Showing papers in "Reviews of Geophysics in 2000"

Volcanic eruptions and climate

Abstract: Volcanic eruptions are an important natural cause of climate change on many timescales. A new capability to predict the climatic response to a large tropical eruption for the succeeding 2 years will prove valuable to society. In addition, to detect and attribute anthropogenic influences on climate, including effects of greenhouse gases, aerosols, and ozone-depleting chemicals, it is crucial to quantify the natural fluctuations so as to separate them from anthropogenic fluctuations in the climate record. Studying the responses of climate to volcanic eruptions also helps us to better understand important radiative and dynamical processes that respond in the climate system to both natural and anthropogenic forcings. Furthermore, modeling the effects of volcanic eruptions helps us to improve climate models that are needed to study anthropogenic effects. Large volcanic eruptions inject sulfur gases into the stratosphere, which convert to sulfate aerosols with an e-folding residence time of about 1 year. Large ash particles fall out much quicker. The radiative and chemical effects of this aerosol cloud produce responses in the climate system. By scattering some solar radiation back to space, the aerosols cool the surface, but by absorbing both solar and terrestrial radiation, the aerosol layer heats the stratosphere. For a tropical eruption this heating is larger in the tropics than in the high latitudes, producing an enhanced pole-to-equator temperature gradient, especially in winter. In the Northern Hemisphere winter this enhanced gradient produces a stronger polar vortex, and this stronger jet stream produces a characteristic stationary wave pattern of tropospheric circulation, resulting in winter warming of Northern Hemisphere continents. This indirect advective effect on temperature is stronger than the radiative cooling effect that dominates at lower latitudes and in the summer. The volcanic aerosols also serve as surfaces for heterogeneous chemical reactions that destroy stratospheric ozone, which lowers ultraviolet absorption and reduces the radiative heating in the lower stratosphere, but the net effect is still heating. Because this chemical effect depends on the presence of anthropogenic chlorine, it has only become important in recent decades. For a few days after an eruption the amplitude of the diurnal cycle of surface air temperature is reduced under the cloud. On a much longer timescale, volcanic effects played a large role in interdecadal climate change of the Little Ice Age. There is no perfect index of past volcanism, but more ice cores from Greenland and Antarctica will improve the record. There is no evidence that volcanic eruptions produce El Nino events, but the climatic effects of El Nino and volcanic eruptions must be separated to understand the climatic response to each.

Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review

Abstract: This paper reviews the many developments in estimates of the direct and indirect global annual mean radiative forcing due to present-day concentra- tions of anthropogenic tropospheric aerosols since Inter- governmental Panel on Climate Change (1996). The range of estimates of the global mean direct radiative forcing due to six distinct aerosol types is presented. Addition- ally, the indirect effect is split into two components corresponding to the radiative forcing due to modifica- tion of the radiative properties of clouds (cloud albedo effect) and the effects of anthropogenic aerosols upon the lifetime of clouds (cloud lifetime effect). The radia- tive forcing for anthropogenic sulphate aerosol ranges from 20.26 to 20.82 W m 22 . For fossil fuel black carbon the radiative forcing ranges from 10.16 W m 22 for an external mixture to 10.42 W m 22 for where the black carbon is modeled as internally mixed with sulphate aerosol. For fossil fuel organic carbon the two estimates of the likely weakest limit of the direct radiative forcing are 20.02 and 20.04 W m 22 . For biomass-burning sources of black carbon and organic carbon the com-

Organic atmospheric aerosols: Review and state of the science

Abstract: Atmospheric aerosol particles are known to contain organic carbon material in variable amounts, depending on their location. In some parts of the world, organic compounds make up the majority of the total suspended particle mass. This class of particulate matter is important in a wide range of geophysical and environmental problems, ranging from local issues (e.g., pollution toxicity) to the global scale (e.g., climate change). Unfortunately, the richness of organic chemistry and the highly variable physical properties associated with both natural and anthropogenic organic particles lead to great difficulties in sampling and obtaining complete chemical information on these materials. These obstacles result in an incomplete picture of a potentially significant part of atmospheric chemistry and a correspondingly poor understanding of the geophysical and environmental effects of this aerosol. Given the paucity of quantitative molecular data, the purpose of this paper is not to quantitatively describe the importance of organic aerosols in environmental issues, but rather to present a basis for defining what data are needed. With this goal in mind, we begin with an overview of the major environmental issues known to be affected by organic aerosols, followed by a description of the distribution, sources, and chemical and physical properties of organic aerosols as they are currently understood. Methods used to collect and study organic aerosols are provided, followed by a list of outstanding scientific questions and suggestions for future research priorities.

