bluuebunny/crossref_metadata_2025 huggingface dataset:
Fractal description of wetting front instability in layered soils
Authors: Chang Wen‐Lian , Biggar J. W. , Nielsen D. R.
Published: February 2004

Fractal theory was applied to describe fingering structure and to estimate the effective surface tension at the wetting front during infiltration. A scale length of mean pore size was used to estimate the microscopic system length of fingering. After bulk surface tension was replaced by effective surface tension in a linear stability theory, theoretical maximum wavelengths agreed with those of experimental results when fingering developed in quartz sand beneath Oakley sand, but not beneath Yolo clay.
A stochastic model relating rainfall intensity to raindrop processes
Authors: Smith James A. , De Veaux Richard D.
Published: February 2004

The temporal variability of rainfall and raindrop processes is examined at time scales ranging from less than 1 min to 1 hour. Raindrop processes are represented in terms of drop arrival rate, mean diameter, and coefficient of variation of drop diameter and modeled as time‐varying stochastic processes. It is shown that rainfall rate and accumulated rainfall have simple and accurate representations in terms of raindrop processes. Using these results the temporal variability of rainfall rate is examined in terms of temporal variability of raindrop processes. It is shown that the temporal variability of rainfall rate varies systematically across a range of climatic settings and, more importantly, that these climatic contrasts in rainfall rate can be related to contrasting properties of raindrop processes. Two statistical models of rainfall rate and raindrop processes are examined in detail: a lognormal model with fixed parameters and a lognormal model with parameters that vary from storm to storm. Temporal correlation structure of rainfall rate exhibits qualitatively different behaviour under the two models. Lognormal models of rainfall rate are extended to models in which dependence on the averaging time interval is explicitly represented. Scale properties of rainfall rate are examined empirically for averaging time intervals ranging from 1 min to 30 min. Empirical analyses are based on drop‐size data from North Carolina, New Jersey, Oregon, Alaska, and the Marshall Islands.
A forced‐gradient experiment on solute transport in the Borden aquifer: 2. Transport and dispersion of the conservative tracer
Authors: Thorbjarnarson Kathryn W. , Mackay Douglas M.
Published: February 2004

Conservative tracer transport was monitored at 42 multilevel points and a partially penetrating (PP) well during a forced‐gradient test in the Borden aquifer. Point breakthrough curves were adequately simulated by the advection‐dispersion model indicating negligible or nondetectable physical nonequilibrium effects. Relative hydraulic conductivities estimated from fitted flow rates for each point varied by almost an order of magnitude, similar to the variation previously reported for permeameter analyses of samples from a nearby core transect. Longitudinal dispersivities were 16 cm for the PP well data and ranged from approximately 1 to 11 cm for the point data. These values are within the range of local dispersivities reported for point data in previous tracer tests at the Borden site and other experiments. Composite layer simulations of the PP well utilizing flow and dispersivity estimates from multilevel data matched the general shape of the observed response but not its position.
A model of organic solute uptake from aqueous solutions by soils
Authors: Yiacoumi Sotira , Tien Chi
Published: February 2004

A model is proposed for the sorption of organic solutes from aqueous solutions by soils. This model is formulated by assuming that the uptake process is controlled by the pore diffusion of solutes within the soil pellets as well as the partition of solutes into the soil organic matter. A general expression of the solute uptake in the Laplace domain is obtained, which together with appropriate macroscopic conservation equations and standard Laplace transform inversion techniques, may be used to obtain the extent of sorption and the distribution of organics under a variety of conditions. Comparisons between model predictions and experiments indicate that the inclusion of the partition effect is indeed important in most of the cases.
Stochastic analysis of solute transport in partially saturated heterogeneous soil: 1. Numerical experiments
Authors: Russo David , Zaidel Jacob , Laufer Asher
Published: February 2004

The effect of water saturation on the transport of a conservative, nonreactive solute pulse by quasi steady state and transient flows in a vertical cross section of a hypothetical, yet realistic heterogeneous, partially saturated soil has been analyzed here by a series of numerical experiments. Mean water saturation in the flow domain was controlled by predetermined capillary pressure head at the entry zone, or by predetermined time interval between successive water applications. Analyses of the simulation results show that under both quasi steady state and transient flow regimes, lower degree of water saturation enhances solute spreading, in qualitative agreement with the results of Lagrangian‐stochastic analyses of vadose‐zone transport [Russo, 1993a, b]. The pattern of the velocity field, and, consequently, the spreading of the solute body, were affected considerably by the boundary conditions imposed on the flow at the entry zone. In the case of the quasi steady state flows under predetermined capillary pressure head at the inlet zone, the flow was essentially unidirectional vertical, with slight local derivations and, consequently, solute spreading took place mainly in the longitudinal direction. In the case of transient flows under periodic influx, the flow pattern was more complicated, essentially two dimensional, thereby restricting the longitudinal spreading of the solute plume and enhancing its transverse spreading. In the companion paper (Russo et al., this issue), integrated measures of the simulated solute plumes are compared with their predicted values, based on the first‐order Lagrangian‐stochastic analysis.
Two‐dimensional stochastic analysis and optimal estimation in aquifers: Random recharge
Authors: Hantush Mohamed M. , Mariño Miguel A.
Published: February 2004

