The effects of different parameter regimes in geodynamo simulations     

Darcy E. Ogden  Gary A. Glatzmaier  Robert S. Coe 《地球物理与天体物理流体动力学》2013,107(2):107-120

Over the past 10 years, geodynamo simulations have grown rapidly in sophistication. However, it is still necessary to make certain approximations in order to maintain numerical stability. In addition, models are forced to make assumptions about poorly known parameters for the Earth's core. Different magnetic Prandtl numbers have been used and different assumptions about the presence of radiogenic heating have been made. This study examines some of the consequences of different approximations and assumptions using the Glatzmaier–Roberts geodynamo model. Here, we show that the choice of magnetic Prandtl number has a greater influence on the character of the magnetic field produced than the addition of a plausible amount of radiogenic heating. In particular, we find that prescribing a magnetic Prandtl number of unity with Ekman number limited by current computing resources, results in magnetic fields with significantly smaller intensities and variabilities compared with the much more Earth-like results obtained from simulations with large magnetic Prandtl numbers. A magnetic Prandtl number of unity, with both the viscous and magnetic diffusivities set to the Earth's magnetic diffusivity, requires a rotation rate much smaller than that of the Earth for currently reachable Ekman numbers. This results in a reduced dominance of the Coriolis forces relative to the buoyancy forces, and therefore, a reduction in the magnetic field intensity and the variability compared to the large Prandtl number cases.  相似文献   

Coupling the terrestrial hydrology model with biogeochemistry to the integrated LAND surface model: Amazon Basin applications     

Aline Anderson de Castro  Luz Adriana Cuartas  Michael T. Coe  Celso Von Randow  Andrea Castanho  Alex Ovando 《水文科学杂志》2013,58(13-14):1954-1966

ABSTRACT

We coupled the hydrologic routing and flood dynamics model Terrestrial Hydrology Model with Biogeochemistry (THMB) to the Integrated LAND Surface Model (INLAND) and compared simulations of the discharge and flood extent area against gauge station and satellite-based information in the Amazon Basin. The coupled model represents well the seasonality of the flooding and discharge, but underestimates both of them. This can be related to an already discussed underestimate of the precipitation in the east of the Andes Mountains. A photosynthesis limitation on the flooded area was also included, showing changes in plant productivity and reduction in vegetation carbon stocks. Despite its limitations, the model proves to be a valuable tool for studies of the hydrological cycle and flood dynamics response to climate change projections, allowing it to be used to represent the feedbacks between continental surface water cycle and vegetation.  相似文献   

Initiation conditions for debris flows generated by runoff at Chalk Cliffs, central Colorado   总被引：7，自引：1，他引：6  

Jeffrey A. Coe  David A. Kinner  Jonathan W. Godt 《Geomorphology》2008,96(3-4):270

We have monitored initiation conditions for six debris flows between May 2004 and July 2006 in a 0.3 km² drainage basin at Chalk Cliffs; a band of hydrothermally-altered quartz monzonite in central Colorado. Debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14° to 45°. The availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes. Rainfall exceeding I = 6.61(D)^− 0.77, where I is rainfall intensity (mm/h), and D is duration (h), was required for the initiation of debris flows in the drainage basin. The approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0.15 m³/s. Colluvium in the basin was unsaturated immediately prior to (antecedent) and during debris flows. Antecedent, volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4–9%, and 4–7%, respectively. During debris flows, peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10–20%, and 4–12%, respectively. Channel sediment at a depth of 45 cm was unsaturated before and during debris flows; antecedent moisture ranged from 20–22%, and peak moisture ranged from 24–38%. Although we have no measurements from shallow rill or channel sediment, we infer that it was unsaturated before debris flows, and saturated by surface-water runoff during debris flows.Our results allow us to make the following general statements with regard to debris flows generated by runoff in semi-arid to arid mountainous regions: 1) high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff, 2) locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows, and 3) rainfall and simulated surface-water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment.  相似文献   

The Chandra Small Magellanic Cloud Wing Survey – the search for X-ray binaries     

K. E. McGowan  M. J. Coe  M. P. E. Schurch  V. A. McBride  J. L. Galache  W. R. T. Edge  R. H. D. Corbet  S. Laycock  D. A. H. Buckley 《Monthly notices of the Royal Astronomical Society》2008,383(1):330-338

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An X-ray and optical study of the new SMC X-ray binary pulsar system SXP7.92 and its probable optical counterpart, AzV285     

M. J. Coe  M. Schurch  V. A. McBride  R. H. D. Corbet  L. J. Townsend  A. Udalski  J. L. Galache 《Monthly notices of the Royal Astronomical Society》2009,394(4):2191-2196

