Showing papers in "Reviews of Geophysics in 2003"

Gravity wave dynamics and effects in the middle atmosphere

Abstract: [1] Atmospheric gravity waves have been a subject of intense research activity in recent years because of their myriad effects and their major contributions to atmospheric circulation, structure, and variability. Apart from occasionally strong lower-atmospheric effects, the major wave influences occur in the middle atmosphere, between ∼ 10 and 110 km altitudes because of decreasing density and increasing wave amplitudes with altitude. Theoretical, numerical, and observational studies have advanced our understanding of gravity waves on many fronts since the review by Fritts [1984a]; the present review will focus on these more recent contributions. Progress includes a better appreciation of gravity wave sources and characteristics, the evolution of the gravity wave spectrum with altitude and with variations of wind and stability, the character and implications of observed climatologies, and the wave interaction and instability processes that constrain wave amplitudes and spectral shape. Recent studies have also expanded dramatically our understanding of gravity wave influences on the large-scale circulation and the thermal and constituent structures of the middle atmosphere. These advances have led to a number of parameterizations of gravity wave effects which are enabling ever more realistic descriptions of gravity wave forcing in large-scale models. There remain, nevertheless, a number of areas in which further progress is needed in refining our understanding of and our ability to describe and predict gravity wave influences in the middle atmosphere. Our view of these unknowns and needs is also offered.

Snow avalanche formation

Abstract: [1] Snow avalanches are a major natural hazard, endangering human life and infrastructure in mountainous areas throughout the world. In many countries with seasonally snow-covered mountains, avalanche-forecasting services reliably warn the public by issuing occurrence probabilities for a certain region. However, at present, a single avalanche event cannot be predicted in time and space. Much about the release process remains unknown, mainly because of the highly variable, layered character of the snowpack, a highly porous material that exists close to its melting point. The complex interaction between terrain, snowpack, and meteorological conditions leading to avalanche release is commonly described as avalanche formation. It is relevant to hazard mapping and essential to short-term forecasting, which involves weighting many contributory factors. Alternatively, the release process can be studied and modeled. This approach relies heavily on snow mechanics and snow properties, including texture. While the effect of meteorological conditions or changes on the deformational behavior of snow is known in qualitative or semiquantitative manner, the knowledge of the quantitative relation between snow texture and mechanical properties is limited, but promising developments are under way. Fracture mechanical models have been applied to explain the fracture propagation, and micromechanical models including the two competing processes (damage and sintering) have been applied to explain snow failure. There are knowledge gaps between the sequence of processes that lead to the release of the snow slab: snow deformation and failure, damage accumulation, fracture initiation, and fracture propagation. Simultaneously, the spatial variability that affects damage, fracture initiation, and fracture propagation has to be considered. This review focuses on dry snow slab avalanches and shows that dealing with a highly porous media close to its melting point and processes covering several orders of scale, from the size of a bond between snow grains to the size of a mountain slope, will continue to be very challenging.

Sea breeze: Structure, forecasting, and impacts

Abstract: [1] The sea breeze system (SBS) occurs at coastal locations throughout the world and consists of many spatially and temporally nested phenomena. Cool marine air propagates inland when a cross-shore mesoscale (2–2000 km) pressure gradient is created by daytime differential heating. The circulation is also characterized by rising currents at the sea breeze front and diffuse sinking currents well out to sea and is usually closed by seaward flow aloft. Coastal impacts include relief from oppressive hot weather, development of thunderstorms, and changes in air quality. This paper provides a review of SBS research extending back 2500 years but focuses primarily on recent discoveries. We address SBS forcing mechanisms, structure and related phenomena, life cycle, forecasting, and impacts on air quality.

The second law of thermodynamics and the global climate system: A review of the maximum entropy production principle

Abstract: [1] The long-term mean properties of the global climate system and those of turbulent fluid systems are reviewed from a thermodynamic viewpoint. Two general expressions are derived for a rate of entropy production due to thermal and viscous dissipation (turbulent dissipation) in a fluid system. It is shown with these expressions that maximum entropy production in the Earth's climate system suggested by Paltridge, as well as maximum transport properties of heat or momentum in a turbulent system suggested by Malkus and Busse, correspond to a state in which the rate of entropy production due to the turbulent dissipation is at a maximum. Entropy production due to absorption of solar radiation in the climate system is found to be irrelevant to the maximized properties associated with turbulence. The hypothesis of maximum entropy production also seems to be applicable to the planetary atmospheres of Mars and Titan and perhaps to mantle convection. Lorenz's conjecture on maximum generation of available potential energy is shown to be akin to this hypothesis with a few minor approximations. A possible mechanism by which turbulent fluid systems adjust themselves to the states of maximum entropy production is presented as a self-feedback mechanism for the generation of available potential energy. These results tend to support the hypothesis of maximum entropy production that underlies a wide variety of nonlinear fluid systems, including our planet as well as other planets and stars.

