bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the spatial variability of natural trace element concentrations in groundwater
Authors: Ramaswami Anuradha , Small Mitchell J.
Published: February 2004

A conceptual stochastic model is developed that describes the natural spatial variability of nonreactive solute concentrations in large groundwater systems. The spatial variation in aqueous concentration is associated with dissolution from source areas of high mineral enrichment in the aquifer matrix. The stochastic model considers randomly varying inputs of a solute species from source deposits that occur as a two‐dimensional spatial Poisson process. A steady state advective‐dispersive transport equation is utilized to predict the downgradient movement of the solute from the source areas. The total groundwater concentration at any location is calculated from the superposition of the individual contributions from each source area in the aquifer. A spatially varying concentration field results, described mathematically by a filtered Poisson process model. The theoretical concentration field is nonstationary, with the mean and variance increasing, and the coefficient of variation decreasing, in the direction of groundwater flow. Gaussian fields for abundant elements and highly skewed probability distributions for trace elements are indicated by the filtered Poisson process model. Evaluation of elemental concentration data from the Sherwood aquifer in England demonstrates how field data may be analyzed in the context of the stochastic model.
Solving three‐dimensional hexahedral finite element groundwater models by preconditioned conjugate gradient methods
Authors: Larabi A. , De Smedt F.
Published: February 2004

Practical aspects of three‐dimensional modeling of groundwater flow in heterogeneous aquifer systems are investigated using a finite element approach. Particular attention is given to the properties of the conductance matrix and the efficiency of the conjugate gradient method with different preconditioners: diagonal scaling, incomplete Cholesky decomposition, incomplete factorization, and modified incomplete factorization. It is shown that for hexahedral trilinear finite elements the resulting matrix is, except for cube‐shaped elements, never diagonally dominant, which restricts the existence of several preconditioners. Numerical comparison of several test problems, including hypothetical and field applications with different degrees of heterogeneity, show that the incomplete Cholesky and the incomplete factorization preconditioners, if they exist, are more efficient than diagonal scaling with respect to both rate of convergence and overall computing time, but diagonal scaling can be considered superior because it is always possible. An M matrix transformation is proposed which guarantees the existence of all preconditioners. Numerical comparison of the test problems shows that this technique is very effective. From the resulting preconditioners, the incomplete Cholesky and the incomplete factorization are shown to be the most efficient, but the latter is superior from the point of view of computer storage and is recommended for all practical applications.
Spatial averaging of unsaturated flow equations under infiltration conditions over areally heterogeneous fields 2. Numerical simulations
Authors: Chen ZhiQiang , Govindaraju Rao S. , Kavvas M. Levent
Published: February 2004

Two models for horizontally averaged unsaturated flow have been developed from two different approaches in the first (Chen et al., this issue) of these companion papers. In this paper the results from both the spatially horizontally averaged Richards equation (SHARE) model and the averaged Green‐Ampt model are compared with the results from a three‐dimensional finite difference model of unsaturated flow which is perceived as the reference solution. The results of the averaged Green‐Ampt model show very good agreement with the averaged results from the three‐dimensional model, while SHARE model results are applicable only when fluctuations in soil parameters are small with respect to their mean values. It is also shown that methods of simple parameter averaging (arithmetic or geometric averages) with the local Richards equation does not yield meaningful results in heterogeneous soils. This study suggests that spatially horizontally averaged simplified models (such as the averaged Green‐Ampt model) are attractive alternatives to perturbation models (such as the SHARE model) in heterogeneous fields. Due to their simplicity in formulation, accuracy in predicting average behaviors, and minimal requirement of computer effort, the spatially horizontally averaged simplified models can be easily implemented in large‐scale models, such as atmospheric mesoscale models.
Spatial averaging of unsaturated flow equations under infiltration conditions over areally heterogeneous fields: 1. Development of models
Authors: Chen ZhiQiang , Govindaraju Rao S. , Kavvas M. Levent
Published: February 2004

Unsaturated flows within subsurface regions control many large‐scale hydrological and environmental processes. This study addresses the issue of spatial averaging of unsaturated flow equations at field scales. Two models for horizontally averaged unsaturated flow have been developed from two different approaches in this study. The spatially horizontally averaged Richards equation (SHARE) model is represented by a system of two coupled partial differential equations for the mean water saturation in each horizontal soil layer and the cross‐covariance of the saturated hydraulic conductivity and the water saturation in each horizontal soil layer in a heterogeneous field. As an alternative to the spatial averaging/perturbation approach which was used for SHARE, a spatially horizontally averaged form of Green‐Ampt model is obtained by field scale spatial horizontal averaging of the local soil water dynamics which are represented approximately by rectangular profiles. This strategy leads to analytical solutions for average water content and the results can be upscaled to large spatial areas. The computational effort required to evaluate such analytic expressions is trivial in comparison with that of the numerical solution of Richards equation. The averaged Green‐Ampt model, though approximate, yields good results when large variations exist in the soil properties in the horizontal directions.
Effect of saturated areas on backscattering coefficient of the ERS 1 synthetic aperture radar: First results
Authors: Merot P. , Crave A. , Gascuel‐Odoux C. , Louhala S.
Published: February 2004