This paper investigates two‐dimensional aquifer flow under naturally variable recharge and its application to optimal estimation in groundwater. The adopted framework is a stochastic one in which the governing stochastic equation is solved quasi‐analytically using the Galerkin finite element method and matrix exponentials. The continuous‐time stochastic solution relates head perturbations to random initial bead and a convolution of the pertinent stochastic recharge process. Based on the quasi‐analytical solution, continuous time autocorrelation matrices for aquifer head are developed conforming to (1) white noise recharge fluctuations in time and (2) fully correlated recharge fluctuations in time. From a stochastic point of view, transient aquifer flow in its conventional meaning is only inferred when recharge fluctuations are fully correlated in time, provided that the mean flow approaches steady state. Application to optimal state feedback estimation is demonstrated by adopting Kalman filtering and forecasting to a numerical experiment consisting of two‐dimensional aquifer flow instigated by variable leakage. Filtering results demonstrate that under scarce measurements, statistical conditioning on available measurements can result in an estimated aquifer hydraulic response that captures the actual variability that may exist under natural field conditions. Forecasted aquifer heads, however, are sufficient for a relatively short period; they converge asymptotically to their respective average values.
Frequency domain analysis of time domain reflectometry waveforms: 2. A four‐component complex dielectric mixing model for soils
Authors: Heimovaara T. J. , Bouten W. , Verstraten J. M.
Published: February 2004

Although time domain reflectometry (TDR) is becoming accepted as an important tool for the measurement of soil water content and bulk soil electrical conductivity, a major part of the method is based on empirical relationships. An improved understanding of dielectric measurements on soils may give more insight into soil properties other than soil water content and bulk soil electrical conductivity. Frequency domain analysis of TDR waveforms enables the measurement of the frequency dependent complex dielectric permittivity of soils. The frequency dependent complex dielectric permittivity of soils can be described with a four‐component complex dielectric mixing model based on the volumetric mixing of the refractive indices of the soil components. The four soil components in the model are air, solids, bound water, and free water. Results indicate that the apparent dielectric permittivity obtained from the travel time of the TDR pulse in the soil is the dielectric permittivity at the highest measurement frequency of the cable tester, probe, and soil system. The model based on the volumetric mixing of real permittivities underestimates the measurements in situations with high values of the imaginary part of the dielectric permittivity. Because the model based on the mixing of the complex dielectric permittivities can describe the data, we conclude that the apparent dielectric permittivity is influenced by the imaginary parts in the dielectric, permittivities of the soil components. Combination of the four‐component complex dielectric mixing model with the complex dielectric permittivity obtained from the frequency domain analysis of TDR waveforms gives a tool for modeling the bulk soil electrical conductivity by separating the conductivity of the soil water into a bound water conductivity and a free water conductivity.
A class of stochastic models for relating synoptic atmospheric patterns to regional hydrologic phenomena
Authors: Hughes James P. , Guttorp Peter
Published: February 2004

A model for multistation precipitation, conditional on synoptic atmospheric patterns, is presented. The model, which we call the nonhomogeneous hidden Markov model (NHMM), postulates the existence of an unobserved weather state, which serves as a link between the large‐scale atmospheric measures and the small‐scale spatially discontinuous precipitation field. The weather state effectively acts as an automatic classifier of atmospheric patterns. The weather state process is assumed to be conditionally Markov, given the atmospheric data. The rainfall process is then assumed to be conditionally independent given the weather state. Various parameterizations for the weather state process and the rainfall process are discussed, and a likelihood‐based estimation procedure is described. Model‐based estimates of the storm duration distribution and first and second moments of the rainfall process are derived. As an example the model is fit to a four‐station network of rain gauge stations in Washington state. The observed first and second moments are reproduced very closely. The fitted duration distributions are somewhat lighter tailed than the observed distribution at two of the four stations but provide a good fit at the other two. We conclude that the NHMM has promise as a method of relating synoptic atmospheric data to rainfall and other regional or local hydrologic processes.
Modeling of soil water retention from saturation to oven dryness
Authors: Rossi Cinzia , Nimmo John R.
Published: February 2004

Most analytical formulas used to model moisture retention in unsaturated porous media have been developed for the wet range and are unsuitable for applications in which low water contents are important. We have developed two models that fit the entire range from saturation to oven dryness in a practical and physically realistic way with smooth, continuous functions that have few parameters. Both models incorporate a power law and a logarithmic dependence of water content on suction, differing in how these two components are combined. In one model, functions are added together (model “sum”); in the other they are joined smoothly together at a discrete point (model “junction”). Both models also incorporate recent developments that assure a continuous derivative and force the function to reach zero water content at a finite value of suction that corresponds to oven dryness. The models have been tested with seven sets of water retention data that each cover nearly the entire range. The three‐parameter sum model fits all data well and is useful for extrapolation into the dry range when data for it are unavailable. The two‐parameter junction model fits most data sets almost as well as the sum model and has the advantage of being analytically integrable for convenient use with capillary‐bundle models to obtain the unsaturated hydraulic conductivity.
Statistical moments of reactive solute concentration in a heterogeneous aquifer
Authors: Kabala Z. J. , Sposito Garrison
Published: February 2004

