Published in NASA Science Information Systems Newsletter, Issue 48, 1998
A critical tool for Earth and space sciences research is high-performance computing and communications (HPCC). Simulations on the most powerful computers provide evidence for how phenomena evolved by filling in gaps left by observations and also describe the behavior of as-yet unobserved objects whose existence is surmised by theory.
In August 1996, NASA HPCC's Earth and Space Sciences (ESS) Project awarded three-year cooperative agreements to nine "Grand Challenge" investigator teams from across the United States. A simultaneous agreement with Silicon Graphics, Inc., supplies world-class computational resources in the form of a 512-processor CRAY T3E at Goddard Space Flight Center. In the first-ever Science Team Symposium, team principal investigators and co-investigators presented their latest findings at NASA Headquarters on April 2.
Testing general relativity
Einstein's theory of general relativity insists that space and time are dynamic. A strong test for the theory is neutron star mergers, involving pairs of objects packing our sun's mass into a city-sized space! The principal investigator is Paul Saylor, University of Illinois at Urbana-Champaign. Presenting was co-investigator Doug Swesty, also of the University of Illinois. Swesty related a major accomplishment: the first model of a neutron star's evolution with full relativisitic behavior was completed by team members from Washington University in St. Louis.
Swesty also explained how the team is challenged by the new generation of orbiting astronomical observatories. Last September, the Hubble Space Telescope captured a gamma-ray burst in an optical wavelength for the first time. The observation showed that bursts can originate in other galaxies besides our own. Because of the distance and the tremendous energy involved, some scientists believe neutron star mergers cause the bursts. The difficulty is that mergers occur in a few milliseconds while a burst can last as long as 1,500 seconds, a mystery the team is trying to unravel.
A complicated sun
Strong magnetic activity in the sun and sun-like stars leads to many spectacular and well-observed phenomena like sunspots, solar flares, the solar cycle and the solar wind. The University of Chicago's Andrea Malagoli, principal investigator, said that the most likely underlying mechanism is turbulent convection interacting with magnetic fields below the sun's surface.
The team has carried out large-scale simulations of this magneto-convection as well as dynamo processes, which convert kinetic energy into magnetic energy. Malagoli said they use three computer codes because the sun is so complicated that it is necessary to model a local zone applying as much resolution as possible to get credible answers. Together with observations, the models suggest that the sun rotates differentially, convection occurs in the star's upper third, and energy is generated in the core. Malagoli called the results "novel and unique" in that they explore complex physical regimes where the underlying equations are strongly nonlinear.
Modeling solar activity
Studying the sun's outer atmosphere, or corona, is a Naval Research Laboratory (NRL)-based team, which was represented by Spiro Antiochos. Led by NRL's John Gardner, they have developed three codes for modeling coronal activity. Antiochos presented results from simulations of solar prominences and the initiation of coronal mass ejections (CMEs). The key features of the phenomena lie in the 3-D complexity of the solar magnetic field, he said.
Scientists had been baffled as to how material can be held far above the sun's surface in a prominence. NRL models show a tug of war between two sets of magnetic field lines forcing material to bulge outwards. The resulting s-shaped structure agrees with observations. In the CME studies, the team found that magnetic field lines can travel in four different directions. Antiochos likened coronal masses to helium balloons held down by tethers. As "tether" magnetic field lines are moved out of the way through breaking up and reconnecting with other field lines, a CME can be released and erupt through the surface.
The streaming solar wind
CMEs travel out from the sun riding the solar wind, a gaseous mixture of protons, helium atoms and ions. As a CME interacts with Earth's magnetic field, it can fuel magnetic storms that knock out satellites and power grids. A University of Michigan team, headed by Tamas Gombosi, has built a versatile code to simulate CMEs and a variety of other solar wind phenomena, including interactions with comets, Mars and Venus. Their largest model to date follows a CME's path from formation for 40 hours.
Co-investigator Ken Powell explained how their code uses adaptive mesh refinement. The mesh on which the governing equations are being solved adapts, on the fly, to the gas flow. In this way, under-resolution of high-gradient regions and over-resolution of low-gradient regions are avoided, leading to highly efficient use of computer resources. In addition, they took a "start from scratch" approach with their model code, designing with maximum parallel performance in mind. The resulting code has high single-processor performance and scales with very high efficiency.