One way to estimate spatial distribution of water content at the soil surface consists of using active microwave remote sensing. It has been theoretically and experimentally demonstrated for unsaturated conditions. Nevertheless, radar data are ambiguous when ponding conditions occur, as in variable source areas, due to the contradictory influence of the dielectric effect and the specular effect on the baekscattering attenuation coefficient, σ0. A procedure is considered, based on a topographic analysis, to take into account the influence of the two effects on radar response. The present results stress the capabilities of the ERS (European Remote Sensing Satellite) radar to survey saturated areas in time and space.
The estimation of surface momentum flux under unstable conditions from the atmospheric pressure field
Authors: Crago Richard D. , Brutsaert Wilfried
Published: February 2004

Data from FIFE (the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment) were used in an atmospheric boundary layer bulk similarity formulation to derive values of the friction velocity, u* (i.e., the shear stress), from surface geostrophic wind measurements under unstable conditions. Several stability‐dependent forms for the similarity functions B (corresponding to the x direction geostrophic wind, ug) and Bw (corresponding to the scalar geostrophic wind speed, G) were tested. Formulations to calculate u* from G were more successful than those which used only ug. However, the dependency of B, Bw, or the geostrophic drag coefficient, CG = u*/G, on either stability or baroclinicity was observed to be relatively weak. For FIFE, a constant value of Bw = 1.99 agreed with the reference u* values almost as well as the best stability‐dependent formulations. A constant value of CG = 0.0883, used with measured values of G, gave an unbiased estimate of u*, on average, with a correlation r = 0.80.
How permeable are clays and shales?
Authors: Neuzil C. E.
Published: February 2004

The permeability of argillaceous formations, although rarely measured and poorly understood, is commonly a critical parameter in analyses of subsurface flow. Data now available suggest a regular relation between permeability and porosity in clays and shales and permeabilities that, even at large scales, are significantly lower than usually assumed. Permeabilities between 10−23 and 10−17 m2 have been obtained at porosities between 0.1 and 0.4 in both laboratory and regional studies. Although it is clear that transmissive fractures or other heterogeneities control the large‐scale hydraulic behavior of certain argillaceous units, the permeability of many others is apparently scale independent. These results have significant implications for understanding fluid transport rates and abnormal pressure generation in basins, and could prove important for waste isolation efforts.
Predicting solute plume evolution in heterogeneous porous formations
Authors: Sposito Garrison , Dagan Gedeon
Published: February 2004

Modeling of the time evolution of a solute plume in a heterogeneous porous formation has often focused on low‐order spatial concentration moments calculated in comprehensive field experiments in which a plume is sampled synoptically at successive stages of its postinjection development. Assumptions of a stochastic nature usually have been invoked to derive equations that predict the time dependence of these spatial moments and permit their interpretation in terms of field concentration data. In this paper, expressions for the low‐order spatial moments are derived on the basis of a purely dynamical formulation of plume motion in terms of the solution of the autonomous ordinary differential equation determined by a steady fluid velocity field. The equations derived stress the importance of spatial interdependence between the (arbitrary) initial state of a solute plume, as defined by its concentration field, and the subsequent displacements of moving spatial points determined by a groundwater velocity field.
Flow channeling in strongly heterogeneous porous media: A numerical study
Authors: Moreno Luis , Tsang Chin‐Fu
Published: February 2004

Flow and solute transport through porous media having strongly variable permeability were studied for parallel and convergent/divergent flows. The variation in hydraulic conductivity K causes (1) the fluid to flow through a porous medium along least resistive pathways and (2) solute dissolved in the fluid to be transported with widely differing velocities. Numerical simulations were performed to study flow and solute transport in a three‐dimensional heterogeneous porous block. It is found that for a strongly heterogeneous medium the particles (or solutes) travel through the medium along preferred flow paths, which we call channels. These channels possess hydraulic properties that are different from those of the global porous medium and which are invariant regardless of the direction from which the hydraulic gradient is applied to the porous block. The log‐hydraulic conductivities along these channels have a greater mean value and a smaller standard deviation than for the global porous medium. These differences or “shifts” were calculated as a function of the hydraulic conductivity variance of the global porous medium. Tracer breakthrough curves for a pulse injection were also calculated. For small standard deviations of the global hydraulic conductivity distribution, a peak in the breakthrough curve is found which spreads out around its peak value as the standard deviation is increased. However, as the standard deviation is increased further, a new peak emerges at a much earlier time. This may be the result of increasing channeling effects at large standard deviations. For the case of a spherical pressure boundary around point tracer injection, the flow follows the usual divergent pattern only for small variations in hydraulic conductivity. When the standard deviation in log K is large, a significant portion of the flow becomes channelized; i.e., it tends toward a linear flow pattern.
Geostatistical analysis of aquifer heterogeneity from the core scale to the basin scale: A case study
Authors: Desbarats A. J. , Bachu S.
Published: February 2004

