bluuebunny/crossref_metadata_2025 huggingface dataset:
Sea surface temperature variability in coastal areas of the northeastern Pacific related to the El Niño‐Southern Oscillation and the Pacific Decadal Oscillation
Authors: Lluch‐Cota Daniel B. , Wooster Warren S. , Hare Steven R.
Published: February 2004

We examined monthly time‐series (1950 to 1999) of sea surface temperature (SST) anomalies in 47 quadrants (2° × 2°) along the coast of North America and their relation to the El Niño‐Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). We used the Multivariate ENSO Index (MEI) and the PDO Index (PDOI) and assumed a linear interaction of the mechanisms behind their interannual variations. Then, we examined SST anomalies as functions of MEI and PDOI using multiple linear regression (MLR). On average, ENSO influence decreases poleward but is still dominant at 31°N. Northward, the PDO signal dominates. MLR was also applied to selected sea level height (SLH) series. ENSO effects on SLH were clear up to 48°N, while PDO showed a weak effect only in the Gulf of Alaska. This suggests that, poleward of Southern California, ENSO effects may be largely restricted to the inshore coastal environment.
The two behaviours of Mt. Etna Volcano before and after a large intrusive episode: Evidences from b value and fractal dimension of seismicity
Authors: Vinciguerra Sergio , Gresta Stefano , Barbano Maria Serafina , Distefano Giovanni
Published: February 2004

Seismic activity of Mt. Etna volcano has been investigated by analysing time‐spatial fractal dimensions and the b value of the Gutenberg‐Richter's law relatively to two differing eruptive style periods, i.e. 1983–1988 and 1993–1996. The obtained results evidence two different volcano stress conditions before and after the large intrusive episode occurred during the time span 1989–1992. Fractal dimensions and the b value seem to be a promising tool to understand the internal current state of the volcano, allowing to evaluate the short‐middle term volcanic hazard.
Absolute gravity measurements at UK tide gauges
Authors: Williams Simon D. P. , Baker Trevor F. , Jeffries Graham
Published: February 2004

Absolute gravity near 3 of the United Kingdom's (UK) core tide gauges was repeatedly measured using an FG5 gravimeter over a period of 3–4 years to determine vertical land movements at those gauges. The absolute gravity sites were established at Newlyn and Aberdeen in 1995 and in Lerwick in 1996. Assuming a height change of 1 mm causes a change in gravity of 0.2 µgal we see vertical land movements of 1.0±1.4 mm/yr at Newlyn, −3.8±1.6 mm/yr at Lerwick and 0.9±3.1 mm/yr at Aberdeen. These land movements are, within the error estimates, in agreement with land movement predicted by a model of post‐glacial rebound/subsidence. To ensure that our absolute gravimeter is giving accurate results it has been regularly intercompared with other absolute gravimeters and was verified to be in agreement at the 2 µgal level.
North Atlantic Oscillation impact on tropical North Atlantic winter atmospheric variability
Authors: George Steve E. , Saunders Mark A.
Published: February 2004

The dominant mode of windspeed variability in the wintertime tropical north Atlantic (TNA) is represented by the North Atlantic Oscillation (NAO). For the December–January–February (DJF) season the leading principal component of TNA windspeed (representing 46% of the total variance) exhibits a 0.68 correlation with the NAO time‐series. We show that the NAO impact on TNA trade winds peaks in January and is statistically significant at the 99% level for each month from November through to April. This association arises through the meridional pressure gradient equatorward of the Azores high pressure covarying with the NAO. We also show that the winter NAO index determines monthly precipitation levels across the northern Caribbean throughout the following year. We suggest this rainfall impact is due to long lasting, DJF forced perturbations to the north Atlantic sea surface temperature tripole characteristic of the NAO signal.
Biogeophysical versus biogeochemical feedbacks of large‐scale land cover change
Authors: Claussen Martin , Brovkin Victor , Ganopolski Andrey
Published: February 2004

Large‐scale changes in land cover affect near‐surface energy, moisture and momentum fluxes owing to changes in surface structure (referred to as biogeophysical effects) and the atmospheric CO2 concentration owing to changes in biomass (biogeochemical effects). Here we quantify the relative magnitude of these processes as well as their synergisms by using a coupled atmosphere‐biosphere‐ocean model of intermediate complexity. Our sensitivity studies show that tropical deforestation tends to warm the planet because the increase in atmospheric CO2 and hence, atmospheric radiation, outweighs the biogeophysical effects. In mid and high northern latitudes, however, biogeophysical processes, mainly the snow‐vegetation‐albedo feedback through its synergism with the sea‐ice‐albedo feedback, win over biogeochemical processes, thereby eventually leading to a global cooling in the case of deforestation and to a global warming, in the case of afforestation.
Linking oil production to surface subsidence from satellite radar interferometry
Authors: Xu Haibin , Dvorkin Jack , Nur Amos
Published: February 2004