Fluid behavior in microgravity
Another original code was developed by University of Texas at Austin researchers for studying coupled viscous fluid flow and heat transfer in microgravity environments. Surface tension rather than buoyancy becomes a dominant factor, and there are interesting nonlinear free surface effects that still are not understood, explained Principal investigator Graham Carey. For example, nonlinear instabilities can lead to flow patterns that depress or raise the surface. In a related problem called the "liquid bridge," a thermal band rotates around the surface like the stripe on a barber pole; a thermal probe applied near the surface can control the behavior.
Carey said their primary interests are manufacturing and life support processes on the International Space Station, NASA's space shuttle, and future space projects. The simulations also have close interplay with terrestrial experiments involving free surface phenomena on thin films, which are used to manufacture computer chips. Carey also mentioned the possibility of modeling fires with the code, particularly oil fires on water, where surface tension plays a role.
Probing the geodynamo
Like the sun, the Earth is among the majority of solar system planets possessing magnetic fields that originate from self-sustaining dynamos. Principal investigator Peter Olson, Johns Hopkins University, said the Earth has the three necessary ingredients for a dynamo: a sufficiently large volume of electrically conducting fluid in its iron-rich outer core, energy from core-to-mantle heat transfer to circulate the fluid and planetary rotation to impart helicity to the fluid motions.
A major finding is that the Earth's inner core rotates faster than the outer core, matching seismological observations for the first time. Simulations show the inner core setting outer core fluids in motion to create the dynamo. The team's dynamo models exhibit occasional reversals in magnetic polarity when the heat flow on the boundary between the liquid core and solid mantle is spatially heterogeneous. These results suggest that thermal coupling between the core and the mantle may be the ultimate cause of geomagnetic polarity reversals. Olson said they will be investigating this "holy grail of magnetic fields" more closely.
Observing Earth with radar
Synthetic aperture radar (SAR) is an anytime, all-weather instrument. The sheer mass of collected data makes for a computational challenge. For instance, it has taken five years to process the ten-day SIR-C mission flown on the space shuttle, according to principal investigator Dave Curkendall, Jet Propulsion Laboratory (JPL). His team is moving towards giving scientists real-time processing capabilities with a Scalable SAR Software Suite that works with multiple types of SAR data and in a heterogeneous computing environment.
Advancing the Suite are three critical SAR applications. One involves Southern California tectonic plate movements during and between earthquakes. Scripps Institution of Oceanography researchers developed a technique to better interpret and average data from multiple SAR satellite passes. University of California, Santa Barbara scientists recently found multiple satellites useful for globally forecasting water produced by melted snow. JPL made significant progress in identifying flooding and deforestation in the Amazon rain forest and built the Digital Light Table software to display very large SAR mosaics.
Simulating the climate system
Roberto Mechoso, University of California, Los Angeles (UCLA), leads a team developing an Earth system model, including chemical tracers that are found in, and may be exchanged between, the atmosphere and the oceans.
UCLA colleague John Farrara described their atmospheric general circulation model (AGCM) and its coupling with an oceanic general circulation model (OGCM). A 50-year run of the coupled model produced El Niño conditions every three to four years, although their magnitude was not as big as reality. A higher-resolution AGCM, with prescribed sea surface temperatures, agreed reasonably well with United States precipitation amounts observed during the recent El Niño.
JPL's Yi Chao discussed a North Atlantic version of the OGCM, run at 1/6-degree resolution for 40 years, the longest simulation of its kind. Additional unique aspects are the production of a realistic Gulf Stream and narrow ocean currents known as eddies. The correct ocean heat transport is impossible without resolving eddies, which also bring nutrients up from the deep ocean. Chao said the team probably can do a 1/6-degree model of the global ocean.
Better atmospheric data assimilation
Data assimilation combines actual observations with climate model simulations to produce a more accurate description of the Earth system than the observations alone provide. Principal investigator Peter Lyster, University of Maryland, related that 80 percent of atmospheric observations come from satellites; other sources include weather balloons, aircraft, ships and surface instruments. This team aids NASA's Data Assimilation Office in its mission to provide data sets for climate research and to support agency satellite and aircraft missions.
Team research falls into two areas: a large-scale data assimilation system and the Kalman filter. Lyster described performance gains with the system, which includes an analysis routine and a climate model. The system currently can process six days of data in 24 hours; the goal for 1999 is 30 days. The more computationally demanding Kalman filter evolves observations and their errors through space and time. A study of methane gas showed less-than-expected mixing occurring in the stratosphere.