大数据时代的地球动力学数值计算方法回顾与展望

随着大数据时代的到来,计算地球动力学数值计算方法体系更加完善.本文系统地回顾了传统数值模拟方法在计算地球动力学领域的应用进展,包括:有限差分法、有限单元法、谱方法和谱元法;并对近年来一些新发展的算法和应用前景进行了综述,如:不连续Galerkin法、小波方法和格子玻尔兹曼方法等.本综述有助于读者以整体视角了解地球动力学数值计算方法的发展脉络,并对大数据时代下研究适应日益丰富的数据和新算法提供有益参考.     

Hydrometeorological analysis of the December 2008 flood in Rome

Abstract

Rome has been plagued by flooding since its foundation, and, in December 2008, the largest flood event over the past 20 years caused a fatality and more than €150 million in economic damage. Meteorological conditions associated with the December 2008 flooding are shown to be typical of flooding in the Tiber. The long record of discharge measurements of the Tiber River at the Ripetta station in downtown Rome was used to examine flood frequency for the Tiber, including assessment of the return interval of the December 2008 flood. Particular attention is given to examination of the stationarity assumption for flood peaks through change-point and trend analyses, quantile regression, and statistical modelling of the flood-peak distribution. Once anthropogenic changes linked to reservoir regulation of the Tiber River have been accounted for, the stationarity assumption holds and can be used for flood frequency analysis. We highlight the difficulties in detecting departures from the stationarity assumption due to climate change. In the current regime, the December 2008 flood event has a return period of the order of 10–20 years.

Citation Villarini, G., Smith, J.A., Napolitano, F. & Baeck, M.L. (2011) Hydrometeorological analyses of the December 2008 flood in Rome. Hydrol. Sci. J. 56(7), 1150–1165.     

The evolution of barium and europium in local dwarf spheroidal galaxies

By means of a detailed chemical evolution model, we follow the evolution of barium (Ba) and europium (Eu) in four Local Group Dwarf Spheroidal (dSph) galaxies, in order to set constraints on the nucleosynthesis of these elements and on the evolution of this type of galaxies compared with the Milky Way. The model, which is able to reproduce several observed abundance ratios and the present-day total mass and gas mass content of these galaxies, adopts up-to-date nucleosynthesis and takes into account the role played by supernovae (SNe) of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba and Eu). By assuming that Ba is a neutron-capture element produced in low-mass asymptotic giant branch stars by s-process but also in massive stars (in the mass range 10–30 M_⊙) by r-process, during the explosive event of SNe of Type II, and that Eu is a pure r-process element synthesized in massive stars also in the range of masses 10–30 M_⊙, we are able to reproduce the observed [Ba/Fe] and [Eu/Fe] as functions of [Fe/H] in all four galaxies studied. We confirm also the important role played by the very low star formation (SF) efficiencies (ν= 0.005–0.5 Gyr⁻¹) and by the intense galactic winds (6–13 times the star formation rate) in the evolution of these galaxies. These low SF efficiencies (compared to the one for the Milky Way disc) adopted for the dSph galaxies are the main reason for the differences between the trends of [Ba/Fe] and [Eu/Fe] predicted and observed in these galaxies and in the metal-poor stars of our Galaxy. Finally, we provide predictions for Sagittarius galaxy for which data of only two stars are available.     

Internal shocks in the jets of radio-loud quasars

The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. We propose that this internal shock scenario , which is the scenario generally thought to explain the observed gamma-ray burst radiation, can also work for radio sources in general, and for blazars in particular. We investigate in detail this idea, simulating the birth, propagation and collision of shells, calculating the spectrum produced in each collision, and summing the locally produced spectra from those regions of the jet which are simultaneously active in the observer's frame. We can thus construct snapshots of the overall spectral energy distribution, time-dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations between the emission at different frequencies, specify the contribution of each region of the jet to the total emission, and find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to reproduce qualitatively the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we will explore the constraints that this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars.     

Crustal accretion and dynamic feedback on mantle melting of a ridge centred plume: The Iceland case

A major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation. In the case of Iceland crustal thickness as large as 20 to 40 km has been reported. Crustal seismic velocities are high, and have to be explained by thermal or chemical effects. In the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting. To study this question we solve the convection equations for a ridge centred plume with non-Newtonian rheology including melting, melt extraction associated with deposition of cold crust at the surface of the model, and using a simplified approach for compaction. The assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km. Depending on the buoyancy flux of the plume, crustal thicknesses between 10 and 40 km are modelled, showing characteristic dipping structures resembling the rift-ward dipping basaltic layers of East- and Western Iceland. Comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates, that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg/s. Based on the observed crustal thickness of Iceland our models predict a plume buoyancy flux of 1140 kg/s.In the second part we study the accretion of the Icelandic crust by a thermo-mechanical model in more detail based on the Navier–Stokes-, the heat transport and the mass conservation equations including volumetric sources. Hot (1200 °C) molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions: a) deep, widespread emplacement of dykes and sills including crustal underplating, b) magma chambers at shallow to mid-crustal level, and c) surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres. We identify the material from the different source regions by a marker approach. Varying the relative dominance of these source regions, characteristic crustal structures evolve, showing shallow dipping upper crustal layers with dip angles between 10 and 15°. The thermal structure of the crust varies between cold crust (shallow-source region dominating) and hot crust (deep-source region dominating). We use observations of maximum depth of seismicity to constrain the depth of the 650 °C isotherm and seismological inferences on the lower crust to constrain temperatures in that region. The best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion.     