Land subsidence over the Belridge and Lost Hills oil fields, Southern California, was measured using spaceborne interferometric synthetic aperture radar (InSAR). During the 105‐day period between 11/5/95 and 2/17/96, the subsidence in the center of the Lost Hills field reached 15 cm. We assume that this surface subsidence resulted from the vertical shrinkage of the reservoir, which in turn was due to oil production and the resulting pore pressure drop. We model this mechanical effect using an elastic deformation theoretical solution with input constants taken from relevant experiments. The modeled surface deformation matches the InSAR measured values. This result indicates that it is possible, in principle, to monitor hydrocarbon production using satellite‐based measurements of earth deformation.
Interplanetary magnetic field variations and slow mode transitions in the Earth's magnetosheath
Authors: Hubert Daniel
Published: February 2004

The event observed on September 17, 1978 on ISEE 1–2, which led to the concept of a stationary slow mode transition region (SMT) in the magnetosheath in front of the magnetopause, is revisited. We establish that the two edges of this SMT have an exogenous origin induced by two discontinuities of the interplanetary magnetic field. The key of our analysis is that the outer edge of the SMT is built up by a tangential interplanetary discontinuity which is observed on ISEE‐3 at a large distance from the Sun‐Earth line and which has an unusual direction. In this SMT the subsolar magnetosheath is entirely downstream of a quasi‐parallel bow shock, while upstream this SMT the subsolar magnetosheath is downstream of a quasi‐perpendicular shock. We identify three effects at the origin of the density enhancement in this SMT. We extend this approach to the original statistical study and we find that any SMT is connected to interplanetary magnetic field variations. This corroborates our hypothesis that SMTs have an exogeneous origin driven by interplanetary magnetic field variations.
Liquid CO2 breakout and the formation of recent small gullies on Mars
Authors: Musselwhite Donald S. , Swindle Timothy D. , Lunine Jonathan I.
Published: February 2004

We show that the action of a CO2 suspended flow could have produced the recent small gullies on Mars, and, hence, that liquid water is not required. The model involves the build‐up of a liquid‐CO2 aquifer behind and below a dry‐ice barrier (dam/cap rock) in the pore spaces a few meters into the rock from the cliff face and on order of a hundred meters below the top of the cliff brink surface. Seasonal (or obliquity‐cycle‐seasonal) heating causes pinching out of the dry‐ice barrier and rapid release of the liquid CO2. Erosion of the gullies occurs as the rapid vaporization of the liquid CO2 with entrainment of rock and clathrate‐hydrate ice produces a density flow analogous to a terrestrial nue ardente.
Implications of meteor observations by the MU Radar
Authors: Zhou Qihou H. , Mathews John D. , Nakamura Takuji
Published: February 2004

We report high resolution meteor echo observations using the Kyoto University Middle and Upper (MU) Atmosphere 46.5 MHz Radar. When the MU radar was pointed perpendicular to the geomagnetic field lines (B), numerous long‐lived range spread trail echoes were observed which were largely absent when the beam was pointed in the vertical and parallel‐to‐B directions. This shows that this type of trail echo is largely due to scattering structures aligned along B. Additionally, nearly all the head echoes displaying an along‐the‐beam velocity component were followed by range spread echoes in the perpendicular‐to‐B pointing geometry. This demonstrates that meteoric field aligned irregularity is present in essentially all meteors up to the detection limit of the MU radar. Practically all the spectra are limited within a bandwidth corresponding to a Doppler shift of 320 m/s, suggesting that the two stream instability is absent most of the time. Meteoric field aligned structures can be a potential error source for aeronomical applications if they are not appropriately considered.
Correlation of upper‐atmospheric 7Be with solar energetic particle events
Authors: Phillips G. W. , Share G. H. , King S. E. , August R. A. , Tylka A. J. , Adams J. H. , Panasyuk M. I. , Nymmik R. A. , Kuzhevskij B. M. , Kulikauskas V. S. , Zhuravlev D. A. , Smith A. R. , Hurley D. L. , McDonald R. J.
Published: February 2004

A surprisingly large concentration of radioactive 7Be was observed in the upper atmosphere at altitudes above 320 km on the LDEF satellite that was recovered in January 1990. We report on follow‐up experiments on Russian spacecraft at altitudes of 167 to 370 km during the period of 1996 to 1999, specifically designed to measure 7Be concentrations in low earth orbit. Our data show a significant correlation between the 7Be concentration and the solar energetic proton fluence at Earth, but not with the overall solar activity. During periods of low solar proton fluence, the concentration is correlated with the galactic cosmic ray fluence. This indicates that spallation of atmospheric N by both solar energetic particles and cosmic rays is the primary source of 7Be in the ionosphere.
An ENSO signal in the North Atlantic subtropical area
Authors: Gallego D. , Garcia R. , Hernandez E. , Gimeno L. , Ribera P.
Published: February 2004

Precipitation in the Canary Islands is studied from a synoptic point of view for the 1955–1998 period, using data from the NCEP‐NCAR reanalysis. The use of automatic objective techniques allows the identification of five patterns leading to precipitation in the area. The influence of intense El Niño and La Niña events on these patterns is then studied. The results show that La Niña conditions are characterized by more intense and frequent precipitation episodes as result of deeper and more frequent geopotential anomalies at the 500 hPa level.
A calibration of the oxygen isotope paleothermometer of coral aragonite from porites
Authors: Juillet‐Leclerc Anne , Schmidt Gavin
Published: February 2004