Neglecting local solute dispersion, we prove that under realistic conditions the ensemble moments of the local concentration satisfy a mean convection dispersion equation (CDE) for both conservative and instantaneously adsorbing solutes, provided that the solute velocity field is stochastically stationary. Thus the same algorithm can be used to model the ensemble mean concentration and its uncertainty. For uniform initial conditions the solution of the mean CDE contains all the information about the local concentration probability distribution function, which is not Gaussian. The governing equation for k‐point ensemble moments of the concentration is similar in form to the mean CDE, with the same effective dispersion coefficient and convective velocity. The special case of an impulse initial condition is used to illustrate that volume averaging can significantly change the concentration coefficient of variation except at the plume front. The persistence in the longitudinal direction of concentration moments and concentration k‐point moments is caused by large longitudinal megadispersion. Finally, it is suggested that “ergodicity” should be interpreted operationally in terms of an acceptably small coefficient of variation of the concentration field so that this concept can be applied to experimental field data.
A vertical equilibrium model for assessing nonaqueous phase liquid contamination and remediation of groundwater systems
Authors: Wu Yu‐Shu , Huyakorn P. S. , Park N. S.
Published: February 2004

A numerical model has been developed to provide areal analyses of the three‐dimensional spreading of immiscible liquids in groundwater systems. The model is intended for specific use in prediction of hydrocarbon spreading from subsurface leaks and spills and in design and performance evaluation of remedial schemes. The mathematical formulation is based on vertical integration of the three‐dimensional two‐phase flow equations and incorporation of the concept of gravity‐capillary vertical equilibrium (GCVE) in which a vertical balance of gravitational and capillary forces is assumed. History‐dependent pseudo functions of capillary pressure and relative permeabilities are introduced for the present GCVE model. A direct numerical procedure for evaluating the pseudo functions is derived and verified using an example given in the petroleum literature. An overview of numerical techniques for solving the nonlinear governing equations is presented. Special schemes for handling production wells are derived. Simulation examples are provided to verify, validate, and demonstrate utility of the model. Numerical results obtained from the GCVE model are compared with analytical and rigorous multiphase three‐dimensional numerical solutions. The model is further validated using data from a laboratory investigation on waterflood, five‐spot well performance. The results of these comparisons show the validity of the GCVE modeling assumptions and accuracy and robustness of the proposed formulations and computational schemes. The numerical study also indicates that the present model is highly efficient and suitable for field simulations on personal computers or workstations.
A cubic‐spline technique to calculate nodal Darcian velocities in aquifers
Authors: Zhang Zhihui , Xue Yuqun , Wu Jichun
Published: February 2004

A continuous and reliable velocity field is required for the simulation of mass and heat transport in an aquifer system. In this paper, a cubic‐spline technique is proposed to calculate nodal Darcian velocities after the flow field is obtained. With this technique, a continuous velocity field is produced, and mass conservation is satisfied to an acceptable degree. Its requirements for computer storage are very small, and the calculation speed is very fast; it can be implemented in parallel. A synthetic diffusion problem is used to examine the accuracy of this technique. The results agree very well with the analytical solution. Application to another problem shows that this technique can be applied to any heterogeneous and anisotropic aquifer. The results of the two sample problems are also compared with the Galerkin finite element method [Yeh, 1981] and the dual mesh method [Batu, 1984]. It is observed that its agreement with the Galerkin method is much better than the dual mesh method.
The effects of recharge on flow nonuniformity and macrodispersion
Authors: Rubin Yoram , Bellin Alberto
Published: February 2004

The spatial, statistical structure of the fluid velocity field in the case of uniformly recharged heterogeneous aquifers is investigated, and the spatial covariances of the velocity field are derived. This information is necesary for an investigation of flow and macrodispersion using the Lagrangian formalism proposed by Dagan (1984). The resulting first two moments of the velocity are nonstationary. They are functions of a parameter β which characterizes the degree of flow nonuniformity and is related to the recharge. The displacement variances are computed and tested favorably using numerical simulations. Simple relations are developed which relate the transport parameters found for the case of uniform‐in‐the‐average flows to nonuniform flows using a simple, nonlinear transformation of the travel time, based on β.
Reliability analysis of contaminant transport in saturated porous media
Authors: Jang Yeon‐Soo , Sitar Nicholas , Der Kiureghian Armen
Published: February 2004

An approach to probabilistic modeling of contaminant transport based on the first‐ and second‐order reliability methods (FORM and SORM) is presented. FORM and SORM were initially developed for structural reliability applications to estimate the occurrence of low‐probability events. They can be readily used with both analytical and numerical models and do not require restrictive assumptions about the problem geometry or about the properties of the media. Sensitivity information is obtained as an integral part of these analyses and is used to identify the variables or parameters which have a major influence on the estimate of probability. Example reliability analyses of one‐ and two‐dimensional transport are used to illustrate the approach, and the accuracy of the reliability methods is evaluated in comparison with Monte Carlo simulations. The results show that FORM increasingly overestimates the probability of exceedance as the spatial variability of the domain increases. SORM, on the other hand, accounts for the nonlinearity of the limit state surface and gives results consistent with Monte Carlo simulation over a range of coefficient of variation of K from 0.1 to 0.7. In addition, the FORM/SORM analyses are shown to provide a computational advantage over the Monte Carlo simulation for low‐probability events, because the computational effort is independent of the probability and the results also include sensitivity information. Finally, an example application of system reliability using FORM and SORM shows that problems with multiple limit state surfaces can be readily analyzed and the computational effort is proportional to the number and complexity of the limit state functions.
Potential distribution of methane hydrates in the world's oceans
Authors: Gornitz V. , Fung I.
Published: February 2004