What caused the glacial/interglacial atmospheric pCO2 cycles?

Abstract: Fifteen years after the discovery of major glacial/interglacial cycles in the CO2 concentration of the atmosphere, it seems that all of the simple mechanisms for lowering pCO2 have been eliminated. We use a model of ocean and sediment geochemistry, which in- cludes new developments of iron limitation of biological production at the sea surface and anoxic diagenesis and its effect on CaCO3 preservation in the sediments, to evaluate the current proposals for explaining the glacial/ interglacial pCO2 cycles within the context of the ocean carbon cycle. After equilibration with CaCO3 the model is unable to generate glacial pCO2 by increasing ocean NO3 but predicts that a doubling of ocean H4SiO4 might suffice. However, the model is unable to generate a doubling of ocean H4SiO4 by any reasonable changes in SiO2 weathering or production. Our conclusions force us to challenge one or more of the assumptions at the foundations of chemical oceanography. We can abandon the stability of the "Redfield ratio" of nitrogen to phos- phorus in living marine phytoplankton and the ultimate limitation of marine photosynthesis by phosphorus. We can challenge the idea that the pH of the deep ocean is held relatively invariant by equilibrium with CaCO3 .A third possibility, which challenges physical oceanogra- phers, is that diapycnal mixing in ocean circulation mod- els exceeds the rate of mixing in the real ocean, dimin- ishing the model pCO2 sensitivity to biological carbon uptake.

High pressure experiments and the phase diagram of lower mantle and core materials

Abstract: The interpretation of seismic data and computer modeling requires increased accuracy in relevant material properties in order to improve our knowledge of the structure and dynamics of the Earth's deep interior. To obtain such properties, a complementary method to classic shock compression experiments and theoretical calculations is the use of laser-heated diamond cells, which are now producing accurate data on phase diagrams, equations of state, and melting. Data on one of the most important measurements, the melting temperatures of iron at very high pressure, are now converging. Two other issues linking core properties to those of iron are investigated in the diamond cell: One is the melting point depression of iron in the presence of light elements, and the other is the structure of iron at the conditions of the inner core. First measurements on eutectic systems indicate a significant decrease in the melting point depression with increasing pressure, which is in contrast to previous predictions. X-ray diffraction measurements at simultaneously high pressure and high temperature have improved significantly with the installation of high-pressure “beam lines” at synchrotron facilities, and structural measurements on iron are in progress. Considerable efforts have been made to develop new techniques to heat minerals at the conditions of the deep mantle in the diamond cell and to measure their phase relations reliably. Even measurements of the melting behavior of realistic rock compositions at high pressure, previously considered to be impossible in the diamond cell, have been reported. The extrapolated solidus of the lower mantle intersects the geotherm at the core-mantle boundary, which may explain the seismically observed ultra low velocity zone. The diamond cell has great potential for future development and broad application, as new measurements on high-pressure geochemistry at deep mantle and core conditions have opened a new field of research. There are, however, strict experimental requirements for obtaining reliable data, which are summarized in the present paper. Results from recent measurements of melting temperatures and phase diagrams of lower mantle and core materials at very high pressure are reviewed.