This study examines the spatial variability of conductivity within the sandstone Wabiskaw member of the Lower Cretaceous, Middle Mannville in the Alberta Basin. The Wabiskaw is an important regional hydrostratigraphic unit. Vertical variability within the aquifer is characterized by downhole variograms of log conductivity from core measurements. They are found to exhibit pseudoperiodic structures due to alternating sand and shale layers. Horizontal spatial variability is characterized using variograms of log transmissivity and indicator data. These variograms show a steady increase in spatial variability over length scales ranging from 400 m to 100 km, characteristic of self‐similar or fractal behavior. They also reveal distinct correlation structures related to both high and low transmissivity depositional features. This study also considers the upscaling of conductivity vertically, from the core to the well scale, and horizontally, from the well to the basin scale. It is shown that traditional vertical upscaling of conductivities by arithmetic averaging causes an overestimation of transmissivity in two‐dimensional areal flow models. An alternative approach, based on power averaging, is proposed. Transmissivities derived by vertical upscaling of core measurements are compared with values derived from drill‐stem tests in the same well. No correlation between these quantities is observed. An average transmissivity for the Wabiskaw in the study area is determined by geometric spatial averaging; however, the highly clustered spatial distribution of well locations requires a weighting of sample values using ordinary kriging.
Push broom microwave radiometer observations of surface soil moisture in Monsoon '90
Authors: Schmugge T. , Jackson T. J. , Kustas W. P. , Roberts R. , Parry R. , Goodrich D. C. , Amer S. A. , Weltz M. A.
Published: February 2004

The push broom microwave radiometer (PBMR) was flown on six flights of the NASA C‐130 to map the surface soil moisture over the U.S. Department of Agriculture's Agricultural Research Service Walnut Gulch experimental watershed in southeastern Arizona. The PBMR operates at a wavelength of 21 cm and has four horizontally polarized beams which cover a swath of 1.2 times the aircraft altitude. By flying a series of parallel flight lines it was possible to map the microwave brightness temperature (TB), and thus the soil moisture, over a large area. In this case the area was approximately 8 by 20 km. The moisture conditions ranged from very dry, <2% by volume, to quite wet, >15%, after a heavy rain. The rain amounts ranged from less than 10 mm to more than 50 mm over the area mapped with the PBMR. With the PBMR we were able to observe the spatial variations of the rain amounts and the temporal variation as the soil dried. The TB values were registered to a Universal Transverse Mercator grid so that they could be compared to the rain gage readings and to the ground measurements of soil moisture in the 0‐ to 5‐cm layer. The decreases in TB were well correlated with the rainfall amounts, R2 = 0.9, and the comparison of Tg with soil moisture was also good with an R2 of about 0.8. For the latter, there was some dependence of the relation on location, which may be due to soil or vegetation variations over the area mapped. The application of these data to runoff forecasts and flux estimates will be discussed.
Two‐dimensional transport model for variably saturated porous media with major ion chemistry
Authors: Šimůnek Jiří , Suarez Donald L.
Published: February 2004

We present the development and demonstrate the use of the two‐dimensional finite element code UNSATCHEM‐2D for modeling major ion equilibrium and kinetic nonequilibrium chemistry in variably saturated porous media. The model is intended for prediction of major ion chemistry and water and solute fluxes for soils under transient conditions. Since the solution chemistry in the unsaturated zone is significantly influenced by variations in water content, temperature, and CO2 concentrations in the soil gas, all these variables are also calculated by the model. The major variables of the chemical system are Ca, Mg, Na, K, SO4, Cl, NO3, alkalinity, and CO2. The model accounts for equilibrium chemical reactions between these components such as complexation, cation exchange, and precipitation‐dissolution. For the precipitation‐dissolution of calcite and dissolution of dolomite, either equilibrium or multicomponent kinetic expressions are used which include both forward and back reactions. Other dissolution‐precipitation reactions considered include gypsum, hydromagnesite, and nesquehonite. Since the ionic strength of soil solutions can often reach high values, both modified Debye‐Hückel and Pitzer expressions were incorporated into the model to calculate single ion activities. The need for an iterative coupling procedure between the solute transport and chemical modules is demonstrated with an example which considers root water uptake and irrigation using moderately saline water. The utility of the model is further illustrated with two‐dimensional simulations with surface and subsurface irrigation from a line source.
Optimal groundwater remediation with well location as a decision variable: Model development
Authors: Wang Wen , Ahlfeld David P.
Published: February 2004

A new formulation for the optimal design of aquifer remediation strategies is presented in which the well location problem is solved by explicitly incorporating the spatial coordinates and pumping rates of wells as decision variables. The Hermite interpolation function is used to represent the well location as a continuous function of space and to facilitate the incorporation of a barrier function for containing the well location within the model domain. The management model combines the numerical simulation of groundwater flow and contaminant transport with optimization methods to select the optimal location and pump rate by moving each well within the problem domain unrestricted by nodal location. The details of the numerical implementation required to incorporate well locations as decision variables in a two‐dimensional Galerkin finite element discretization of the groundwater flow and contaminant transport equations are presented. The approach is contrasted with an optimization model for groundwater quality management in which optimal well locations are chosen from a number of preselected candidate locations. A simple hypothetical problem is solved to compare these two approaches and examine the numerical behavior of the new formulation. The results indicate that a formulation in which well location is a decision variable can be implemented and that improved solutions are possible. The results are also used to demonstrate how this approach can be used to solve problems in which the initial costs of well installation are included.
Climate controls on temporal variability of methane flux from a poor fen in southeastern New Hampshire: Measurement and modeling
Authors: Frolking Steve , Crill Patrick
Published: February 2004