The oxygen isotopic composition (δ18Oaragonite) of coral aragonite is dependent upon two parameters, temperature and water oxygen isotopic composition. This tracer allows temperature reconstruction in tropical zones. Different published δ18O:temperature relationships show a slope derived from the regression δ18O against T(°C) which varies between −0.16 and −0.23 for Porites genus. This implies greater uncertainty for paleoclimatic reconstruction. The calibrations were generated differently. They were obtained from several colonies, collected in different locations or the formula was derived from the same coral head following seasonal variations of temperature. The relationships took into account of δ18OSw fluctuation or δ18OSw were estimated from salinity or were not considered. We show by using isotopic data from Weber and Woodhead (1972) and assessing the δ18OSw value by two different methods that a thermodynamically significant formula is generated.
A comparison of mean winds and gravity wave activity in the northern and southern polar MLT
Authors: Dowdy Andrew , Vincent Robert A. , Igarashi Kiyoshi , Murayama Yasuhiro , Murphy Damian J.
Published: February 2004

Mean winds and waves observed in the mesosphere and lower thermosphere with MF radars located at Davis (69°S, 78°E) and Poker Flat (65°N, 147°W) are compared. Measurements covering the period from 1999 to mid 2000 show differences in the strength of the horizontal wind fields. In the southern hemisphere the zonal and meridional winds reach their maximum values near the summer solstice, but are delayed by 2–3 weeks in the northern hemisphere. Gravity wave variances also show significant differences, as do the strength of vertical velocities.
The 8.2 kyr BP event simulated by a Global Atmosphere—Sea‐Ice—Ocean Model
Authors: Renssen H. , Goosse H. , Fichefet T. , Campin J.‐M.
Published: February 2004

Seven freshwater perturbation experiments were performed with a global atmosphere—sea‐ice—ocean model to study the mechanism behind the 8.2 kyr BP Holocene cooling event. These experiments differed in initial state and duration of the applied freshwater pulse, while the amount of freshwater was kept constant (4.67 × 1014 m³). One of the scenarios, with freshwater added to the Labrador Sea at a rate of 0.75 Sv during 20 years, resulted in weakening of the North Atlantic thermohaline circulation during 320 years and surface cooling varying from 1 to 5°C over adjacent continents. This result is consistent with proxy data, suggesting that a meltwater‐induced weakening of the thermohaline circulation caused the event. Moreover, our results indicate that the time‐scale of the meltwater release and the initial state are important, as both have a strong effect on the magnitude and duration of the produced model response.
Stability and equation of state of Fe3C to 73 GPa: Implications for carbon in the Earth's core
Authors: Scott Henry P. , Williams Quentin , Knittle Elise
Published: February 2004

We have measured the volume and lattice parameters of Fe3C‐cementite as a function of pressure to 73 GPa using synchrotron‐based x‐ray diffraction. Several samples were laser heated and temperature‐quenched from >1,500 K while held at high‐pressure. No pressure or temperature induced phase transitions are observed in this study, and the 300 K isothermal equation of state of Fe3C yields a best‐fit bulk modulus, K0T, of 175±4 GPa and a
of 5.2±0.3. The bulk sound speed of Fe3C under outer core conditions is calculated: barring higher pressure polymorphism or marked changes in compressibility on melting, carbon could produce some or all of the offset between seismic observations (PREM) and measurements on pure iron and iron‐nickel alloys. Additionally, we model the thermodynamic stability of this phase at the conditions of Earth's mantle, and determine that in the presence of free iron (i.e. during core formation) the formation of Fe3C is strongly favored relative to carbonates.
Light δ13C events during deglaciation of the East Greenland Continental Shelf attributed to methane release from gas hydrates
Authors: Smith L. M. , Sachs J. P. , Jennings A. E. , Anderson D. M. , deVernal A.
Published: February 2004

We have documented high‐resolution stable isotope records from three marine cores on the East Greenland shelf. These records exhibit three rapid light δ13C events (−3 to −7‰) in benthic and planktic foraminifera during deglaciation that are spatially and temporally transgressive. The light δ13C events are associated with light planktic δ18O, indicative of meltwater. In Kangerlussuaq Trough, the first δ13C event occurs at initial deglaciation, 13.94–14.0 cal ka and the second occurs at 12.85–12.9 cal ka. A younger event at 10.3–9.3 cal ka is recorded near the mouth of Nansen Fjord during final deglaciation. The hypothesized mechanism for the light δ13C is expulsion of methane from gas hydrates in the seafloor from pressure release during ice sheet retreat.
Correlation between surface heat flow and elevation and its geophysical implication
Authors: Fukahata Yukitoshi , Matsu'ura Mitsuhiro
Published: February 2004