Land surface processes and Sahel climate

Abstract: This paper examines the question of land surface-atmosphere interactions in the West African Sahel and their role in the interannual variability of rainfall. In the Sahel, mean rainfall decreased by 25–40% between 1931–1960 and 1968–1997; every year in the 1950s was wet, and nearly every year since 1970 has been anomalously dry. Thus the intensity and multiyear persistence of drought conditions are unusual and perhaps unique features of Sahel climate. This article presents arguments for the role of land surface feedback in producing these features and reviews research relevant to land surface processes in the region, such as results from the 1992 Hydrologic Atmospheric Pilot Experiment (HAPEX)-Sahel experiment and recent studies on aerosols and on the issue of desertification in the region, a factor implicated by some as a cause of the changes in rainfall. Included also is a summary of evidence of feedback on meteorological processes, presented from both model results and observations. The reviewed studies demonstrate numerous ways in which the state of the land surface can influence interactions with the atmosphere. Surface hydrology essentially acts to delay and prolong the effects of meteorological drought. Each evaporative component of the surface water balance has its own timescale, with the presence of vegetation affecting the process both by delaying and prolonging the return of soil moisture to the atmosphere but at the same time accelerating the process through the evaporation of canopy-intercepted water. Hence the vegetation structure, including rooting depth, can modulate the land-atmosphere interaction. Such processes take on particular significance in the Sahel, where there is a high degree of recycling of atmospheric moisture and where the meteorological processes from the scale of boundary layer development to mesoscale disturbance generation are strongly influenced by moisture. Simple models of these feedback processes and their various timescales have demonstrated that the net feedback to the atmosphere is positive for both wet and dry surface anomalies. Hence the role of the surface is to reinforce meteorologically induced changes. Recovery from the dry state is slower than from the wet state, suggesting that dry conditions would tend to persist longer, as is actually observed in the Sahel. These simple models suggest that the surface hydrology locks the system into a drought mode that persists for several years, until the system randomly slips into a persistent wet mode. The hypothesis that desertification in the Sahel might likewise be responsible for the persistent drought is found to be untenable. Rather than a progressive encroachment of the desert onto the savanna, the vegetation cover responds dramatically to interannual fluctuations in rainfall. There is little evidence of large-scale denudation of soils, increase in surface albedo, or reduction of the productivity of the land, although degradation has probably occurred in some areas. There has, however, been a steady buildup of dust in the region over the last half a century. Significant radiative effects of the dust have been demonstrated; therefore the dust has probably influenced large-scale climate. The buildup is probably mainly a result of changes in the land surface that accompanied the shift to drier conditions, but it may have been exacerbated by anthropogenic factors. Complex general circulation models nearly universally underscore the importance of feedback processes in the region. Although it has not been unequivocally demonstrated that the rainfall regime of the Sahel is modulated by surface processes, there is recent observational evidence that this is case.

Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit

Abstract: Over the past decade the analysis of sedimentary cyclic rhythmites of tidal origin, i.e., stacked thin beds or laminae usually of sandstone, siltstone, and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation, has provided information on Earth's paleorotation and the evolving lunar orbit for Precambrian time (before 540 Ma). Depositional environments of tidal rhythmites range from estuarine to tidal delta, with a wave-protected, distal ebb tidal delta setting being particularly favorable for the deposition and preservation of long, detailed rhythmite records. The potential sediment load of nearshore tidal currents and the effectiveness of the tide as an agent of sediment entrainment and deposition are related directly to tidal range (or maximum tidal height) and consequent current speed. Hence the thickness of successive laminae deposited by tidal currents can be a proxy tidal record, with paleotidal and paleorotational values being determined by analysis of measured records of lamina and cycle thickness. The validity of the findings can be investigated by testing the primary, observed values for internal self-consistency through application of the laws of celestial mechanics. Paleotidal and paleorotational values provided by late Neoproterozoic (∼620 Ma) tidal rhythmites in South Australia are validated by these tests and indicate 13.1±0.1 synodic (lunar) months/yr, 400±7 solar days/yr, a length of day of 21.9±0.4 h, and a relative Earth-Moon distance a/a0 of 0.965±0.005. The mean rate of lunar recession since that time is 2.17±0.31 cm/yr, which is little more than half the present rate of lunar recession of 3.82±0.07 cm/yr obtained by lunar laser ranging. The late Neoproterozoic data militate against significant overall change in Earth's moment of inertia and radius at least since 620 Ma. Cyclicity displayed by Paleoproterozoic (2450 Ma) banded iron formation in Western Australia may record tidal influences on the discharge and/or dispersal of submarine hydrothermal plumes and suggests 14.5±0.5 synodic months/yr and a/a0 = 0.906±0.029. The combined rhythmite data give a mean rate of lunar recession of 1.24±0.71 cm/yr during most of the Proterozoic (2450–620 Ma), suggesting that a close approach of the Moon did not occur during earlier time. Concentrated study of Precambrian tidal rhythmites promises to illuminate the evolving dynamics of the early Earth-Moon system and may permit the lunar orbit to be traced back to near the time of the Moon's origin.