Three scales of temporal variability were present in methane (CH4) flux data collected during a 2.5 year (mid‐1990–1992) study at a small, poor fen in southeastern New Hampshire. (1) There was a strong seasonality to the fluxes (high in summer); monthly average fluxes range from 21.4 mg CH4 m−2 d−1 (February 1992) to 639.0 mg CH4 m−2 d−1 (July 1991). Annual fluxes were 68.8 g CH4 m−2 (1991) and 69.8 g CH4 m−2 (1992). (2) There was interannual variability; distribution of flux intensity was very different from 1991 to 1992, particularly the timing and rapidity of the onset of higher fluxes in the spring. (3) There was a high degree of variability in CH4 flux during the warm season; four successive weekly flux rates in July 1991 were 957, 1044, 170, and 491 mg CH4 m−2 d−1. Fluxes were correlated with peat temperature (r2=0.44) but only weakly with depth to water table (r2 = 0.14 for warm season data). Warm season fluxes appeared to be suppressed by rainstorms. Along with methane flux data we present an analysis of this temporal variability in flux, using a peatland soil climate model developed for this site. The model was driven by daily air temperature, precipitation, and net radiation; it calculated daily soil temperature and moisture profiles, water table location, and ice layer thickness. Temperature profiles were generally in good agreement with field data. Depth to water table simulations were good in 1992, fair in 1990, and poor in the summer of 1991. Using model‐simulated peat climate and correlations to methane flux developed from the field data, simulated methane fluxes exhibited the same three modes of temporal variability that were present in the field flux data, though the model underestimated peak fluxes in 1990 and 1991. We conclude that temporal variability in flux is significantly influenced by climate/weather variability at all three scales and that rainfall appears to suppress methane flux for at least several days at this site.
Distribution of radiocarbon as a test of global carbon cycle models
Authors: Jain Atul K. , Kheshgi Haroon S. , Hoffert Martin I. , Wuebbles Donald J.
Published: February 2004

Accurate global carbon cycle models are needed to estimate the future change of atmospheric CO2 for specified scenarios of CO2 emissions. Model accuracy cannot be tested directly because of the difficulty in estimating the carbon flux to the oceans and the terrestrial biosphere. However, one test of model consistency is the requirement that the model reproduce past changes and spatial distributions of 14C. A model for carbon exchange within and among the atmosphere, oceans, and terrestrial biosphere is found to satisfy this test. The ocean is modeled as an upwelling‐diffusion column capped by a mixed layer with recirculation of the polar bottom water to complete the thermohaline circulation. This ocean advection scheme contains only two key dynamic parameters, the vertical eddy diffusivity κ and the upwelling velocity w, which are calibrated to match the vertical distribution of preanthropogenic 14C. The thermocline depth scale κ/w = 1343 m found by calibration is considerably deeper than that required to match the steady vertical temperature profile (500 m). This is consistent with the hypothesis that isopycnal mixing, which is much more rapid than diapycnal mixing, has a stronger effect on 14C than on temperature since isopycnals are nearly isothermal. This model is found to match measured values, within measurement error, of the prebomb decrease in 14C in the atmosphere and the mixed layer due to the Suess effect, the bomb 14C in the mixed layer, the bomb 14C penetration depth, the bomb 14C ocean inventory, and the vertical distribution of total carbon. Results are compared to those of other schematic carbon cycle models as well as those of ocean general circulation models.
Light nonmethane hydrocarbons in seawater
Authors: Plass‐Dülmer C. , Koppmann R. , Ratte M. , Rudolph J.
Published: February 2004

A database of dissolved C2‐C4 hydrocarbons in the surface water of the oceans is compiled based on more than 1000 measurements. Hydrocarbon emission rates are calculated using a diffusive microlayer approach and climatologic wind data. This database is used to calculate averages and ranges of variation, and an attempt is made to identify the environmental factors which have an impact on the hydrocarbons dissolved in seawater. The paper focuses on data obtained in situ since other techniques generally contain larger uncertainties. Mean concentrations are 134 pmol/L for ethene, 59 pmol/L for propene, and 37 pmol/L for 1‐butene. Alkane concentrations are lower with an average value of 22 pmol/L for ethane and less than 14 pmol/L for the other alkanes and acetylene. Ninety percent of the concentrations of an individual compound generally ranges within an order of magnitude. Ethene concentrations are significantly anticorrelated with the transfer velocities of the sea‐air exchange (r=−0.49; r0.01=0.29). Ethene concentrations are not correlated with the solar radiation, chlorophyll a, and the water temperature. Averaged emissions of C2‐C4 hydrocarbons extrapolated to the global ocean of 2.1 × 1012 g/yr are calculated, with ethene alone contributing about 40% to the total. Thus the oceanic source is on the low side of previous estimates and plays a minor role in global budgets compared to continental sources.
Factors controlling atmospheric methane consumption by temperate forest soils
Authors: Castro Mark S. , Steudler Paul A. , Melillo Jerry M. , Aber John D. , Bowden Richard D.
Published: February 2004