Surface heat flow gives direct information about geothermal gradient, but it is significantly affected by erosion and sedimentation. The rate of erosion (sedimentation) is generally in proportion to elevation. Following such an idea, first, we examined the relation between observed surface heat flow and elevation along each of three lines crossing the northernmost, northeastern and southwestern parts of Japan, and found a clear positive correlation between them. The good correlation indicates essential importance of the effects of erosion and sedimentation on surface heat flow. Next, assuming the linear dependence of erosion rates on elevation, we calculated the profiles of surface heat flow expected from the surface topography with a 2‐D thermal conduction model. The results of calculation well explain gross features of the observed heat flow profiles, but some systematic differences still remain. From these systematic differences we can extract unbiased information about thermal state at depth.
Inflation or deflation? New results for Mayon Volcano applying elastic‐gravitational modeling
Authors: Fernández J. , Tiampo K. F. , Jentzsch G. , Charco M. , Rundle J. B.
Published: February 2004

Volcanic activity produces deformation and gravity changes that many times can be used as precursors of future eruptions. Applying geodetic techniques to monitoring activity involves interpretation using deformation models. Usually, the observed changes of the deformation and gravity fields are interpreted seperately, not in a joint inversion. It can be difficult, if not impossible, to interpret the data coherently or correctly in terms of the characteristics of the intrusion or the deflation derived from the gravity changes with purely elastic models, as in the case of Mayon Volcano, Phillipines. We show that elastic‐gravitational models can be used to interpret these cases simultaneously leading to a result that is more plausible on the basis of the available information. Thus, we may need to change the philosophy normally used to interpret geodetic observations. Interpretation as proposed in this work can significantly improve the possibility of predicting future eruptions.
Boundary layer roll statistics from SAR
Authors: Levy Gad
Published: February 2004

The Atmospheric Boundary Layer (ABL) mean flow frequently consists of an organized secondary circulation in the form of counter rotating rolls embedded in it. Theory predicts this is the case for a range of thermodynamic conditions. Although generally associated with the formation of “cloud streets”, numerical and theoretical studies suggest that rolls should be present whenever there is sufficient shear to organize the convection, including cloud free conditions. Through their association with clouds and their modification of the mean low level flow, ABL rolls alter air‐sea interaction processes, surface energy fluxes, and the earth radiational balance. Despite their potential importance to climate there exist no prior studies that can establish the extent and statistical frequency of roll occurrence. Synthetic Aperture Radar (SAR) can detect ABL roll signature on the ocean surface. A first regional climate scale attempt to construct ABL roll frequency statistics from the RADARSAT SAR observations is described.
Identification and characterization of internal waves in SAR images along the coast of Norway
Authors: Dokken Sverre Thune , Olsen Richard , Wahl Terje , Tantillo Marianne Vinje
Published: February 2004

The Synthetic Aperture Radars (SARs) on ERS and RADARSAT are sensitive to changes in the small‐scale surface roughness present on the ocean surface, which is altered by the velocity field associated with internal waves. More than 2600 SAR images from Norwegian waters were analyzed to obtain geometric and radiometric signature statistics of internal waves. The statistical analysis shows that the dispersion properties can be described by a single wavelength scaling parameter of 1.4, allowing internal waves to be detected and characterized with a wavelet based algorithm. Internal waves are identified mainly in four “hot‐spot” areas with somewhat distinctive characteristics. Information on internal waves is useful for the offshore oil industry and for the military, both for planning purposes and operationally in near real time.
The Laptev Sea as a source for recent Arctic Ocean salinity changes
Authors: Johnson Mark A. , Polyakov Igor V.
Published: February 2004

This study was motivated by observations of significant salinification of the upper Eurasian Basin that began around 1989. Observational data and modeling results provide evidence that increased arctic atmospheric cyclonicity in the 1990s resulted in a dramatic increase of the salinity in the Eurasian Basin. Two mechanisms account for the Laptev Sea salinization: eastward diversion of Russian rivers and increased brine formation due to enhanced ice production in numerous leads in the Laptev Sea ice cover. Both these mechanisms are linked to changes in wind patterns and were essential to the formation of salinity anomalies. The resulting Laptev Sea salinity anomaly was advected to the central Eurasian Basin. The strong salinization over the Eurasian Basin altered the formation of cold halocline waters, weakened vertical stratification, and released heat upward from below the cold halocline layer. Our analysis suggests that local processes in the Laptev Sea may have a basin‐wide impact on the thermohaline structure of the Arctic Ocean.
Statistical self‐similarity of hotspot seamount volumes modeled as self‐similar criticality
Authors: Tebbens S. F. , Burroughs S. M. , Barton C. C. , Naar D. F.
Published: February 2004

The processes responsible for hotspot seamount formation are complex, yet the cumulative frequency‐volume distribution of hotspot seamounts in the Easter Island/Salas y Gomez Chain (ESC) is found to be well‐described by an upper‐truncated power law. We develop a model for hotspot seamount formation where uniform energy input produces events initiated on a self‐similar distribution of critical cells. We call this model Self‐Similar Criticality (SSC). By allowing the spatial distribution of magma migration to be self‐similar, the SSC model recreates the observed ESC seamount volume distribution. The SSC model may have broad applicability to other natural systems.
On ion outflows from Titan's exosphere
Authors: Chiu Wan‐Ting , Hsu Hui‐Chun , Kopp Andreas , Ip Wing‐Huen
Published: February 2004