Estimates of the magnitudes and spatial distribution of potential oceanic methane hydrate reservoirs have been made from pressure‐temperature phase relations and a plausible range of thermal gradients, sediment porosities, and pore fillings taken from published sources, based on two major theories of gas hydrate formation (1) in situ bacterial production and (2) pore fluid expulsion models. The implications of these two models on eventual atmospheric methane release, due to global warming, are briefly examined. The calculated range of methane volumes in oceanic gas hydrates is 26.4 to 139.1 × 1015 m3, with the most likely value on the lower end of this range. The results for the bacterial model show a preferential distribution of hydrates at mid‐ to high latitudes, with an equatorial enhancement in the case of the fluid migration model. The latter model also generates a deeper and thicker hydrate stability zone at most latitudes than does the former. Preliminary results suggest that the hydrate distribution predicted by the fluid migration model may be more consistent with observations. However, this preliminary finding is based on a very limited sample size, and there are high uncertainties in the assumptions. The volume of methane hydrate within the uppermost l m of the hydrate stability zone and within 1°‐2°C of the equilibrium curve, assuming in situ bacterial generation, is 0.93–6.32 × 1012 m3, or 0.0035–0.012% of the maximal estimated hydrate reservoir. Nevertheless this volume, if released uniformly over the next 100 years, is comparable to current CH4 release rates for several important CH4 sources. Corresponding CH4 volumes calculated using the fluid migration model are nearly 2 orders of magnitude lower.
Influence of water table on carbon dioxide, carbon monoxide, and methane fluxes from Taiga Bog microcosms
Authors: Funk Dale W. , Pullman Erik R. , Peterson Kim M. , Crill Patrick M. , Billings W. D.
Published: February 2004

Hydrological changes, particularly alterations in water table level, may largely overshadow the more direct effects of global temperature increase upon carbon cycling in arctic and subarctic wetlands. Frozen cores (n=40) of intact soils and vegetation were collected from a bog near Fairbanks, Alaska, and fluxes of CO2, CH4, and CO in response to water table variation were studied under controlled conditions in the Duke University phytotron. Core microcosms thawed to a 20‐cm depth over 30 days under a 20 hour photoperiod with a day/night temperature regime of 20/10°C. After 30 days the water table in 20 microcosms was decreased from the soil surface to −15 cm and maintained at the soil surface in 20 control cores. Outward fluxes of CO2 (9–16 g m−2 d−1) and CO (3–4 mg m−2 d−1) were greatest during early thaw and decreased to near zero for both gases before the water table treatment started. Lower water table tripled CO2 flux to the atmosphere when compared with control cores. Carbon monoxide was emitted at low rates from high water table cores and consumed by low water table cores. Methane fluxes were low (<1 mg m−2 d−1) in all cores during thaw. High water table cores increased CH4 flux to 8‐9 mg m−2 d−1over 70 days and remained high relative to the low water table cores (<0.74 mg m−2 d−1). Although drying of wetland taiga soils may decrease CH4 emissions to the atmosphere, the associated increase in CO2 due to aerobic respiration will likely increase the global warming potential of gas emissions from these soils.
Sulfur and reactive nitrogen oxide fluxes in the North Atlantic atmosphere
Authors: Whelpdale D. M. , Galloway J. N.
Published: February 2004

This paper compiles atmospheric budgets for the large‐scale cycling of sulfur and reactive nitrogen oxides in the atmosphere of the North Atlantic Ocean. Using model results and measurements reported in the literature, estimates are made of the following large‐scale fluxes: eastward from North America into the North Atlantic atmosphere, northward into the Arctic, eastward into and westward out of Europe, westward from Africa, and westward at the Caribbean. Marine emissions into the North Atlantic atmosphere and deposition to the ocean are estimated. Atmospheric transport from continental sources contributes 4.4–7.5 Tg S/yr and 1.7–3.4 Tg N/yr to the North Atlantic Ocean atmosphere, while 0.6–3.6 Tg S/yr comes from marine emissions. Atmospheric transport removes 1.4–1.9 Tg S/yr and 0.5–0.8 Tg N/yr, and deposition to the ocean accounts for 5.2–15.7 Tg S/yr and 1.6–5.7 Tg N/yr. The main input terms for sulfur are flow from North America, followed by marine dimethylsulfide emissions and flow from Africa; for nitrogen, North American and African inputs are comparable. Although outputs exceed inputs for both sulfur and nitrogen, the ranges of estimates overlap by approximately 75%. The spatial distribution of total sulfur deposition flux is generally consistent with known sources and flow regimes except for high values which occur near the European west coast and off the east coast of the Americas south of 30°N. The spatial distribution of total nitrogen deposition flux is more consistent with current knowledge, with the exception of high values across the 0–10° latitude band. The molar sulfur to nitrogen ratio of total deposition exceeds 2 north of 30°N but is close to or below 1 to the south. Both sulfur and nitrogen outputs exceed inputs in the northeastern and southwestern parts of the region, indicating that either inputs from adjacent continents are underestimated or that the deposition is overestimated.
Influence of organic matter incorporation on the methane emission from a wetland rice field
Authors: van der Gon H. A. C. Denier , Neue H. U.
Published: February 2004