Topography of the transition zone seismic discontinuities

Abstract: Phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities, nominally at 410, 520, and 660 km depth. Thermodynamic principles govern phase transformations, making them sensitive to changes in the mantle's ambient conditions through the thermodynamic response: Changes in temperature or composition shift the transformation to a different pressure, creating topography on a level discontinuity. With this use as an exploratory tool for the mantle in mind, we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities. Reviewing the evidence for and against the phase transition model, we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone.

Auroral emissions of the giant planets

Abstract: Auroras are (generally) high-latitude atmospheric emissions that result from the precipitation of energetic charged particles from a planet's magnetosphere. Auroral emissions from the giant planets have been observed from ground-based observatories, Earth-orbiting satellites (e.g., International Ultraviolet Explorer (IUE), Hubble Space Telescope (HST), and Roentgensatellit (ROSAT)), flyby spacecraft (e.g., Voyager 1 and 2), and orbiting spacecraft platforms (e.g., Galileo) at X-ray, ultraviolet (UV), visible, infrared (IR), and radio wavelengths. UV, visible, and IR auroras are atmospheric emissions, produced or initiated when ambient atmospheric species are excited through collisions with the precipitating particles, while radio and X-ray auroras are beam emissions, produced by the precipitating species themselves. The emissions at different wavelengths provide unique and complementary information, accessible to remote sensing, about the key physical processes operating in the atmospheric and magnetospheric regions where they originate. This paper reviews the development of our current understanding of auroral emissions from Jupiter, Saturn, Uranus, and Neptune, as revealed through multispectral observations and supplemented by plasma measurements.

Observed distributions of nitrogen oxides in the remote free troposphere from the Nasa Global Tropospheric Experiment Programs

Abstract: Reactive oxides of nitrogen are critical catalysts that can limit the formation rates of tropospheric oxidants. As a result, they control the efficiency with which the troposphere cleanses itself of the many natural and artificial compounds that would otherwise reach toxic levels. They play a pivotal role in the troposphere's capacity to generate O3 via photochemical processes. Even at concentrations of a few parts per trillion by volume, as are typically seen in the remote tropical Pacific and Arctic/Antarctic regions, reactive nitrogen oxides lead to ozone formation at rates estimated to be several times larger than the influx of ozone from the stratosphere. Reflecting its multilevel importance in our evolving chemical environment, significant effort has been expended toward defining the factors that control the global distribution of reactive nitrogen oxides. Developing this understanding, however, has not been a simple task. The chemical reactivities that are inherently associated with these compounds allow them to undergo transformations on timescales that range from minutes to months. In addition, a wide variety of sources exist for this family. Some of these are near the Earth's surface (e.g., by-products of emissions from fossil fuel combustion, biomass burning, and nitrification/denitrification of soils), whereas others are within the troposphere itself (i.e., lightning, emissions from subsonic aircraft, stratospheric intrusions, and the oxidation of reduced nitrogen compounds). As a result, large temporal and regional variations can be expected and in fact are found from this array of sources. The variations are dependent on factors ranging from the effects of soil moisture on microbial activity to the convective potential of cumulus clouds. Recent airborne field measurement campaigns have made significant progress toward unraveling many of the NOx distribution controlling factors, particularly as they relate to the remote troposphere. This review concentrates on the progress that has been made during the last 15 years. The major focus is on compounds within the reactive nitrogen oxide family, together with associated chemically coupled species. The measurements reported are predominately those from airborne platforms that have been part of NASA's Global Tropospheric Experiment (GTE) program. However, comparison is also made with other prominent airborne studies including NASA's Airborne Arctic Stratospheric Experiment (AASE) I and II as well as the non-U.S. programs Stratospheric Ozone Experiment (STRATOZ) II and III and International Stratospheric Chemistry (INSTAC) 1. Major findings from these programs as reflected in this review can be summarized as follows: (1) Throughout much of the remote troposphere, NOx levels (inferred from NO observations) are at sufficient levels to significantly impact on the photochemistry of this region. This was found to be particularly true in regard to the O3 production. Thus average column production of O3 in the remote troposphere is estimated to be several times the average stratospheric intrusion flux of O3. (2) A persistent characteristic of observed NO is that mixing ratios in the remote upper troposphere are generally enhanced by a factor of 3 or more compared with middle and lower altitudes. This typically results in net O3 production in the upper troposphere and net loss at the lowest tropospheric altitudes. (3) The largest source of NOx in the upper troposphere appears to be from lightning. However, convection of surface sources such as fossil fuel combustion and biomass burning were also found to be important. In addition, recycling of NOx through other odd nitrogen species may contribute significantly to maintaining the level of NOx, particularly in the upper troposphere, but much uncertainty remains concerning the mechanisms responsible for this. (4) Substantial uncertainties also still exist in the NOx production rate from lightning and from oxidation of NH3.