A comprehensive chemical model is applied to Titan's ionosphere in combination with a three dimensional magnetohydrodynamic model of the plasma flow in Titan's vicinity. The calculation shows that in the wake region the ionospheric plasma should concentrate in a thin sheet of a vertical thickness of 0.1–0.2 Titan radius. The major ions are
and
. These carbon‐ and nitrogen‐based molecular ions can be used as a tracer of the magnetospheric dynamical processes.
Solute transport in variable‐aperture fractures: An investigation of the relative importance of Taylor dispersion and macrodispersion
Authors: Detwiler Russell L. , Rajaram Harihar , Glass Robert J.
Published: February 2004

Dispersion of solutes in a variable aperture fracture results from a combination of molecular diffusion and velocity variations in both the plane of the fracture (macrodispersion) and across the fracture aperture (Taylor dispersion). We use a combination of physical experiments and computational simulations to test a theoretical model in which the effective longitudinal dispersion coefficient DL is expressed as a sum of the contributions of these three dispersive mechanisms. The combined influence of Taylor dispersion and macrodispersion results in a nonlinear dependence of DL on the Peclet number (Pe = V〈b〉/Dm, where V is the mean solute velocity, 〈b〉 is the mean aperture, and Dm is the molecular diffusion coefficient). Three distinct dispersion regimes become evident: For small Pe (Pe ≪1), molecular diffusion dominates resulting in DL ∝ Pe0; for intermediate Pe, macrodispersion dominates (DL ∝ Pe); and for large Pe, Taylor dispersion dominates (DL ∝ Pe2). The Pe range corresponding to these different regimes is controlled by the statistics of the aperture field. In particular, the upper limit of Pe corresponding to the macrodispersion regime increases as the macrodispersivity increases. Physical experiments in an analog, rough‐walled fracture confirm the nonlinear Pe dependence of DL predicted by the theoretical model. However, the theoretical model underestimates the magnitude of DL. Computational simulations, using a particle‐tracking algorithm that incorporates all three dispersive mechanisms, agree very closely with the theoretical model predictions. The close agreement between the theoretical model and computational simulations is largely because, in both cases, the Reynolds equation describes the flow field in the fracture. The discrepancy between theoretical model predictions and DL estimated from the physical experiments appears to be largely due to deviations from the local cubic law assumed by the Reynolds equation.
Application and improvement of a recent approximate analytical solution of Richards' Equation
Authors: Hogarth W. L. , Parlange J. Y.
Published: February 2004

A method for developing wetting front profiles for the one‐dimensional Richards' equation is given. The method is obtained by simplifying and extending a recent approximate solution and brings together features from several different studies. Difficulties associated with profile development are discussed by applying the method to several examples, providing a deeper understanding of the solutions of Richards' equation. For simplicity, we illustrate the improvements for soil‐water diffusivities and conductivities which have a power law dependence on the water content although they should apply in general, and the appropriate generalization is given in the conclusion.
Simulation of snowmelt in a subarctic spruce woodland: 2. Open woodland model
Authors: Giesbrecht Mark A. , Woo Ming‐ko
Published: February 2004

A model is presented to simulate snowmelt in a subarctic woodland using a Geographic Information System to express the spatial distribution of snow and the pattern of trees and their shifting shadows during the day. The woodland is distinguished into several zone types, including openings under the Sun and in the shade, zones beneath the tree canopy, and the tree trunks. Meteorological data obtained at an open site are transposed to each zone for the calculation of melt rates. The wooded experimental plot is subdivided into 2×2 m2 grid cells, each with different fractional areas occupied by various zone types. Melt rates at each cell are obtained by weighting the zonal melt with these fractional areas. Despite some limitations the model provides a spatial dimension to snowmelt in the woodland and yields mean melt values that enhance the representation of the forest melt conditions conventionally obtained using calculations for single points.
Hydraulic resistance in overland flow during partial and marginal surface inundation: Experimental observations and modeling
Authors: Lawrence D. S. L.
Published: February 2004

Hydraulic resistance as a function of surface roughness inundation was evaluated in a set of experiments designed to simulate overland flow on a rough, granular surface. The data are compared with an additive drag model based on the contribution of individual elements to flow resistance and a mixing length model for estimating bulk flow resistance. During partial inundation of the surface roughness the observed coefficient of drag per element is much higher than for an isolated element, and a model based on element form drag alone underestimates the observed friction. These high resistance values are strongly correlated with the hydrostatic wave drag estimated from the free surface deformation around elements. The mixing length model, incorporating a multiplier setting the hydraulically effective roughness height, reliably reproduces the trends in resistance during marginal inundation. This multiplier is shown to have a value similar to the root‐mean‐square of the surface height.
Macroscopic behavior and random‐walk particle tracking of kinetically sorbing solutes
Authors: Michalak Anna M. , Kitanidis Peter K.
Published: February 2004