Methane (CH4) emission from Philippine rice paddies was monitored with a closed chamber technique during the 1992 dry and wet season. CH4 emissions were significantly higher in the dry season. Application of green manure stimulated CH4 emissions. In plots that received more than 11 t ha−1 of fresh green manure, CH4 emission was highest during the first half of the growing season. Significant amounts of CH4 may evolve during or immediately after transplanting, if the organic amendments are incorporated 1 to 3 weeks before transplanting. Laboratory incubations of soil cores show that CH4 production is highest near the soil surface. CH4 production in green manure treated fields is higher than in urea‐fertilized fields, but toward the end of the season this difference is less pronounced. Around panicle initiation, the fraction of CH4 produced, which was emitted to the atmosphere, is lower than at tillering or ripening. The impact of organic amendments on CH4 emissions at different locations of the world can be described by a dose response curve, if CH4 emission from organically amended plots is expressed relative to CH4 emission from mineral fertilized plots of the same location and season. Various organic amendments (e.g., straw, fermented residues) have a similar effect on CH4 emissions after correction for differences in easily decomposable carbon content.
A forward‐reverse shock pair in the solar wind driven by over‐expansion of a coronal mass ejection: Ulysses observations
Authors: Gosling J. T. , Bame S. J. , McComas D. J. , Phillips J. L. , Scime E. E. , Pizzo V. J. , Goldstein B. E. , Balogh A.
Published: February 2004

A previously unidentified type of solar wind forward‐reverse shock pair has been observed by Ulysses at 4.64 AU and S32.5°. In contrast to most solar wind forward‐reverse shock pairs, which are driven by the speed difference between fast solar wind plasma and slower plasma ahead, this particular shock pair was driven purely by the over‐expansion of a coronal mass ejection, CME, in transit from the Sun. A simple numerical simulation indicates that the overexpansion was a result of a high initial internal plasma and magnetic field pressure within the CME. The CME observed at 4.64 AU had the internal field structure of a magnetic flux rope. This event was associated with a solar disturbance in which new magnetic loops formed in the corona almost directly beneath Ulysses ∼11 days earlier. This association suggests that the flux rope was created as a result of reconnection between the “legs” of neighboring magnetic loops within the rising CME.
Extended‐range hindcasts of tropical‐origin Atlantic hurricane activity
Authors: Hess J. C. , Elsner J. B.
Published: February 2004

Annual tropical storm activity in the Atlantic basin is divided into two groups: 1) all storms and 2) storms having formed exclusively from tropical disturbances. It is demonstrated that, using ordinary least‐squares regression, cross‐validated hindcasts of tropical storm activity from the latter group are more skillful than hindcasts of total hurricane activity. Differences in hindcast skill are above what is expected from a random reduction of the total numbers. Improvements are greatest for yearly numbers of hurricanes and hurricane days.
The compression of diaspore, AlO(OH) at room temperature up to 27 GPa
Authors: Xu Ji‐an , Hu Jingzhu , Ming Li‐chuang , Huang Eugene , Xie Hongsen
Published: February 2004

Diaspore AlO(OH) was compressed in a diamond anvil cell up to 27 GPa at room temperature. X‐ray diffraction experimental data on this sample show that the bulk modulus KT is 230 GPa assuming its pressure derivative Ko′ is 4.
On the problem of measuring interannual wind speed variations using SSMI data
Authors: Halpern David , Wentz Frank
Published: February 2004

The first Special Sensor Microwave Imager (SSMI) was launched on the Defence Meteorological Satellite Program (DMSP) F8 spacecraft in July 1987, and wind speed was no longer retrieved after December 1991. A second SSMI was launched on DMSP F10 in December 1990. Interpretation of the 1987–1993 (or longer) SSMI wind speed time series is dependent upon the space and time characteristics of the differences between F8 and F10 SSMI measurements. The 10°‐zonal averaged monthly mean F8–F10 wind speed difference was negative (positive) for wind speeds less (greater) than 7.9 m s−1, reaching −0.43 (0.32) m s−1 at 5 (10) m s−1. Between 60°S and 60°N the 10°‐zonal averaged monthly mean F8‐F10 wind speed bias was greater than ±0.5 m s−1 on several occasions. From 60°S–60°N the 1991 average value of the monthly mean root‐mean‐square difference between daily F8 and F10 wind speeds in 10°‐longitudinal bands was 2.0 m s−1. In the 60°S–60°N region, about 50% of the daily F8 and F10 wind speed differences was caused by measurement non‐simultaneity and about 50% of the difference was attributed to other factors, such as instrument noise and the different azimuthal orientations of each SSMI. Caution is advised in the interpretation of large‐scale monthly mean analyses obtained from a combined F8 and F10 wind speed time series.
Downward propagating internal waves generated at the base of the surface layer of a stratified fluid
Authors: Stevens Craig , Imberger Jörg
Published: February 2004