Abundances and charge states of particles in the solar wind

Abstract: The Sun is the only star from which matter can be collected in order to investigate its elemental and isotopic composition. Solar elemental abundances pro- vide the most important benchmark for the chemical evolution of the galaxy. They can be derived from pho- tospheric observations, from in situ investigations of the solar wind, and from solar energetic particles. Solar isotopic abundances provide an important reference for the galactic evolution and if available with sufficient precision, also for the chemical and physical evolution of the solar system. The abundances of isotopes in the solar atmosphere can only be inferred from in situ observa- tions of solar particles. This review makes an attempt to summarize current knowledge about the composition of the solar wind and shows how the elemental, isotopic, and charge state composition of solar wind particles is shaped as the solar corona expands throughout the he- liosphere.

The mathematics of early diagenesis: From worms to waves

Abstract: The changes that sediments undergo after deposition are collectively known as diagenesis. Diagenesis is not widely recognized as a source for mathematical ideas; however, the myriad processes responsible for these changes lead to a wide variety of mathematical models. In fact, most of the classical models and methods of applied mathematics emerge naturally from quantification of diagenesis. For example, small-scale sediment mixing by bottom-dwelling animals can be described by the diffusion equation; the dissolution of biogenic opal in sediments leads to sets of coupled, nonlinear, ordinary differential equations; and modeling organisms that eat at depth in the sediment and defecate at the surface suggests the one-dimensional wave equation, while the effect of waves on pore waters is governed by the two- or three-dimensional wave equation. Diagenetic modeling, however, is not restricted to classical methods. Diagenetic problems of concern to modern mathematics exist in abundance; these include free-boundary problems that predict the depth of biological mixing or the penetration of O2 into sediments, algebraic-differential equations that result from the fast-reversible reactions that regulate pH in pore waters, inverse calculations of input functions (histories), and the determination of the optimum choice in a hierarchy of possible diagenetic models. This review highlights and explores these topics with the hope of encouraging further modeling and analysis of diagenetic phenomena.

The role of cumulus convection in hurricanes and its representation in hurricane models