Over the past 6 years (1988–1993), we have examined the effects of soil temperature, soil moisture, site fertility, and nitrogen fertilization on the consumption of atmospheric CH4 by temperate forest soils located at the Harvard Forest in Petersham, Massachusetts. We found that soil temperature is an important controller of CH4 consumption at temperatures between −5° and 10°C but had no effect on CH4 consumption at temperatures between 10° and 20°C. Soil moisture exerts strong control on CH4 consumption over a range of 60 to 100% water‐filled pore space (% WFPS). As moisture increased from 60 to 100% WFPS, CH4 consumption decreased from 0.1 to 0 mg CH4‐C m−2 h−1 because of gas transport limitations. At 20 to 60% WFPS, site fertility was a strong controller of CH4 consumption. High‐fertility sites had 2 to 3 times greater CH4 consumption rates than low‐fertility sites. Nitrogen‐fertilized soils (50 and 150 kg NH4NO3‐N ha−1 yr−1 ) had annually averaged CH4 consumption rates that were 15 to 64% lower than annually averaged CH4 consumption by control soils. The decrease in CH4 consumption was related to both the years of application and quantity of nitrogen fertilizer added to these soils.
Modeling the inorganic phosphorus cycle of the North Pacific using an adjoint data assimilation model to assess the role of dissolved organic phosphorus
Authors: Matear Richard J. , Holloway Greg
Published: February 2004

An adjoint model was developed for the conservation of a tracer and used to assimilate phosphate observations from the North Pacific. The adjoint model estimated optimal values for new production, remineralization length scale of particulate organic matter (POM), phosphate field, and circulation field. Allowing no modifications to the circulation field computed by the Hamburg large‐scale geostrophic (LSG) model, we could not produce optimal estimates of new production and phosphate concentrations that were consistent with the observations. However, by allowing modifications to the LSG model's circulation field the adjoint model demonstrated that a model with only POM transport of organic matter produced results which were consistent with observations of new production and phosphate concentrations. In this model only small modifications to the circulation field were required to produce consistent estimates of new production and phosphate concentrations. This showed that the modeled phosphate field and new production were sensitive to small changes in the circulation. Furthermore, the data assimilation model implied that dissolved organic phosphorus did not necessarily play an important role in the cycling of phosphate in the North Pacific.
Mixing the Earth's mantle by thermal convection: A scale dependent phenomenon
Authors: Schmalzl Jörg , Hansen Ulrich
Published: February 2004

We have investigated the mixing properties of 2D Rayleigh Bénard convection in infinite Prandtl number flows. The mixing properties were monitored by injecting tracer particles into previously calculated flow fields. A particle correlation function H(r) and the corresponding correlation dimension have been used to characterize the mixing efficiency of the flow as function of its vigor and it's structure. We demonstrate that the chosen method captures the mixing process in a detailed manner. Our study suggests that mixing properties of the flow depend on the spatial scale. At Rayleighnumbers 106 < Ra < 108 heterogeneities within one circulation cell are destroyed rapidly while two adjacent cells can remain unmixed substantially longer.
The potential role of the ocean in regulating atmospheric CH3Br
Authors: Butler James H.
Published: February 2004

An increase or decrease in anthropogenic emissions of methyl bromide (CH3Br) will induce a compensating flux of this gas from the ocean to the atmosphere. This, in turn, will tend to lessen the expected change in atmospheric mixing ratio from that calculated in models that incorporate a constant oceanic source. To a first degree, this buffering effect by the ocean is independent of in situ oceanic production and land based sources. The partial lifetime of atmospheric CH3Br with respect to the ocean could reasonably range from 1.3 to 14 y, with a current best estimate of 3.7 y. This yields an effective atmospheric lifetime, which incorporates oceanic losses and thus can be used in simple, mass balance calculations of atmospheric CH3Br, of 1.2 y, with a probable range of 0.7 to 1.8 y.
Asymmetric spectral broadening of modulated electrostatic ion‐cyclotron waves
Authors: Koepke M. E. , Alport M. J. , Sheridan T. E. , Amatucci W. E. , Carroll J. J.
Published: February 2004

Modulated, current‐driven, electrostatic ion‐cyclotron (CDEIC) waves are shown to exhibit amplitude and frequency modulation, spectral broadening, and time‐averaged frequency pulling. The observed spectral broadening is asymmetric and sensitively dependent on the driving frequency. Qualitative features of the experimental data are reproduced by the forced van der Pol equation and are explainable using processes associated with driven self oscillations. These results may be relevant to ionospheric modification experiments involving the controlled modulation of the natural electrojet.
Global increase of SF6 observed in the atmosphere
Authors: Maiss Manfred , Levin Ingeborg
Published: February 2004