Analytical expressions are derived for the zeroth, first, and second spatial moments of sorbing solutes that follow a linear reversible kinetic mass transfer model. We determine phase‐transition probabilities and closed‐form expressions for the spatial moments of a plume in both the sorbed and aqueous phases resulting from an arbitrary initial distribution of solute between the phases. This allows for the evaluation of the effective velocity and dispersion coefficient for a homogeneous domain without resorting to numerical modeling. The equations for the spatial moments and the phase‐transition probabilities are used for the development of a new random‐walk particle‐tracking method. The method is tested against three alternate formulations and is found to be computationally efficient without sacrificing accuracy. We apply the new random‐walk method to investigate the possibility of a double peak in the aqueous solute concentration resulting from kinetic sorption. The occurrence of a double peak is found to be dependent on the value of the Damköhler number, and the timing of its appearance is controlled by the mass transfer rate and the retardation factor. Two ranges of the Damköhler number leading to double peaking are identified. In the first range (DaI ≤ 1), double peaking occurs for all retardation factors, while in the second range (1 ≤ DaI ≤ 3), this behavior is most significant for R > 12.
A compartmentalized solute transport model for redox zones in contaminated aquifers: 1. Theory and development
Authors: Abrams Robert H. , Loague Keith
Published: February 2004

This paper, the first of two parts [see Abrams and Loague, this issue], takes the compartmentalized approach for the geochemical evolution of redox zones presented by Abrams et al. [1998] and embeds it within a solute transport framework. In this paper the compartmentalized approach is generalized to facilitate the description of its incorporation into a solute transport simulator. An equivalent formulation is developed which removes any discontinuities that may occur when switching compartments. Rate‐limited redox reactions are modeled with a modified Monod relationship that allows either the organic substrate or the electron acceptor to be the rate‐limiting reactant. Thermodynamic constraints are used to inhibit lower‐energy redox reactions from occurring under infeasible geochemical conditions without imposing equilibrium on the lower‐energy reactions. The procedure used allows any redox reaction to be simulated as being kinetically limited or thermodynamically limited, depending on local geochemical conditions. Empirical reaction inhibition methods are not needed. The sequential iteration approach (SIA), a technique which allows the number of solute transport equations to be reduced, is adopted to solve the coupled geochemical/solute transport problem. When the compartmentalized approach is embedded within the SIA, with the total analytical concentration of each component as the dependent variable in the transport equation, it is possible to reduce the number of transport equations even further than with the unmodified SIA. A one‐dimensional, coupled geochemical/solute transport simulation is presented in which redox zones evolve dynamically in time and space. The compartmentalized solute transport (COMPTRAN) model described in this paper enables the development of redox zones to be simulated under both kinetic and thermodynamic constraints. The modular design of COMPTRAN facilitates the use of many different, preexisting solute transport and geochemical codes. The companion paper [Abrams and Loague, this issue] presents examples of the application of COMPTRAN to field‐scale problems.
Inferring the relation between seismic slowness and hydraulic conductivity in heterogeneous aquifers
Authors: Hyndman David W. , Harris Jerry M. , Gorelick Steven M.
Published: February 2004

Cross‐well seismic tomography can be used to develop high‐resolution seismic slowness (1/velocity) estimates along planes through aquifers. Unfortunately, the relation between seismic slowness and hydraulic conductivity is poorly understood, resulting in poor characterization of hydraulic properties from seismic data. This relation is generally developed from laboratory measurements, but slowness values measured with very high frequencies in the lab are often poorly correlated with lower frequency cross‐well and surface seismic slowness values. To address this problem, we developed an approach to infer the relation between slowness and hydraulic conductivity using field scale geophysical and hydrogeologic measurements. We first develop an a priori relation between the conductivity measurements and the cross‐well slowness estimates. Multiple three‐dimensional slowness realizations, conditioned on the cross‐well estimates, are then generated and remapped into log conductivity fields using the a priori slowness to log conductivity relation. We simulate groundwater flow and tracer transport through these conductivity fields and calculate the residuals between measured and simulated concentration arrival time quantiles and drawdown. The slope and intercept of the relation between slowness and log hydraulic conductivity and the dispersivity are then estimated for each slowness realization to minimize the sum of these squared residuals. We demonstrate this approach for the Kesterson aquifer, California, where seismic tomography provided valuable information about aquifer properties. The groundwater flow and tracer transport simulations, through the estimated conductivity fields, yield reasonable fits to the observed tracer concentration histories for two multiple‐well tracer tests (one of which was not used in the inversion) and to the measured drawdown. This approach provides estimates of seismic slowness and hydraulic conductivity, and information about the relation between slowness and log conductivity for a field site.
Shallow subsurface storm flow in a forested headwater catchment: Observations and modeling using a modified TOPMODEL
Authors: Scanlon Todd M. , Raffensperger Jeff P. , Hornberger George M. , Clapp Roger B.
Published: February 2004