Statistical physics approach to understanding the multiscale dynamics of earthquake fault systems

Abstract: [1] Earthquakes and the faults upon which they occur interact over a wide range of spatial and temporal scales. In addition, many aspects of regional seismicity appear to be stochastic both in space and time. However, within this complexity, there is considerable self-organization. We argue that the occurrence of earthquakes is a problem that can be attacked using the fundamentals of statistical physics. Concepts of statistical physics associated with phase changes and critical points have been successfully applied to a variety of cellular automata models. Examples include sandpile models, forest fire models, and, particularly, slider block models. These models exhibit avalanche behavior very similar to observed seismicity. A fundamental question is whether variations in seismicity can be used to successfully forecast the occurrence of earthquakes. Several attempts have been made to utilize precursory seismic activation and quiescence to make earthquake forecasts, some of which show promise.

Ion-aerosol-cloud processes in the lower atmosphere

Abstract: [1] Natural terrestrial radioactivity and cosmic ray ionization lead to the formation of air ions and charged aerosol particles even away from regions of active charge separation, such as in thunderstorms. The natural electrified state of the atmosphere has been studied for over a century; however, the effect of ionization on the physical properties of aerosols and clouds has rarely been studied in its own right except in thunderstorms. Here we review the status of our understanding of atmospheric charged particles and their influence on aerosol and cloud microphysical processes. We also review mechanisms that have been recently proposed to connect variations in the atmospheric ionization rate with variations in global cloudiness and weather systems. We conclude that a mechanism linking cosmic ray ionization and cloud properties cannot be excluded and that there are established electrical effects on aerosol and cloud microphysics. Necessary further work includes measurements of cloud, droplet, and aerosol charging and ion-aerosol conversion, together with modeling of the electrical aspects of nonthunderstorm cloud microphysics.

Dyes as tracers for vadose zone hydrology

Abstract: [1] Dyes are important tracers to investigate subsurface water movement. For more than a century, dye tracers have provided clues about the hydrological cycle as well as flow and transport processes in the subsurface. Groundwater contamination often originates in the vadose zone. Agrochemicals applied to the soil surface, toxic compounds accidentally spilled by human activities, and contaminants released from waste repositories leach through the vadose zone and can ultimately pollute groundwater resources. Dyes are an important tool to assess flow pathways of such contaminants. This review compiles information on dyes used as hydrological tracers, with particular emphasis on vadose zone hydrology. We summarize briefly different human-applied tracers, including nondye tracers. We then provide a historical sketch of the use of dyes as tracers and describe newer developments in visualization and quantification of tracer experiments. Relevant chemical properties of dyes used as tracers are discussed and illustrated with dye intermediates and selected dye tracers. The types of dyes used as tracers in subsurface hydrology are summarized, and recommendations are made regarding the use of dye tracers. The review concludes with a toxicological assessment of dyes used as hydrological tracers. Many different dyes have been proposed as tracers for water movement in the subsurface. All of these compounds, however, are to some degree retarded by the subsurface medium. Nevertheless, dyes are useful tracers to visualize flow pathways.