High precision long‐term observations of the trace gas sulphur hexafluoride (SF6) in background air at Neumayer station, Antarctica (1986–1991), and at Izaña observatory, Tenerife (1991–1992), are presented. Since the very first measurements in 1970 (0.03 pptv), the purely anthropogenic greenhouse gas SF6 has increased by two orders of magnitude to a global mean value of 2.8 pptv in 1992. The observations can best be fitted by a quadratic curve with a recent increase rate of 8.3% yr−1. A significant north‐south gradient of 0.29 pptv is observed. From this gradient an interhemispheric exchange time of 1.4 years is derived. A modeled atmospheric budget history agrees reasonably well with estimates of global SF6 production rates and leads to an extrapolated SF6 concentration of about 20 pptv for the year 2030.
A 27‐month periodicity in the low latitude geomagnetic field and its connection to the stratospheric QBO
Authors: Olsen Nils
Published: February 2004

Geomagnetic daily variations are produced at a height of about 100 to 130 km due to dynamo action of thermospheric winds in combination with the ionospheric conductivity. They are influenced by longterm variability of thermospheric dynamics and are used as tool for investigating a QBO in thermospheric winds.

Power spectrum analysis has been applied to monthly means of the daily range of the geomagnetic field. In addition to an annual modulation (and subharmonics) there is clear evidence for a sharp peak at about 27 months in the H‐component at low‐latitude observatories (ΔH ≈ 0.5 nT). At single observatories, statistical significance is at the 99% confidence level, squared coherency between distant observatories is highly significant, too. However, there is no global modulation of the Sq current system because (1) the D‐component contain no 27‐month periodicity, and (2) the oscillation is not present at mid‐latitude observatories.

In order to explain the observations, a 27‐month oscillation in the low‐latitude prevailing zonal winds with an amplitude of few m/s at a height of about 100‐130 km is postulated. Squared Coherency between geomagnetic variations and prevailing winds in the equatorial stratosphere is significant at the 95%‐level.
Multi‐wavelength profiles of aerosol backscatter over Lauder, New Zealand, 24 November 1992
Authors: McKenzie R. L. , Rosen J. M. , Kjome N. T. , McGee T. J. , Gross M. R. , Singh U. N. , Ferrare R. F. , Kimvilakani P. , Uchino O. , Nagai T.
Published: February 2004

Simultaneous profiles of aerosol backscatter ratio were measured over Lauder, New Zealand (45°S, 170°E) on the night of November 24, 1992. Instrumentation comprised two complementary lidar systems and a backscattersonde, to give measurements at wavelengths 351, 490, 532 and 940 nm. The data from the lidars and the backscattersonde were self‐consistent, enabling the wavelength dependence of aerosol backscatter to be determined as a function of altitude. This wavelength‐dependence is a useful parameter in radiative transfer calculations. In the stratosphere, the average wavelength exponent between 351 and 940 nm was −1.23±0.1, which was in good agreement with values derived from measured physical properties of aerosols.
Three‐dimensional numerical simulation of current collection by a probe in a magnetized plasma
Authors: Singh Nagendra , Vashi B. I. , Leung L. C.
Published: February 2004

A three‐dimensional numerical model for current collection in a magnetized plasma is reported. The model is based on an electrostatic particle‐in‐cell code. The model yields self‐consistent sheath structure including distributions of plasma and the electric potential around the body and the collection of electrons. The analytical theory of current collection by a body in a magnetized plasma yields an upper bound for the collected current determined by the conservation of energy and canonical angular momentum. The theory shows that the collected charged particles come from a cylindrical volume aligned with the magnetic shadow of the body; the maximum radius ro of this volume is determined by the body size, body potential, and the ambient magnetic field. This theory does not deal with the sheath structure around the body. The condition for the actual current to approach the upper‐bound value has been a matter of debate. Our simulations reveal when and why the collected current becomes equal to its upper‐bound value. Sheath size in the radial direction perpendicular to the axial ambient magnetic field is determined by the current‐limiting radius ro. Our simulation yields time‐average current in good agreement with its upper bound. This feature of the current collection is explained as follows: Once electrons enter the sheath, some of them are freely accelerated perpendicular to the magnetic field because they are demagnetized by the large gradients in the perpendicular electric fields. Simulations show a large perpendicular acceleration, producing perpendicular energy as large as that determined by the potential on the body, especially in the region where perpendicular electric fields (E⊥) are the strongest. An analysis shows that the demagnetization of electrons occurs above a threshold potential on the body. This threshold condition follows from the breakdown of the adiabaticity of the electron dynamics inside the sheath.
Small‐scale plasma irregularities produced during electron attachment chemical releases
Authors: Scales W. A. , Bernhardt P. A. , Ganguli G. , Siefring C. L. , Rodriguez P.
Published: February 2004