Transient, perched water tables in the shallow subsurface are observed at the South Fork Brokenback Run catchment in Shenandoah National Park, Virginia. Crest piezometers installed along a hillslope transect show that the development of saturated conditions in the upper 1.5 m of the subsurface is controlled by total precipitation and antecedent conditions, not precipitation intensity, although soil heterogeneities strongly influence local response. The macroporous subsurface storm flow zone provides a hydrological pathway for rapid runoff generation apart from the underlying groundwater zone, a conceptualization supported by the two‐storage system exhibited by hydrograph recession analysis. A modified version of TOPMODEL is used to simulate the observed catchment dynamics. In this model, generalized topographic index theory is applied to the subsurface storm flow zone to account for logarithmic storm flow recessions, indicative of linearly decreasing transmissivity with depth. Vertical drainage to the groundwater zone is required, and both subsurface reservoirs are considered to contribute to surface saturation.
Soil gas transport above a jet fuel/solvent spill at Plattsburgh Air Force Base
Authors: Ostendorf David W. , Hinlein Erich S. , Lutenegger Alan J. , Kelley Shawn P.
Published: February 2004

We calibrate a stoichiometrically coupled soil gas diffusion model with spatially resolved observations of oxygen, carbon dioxide, total hydrocarbon, and trichloroethylene vapor concentrations in the unsaturated zone above a weathered jet fuel/solvent spill at Plattsburgh Air Force Base in upstate New York. The calibration suggests that aerobic microorganisms in the capillary fringe degrade jet fuel vapor at a steady rate of 9.5 μg hydrocarbons (m−2 s−1). The solvent does not degrade in the fringe, however, and the model and data estimate a steady evaporation rate of 1.2×10−2 μg TCE (m−2 s−1). Barometric pumping slightly alters the steady concentration profile at Plattsburgh, although the transient advective flux is the same order of magnitude as the steady diffusive flux. We derive a simple perturbation theory for the second‐order transient concentration corrections and include it in the calibration. The perturbation theory is valid at Plattsburgh because the soil is uniform and permeable with a relatively deep capillary fringe.
Partitioning total erosion on unpaved roads into splash and hydraulic components: The roles of interstorm surface preparation and dynamic erodibility
Authors: Ziegler Alan D. , Sutherland Ross A. , Giambelluca Thomas W.
Published: February 2004

Field rainfall simulation experiments at two sites are used to partition sediment transport on unpaved roads into splash and hydraulic erosion components. Rain splash processes contributed 38–45% of total sediment output, with instantaneous contributions being variable throughout 60‐min high‐energy events. For low‐ and medium‐magnitude rainstorms, splash erosion on roads is initially controlled by the removal of easily erodible material, followed by a dramatic reduction in sediment output associated with limited detachment from the resistant, highly compacted road surface. A conceptual model explaining temporal variations in splash and hydraulic erosion as functions of prestorm surface preparation (via traffic, maintenance, and mass wasting processes) is presented. For situations where loose sediment is readily available, rain splash energy is less important to sediment detachment. If the loose layer is diminished (e.g., following an overland flow event) or protected by a surface crust, splash energy is needed to detach material from the road surface. Equations in most physically based erosion models do not predict temporal variations in road sediment transport that result from the removal of a loose surface layer of finite depth. A strategy that successfully treats this removal as changes in road erodibility is introduced.
A simplified model for the transient water budget of a shallow unsaturated zone
Authors: Simmons Carver S. , Meyer Philip D.
Published: February 2004

A simplified model describing the transient water budget of a shallow unsaturated soil profile is presented and applied to an example of long‐term climatic conditions for a site near Coshocton, Ohio. Its conceptualization is simplified because the instantaneous redistribution of infiltrated water in a uniform soil profile is imposed. The model's formulation is a revision of that originally presented by Kim et al. [1996]. Runoff from a soil profile with uniform hydraulic properties is described by using an infiltration equation derived by Salvucci [1996] for ponded conditions. During storm periods the infiltration rate equals the rainfall intensity, unless that rate exceeds the saturated hydraulic conductivity, and then the input rate is determined by the infiltration equation for ponded conditions, with the excess applied water being accounted for as runoff. During interstorm periods, when the soil profile is drying due to evapotranspiration (ET) and drainage, the ET rate equals the potential evapotranspiration rate Ep, until the soil profile relative saturation s drops below a certain falling value of saturation Sf. When s < Sf, then the ET rate is linearly proportional to s as given by (s/Sf) Ep. Drainage flux from a profile equals the hydraulic conductivity under a unit hydraulic gradient. Drainage for a 50‐year‐long record of climate on a daily basis for the Coshocton site compared well with measurements from four lysimeters. It is shown that the effective saturated hydraulic conductivity for the profile is determined by the runoff amount over the simulation period, and the value of Sf can be set so that the predicted drainage approximates that measured by lysimeters. This determines the appropriate cumulative ET for the site consistent with the climate data. The appropriate value of Sf also depends on the presumed profile depth, which is the plant root depth over which ET is extracted.
Examination of the influence of data aggregation and sampling density on spatial estimation
Authors: Vucetic Slobodan , Fiez Tim , Obradovic Zoran
Published: February 2004

Spatial processes may be sampled by point sampling or by aggregate sampling. If aggregate samples are collected over a regular grid and used to represent the central point of each aggregation area, the aggregate sampling functions as a low‐pass filter and may eliminate aliasing during spatial estimation. To assess potential accuracy improvements, a numerical procedure for calculating the estimation error variance was developed. Analysis of point and block sampling techniques for kriging and inverse distance interpolation showed that for the same sampling density, block sampling provides better estimation. To achieve the same error levels, over 30%–50% more point samples were required than block samples. Furthermore, interpolation of block sampled data resulted in lower error variability and surfaces with more visual appeal.
Fractional advection‐dispersion equation: A classical mass balance with convolution‐Fickian Flux
Authors: Cushman John H. , Ginn Tim R.
Published: February 2004