Review of mesospheric temperature trends

Abstract: In recent times it has become increasingly clear that releases of trace gases from human activity have a potential for causing change in the upper atmosphere. However, our knowledge of systematic changes and trends in the temperature of the mesosphere and lower thermosphere is relatively limited compared to the Earths lower atmosphere, and not much effort has been made to synthesize these results so far. In this article, a comprehensive review of long-term trends in the temperature of the region from 50 to 100 km is made on the basis of the available up-to-date understanding of measurements and model calculations. An objective evaluation of the available data sets is attempted, and important uncertainly factors are discussed. Some natural variability factors, which are likely to play a role in modulating temperature trends, are also briefly touched upon. There are a growing number of experimental results centered on, or consistent with, zero temperature trend in the mesopause region (80–100 km). The most reliable data sets show no significant trend but an uncertainty of at least 2 K/decade. On the other hand, a majority of studies indicate negative trends in the lower and middle mesosphere with an amplitude of a few degrees (2–3 K) per decade. In tropical latitudes the cooling trend increases in the upper mesosphere. The most recent general circulation models indicate increased cooling closer to both poles in the middle mesosphere and a decrease in cooling toward the summer pole in the upper mesosphere. Quantitatively, the simulated cooling trend in the middle mesosphere produced only by CO 2 increase is usually below the observed level. However, including other greenhouse gases and taking into account a “thermal shrinking” of the upper atmosphere result in a cooling of a few degrees per decade. This is close to the lower limit of the observed nonzero trends. In the mesopause region, recent model simulations produce trends, usually below 1 K/decade, that appear to be consistent with most observations in this region

Time variations in geomagnetic intensity

Abstract: [1] After many years spent by paleomagnetists studying the directional behavior of the Earth’s magnetic field at all possible timescales, detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components. A significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr. A second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field. A third step was the juxtaposition of these signals with those derived from cosmogenic isotopes. Contemporaneous with the acquisition of records, new techniques have been developed to constrain the geomagnetic origin of the signals. Much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials, proper selection of samples, and investigations of complex changes in magnetization during laboratory experiments. Altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 Myr. On longer timescales the field variability and its average behavior is relatively well documented for the past 400 Myr. Section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past. In each case, current limits and potential promises are discussed. The section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records. Preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features. Special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and/or materials.

Multifaceted nature of hydrogeologic scaling and its interpretation

Abstract: [1] Geology is ubiquitously heterogeneous, exhibiting both discrete and continuous spatial variations on a multiplicity of scales. It is therefore natural to expect that hydrogeologic and other geophysical variables would do likewise. We present evidence that hydrogeologic variables exhibit isotropic and directional dependencies on scales of measurement (data support), observation (extent of phenomena such as a dispersing plume), sampling window (domain of investigation), spatial correlation (structural coherence), and spatial resolution (descriptive detail). We then show that it is possible to interpret these multiple scale dependencies within a unified theoretical framework. This and similar theoretical frameworks may be applicable to a wider range of geophysical scale issues.

A comprehensive approach to monitoring volcano deformation as a window on the eruption cycle

Abstract: [1] Since the 1980 eruption of Mount St. Helens, volcanologists have made considerable progress toward predicting eruptions on the basis of precursors that typically start a few days to several months in advance. Although accurate eruption prediction is by no means routine, it may now be possible in some cases to extend the effective warning period by anticipating the onset of short-term precursors. Three promising indicators of deep magmatic processes are (1) deep, long-period earthquakes and tremor that indicate the ascent of magma through the crust, (2) magmatic CO2 emission rate as a proxy for magma supply rate, and (3) relatively broad, generally aseismic surface uplift caused by magmatic intrusions. In the latter case it is essential to sample the deformation field thoroughly in both time and space to adequately constrain source models. Until recently, this has been nearly impossible because high-precision sensors could not be deployed in sufficient numbers, nor could extensive geodetic surveys be conducted often enough. Advances in instrumentation, interferometric synthetic aperture radar (InSAR), and telecommunications are helping to overcome these limitations. As a result, comprehensive geodetic monitoring of selected volcanoes is now feasible. A combination of InSAR, large-aperture GPS surveys, microgravity surveys, and dense arrays of continuous GPS stations, strain meters, and tiltmeters can reveal both spatial and temporal patterns of ground deformation throughout the eruption cycle. Improved geodetic monitoring of many of the world's volcanoes would be a major stride toward better understanding of magmatic processes and longer-term eruption forecasts.

Spatial relation between main earthquake slip and its aftershock distribution

Abstract: [1] We examine where aftershocks occur relative to the spatial distribution of the main shock slip using data from several recent large earthquakes. No universal relation between high- and low-moment regions and high or low aftershock occurrence, or vice versa, is found. We generally find that few, and usually the smaller, aftershocks occur in the high-slip regions of the fault, a notable exception to this being the great 1996 Biak, Indonesia, subduction zone earthquake. In all cases, aftershocks occur on favorably oriented planes of weakness in regions of increased postseismic stress. Generally, they are clustered at both ends of faults, but examples where aftershocks occur only at one end or where there is no clustering at the ends are found. Aftershock clusters are also found at the edge of unbroken barriers, and regions of rapid transition from high to low slip, within the main fault area. We identify examples of geometrical and inhomogeneous barriers and sharp and dull stress concentrations. Rupture in the main shock is generally found to nucleate in the region of low slip or at the edge of high-slip regions, the 1996 Biak earthquake again being the only exception, nucleating in a very high slip region. Off-fault aftershocks are found for all earthquakes in this study, and they sometimes rupture the nodal plane conjugate to the main shock fault plane.