Downward propagating internal waves are observed in laboratory experiments. They appear to be generated by the ‘Obstacle effect’ beneath a turbulent surface layer in a surface stress driven stratified flow experiment in a rectangular cavity. The obstacles are the perturbations at the base of the surface layer and the shear is created by a combination of an internal seiche and the circulation in the upper layer. The proportion of energy lost from the surface layer is comparable to increases in potential energy due to vertical entrainment Consequently, if conditions are favourable, a reduction in vertical entrainment may occur.
Shock induced formation of MgAl2O4 spinel from oxides
Authors: Potter David K. , Ahrens Thomas J.
Published: February 2004

The physics of mineral grain sliding, which occurs upon dynamic compression of rocks, is investigated by shock loading single crystals of corundum (Al2O3) and periclase (MgO) in contact obliquely in impact experiments. Energy dispersive X‐ray analysis and X‐ray diffraction studies of samples recovered from 26–36 GPa, 800 ns experiments indicated that under certain conditions a spinel phase of composition MgAl2O4 and thickness ≤20 µm was produced at the interface between the two crystals. Although the computed shock (continuum) temperatures were below those necessary to melt the initial oxides, the spinel nonetheless appears to have formed as a result of localised melting, via grain boundary sliding friction, followed by rapid quenching. Scanning electron microscopy (SEM) revealed some evidence for such melting. Moreover, the timescale of the experiments is too short for solid state diffusion (during the shock state) to explain the observed spinel thickness, although defect enhanced solid state diffusion, subsequent to loading and unloading, remains a possibility. The results also reinforce other recent observations and theories of heterogeneous deformation in minerals.
Bond13C/12C ratios reflect (palaeo‐)climatic variations
Authors: Van Klinken Gert J. , van der Plicht Hans , Hedges Robert E. M.
Published: February 2004

Stable isotope ratios of hydrogen, oxygen, nitrogen and carbon can serve as (palaeo‐) environmental indicators [eg, Ramesh et al., 1986].2H and18O have the clearest relationship with precipitation and temperature, while the sensitivity of carbon is thought to be far less pronounced, and understanding is not complete. Carbon isotopic variation in ecosystems is mainly due to photosynthesis in plants, and passed on in the foodweb without much overall modification (this enables palaeodiet reconstructions using archaeological bone [Schwarcz, 1991]). Using radiocarbon databases [Van der Plicht, 1992] which also contain13C data, we have compared a number of European countries for geographical variation in13C/12C ratio of archaeological wood, charcoal and bone samples. We find similar trends for all three materials. A significant trend from northwestern to southern Europe exists in the plant samples, which we relate to climatic differences influencing13C/12C ratios during carbon fixation. This shift passes through the food web, and is thus found in the bone samples, which makes it possible to use accumulated bone stable isotopic data for palaeoclimatic reconstructions.
A comparison of the performance of the ice and ocean tracking modes of the ERS‐1 radar altimeter over non‐ocean surfaces
Authors: Scott R. F. , Baker S. G. , Birkett C. M. , Cudlip W. , Laxon S. W. , Mantripp D. R. , Mansley J. A. , Morley J. G. , Rapley C. G. , Ridley J. K. , Strawbridge F. , Wingham D. J.
Published: February 2004

The European Space Agency's ERS‐1 radar altimeter is the first to include separate operating modes to optimise performance over both ocean and non‐ocean surfaces. As part of the ERS‐1 commissioning activities, we have carried out a study of the tracking performance of this instrument over non‐ocean surfaces. Statistics for land ice, sea ice, arid lands, and inland water are presented. Performance in both operating modes is shown to be better than that of previous missions.
Diurnal temperature range for a doubled carbon dioxide concentration experiment: Analysis of possible physical mechanisms
Authors: Verdecchia M. , Visconti G. , Giorgi F. , Marinucci M. R.
Published: February 2004

An analysis of the results of a climate simulation for a doubling of atmospheric carbon dioxide concentration over the European region is reported. Physical mechanisms are sought which could explain possible changes in the diurnal temperature range (DTR) under conditions of increased atmospheric greenhouse gas content. We show that an important contribution to changes in DTR is given by soil moisture. In areas where soil moisture increases due to an increase in precipitation there is a positive change in latent heat flux and a decrease in sensible heat flux. As a result, in areas with increasing soil moisture, the increase in maximum daytime temperature will be smaller than that in minimum temperature, thereby causing a decrease in the DTR. The opposite occurs for areas which undergo soil drying. This process amplifies the effect of cloud changes on surface solar and infrared radiation and dominates the direct effect of downward infrared radiation associated with increasing greenhouse gas concentration. Because the soil water content is largely controlled by precipitation, our results are consistent with early observational findings of negative correlation between changes in precipitation and in diurnal temperature range.
The temperature sensitivity of elastic wave velocity at high pressure: New results for molybdenum
Authors: Duffy Thomas S. , Ahrens Thomas J.
Published: February 2004