In situ measurements of small‐scale plasma density irregularities made during sounding rocket experiments that released electron attachment materials into the ionosphere are presented. A 2D electrostatic simulation model that includes attachment chemistry is used to study the source and evolution of these irregularities. The simulation shows (1) that large electron flow velocity shears develop on the boundary of the electron depletion and (2) these shears drive a plasma instability that is the likely source of the irregularities.
Problems of UV‐B radiation measurements in biological research. Critical remarks on current techniques and suggestions for improvements
Authors: Tüg Helmut , Baumann Marcus E. M.
Published: February 2004

Problems concerning solar UV‐B measurements for biological research are described from the physical point of view. It is shown that current instrumentation and the observing techniques need to be improved. For reliable measurements the spectrophotometric device must be equipped with a multichannel detector system counting single photon events. This technique allows accurate measurements at solar light levels below 300 nm and the use of the sun for instrumental calibrations.
Large amplitude electric and magnetic field signatures in the inner magnetosphere during injection of 15 MeV electron drift echoes
Authors: Wygant J. , Mozer F. , Temerin M. , Blake J. , Maynard N. , Singer H. , Smiddy M.
Published: February 2004

Electric and magnetic fields were measured by the CRRES spacecraft at an L‐value of 2.2 to 2.6 near 0300 magnetic local time during a strong storm sudden commencement (SSC) on March 24, 1991. The electric field signature at the spacecraft at the time of the SSC was characterized by a large amplitude oscillation (80 mV/m peak to peak) with a period corresponding to the 150 second drift echo period of the simultaneously observed 15 MeV electrons. Considerations of previous statistical studies of the magnitude of SSC electric and magnetic fields versus local time and analysis of the energization and cross‐L transport of the particles imply the existence of 200 to 300 mV/m electric fields over much of the dayside magnetosphere. These observations also suggest that the 15 MeV drift echo electrons were selectively energized because their gradient drift velocity allowed them to reside in the region of strong electric fields for the duration of the accelerating phase of the electric field.
A ferroelectric model for the low emissivity highlands on Venus
Authors: Shepard Michael K. , Arvidson Raymond E. , Brackett Robert A. , Fegley Bruce
Published: February 2004

A model to explain the low emissivity venusian highlands is proposed utilizing the temperature‐dependent dielectric constant of ferroelectric minerals. Ferroelectric minerals are known to occur in alkaline and carbonatite rocks, both of which are plausible for Venus. Ferroelectric minerals possess extremely high dielectric constants (105) over small temperature intervals and are only required in minor (≪1%) abundances to explain the observed emissivities. The ferroelectric model can account for: (1) the observed reduction in emissivity with increased altitude, (2) the abrupt return to normal emissivities at highest elevations, and (3) the variations in the critical elevation observed from region to region.
Estimation of oceanic hydrothermal heat flux from heat flow and depths of midocean ridge seismicity and magma chambers
Authors: Pelayo Aristeo M. , Stein Seth , Stein Carol A.
Published: February 2004

The difference between the heat flow predicted by thermal models of the lithosphere and that measured at the seafloor can be used to estimate the cooling by hydrothermal circulation. However, this approach may yield an overestimate because measurements in thinly sedimented young crust are typically made at topographic lows where hydrothermal circulation may systematically lower heat flow. To circumvent this bias we estimate the cooling using the depths of midocean ridge earthquakes and magma chambers in addition to heat flow data. The results indicate deeper hydrothermal circulation at slow spreading ridges, and higher near‐axial hydrothermal heat loss at fast spreading ridges. The predicted global hydrothermal heat flux is ∼80% of that for models constrained only by heat flow.
Subdaily Earth rotation during the Epoch ‘92 campaign
Authors: Freedman A. P. , Ibañez‐Meier R. , Herring T. A. , Lichten S. M. , Dickey J. O.
Published: February 2004

Global Positioning System (GPS) data were used to estimate Earth rotation variations over an 11‐day period during the Epoch ‘92 campaign in the summer of 1992. Earth orientation was measured simultaneously by several very long baseline interferometry (VLBI) networks. GPS and VLBI estimates of UT1 with 3‐hour time resolution were then compared and analyzed. The high frequency behavior of both data sets is similar, although drifts between the two series of ∼0.1 ms over 2‐5 days are evident. The geodetic results were also compared with models for UT1 fluctuations at tidal periods and with estimates of atmospheric angular momentum made at 6‐hour intervals. Most of the geodetic signal in the diurnal and semidiurnal frequency bands can be attributed to tidal processes, whereas UT1 variations over a few days are mostly atmospheric in origin.
The geological origin of long wavelength lateral heterogeneity at depths of 300–400km
Authors: Hara Tatsuhiko , Geller Robert J.
Published: February 2004

On the basis of two three‐dimensional (3‐D) models of S‐wave velocity, PUMO [Hara et at., 1993] and SH425.2 [Su and Dziewonski, 1991], we show that the deep roots of cratons and mid‐ocean ridges provide a satisfactory explanation of the pattern and amplitude of the Earth's long wavelength lateral heterogeneity in the depth range from 300 to 400km.
Electromagnetic waves with frequencies near the local proton gyrofrequency: ISEE‐3 1 AU observations
Authors: Tsurutani Bruce T. , Arballo John K. , Mok John , Smith Edward J. , Mason Glenn M. , Tan Lun C.
Published: February 2004