The spatial extent of the Deep Western Boundary Current into the Bounty Trough: new evidence from parasound sub-bottom profiling

Deep currents such as the Pacific Deep Western Boundary Current (DWBC) are strengthened periodically in Milankovitch cycles. We studied periodic fluctuations in seismic reflection pattern and reflection amplitude in order to detect cycles in the sedimentary layers of Bounty Trough and bounty fan, east of New Zealand. There, the occurrence of the obliquity frequency is caused only by the DWBC. Therefore, it provides direct evidence for the spatial extent of the DWBC. We can confirm the extent of the DWBC west of the outer sill, previously only inferred via erosional features at the outer sill. Further, our data allow an estimation of the extent of the DWBC into the Bounty Trough, limiting the DWBC presence to east of 178.15°E. Using the presented method a larger dataset will allow a chronological and areal mapping of sedimentation processes and hence provide information on glacial/interglacial cycles.     

Unraveling the time-dependent relevance of input model uncertainties for a lumped hydrologic model of a pre-alpine karst system

Uncertainties in hydrologic model outputs can arise for many reasons such as structural, parametric and input uncertainty. Identification of the sources of uncertainties and the quantification of their impacts on model results are important to appropriately reproduce hydrodynamic processes in karst aquifers and to support decision-making. The present study investigates the time-dependent relevance of model input uncertainties, defined as the conceptual uncertainties affecting the representation and parameterization of processes relevant for groundwater recharge, i.e. interception, evapotranspiration and snow dynamic, on the lumped karst model LuKARS. A total of nine different models are applied, three to compute interception (DVWK, Gash and Liu), three to compute evapotranspiration (Thornthwaite, Hamon and Oudin) and three to compute snow processes (Martinec, Girons Lopez and Magnusson). All the input model combinations are tested for the case study of the Kerschbaum spring in Austria. The model parameters are kept constant for all combinations. While parametric uncertainties computed for the same model in previous studies do not show pronounced temporal variations, the results of the present work show that input uncertainties are seasonally varying. Moreover, the input uncertainties of evapotranspiration and snowmelt are higher than the interception uncertainties. The results show that the importance of a specific process for groundwater recharge can be estimated from the respective input uncertainties. These findings have practical implications as they can guide researchers to obtain relevant field data to improve the representation of different processes in lumped parameter models and to support model calibration.

     

Radiative transfer in the atmosphere of Venus and application to surface emissivity retrieval from VIRTIS/VEX measurements

The radiation measurements of VIRTIS-M-IR (1-5 μm) on Venus Express provide a valuable database for systematic studies of the atmosphere and surface of the Earth’s sister planet. The present paper focuses on the investigation of physical parameters that determine the retrieval accuracy of deep atmosphere and surface features of Venus including compositional conditions, continuum absorption effects, and spectroscopic input data required for radiative transfer simulations. The parameter discussion shall serve as a reference for ongoing and future work on methodical and simulation input data improvements.The high variability of the nightside atmospheric and surface emission window radiances with respect to cloud opacity and surface elevation is modeled and discussed in direct comparison with measurements performed over the northern hemisphere. Venus surface elevation is retrieved using the 1.18 and 1.02 μm emission windows where radiance ratios are well suited to de-cloud the measurement data. In general, the ratio-based VIRTIS topography is in good agreement with the Magellan topography, but differences occur in localized areas. The paper discusses possible origins of such differences including surface emissivity “anomalies”. Surface emissivity variations that may be due to changes in the chemical composition (mineralogy) and surface texture are important indicators of the nature of the surface material.Preliminary radiance retrievals along a number of complete northern orbits reveal a trend towards lower values of highland surface emissivity compared to the surrounding lowlands. Already the Magellan radar experiment suggested compositional variations at moderately high altitudes over the tesserae. They probably indicate a more felsic component giving a hint to older surface forming processes.     

GNSS software receiver for the MAGIA (Missione Altimetrica Geofisica GeochImica lunAre) mission

In recent years a number of missions have been conceived to acquire and track signals from GPS satellites at altitudes higher than the GPS constellation itself. The main purpose of SDR is to reduce the number of hardware components, projected for specific unmodifiable uses, and utilize general purpose units: on board CPU, DSP or FPGA. In this paper he study and design of a GNSS Software Receiver for space application, applied to a Lunar Mission, is presented with particular attention on Space Segment device and on Ground Facilities.     

Chemical compositions of impact melt breccias and target rocks from the Tenoumer impact crater,Mauritania

Abstract— The impact melt breccias from the Tenoumer crater (consisting of a fine‐grained intergrowth of plagioclase laths, pyroxene crystals, oxides, and glass) display a wide range of porosity and contain a large amount of target rock clasts. Analyses of major elements in impact melt rocks show lower contents of SiO₂, Al₂O₃, and Na₂O, and higher contents of MgO, Fe₂O₃, and CaO, than the felsic rocks (i.e., granites and gneisses) of the basement. In comparison with the bulk analyses of the impact melt, the glass is strongly enriched in Si‐Al, whereas it is depleted both in Mg and Fe; moreover, the impact melt rocks are variably enriched or depleted in some REE with respect to the felsic and mafic bedrock types. Gold is slightly enriched in the impact melt, and Co, Cr, and Ni abundances are possibly due to a contribution from mafic bedrock. Evidences of silicate‐carbonate liquid immiscibility, mainly as spherules and globules of calcite within the silicate glass, have been highlighted. HMX mixing calculation confirm that the impact melt rocks are derived from a mixing of at least six different target lithologies outcropping in the area of the crater. A large contribution is derived from granitoids (50%) and mica schist (17–19%), although amphibolites (?15%), cherty limestones (?10%), and ultrabasites (?6%) components are also present. The very low abundances of PGE in the melt rock seem to come mainly from some ultrabasic target rocks; therefore, the contamination from the meteoritic projectile appears to have been negligible.