A new experimental technique is described whereby a material is heated to very high temperature (T), shock compressed to high pressure (P) (and higher T), and the compressional elastic wave velocity of the high P and T state is measured. This method has been applied to the high‐pressure standard molybdenum at pressures between 12 and 81 GPa and at an initial temperature of 1400°C. The compressional velocity of Mo at 2450°C and 81 GPa is found to be 7.91 km/s, compared to a calculated value of 8.36 km/s at 81 GPa along the 25°C isotherm. Data for molybdenum, a number of other metals, and a silicate yield a consistent trend which can be used to determine the scaling coefficient between compressional velocity and temperature at geophysically relevant conditions.
Aftershocks of the 13 May 1993 Shumagin Alaska earthquake
Authors: Lu Zhong , Wyss Max , Tytgat Guy , McNutt Steve , Stihler Scott
Published: February 2004

The 13 May 1993 Ms 6.9 Shumagin earthquake had an aftershock sequence of 247 earthquakes with magnitudes greater than or equal to 1.5 by 1 June 1993. Of these aftershocks, 79 were located by using S‐P travel times at the only two stations within 570 km of the mainshock epicenter. The rupture area inferred from the aftershocks is about 600 km2 and we estimate for the mainshock a mean fault displacement of 1.0 m and a 28 bar stress drop. The magnitude‐frequency plots give a b‐value for the aftershock sequence of about 0.4, which is low compared to the background value of approximately 0.8. The decay of the aftershock sequence followed the modified Omori law with a p‐value of 0.79, which is also lower than the typical values of about 1.1 observed in Alaska. Both of these facts can be interpreted as indicating relatively high ambient stress in the Shumagin seismic gap and the possibility that the 13 May earthquake was a foreshock to a larger gap‐filling event to occur within the next few years.
A composite source model for computing realistic synthetic strong ground motions
Authors: Zeng Yuehua , Anderson John G. , Yu Guang
Published: February 2004

A composite source model is presented for convolution with synthetic Green's functions, in order to synthesize strong ground motions due to a complex rupture process of a large earthquake. Subevents with a power‐law distribution of sizes are located randomly on the fault. Each subevent radiates a displacement pulse with the shape of a Brune's pulse in the far field, at a time determined by a constant rupture velocity propagating from the hypocenter. Thus, all the input parameters have a physical basis. We simulate strong ground motions for event‐station pairs that correspond to records obtained in Mexico by the Guerrero accelerograph network. The synthetic accelerations, velocities, and displacements have realistic amplitudes, durations, and Fourier spectra.
Kinematic mechanism of electron heating in shocks: Theory vs observations
Authors: Balikhin M. , Gedalin M.
Published: February 2004

Shock electron heating due to breakdown of adiabaticity in a small‐scale quasistationary electrostatic field in the shock ramp is analyzed analytically and numerically and compared with experimental data. We derive statistical dependencies of the heating on the shock parameters. Relations obtained numerically are in good agreement with experiment.
On the potential of interplanetary scintillation for predicting geomagnetic activity
Authors: Hapgood Mike , Harrison Richard
Published: February 2004

A survey of IPS, geomagnetic and inter‐planetary medium data has been used to interpret the weak correlation between IPS data and geomagnetic activity. Our results indicate that: (i) we can use IPS data to predict the maximum possible level of geomagnetic activity, given the state of the solar wind; but (ii) to predict the actual level of activity we also require an estimate the coupling between the solar wind and the magnetosphere. This is the first time that the potential (and the limitations) of IPS for predicting geomagnetic activity have been demonstrated quantitatively in a comprehensive survey.
Modelling the low‐latitude boundary layer with reconnection entry
Authors: Song P. , Holzer T. E. , Russell C. T. , Wang Z.
Published: February 2004

We develop a one‐dimensional Low‐Latitude Boundary Layer (LLBL) model for northward IMF. The boundary layer in this model is uniform in the direction normal to the magnetopause, a “plateautype” boundary layer. The boundary layer motion is decoupled from the magnetosheath motion and driven by the plasma pressure associated with the incoming solar wind plasma near local noon, which has become entrained on closed field lines as a result of reconnection in the cusp region. Dissipation in the ionosphere at the feet of the boundary layer field lines opposes this motion. There are two physical solutions for the model. In one, the boundary layer reaches a terminal velocity in the tail as the boundary layer plasma effectively joins the solar wind flow. In the other solution, the flow is nearly stopped in the far tail. In combination with other mechanisms, this latter solution may correspond to the case in which the boundary layer plasma participates in magnetospheric convection and returns sunward. The density, velocity, and thickness as functions of distance from local noon are studied, assuming that the magnetopause has elliptical shape and the magnetospheric field is dipolar.
Atmosphere beryllium‐7 concentrations over the Pacific Ocean
Authors: Uematsu Mitsuo , Duce Robert A. , Prospero Joseph M.
Published: February 2004