The classical form of the partial differential equation governing advective‐dispersive transport of a solute in idealized porous media is the advection‐dispersion equation (ADE). This equation is built on a Fickian constitutive theory that gives the local dispersive flux as the inner product of a constant dispersion tensor and the spatial gradient of the solute concentration. Over the past decade, investigators in subsurface transport have been increasingly focused on anomalous (i.e., non‐Fickian) dispersion in natural geologic formations. It has been recognized that this phenomenon involves spatial and possibly temporal nonlocality in the constitutive theory describing the dispersive flux. Most recently, several researchers have modeled anomalous dispersion using a constitutive theory that relies on fractional, as opposed to integer, derivatives of the concentration field. The resulting ADE itself is expressed in terms of fractional derivatives, and they are described as “fractional ADEs.” Here we show that the fractional ADE is obtained as a special case of the authors’ convolution‐Fickian nonlocal ADE [Cushman and Ginn, 1993]. To obtain the fractional ADE from the convolution‐Fickian model requires only a judicial choice of the wave vector and frequency‐dependent dispersion tensor.
Observation of pollution plume capping by a tropopause fold
Authors: Cho John Y. N. , Newell Reginald E. , Browell Edward V. , Grant William B. , Butler Carolyn F. , Fenn Marta A.
Published: February 2004

Airborne lidar measurements reveal a case in which a layer of high‐ozone air extruding from a tropopause fold appears to cap a pollution plume and force it to spread out in the lower troposphere. The morphology of the high‐ozone layer resembles a three‐dimensional model of tropopause fold evolution that produces a low‐altitude potential vorticity tube. This is a mechanism that can complete the irreversible transfer of air from the stratosphere, and can also affect pollution levels at the surface if the capping layer reaches the top of the boundary layer.
Galileo Probe Doppler residuals as the wave‐dynamical signature of weakly stable, downward‐increasing stratification in Jupiter's deep wind layer
Authors: Allison Michael , Atkinson David H.
Published: February 2004

Doppler radio tracking of the Galileo probe‐to‐orbiter relay, previously analyzed for its in situ measure of Jupiter's zonal wind at the equatorial entry site, also shows a record of significant residual fluctuations apparently indicative of varying vertical motions. Regular oscillations over pressure depth in the residual Doppler measurements of roughly 1–8 Hz (increasing upward), as filtered over a 134 sec window, are most plausibly interpreted as gravity waves, and imply a weak, but downward increasing static stability within the 5‐20 bar region of Jupiter's atmosphere. A matched extensio to deeper levels of an independent inertial stability constraint from the measured vertical wind shear at 1‐ 4 bars is roughly consistent with a static stability of ∼0.5 K/km near the 20 bar level, as independently detected by the probe Atmospheric Structure Instrument.
Intermittent‐type temporal fluctuations in seismicity of the Irpinia (southern Italy) Region
Authors: Telesca Luciano , Cuomo Vincenzo , Lapenna Vincenzo , Macchiato Maria
Published: February 2004

We present a new approach to investigate the temporal fluctuations of seismicity in terms of intermittency as the underlying dynamical mechanism for the variability of seismic activity. Analyzing the 1983–1997 Irpinia (southern Italy) seismicity, we base our analysis on the multifractal formalism combined with the estimate of the correlation codimension. Our findings show that the seismic activity of the area investigated is strongly intermittent, and the degree of intermittency lowers if aftershock clusters, associated to the larger events, are removed.
Evidence for azimuthal and temporal variations of the rupture propagation of the 1999 Chi‐Chi, Taiwan Earthquake from dense seismic array observations
Authors: Huang Bor‐Shouh
Published: February 2004

Data recorded by the SMART‐2 array and its nearby accelerometers in eastern Taiwan were used to make direct measurements of the rupture propagation of the 1999, Mw=7.6 Chi‐Chi, Taiwan earthquake. The N‐th root stack method was employed to window and stack seismic waveforms, and it revealed that coherent seismic energy showed clear temporal changes with regard to azimuth. The results of this study elicited unambiguous observations as to rupture propagation direction, rupture duration time and spatial distribution of the major asperities on the fault plane. The rupture propagation of the Chi‐Chi earthquake was reconstructed and demonstrated that the source of the early arrived coherent energy was close to the epicenter and that the subsequent major energy moved southward. Twelve seconds after the initial rupture, clear arrival energy was resolved and then propagated northward. The horizontal apparent rupture velocity is estimated at 2.1 km/sec for its northward propagation. The largest source of radiated energy was identified and its location was near the northern end of the earthquake fault. The strong shaking was continuous for about 24 seconds. The analysed results of this study provide direct observations of the rupture properties of this earthquake. These properties may serve as valuable constraints for other Chi‐Chi earthquake rupture propagation studies based on other indirect measurement methods.