NERSC Initiative for Scientific Exploration (NISE) 2010 Awards

Modeling the Energy Flow in the Ozone Recombination Reaction

Dmitri Babikov, Marquette University

Associated NERSC Project: Coherent Control of the Ground Electronic State Molecular Dynamics (m409), Principal Investigator: Dmitri Babikov

By resolving the remaining mysteries of what determines the isotopic composition of atmospheric ozone and by investigating unusual isotope effects in other atmospheric species, this work will make a large impact on the fields of Atmospheric Chemistry, Chemical Physics, Climate Science and Geochemistry. Explaining the anomalous isotope effects in O₃, NO₂ and CO₂ will significantly improve our understanding of their production, chemistry, lifetime and loss in the atmosphere. That knowledge will help to identify and remove pollution sources as well as monitor the ozone hole, with the possible impact on enhancing the security of all life on the planet. It will allow the isotopic composition of oxygen to be used as a reliable probe of its source and history and provide information for studying atmospheric chemistry, global climate change, atmospheres of other planets and the history of the solar system.

To do this, we will develop a massively parallel code that should allow us to model very efficiently a flow of rovibrational energy in atom-molecule collisions typical for many recombination reactions. Such chemical processes proceed through formation of an intermediate long-lived metastable state (scattering resonance).

We develop an efficient theoretical framework which should allow us to make this problem treatable computationally with emphasis on massive scaling. Our approach is to keep quantum mechanics for description of the vibrational motion in O₃* (using 3D-wavepacket formalism) but to treat the overall rotation of O₃* and the M + O₃* collisional motion using classical trajectories. In such a mixed quantum-classical approach the quantum physics of the process (zero-point energy, scattering resonances, and symmetry rules for state-to-state transitions) is captured by the vibrational wavepacket, while the classical trajectory part of the system allows sampling initial conditions efficiently by running a set of independent calculations on different processors. For typical calculations, the number of classical trajectories needed is between 10,000 and 100,000, which can be propagated on different processors. With such approach we can easily employ thousands of processors simultaneously.

The fully quantum treatment of such processes is unaffordable due to poor scalability of the intrinsically global quantum mechanics, while the fully classical treatment looses all quantum physics of the process. It was shown, however, that quantum effects in the recombination reaction given above are responsible for the famous mystery -- the anomalous isotope effect in ozone formation. This effect still remains poorly understood, mainly due to computational difficulties associated with quantum dynamics treatment of polyatomic systems. Our mixed quantum-classical approach should help to solve this problem.

Bridging the Gaps Between Fluid and Kinetic Magnetic Reconnection Simulations in Large Systems

Amitava Bhattacharjee, University of New Hampshire

Associated NERSC Project: Center for Integrated Computation and Analysis of Reconnection and Turbulence (m148), Principal Investigator: Amitava Bhattacharjee

Magnetic reconnection drives the most dramatic, explosive energy releasing processes in the solar system: solar flares and coronal mass ejections. These violent events rapidly convert huge amounts of stored magnetic energy into heat and kinetic energy. An X-class solar flare can release up to 6 x 10**25 Joules of energy in less than an hour. This energy release is comparable to tens of millions of atomic bombs exploding simultaneously. The same process occurs through current disruptions on smaller scales in laboratory devices seeking to magnetically confined plasma for nuclear fusion energy.

The main question driving reconnection research is, "How does reconnection happen so rapidly?" If magnetic energy were dissipated only by collisional plasma resistivity (the way a copper wire dissipates the energy stored in a battery) , a solar flare would take years to release its energy, rather than the sub-hour time scale that is actually observed. This is because collisions between the electrons and ions in astrophysical plasmas are exceedingly rare. Simulations of magnetic reconnection generally employ one of two basic strategies. In the kinetic (or Particle-in-Cell, PIC) approach, one follows the individual particles in the plasma subject to self-consistent electromagnetic fields , while the other approach is to model the plasma as a fluid, or multiple co-existing fluids. Particle based simulations include the most realistic dissipative (energy releasing) physics. However, particle models are too computationally expensive to use to model large systems relevant to space and astrophysical plasmas. Fluid models, however, excel in modeling the gross features of very large systems, although they show significant deviations from the predictions of kinetic models in the dissipation regions.

Particle simulations have shown that the dominant effect responsible for energy release in nearly collisionless plasmas is the electron pressure tensor in the generalized Ohm's law governing weakly collisional plasmas. In recent years, interesting analytic closure approximations that represent the electron pressure tensor in terms of multi-fluid variables have been developed, but they have not been tested sufficiently in global multi-fluid codes. The principal objective of our proposed research is to represent these closure approximations in our global multi-fluid codes, and to test the validity of these closure schemes by comparing the predictions of our global codes with results from smaller-scale kinetic simulations as well as with observations. One of the most challenging aspects of this work is that the simulations need to resolve the smallest physical scales (and associated short time scales) in large systems, necessitating the use of large computational grids, and small simulation time steps. The task of simulations is complicated further by the discovery by the PI and his collaborators (as well as a few others) of the tendency of large and thin current sheets embedded in reconnection layers to break up into a copious number of magnetic islands or plasmoids, which provide an additional mechanism for passage to fast reconnection.

The impact of the research would be to be able to simulate explosive eruptions driven by magnetic reconnection in weakly collisional astrophysical, space, or laboratory plasma systems.

Decadal Predictability in CCSM4

Grant Branstator and Haiyan Teng, National Center for Atmospheric Research

Associated NERSC Project: Climate Change Simulations with CCSM: Moderate and High Resolution Studies (mp9), Principal Investigator: Warren Washington, National Center for Atmospheric Research

We propose to estimate the initial-value decadal predictability in CCSM4 using a 40-member ensemble run with each of two perturbed atmospheric initial condition strategies. The runs will be integrated from year 2005 to 2025 under the IPCC RCP4.5 scenario external forcing. We have done similar experiments using CCSM3 and now plan to examine whether and how much the predictability property has changed in CCSM4.

The experiments will help to determine to what extent the ocean initial states can contribute to predictive skills on the decadal time scale. Whether there is initial-value decadal predictability must be addressed before the climate modeling community carries out decadal prediction experiments using initialized ocean observations.