Low Frequency (LF) electromagnetic waves with periods near the local proton gyrofrequency have been detected in interplanetary space by the magnetometer onboard ISEE‐3. Transverse peak‐to‐peak amplitudes as large as
have been noted with compressional components (Δ |B|/ |B|) typically ≤ 0.1. Generally, the waves have even smaller amplitudes, or are not detectable within the solar wind turbulence. The waves are elliptically/linearly polarized and are often, but not always, found to propagate nearly along

. Both right‐ and left‐hand polarizations in the spacecraft‐frame have been detected. The waves are observed during all orientations of the interplanetary magnetic field, with the Parker spiral orientation being the most common case. Because the waves are detected at and near the local proton cyclotron frequency, the generation mechanism must almost certainly be solar wind pickup of freshly created hydrogen ions. Possible sources for the hydrogen are the Earth's atmosphere, coronal mass ejections from the Sun, comets and interstellar neutral atoms. At this time it is not obvious which potential source is the correct one. Statistical tests employing over one year of ISEE‐3 data will be done in the near future to eliminate/confirm some of these possibilities.
Asymmetric diurnal temperature change in the Alpine Region
Authors: Weber Rudolf O. , Talkner Peter , Stefanicki Gérard
Published: February 2004

By now there is general agreement that the annual mean temperature of earth's surface has increased during the last century. Recently, it has become obvious that this warming is quite inhomogeneous in various respects. Besides the spatial and seasonal variability of the temperature trend a diurnal asymmetry of increase has been observed. In large continental regions the annual mean of the daily minimum temperature has increased noticeably faster than the annual mean of the daily maximum. The same behaviour is found in the present study for low‐lying stations in Central Europe. However, data from mountain top stations show a similar increase for both minimum and maximum of daily temperatures. No diurnal asymmetry was observed for these stations. The good agreement of the time series from different mountain stations leads us to believe that the observed trends of minimum and maximum temperature are not caused by particular local influences or observation errors. An analysis of monthly and seasonal means shows that most of the warming took place in fall.
Relationship between ozone and temperature trends in the lower stratosphere: Latitude and seasonal dependences
Authors: McCormack John P. , Hood Lon L.
Published: February 2004

A one‐dimensional radiative transfer model with fixed dynamical heating is used to calculate the approximate latitude and seasonal dependences of lower stratospheric temperature changes associated with observed ozone trends. The spatial and temporal distribution of ozone profile trends in the lower stratosphere is estimated from a combination of Nimbus 7 Solar Backscattered Ultraviolet (SBUV) global measurements of the ozone column below 32 mbar for the period 1979–1990 and balloon ozonesonde profile trends at northern middle latitudes. The calculated temperature trends near 100 mbar compare favorably with those recently derived by Randel and Cobb [1994] using data from Channel 4 of the Microwave Sounding Unit (MSU) on the NOAA operational satellites, although a number of quantitative differences are found. An independent analysis reported here of 100 mbar temperatures derived from northern hemisphere radiosonde data at the Free University of Berlin supports the validity of the satellite‐derived lower stratospheric temperature trends. These results are therefore generally consistent with the hypothesis that observed lower stratospheric cooling trends are predominantly determined by reductions in radiative heating associated with stratospheric ozone depletion.
Lifetimes and eigenstates in atmospheric chemistry
Authors: Prather Michael J.
Published: February 2004

The time scales and mode of the atmosphere's response to chemical perturbations are defined by the eigenvalues and eigenvectors of the system. The eigenstates of a simplified one‐box CH4‐CO‐OH system are analyzed. The longest time constant (smallest eigenvalue) always exceeds the lifetime defined by the steady‐state loss frequency for CH4, the longest lived gas. Thus, the extent of a CH4 perturbation—the methane response time—is always longer than predicted by the steady‐state lifetime and is independent of size of the perturbation in the linear limit. This lengthening of the atmospheric recovery time can be diagnosed by how close we are to a chemically unstable troposphere, i.e., how much OH production exceeds that minimum needed to oxidize just the global emissions of CH4, CO, and other hydrocarbons and species.
The effect of the Mt. Pinatubo aerosol on the HNO3 column over Mauna Loa, Hawaii
Authors: David Shelle J. , Murcray Frank J. , Goldman Aaron , Rinsland Curtis P. , Murcray David G.
Published: February 2004

A very high resolution infrared solar spectrometer system for the Network for Detection of Stratospheric Change has been routinely operated at the Mauna Loa Observatory in Hawaii since November, 1991. Solar absorption spectra are normally taken one day a week at sunrise. We report the analysis of these spectra for the vertical column amount of nitric acid (HNO3) vapor. The observations began four months after the arrival of volcanic aerosols from Mt. Pinatubo over the site. Although quite variable, total HNO3 columns were initially about 7×1015 molecules/cm² and have decreased about 30% in the following two years.