Observational and theoretical studies of the dynamics of mantle plume–mid-ocean ridge interaction

Abstract: [1] Hot spot–mid-ocean ridge interactions cause many of the largest structural and chemical anomalies in Earth's ocean basins. Correlated geophysical and geochemical anomalies are widely explained by mantle plumes that deliver hot and compositionally distinct material toward and along mid-ocean ridges. Compositional anomalies are seen in trace element and isotope ratios, while elevated mantle temperatures are suggested by anomalously thick crust, low-density mantle, low mantle seismic velocities, and elevated degrees and pressures of melting. Several geodynamic laboratory and modeling studies predict that the width over which plumes expand along the ridge axis increases with plume flux and excess buoyancy and decreases with plate spreading rate, plume viscosity, and plume-ridge separation. Key aspects of the theoretical predictions are supported by observations at several prominent hot spot–ridge systems. Still, many basic aspects of plume-ridge interaction remain enigmatic. Outstanding problems pertain to whether plumes flow toward and along mid-ocean ridges in narrow pipe-like channels or as broad expanding gravity currents, the origin of geochemical mixing trends observed along ridges, and how mantle plumes alter the geometry of the mid-ocean ridge plate boundary, as well as the origin of other ridge axis anomalies not obviously related to mantle plumes.

Four centuries of geomagnetic data from historical records

Abstract: [1] We present a comprehensive review of historical geomagnetic data collected over the last 2 decades by many workers, culminating in the largest such compilation in the world. It spans over 4 centuries, from 1510 to 1930 inclusive, and consists of 151,560 declinations, 19,525 inclinations, and 16,219 intensities. The greater part of the new data set comprises unpublished observations recorded by mariners engaged in merchant and naval shipping across the world's oceans. Earlier published compilations and printed accounts of voyages have also been included, in particular when their original sources were inaccessible, missing, or no longer extant. The current effort has brought together an unprecedented number of early observations of the Earth's magnetic field, in standardized format, of potential use in many areas of geophysical research.

Theory and applications of the Analytic Element Method

Abstract: [1] We present a review of the theory and applications of the Analytic Element Method as it exists today. The paper begins with a presentation of analytic elements used for modeling of divergence-free and irrotational flow in both two and three dimensions, including a description of the superblock approach, which makes it possible to deal effectively with very large models both in terms of accuracy and speed. We briefly discuss a particular application of the Analytic Element Method to multiaquifer problems, present the theory of the Analytic Element Method for general vector fields, and discuss the inclusion of source terms (irrotational vector fields with nonzero divergence). We finish the paper with a brief discussion of three major applications on a regional scale.

Why small? The use of small inorganic clusters to understand mineral surface and dissolution reactions in geochemistry

Abstract: [1] Geochemical reactions commonly fall into two classes: (1) ligand exchange reactions where one of the atoms bonded to a metal is exchanged for another and (2) electron exchange reactions where the valence state of a metal changes. Electron exchange reactions cannot be understood without first addressing the role of the coordinated ligands, and we review the use of small aqueous oligomers and complexes to understand mechanisms of ligand exchange reactions at the surfaces of minerals such as metal-(hydr)oxides in soil.

Large-scale sea bed dynamics in off shore morphology. Modelling human intervention

Abstract: We extend the class of simple offshore models that describe large-scale bed evolution in shallow shelf seas. In such seas, shallow water flow interacts with the seabed through bed load and suspended load transport. For arbitrary topographies of small amplitude we derive general bed evolution equations. The initial topographic impulse response (initial sedimentation and erosion patterns around an isolated feature on a flat seabed) provides analytical expressions that provide insight into the inherent instability of the flat seabed, the Coriolis-induced preference for cyclonically oriented features, and bed load transport being a limiting case of suspended load transport. The general evolution equation can be used to describe sandbank formation, known as the result of self-organization. Examples of human intervention at the seabed include applications to a dredged channel and an offshore sandpit. An outlook toward future research is also presented.