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Pouquet A.,University of Colorado at Boulder | Pouquet A.,Computational and Information Systems Laboratory | Marino R.,Computational and Information Systems Laboratory
Physical Review Letters

The ocean and the atmosphere, and hence the climate, are governed at large scale by interactions between pressure gradient and Coriolis and buoyancy forces. This leads to a quasigeostrophic balance in which, in a two-dimensional-like fashion, the energy injected by solar radiation, winds, or tides goes to large scales in what is known as an inverse cascade. Yet, except for Ekman friction, energy dissipation and turbulent mixing occur at a small scale implying the formation of such scales associated with breaking of geostrophic dynamics through wave-eddy interactions or frontogenesis, in opposition to the inverse cascade. Can it be both at the same time? We exemplify here this dual behavior of energy with the help of three-dimensional direct numerical simulations of rotating stratified Boussinesq turbulence. We show that efficient small-scale mixing and large-scale coherence develop simultaneously in such geophysical and astrophysical flows, both with constant flux as required by theoretical arguments, thereby clearly resolving the aforementioned contradiction. © 2013 American Physical Society. Source

Mininni P.D.,Computational and Information Systems Laboratory | Matthaeus W.H.,University of Delaware | Pouquet A.,Computational and Information Systems Laboratory
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

We examine long-time properties of the ideal dynamics of three-dimensional flows, in the presence or not of an imposed solid-body rotation and with or without helicity (velocity-vorticity correlation). In all cases, the results agree with the isotropic predictions stemming from statistical mechanics. No accumulation of excitation occurs in the large scales, although, in the dissipative rotating case, anisotropy and accumulation, in the form of an inverse cascade of energy, are known to occur. We attribute this latter discrepancy to the linearity of the term responsible for the emergence of inertial waves. At intermediate times, inertial energy spectra emerge that differ somewhat from classical wave-turbulence expectations and with a trace of large-scale excitation that goes away for long times. These results are discussed in the context of partial two dimensionalization of the flow undergoing strong rotation as advocated by several authors. © 2011 American Physical Society. Source

News Article
Site: http://www.scientificcomputing.com/rss-feeds/all/rss.xml/all

BOULDER — The National Center for Atmospheric Research (NCAR) announced that it has selected its next supercomputer for advancing atmospheric and Earth science, following a competitive open procurement process. The new machine will help scientists lay the groundwork for improved predictions of a range of phenomena, from hour-by-hour risks associated with thunderstorm outbreaks to the timing of the 11-year solar cycle and its potential impacts on GPS and other sensitive technologies. The new system, named Cheyenne, will be installed this year at the NCAR-Wyoming Supercomputing Center (NWSC) and become operational at the beginning of 2017. Cheyenne will be built by Silicon Graphics International (SGI) in conjunction with centralized file system and data storage components provided by DataDirect Networks (DDN). The SGI high-performance computer will be a 5.34-petaflop system, meaning it can carry out 5.34 quadrillion calculations per second. It will be capable of more than 2.5 times the amount of scientific computing performed by Yellowstone, the current NCAR supercomputer. Funded by the National Science Foundation and the state of Wyoming through an appropriation to the University of Wyoming, Cheyenne will be a critical tool for researchers across the country studying climate change, severe weather, geomagnetic storms, seismic activity, air quality, wildfires and other important geoscience topics. Since the supercomputing facility in Wyoming opened its doors in 2012, more than 2,200 scientists from more than 300 universities and federal labs have used its resources. “We’re excited to bring more supercomputing power to the scientific community,” said Anke Kamrath, director of operations and services at NCAR’s Computational and Information Systems Laboratory. “Whether it’s the threat of solar storms or a heightened risk in certain severe weather events, this new system will help lead to improved predictions and strengthen society’s resilience to potential disasters.” “Researchers at the University of Wyoming will make great use of the new system as they continue their work into better understanding such areas as the surface and subsurface flows of water and other liquids, cloud processes, and the design of wind energy plants,” said William Gern, vice president of research and economic development at the University of Wyoming. “UW’s relationship with NCAR through the NWSC has greatly strengthened our scientific computing and data-centric research. It’s helping us introduce the next generation of scientists and engineers to these endeavors.” The NWSC is located in Cheyenne, and the name of the new system was chosen to honor the support that it has received from the people of that city. It also commemorates the upcoming 150th anniversary of the city, which was founded in 1867 and named for the American Indian Cheyenne nation. The new data storage system for Cheyenne will be integrated with NCAR’s existing GLADE file system. The DDN storage will provide an initial capacity of 20 petabytes, expandable to 40 petabytes with the addition of extra drives. This, combined with the current 16 petabytes of GLADE, will total 36 petabytes of high-speed storage. The new DDN system also will transfer data at the rate of 200 gigabytes per second, which is more than twice as fast as the current file system’s rate of 90 gigabytes per second. The system will include powerful Intel Xeon processors, whose performance will be augmented through optimization work that has been done by NCAR and the University of Colorado Boulder. NCAR and the university performed this work through their participation in the Intel Parallel Computing Centers program. Even with its increased power, Cheyenne will be three times more energy efficient (in floating point operations per second, or flops, per watt) than Yellowstone, its predecessor, which is itself highly efficient. “The new system will have a peak computation rate of over 3 billion calculations per second for every watt of power consumed," said NCAR’s Irfan Elahi, project manager of Cheyenne and section manager for high-end supercomputing services. High-performance computers such as Cheyenne allow researchers to run increasingly detailed models that simulate complex processes and how they might unfold in the future. These predictions give resource managers and policy experts valuable information for planning ahead and mitigating risk. Some of the areas in which Cheyenne is expected to accelerate research include the following: “Supercomputing is vital to NCAR’s scientific research and applications, giving us a virtual laboratory in which we run experiments that would otherwise be impractical or impossible to do,” said NCAR Director James Hurrell. “Cheyenne will be a key component of the research infrastructure of the United States through its provision of supercomputing specifically tailored for the atmospheric, geospace and related sciences. The capabilities of this new system will be central to the continued improvement of our ability to understand and predict changes in weather, climate, air quality and space weather, as well as their impacts on people, ecosystems and society.” Key features of the new Cheyenne supercomputer system: The new Cheyenne supercomputer and the existing file system are complemented by a new centralized parallel file system and data storage components. Key features of the new data storage system: The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Brachet M.E.,French National Center for Scientific Research | Bustamante M.D.,University College Dublin | Krstulovic G.,University of Nice Sophia Antipolis | Mininni P.D.,Computational and Information Systems Laboratory | And 2 more authors.
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

