling S&TR December 2017 power have resulted in higher First Atmospheric Animation for applications such as tracking fidelity climate models that are more From its inception, the Laboratory releases of radioactive and representative of the real world. These pursued numerical approaches to solving other hazardous materials. In computationally intense simulations have problems using cutting-edge computer the late 1950s, Livermore also helped shake down and benchmark systems. “Livermore went all-in with scientist Cecil “Chuck” subsequent generations of the Department computers,” says Glenn Fox, associate Leith developed one of Energy’s (DOE’s) supercomputers director for Physical and Life Sciences. of Livermore’s first- before the machines transition to classified “When the Laboratory’s doors opened, ever numerical models work. “Climate simulation is an application the first big procurement was a state- capable of simulating that can consume the whole machine of-the-art computer.” The room-sized the hydrodynamic and and put it through its paces in a very Univac-1 had 5,600 vacuum tubes and radiative processes demanding way,” explains Hendrickson. 9 kilobytes of memory and ran at a speed in a thermonuclear “The simulations touch every part of the of 1,000 floating-point operations per explosion. Recognizing computer.” (See S&TR, July/August 2015, second (flops). In 2018, Livermore’s fundamental similarities in pp. 3–14.) Sierra supercomputer will use more than the underlying equations and 1 million microprocessors to achieve a interested in demonstrating speed of 150 petaflops—150 trillion times what could be achieved with On July 12, 2017, a 5,801-square-kilometer piece of faster than the Univac-1. more powerful computing, Leith the Larsen C ice shelf broke away from Antarctica, Even in the Laboratory’s early days, turned his attention to creating more as shown in the satellite image. The Energy researchers understood that the same comprehensive weather system models. Exascale Earth System Model (E3SM) is one of computational approaches for simulating Michael MacCracken, a now-retired the first to simulate the movement and evolution of nuclear weapons could be applied to better climate scientist who headed Livermore’s glaciers and ice sheets. (Image courtesy of NASA.) simulate evolution of the weather and atmospheric and geophysical sciences division from 1987 to 1993, came to the Laboratory as one of Leith’s graduate students. MacCracken says, “Using the most advanced computers available in the early 1960s, Leith developed an atmospheric model that was way ahead of its time.” Leith’s Livermore Atmospheric Model (LAM) divided the atmosphere into a three-dimensional (3D) mesh with six vertical layers and a horizontal grid with five-degree intervals in latitude and longitude. LAM was the world’s first global atmospheric circulation model that calculated temperature, winds, humidity, clouds, precipitation, the day-and-night cycle, and weather systems around the globe, all starting from first-principles equations for the conservation of mass, momentum, energy, and water vapor. Leith also created the first animation of atmospheric modeling results by colorizing photographs of a black-and-white video screen and stitching them together into a film. Leith’s atmospheric work also benefited other Livermore programs. For example, 6 Lawrence Livermore National Laboratory S&TR December 2017 Climate Modeling Climate models divide Earth into a grid with vertical and horizontal intervals. The smaller the intervals, the finer the grid, and the better the resolution of the model—that is, the greater detail the model can produce. (Image courtesy of the National Oceanic and Atmospheric Administration.) to these efforts, atmospheric chemistry models were also developed. The first such model contributed to a successful plan to limit rising concentrations of photochemical smog in the San Francisco Bay Area. The second model simulated stratospheric chemistry and was used to calculate the impact of a proposed fleet of supersonic transport aircraft on stratospheric ozone. This modeling also investigated the potential for ozone depletion from atmospheric nuclear testing back in the early 1960s and the much his study of atmospheric turbulence larger depletion that would result from a led to a better understanding of how to global nuclear war with megaton-yield represent turbulence and turbulent flows. nuclear weapons. MacCracken adds, “Although simulations When chlorofluorocarbon (CFC) of astrophysics, plasma physics, and emissions from aerosol spray cans, nuclear weapons address different refrigerators, and other sources came under temperatures, pressures, and timescales, scrutiny in the 1970s, the stratospheric The Livermore Atmospheric Model (LAM), it’s all the same basic physics. So chemistry model was applied to evaluate developed by Cecil “Chuck” Leith in the 1960s, computational advances in one area benefit the ozone depletion potential and develop was the first-ever global climate model to the others and vice-versa.” a metric for calculating depletion that be animated. Shown are screen captures of was later used in the Montreal Protocol to runs for pressure and precipitation (top) and Ozone and Nuclear Winter regulate CFC emissions. After restrictions temperature (bottom). As environmental awareness rose in the were put in place in 1987, growth of the late 1960s, Laboratory programs began to continent-size hole in the ozone layer address regional and global environmental slowed, and after further international and later became its director from 1958 to problems. Derived from LAM, an early agreements, it eventually stopped growing 1960, Edward Teller had made the critical climate model developed by MacCracken and began to slowly shrink. determination at Los Alamos that a nuclear was used to analyze hypotheses about the In the mid-1980s, famed astrophysicist explosion would not ignite the atmosphere. causes of ice age cycles, the effects of Carl Sagan and others suggested the specter Now he questioned the severity of a nuclear volcanic eruptions and changes in land of a “nuclear winter”—that the blasts and winter. Climate scientist Curt Covey, who cover, and the consequences of changes in fires from a global nuclear war could loft retired from Livermore in 2017, remembers atmospheric composition. The pioneering enough smoke and other matter into the Teller saying, “At Livermore, we have the LAM would eventually lead to the global atmosphere to obscure sunlight for months, best computers. Surely we can do the best climate models that today also encompass causing a global vegetation die-off and job in simulations.” In response, Livermore interactive representations of the oceans, a winterlike cooling of the entire planet used its modeling capabilities to investigate land surfaces, ice masses, and biological that could kill billions of people. In 1945, the global effects of nuclear winter and activity in the oceans and on land. Parallel before he cofounded Lawrence Livermore found that although significant cooling Lawrence Livermore National Laboratory 7 Climate Modeling S&TR December 2017 would occur depending on the amount of Chernobyl power plant in Ukraine. A wide range of release scenarios, including smoke lofted, the effects would be less partial meltdown of the reactor’s core large fires or chemical spills, incidents severe than initially conjectured. resulted in a massive explosion

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