Abstract: This paper reviews our understanding of the role of cumulus convection in hurricanes as well as the various convective parameterization schemes that have been used in hurricane models. Elementary principles show that the primary (tangential) circulation of a vortex intensifies as rings of rotating air converge inward while conserving their (absolute) angular momentum. Thus intensification requires a mechanism to produce enough flow convergence above the surface boundary layer to counter the divergence induced there by the boundary layer itself. Typically, such convergence is associated with the inward branch of the secondary circulation and is produced by an unbalanced negative radial gradient of buoyancy above the boundary layer resulting from condensational heating in the inner region of the vortex. The fact that such buoyancy gradients are produced by all the parameterization schemes, as well as by explicit schemes for latent heat release, explains why all the models are able to simulate hurricane intensification with some degree of realism. In a weak vortex the secondary circulation is dominated by buoyant forcing, but as the vortex intensifies the contribution from surface friction increases until in the mature stage, the buoyantly induced convergence must closely balance the frictionally induced divergence just above the boundary layer. Only a handful of the more recent hurricane models and only two of those in which convection is parameterized represent the effects of convective downdrafts. These downdrafts cool and dry the subcloud layer, tending to suppress further convection, so that acting alone they would serve as a brake on hurricane intensification. The strength of downdrafts decreases when the middle-tropospheric relative humidity increases as a result of sustained cumulus convection over an area. If surface wind speeds are high enough, surface fluxes of sensible and latent heat can more than counteract the cooling and drying effects of convective downdrafts, allowing continued warming of the troposphere in the inner region of the vortex so that vortex intensification can proceed. Accordingly, the surface fluxes provide the energy source for the hurricane, while convection transfers this energy vertically through the troposphere, creating a suitable radial gradient of buoyancy to drive the secondary circulation of the vortex.

Oxygen‐isotopic evolution of the solar nebula

Abstract: Studies of the three oxygen isotopes in chondrite meteorites demonstrate that diverse O reservoirs (characterized by their Δ17O values) were present in the solar nebula. The discovery that some chondritic materials have O-isotopic compositions that cannot be explained by mass-dependent fractionation of an initially well mixed reservoir has important implications for the history of the solar nebula. On a plot of δ17O versus δ18O (or 17O/16O versus 18O/16O), terrestrial samples (with the main exception of stratospheric ozone) fall along a single line (the terrestrial fractionation, or TF, line) having a slope ∼0.52, an indication that the parental reservoir was homogenized. A convenient measure of O-isotopic heterogeneity is the deviation from the TF line, Δ17O = δ17O − 0.52 × δ18O. A popular model to explain the evolution of chondritic oxygen compositions during nebular and asteroidal aqueous alteration processes calls for the nebula to have formed from solids having δ18O = −40‰ and δ17O = −41‰ (Δ17O = −20‰) and a gas having a composition roughly estimated to be δ18O = 30‰ and δ17O = 24‰ (Δ17O = 9‰). This model encounters serious difficulties when examined in detail; in particular, it cannot readily account for the O-isotopic compositions of both chondrules and refractory inclusions from individual chondrite groups, or for the differences between groups, particularly in the compositions of chondrules. Magnetite (Fe3O4) is a key phase for O-isotope studies because during its formation by the oxidation of ferrous metal or FeS, all O comes from the oxidant, probably H2O. It appears that most (and, possibly, nearly all) magnetite formed during aqueous alteration processes that occurred in asteroids. In all chondrite groups studied to date, Δ17O of the magnetite is greater than or equal to that of the chondrule silicates. If the H2O originated in the ambient (local) nebula, then at the time of accretion the Δ17O of the nebular gas was generally higher than that of the solids. An alternative view is that the heterogeneity in the O-isotopic composition of chondrites indicates that the nebula formed from diverse batches of presolar materials, the precise O-isotopic composition of the mix varying during the accretion history of the nebula. The large compositional gaps between groups suggest that agglomeration of nebular dust to form chondrites did not proceed at a constant rate but that periods with turbulence levels low enough to allow agglomeration were punctuated by periods of high turbulence during which the composition of the inner nebula sampled by chondrites changed appreciably.