We investigate the ideal and incompressible magnetohydrodynamic (MHD) equations in three space dimensions for the development of potentially singular structures. The methodology consists in implementing the fourfold symmetries of the Taylor-Green vortex generalized to MHD, leading to substantial computer time and memory savings at a given resolution; we also use a regridding method that allows for lower-resolution runs at early times, with no loss of spectral accuracy. One magnetic configuration is examined at an equivalent resolution of 61443 points and three different configurations on grids of 40963 points. At the highest resolution, two different current and vorticity sheet systems are found to collide, producing two successive accelerations in the development of small scales. At the latest time, a convergence of magnetic field lines to the location of maximum current is probably leading locally to a strong bending and directional variability of such lines. A novel analytical method, based on sharp analysis inequalities, is used to assess the validity of the finite-time singularity scenario. This method allows one to rule out spurious singularities by evaluating the rate at which the logarithmic decrement of the analyticity-strip method goes to zero. The result is that the finite-time singularity scenario cannot be ruled out, and the singularity time could be somewhere between t=2.33 and t=2.70. More robust conclusions will require higher resolution runs and grid-point interpolation measurements of maximum current and vorticity. © 2013 American Physical Society. Source

Thalabard S.,Computational and Information Systems Laboratory | Thalabard S.,CEA Saclay Nuclear Research Center | Rosenberg D.,Computational and Information Systems Laboratory | Pouquet A.,Computational and Information Systems Laboratory | Mininni P.D.,Computational and Information Systems Laboratory
Physical Review Letters

We examine turbulent flows in the presence of solid-body rotation and helical forcing in the framework of stochastic Schramm-Löwner evolution (SLE) curves. The data stem from a run with 15363 grid points, with Reynolds and Rossby numbers of, respectively, 5100 and 0.06. We average the parallel component of the vorticity in the direction parallel to that of rotation and examine the resulting ωz field for scaling properties of its zero-value contours. We find for the first time for three-dimensional fluid turbulence evidence of nodal curves being conformal invariant, belonging to a SLE class with associated Brownian diffusivity κ=3.6±0.1. SLE behavior is related to the self-similarity of the direct cascade of energy to small scales and to the partial bidimensionalization of the flow because of rotation. We recover the value of κ with a heuristic argument and show that this is consistent with several nontrivial SLE predictions. © 2011 American Physical Society. Source

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