Optical and X-ray observations are presented here of a newly reported X-ray transient system in the Small Magellanic Cloud (SMC) – SXP7.92. A detailed analysis of the X-ray data reveal a coherent period of 7.9 s. A search through earlier X-ray observations of the SMC reveal a previously unknown earlier detection of this system. Follow-up X-ray observations identified a new transient source within the error circle of the previous observations. An optical counterpart, AzV285, is proposed which reveals clear evidence for a 36.8 d binary period.  相似文献   

The orthoenstatite to clinoenstatite transformation by shearing and reversion by annealing: Mechanism and potential applications   总被引：5，自引：0，他引：5  

Robert S. Coe  Stephen H. Kirby 《Contributions to Mineralogy and Petrology》1975,52(1):29-55

Clinoenstatite (CE) was produced by deforming single-crystal specimens of ortho-enstatite (OE) in several different sorta of experiments. Examination with light and trans-mission electron microscopes shows that the transformation is coherent and involves a macroscopic shear on (100) [001] through an angle of 12.8±1.3 °, in good agreement with the theoretically expected value of 13.3 °, and that the transformation is accomplished by glide on (100) of partial dislocations with b= 0.83[001]. Structural analysis provides further insight into the transformation mechanism. Reversion occurs in specimens annealed under a variety of conditions, and thin lamellae of CE in unconstrained, low-strain specimens recover their original shape during transformation back to OE. Our experiments and thermodynamic estimates both suggest that the equilibrium transition temperature is raised roughly 300 ° C per kilobar of shear stress on (100) [001]. This provides the basis of a method by which it may be possible to determine the magnitude as well as the orientation of the principal stresses that produce CE in nature during deformation of enstatite-bearing rocks.  相似文献   

Absolute palaeointensity of Oligocene (28–30 Ma) lava flows from the Kerguelen Archipelago (southern Indian Ocean)     

G. Plenier  P. Camps  R. S. Coe  M. Perrin 《Geophysical Journal International》2003,154(3):877-890

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Dynamic stress wave reflections/attenuation: Earthquake simulation in centrifuge soil models     

Carlos J. Coe  Jean H. Prevost  Robert H. Scanlan 《地震工程与结构动力学》1985,13(1):109-128

In this investigation, the feasibility of earthquake simulation in centrifuge soil experiments is studied. The strong detrimental effect of standing waves for such an endeavour is clearly shown. A modest degree of success toward producing a model earthquake is reported via two devices—a certain kind of physically tuned internal excitor and an effective absorbent material at the walls.  相似文献   

Archaean crustal evolution in southern West Greenland: A review based on observations in the Buksefjorden region     

Brian Chadwick  Kenneth Coe 《Tectonophysics》1984,105(1-4)

Thermal, metamorphic and tectonic processes associated with the two major crust-forming events expressed as voluminous generation of Amîtsoq and Nûk tonalitic-granitic gneisses, ca. 3750 Ma and ca. 2800 Ma respectively, in southern West Greenland are reviewed briefly in the light of recent studies in the Buksefjorden region.  相似文献   

Anisotropic growth in the olivine-spinel transformation of Mg₂GeO₄ under nonhydrostatic stress   总被引：2，自引：0，他引：2  

Peter J. Vaughan  Harry W. Greeen  Robert S. Coe 《Tectonophysics》1984,108(3-4)

The olivine-spinel phase transformation in Mg₂GeO₄ does not occur by a martensitic mechanism. The evidence, from samples transformed in a Griggs-type solid medium deformation apparatus, are:

1. (1) lack of microstructural features in the olivine phase which can be specifically associated with a martensitic mechanism

2. (2) the orientation relationship between the two phases that is predicted by the martensitic mechanism does not occur nor is there any apparent consistency of relative orientations

3. (3) application of a differential stress to the transforming sample resulted in an anisotropic growth rate for the spinel phase indicating that growth was externally controlled rather than crystallographically controlled.

Anisotropic growth of the spinel phase results in elongation of the residual olivine phase grains in the plane normal to the direction of maximum principal compressive stress. A velocity ratio of 1.7_−0.7^+5.4 has been determined for the growth rate of the spinel from measurements on residual olivine grains. The interphase grain boundary in samples transformed under stress has cusp-shaped fingers of spinel with a blunt end separated by thin spikes of olivine. Samples transformed isostatically do not exhibit this feature providing further confirmation of anisotropic growth of the spinel. The preferred growth of the spinel is consistent with a theory of phase transformation under nonhydrostatic stress. The predicted spinel finger shape based on this theory is generally consistent with observed shapes except for the blunt end. The discrepancy may be due to surface energy which has not been considered here, or to local deviations of the applied macroscopic stress.  相似文献