Mass anomaly structure of the Earth

Abstract: Gravitational interaction is the weakest among the four known forces in the universe. The particular gravity equipotential field that coincides with sea level is called the geoid, and satellite data have revealed anomalies in its pattern that are puzzling to explain. Dynamic topography solutions have offered the preferred explanation for the past 15 years, but problems remain. An alternative explanation presented here contends that very large mass anomalies lie deep in the Earth, presumably produced by topography at the coremantle boundary (CMB), and only much smaller mass anomalies occur at shallower depths. The principal distinction is whether one is willing to accept the possibility that processes within the Earth's core may generate the CMB topography. An analysis of the South American regional geoid high (spherical harmonic degrees 2–10) indicates that only mass anomalies shallower than 1200 km are its cause. This single positive source is contrary to dynamic topography solutions that would generate negative deflections of both the surface and the CMB in response to the positive subducted mass for the Earth's fourth greatest geoid high. Our alternate explanation is tested utilizing a set of 9274 subduction 5° cube slab bloblet center points, developed from reconstructed plate history, that provide estimates of the locations of material subducted into the Earth's mantle. Two global mass solutions are offered utilizing (1) only those bloblets in the outer 800 km and (2) only those bloblets in the outer 1400 km. Four point masses at 3000 km depth to simulate CMB topography, produced by processes within the core unrelated to mantle dynamic topography, complete the two mass models. Both models show reasonable agreement with patterns and magnitudes of the regional geoid and its component degree parts. The model CMB mass anomalies are only a factor of 3 greater than those initially estimated by gravity to geoid ratios from observed values at geoid anomaly center locations. Difficulties with the dynamic topography solutions, matters of the density of the subducted material, the proper hydrostatic flattening value to use for analyses, and possible causes for the CMB topography are discussed. The subduction mass models are also used to estimate the magnitude of the driving force for plate tectonics (2.78 × 1020 to 3.23 × 1021 N), approximately equivalent to 7.0 × 1012 to 8.1 × 1013 N m−1. These force values are comparable to published slab force estimates based on thermal considerations.

Remote sensing of solar magnetic fields

Abstract: New techniques for remote sensing of solar magnetic fields now provide measures of the magnetic field vector within the solar atmosphere with high angular resolution and high precision. These measurements have enabled a much improved physical understanding of magnetic processes and phenomena in the solar atmosphere, processes that drive the variability of the Sun's radiative and particulate output. The new techniques are reviewed here in the context of the scientific advances they have fostered. Emphasis is given to techniques for inferring the field vector. The quantitative nature of the information needed to explore the solar phenomena sharply constrains the needed precision and angular resolution of the observations. These requirements are reviewed here, along with an assessment of how future improvements in observing capabilities will address these requirements. One may also attribute much of the recent advance in our understanding of solar magnetic fields to ongoing progress in techniques for analysis of the polarization measurements that underlie solar magnetometry. The status and prospects of analysis techniques are also reviewed.

Shear instabilities of wave-driven alongshore currents

Abstract: The present state of research into and understanding of shear waves is appraised. In this paper these motions, which result from an instability of an alongshore, wave-driven current, are described, their theoretical explanation is recounted, and the history of their discovery is related. The various investigations into their genesis, which attempted to develop an understanding of when and why they occur, are summarized, and more recent research into their finite amplitude development is discussed, focusing on the understanding gained. Attention is also given to observations of shear waves made in the field and attempts to observe them in the laboratory. Finally, work that still needs to be done is described.

The hydrology of Yucca Mountain

Abstract: Yucca Mountain, located in southern Nevada in the Mojave Desert, is being considered as a geologic repository for high-level radioactive waste. Although the site is arid, previous studies indicate net infiltration rates of 5-10 mm yr(-1) under current climate conditions. Unsaturated flow of water through the mountain generally is vertical and rapid through the fractures of the welded tuffs and slow through the matrix of the nonwelded tuffs. The vitric-zeolitic boundary of the nonwelded tuffs below the potential repository, where it exists, causes perching and substantial lateral flow that eventually flows through faults near the eastern edge of the potential repository and recharges the underlying groundwater system. Fast pathways are located where water flows relatively quickly through the unsaturated zone to the water table. For the bulk of the water a large part of the travel time from land surface to the potential repository horizon (similar to 300 m below land surface) is through the interlayered, low fracture density, nonwelded tuff where flow is predominantly through the matrix. The unsaturated zone at Yucca Mountain is being modeled using a three-dimensional, dual-continuum numerical model to predict the results of measurements and observations in new boreholes and excavations. The interaction between experimentalistsmore » and modelers is providing confidence in the conceptual model and the numerical model and is providing researchers with the ability to plan further testing and to evaluate the usefulness or necessity of further data collection.« less