Atmospheric 7Be is used as a tracer of stratospheric and upper tropospheric air, but most measurements of 7Be in surface air have been restricted to continental regions. We report here the extensive temporal and areal distribution of atmospheric aerosol 7Be concentration in the marine boundary layer. Measurements of 7Be have been carried out for the weekly filter samples collected at the Pacific island stations where are located between 30° N and 30° S in 1981 and 1985. High annual mean concentrations among the stations are observed at latitude between 20° and 30° in both hemispheres. There is little seasonal variation of 7Be at the nine stations. The lowest concentrations are found in the equatorial region due to relatively little subsidence of the air from the upper layer in this area and high precipitation within the ITCZ (intertropical convergence zone) resulting in rapid removal of the aerosol containing the 7Be. The significant correlation between the 7Be concentrations and the nitrate concentrations at the stations in the North Pacific supports the assumption that nitrate is transported from continental sources to the marine boundary layer through the free troposphere by the westerlies.
UARS MLS O3 soundings compared with lidar measurements using the conservative coordinates reconstruction technique
Authors: Redaelli G. , Lait L. R. , Schoeberl M. , Newman P. A. , Visconti G. , D'Altorio A. , Masci F. , Rizi V. , Froidevaux L. , Waters J. W. , Miller A. J.
Published: February 2004

A technique based on conservative properties of certain meteorological fields is used to compare ozone measurements from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) with soundings from a lidar system operated at midlatitudes by the University of L'Aquila, Italy. A few typical cases are analyzed in connection with the position of the vortex relative to the observing station, and it is shown that in general lidar observations taken within the vortex compare well with the UARS data, regardless of whether they are coincident with a satellite overpass. It is shown that such analysis may be useful for comparing measurements of the same quantity taken at different sites using different measurement techniques.
Antarctic snow record of southern hemisphere lead pollution
Authors: Wolff Eric W. , Suttie Edward D.
Published: February 2004

Lead concentrations from an Antarctic snow pit show the pattern of Pb reaching the Antarctic atmosphere over the last 70 years. Between 1920 and 1950, the Pb concentration shows significant variations around a mean of about 2.5 ng kg−1. Between 1950 and 1980, there is a clear increase to 6 ng kg−1, with an apparent reduction after that. A few high concentrations in the late 1970s are probably due to local contamination from aircraft using leaded gasoline (petrol). Excluding these anomalously high values, the chronological pattern in lead concentrations can be reconciled with estimates of emissions from vehicles and metal production processes in the southern hemisphere.
Southern hemisphere ground based measurements of carbonyl fluoride (COF2) and hydrogen fluoride (HF): Partitioning between fluoride reservoir species
Authors: Reisinger Andreas R. , Jones Nicholas B. , Matthews W. Andrew , Rinsland Curtis P.
Published: February 2004

We report infrared ground based total column measurements of the stratospheric fluorine reservoir gases COF2 and HF above Lauder, New Zealand (45°S, 167.8°E) obtained between April 1993 and January 1994. The average retrieved COF2 and HF total columns are 2.81(±0.56)×1014 and 9.91(±1.09)×1014 molecules/cm², respectively. The daily average COF2 and HF columns are correlated; this correlation is likely the result of dynamics. The average HF/COF2 column ratio on days with measurements of both HF and COF2 is 3.63 (±0.55). Comparison of this ratio with model calculations implies that the quantum yield for COF2 photolysis is near unity. Our measured COF2 columns are higher than all previously reported values, but inconsistencies among the earlier measurements and uncertainty in the latitudinal gradient of the COF2 column preclude an accurate determination of the long‐term COF2 trend.
Periodic longitudinal structure of field‐aligned currents in the dawn sector: Large‐scale meandering of an auroral electrojet
Authors: Ohtani S. , Zanetti L. J. , Potemra T. A. , Baker K. B. , Ruohoniemi J. M. , Lui A. T. Y.
Published: February 2004

The azimuthal structure of field‐aligned current (FAC) systems is one of the target subjects of the Freja satellite's magnetometer observation. In the event of May 28, 1993, the satellite observed large‐amplitude fluctuations of the latitudinal, as well as the longitudinal, magnetic field components along a postmidnight‐to‐morning orbit. By combining radar and ground magnetometer data, we inferred that the associated FAC system has a periodic structure consisting of multiple FAC sheets inclined significantly from the azimuthal (east‐west) direction and moving westward and possibly poleward. Such a structure may be interpreted in terms of the meandering of an auroral electrojet, and perhaps as the ionospheric projection of boundary waves.
Present Antarctic aerosol composition: A memory of ice age atmospheric dust?
Authors: Delmas R. J. , Petit J. R.
Published: February 2004

The data obtained from the chemical analysis (Na, Al, and Ca concentrations) of deep Antarctic ice cores has been reexamined. The correlations found between the concentrations of these elements suggest that the Patagonian continental shelf was the dominant aerosol source during the last ice age, the dust being an intimate mixture of marine clay and carbonates. In addition, it is proposed that sea salt deposited on the continental shelf sediments during drop in sea level was emitted and transported jointly with this dust. The present Antarctic aerosol composition is very similar to the one deduced from ice core studies for glacial climatic conditions. It is concluded that present Antarctic aerosol originates mainly from ice age aeolian deposits in South American cold deserts.