bluuebunny/crossref_metadata_2025 huggingface dataset:
Pore Scale Variation in Retardation Factor as a Cause of Nonideal Reactive Breakthrough Curves: 3. Column Investigations
Authors: Sugita Fumi , Gillham Robert W.
Published: February 2004

A series of laboratory column experiments were conducted in a glass bead medium with 36Cl and 85Sr as nonreactive and reactive tracers, respectively. The primary purpose was to compare breakthrough curve (BTC) properties under various conditions with those of the model which accounts for pore scale variation in the retardation factor (R) caused by pore scale heterogeneity. In particular, the purpose was to determine whether pore scale heterogeneity contributes to the nonideality of reactive solute BTCs that cannot be described by the conventional local equilibrium assumption (LEA)‐based advection‐dispersion equation (ADE) for a homogeneous medium. To eliminate other sources of nonideality, it was verified that the LEA was valid for the solute and porous medium used in the study. The following BTC properties were observed: (1) nonreactive BTCs can be described by the ADE (ideal), whereas reactive BTCs are nonideal, (2) materials with larger pore size variation gave greater nonideality in the reactive BTC, (3) although nonreactive BTCs showed equally symmetric ideal shapes regardless of distance, reactive BTCs showed slight travel distance dependence in the degree of nonideality, and (4) the position and shape of dimensionless BTCs are independent of mean fluid velocity. Calculated stochastic dispersivities which accounted for measured pore size variations were significantly larger than those for a nonreactive solute but were smaller than the effective dispersivities actually observed for the reactive BTCs. From these findings, it is concluded that pore scale variation in R caused by pore scale heterogeneity in the medium is at least partially responsible for the nonideality observed for reactive solute transport in homogeneous media. Pore scale variation in R gives larger asymptotic dispersivity and thus longer preasymptotic time and distance for reactive solute transport.
Spatial Pattern of Flow Competence and Bed Load Transport in a Divided Gravel Bed River
Authors: Powell D. Mark , Ashworth Philip J.
Published: February 2004

A 14‐month continuous record of the spatial variation in the incidence and magnitude of bed load transport was provided by nine pit traps sunk into the bed of a 21‐m‐wide upland gravel bed river. The bed load data are related to a simultaneous record of average bed shear stress derived from the depth‐slope product. The study reach is straight and includes a small, low‐relief mid‐channel bar which represents the divide between two zones of markedly different bed structure and stability. he bed of the left channel and bar tail is loose with an open framework, while that of the bar head and right channel is tightly interlocking and well imbricated. Bed surface grain size is similar throughout the reach. The two zones of contrasting bed structure have a strong influence on the threshold of sediment movement (τc) and consequently the spatial pattern of bed load transport. Entrainment thresholds for the tightly structured bed are up to seven times higher than those for the loosely structured bed. Sediment transport is therefore most frequent in the left channel and over the bar tail, where flows with excess shear stresses (τ > τc) peaking at 8τc constitute 19% of the flow record. This is in marked contrast to the right channel and bar head, where excess shear stresses of maximum 1.4τc total only 1.3% of the flow record. However, despite the marked difference in the magnitude and duration of competent flows between the two contrasting bed areas, little difference in bed load yield is observed between the nine traps when averaged over the 14‐month study period. This study shows that different zones of bed structure and stability in a channel will control the incidence and spatial pattern of bed load transport but not necessarily the medium and long‐term bed load yield. This has important implications for local and short‐term bed load sampling programs and the numerical modeling of sediment budgets and zone‐to‐zone bed load transfer.
Hydroclimatology of Continental Watersheds: 2. Spatial Analyses
Authors: Cayan Daniel R. , Georgakakos Konstantine P.
Published: February 2004

We diagnose the spatial patterns and further examine temporal behavior of anomalous monthly‐seasonal precipitation, temperature, and atmospheric circulation in relationship to hydrologic (soil water and potential evapotranspiration) fluctuations at two watersheds in the central United States. The bulk hydrologic balance at each of the two watersheds, Boone River, Iowa (BN), and Bird Creek, Oklahoma (BC), was determined from the rainfall‐runoff‐routing watershed model described in part 1. There are many similarities among the hydroclimatic linkages at the two basins. In both, relationships with precipitation and temperature indicate that the forcing occurs on regional scales, much larger than the individual watersheds. Precipitation exhibits anomaly variability over 500‐km scales, and sometimes larger. Anomalous temperature, which is strongly correlated with potential evapotranspiration, often extends from the Great Plains to the Appalachian Mountains. Seasonally, the temperature and precipitation anomalies tend to have greatest spatial coherence in fall and least in summer. The temperature and precipitation tend to have out‐of‐phase anomalies (e.g., warm associated with dry). Thus low soil water conditions are einforced by low precipitation and high potential evapotranspiration, and vice versa for high soil water. Soil water anomalies in each basin accumulate over a history of significant large‐scale climate forcing that usually appears one or two seasons in advance. These forcing fields are produced by atmospheric circulation anomaly patterns that often take on hemispheric scales. BN and BC have strong similarities in their monthly circulation patterns producing heavy/light monthly precipitation episodes, the primary means of forcing of the watersheds. The patterns exhibit regional high or low geopotential anomalies just upstream over the western United States or near the center of the country. The regional circulation features are often part of a train, with teleconnections upstream over the North Pacific and downstream over the North Atlantic/Eurasia sector. Synoptic scale events exhibit very similar patterns to the monthly circulations, only more intense.
Deducing the Distribution of Terminal Electron‐Accepting Processes in Hydrologically Diverse Groundwater Systems
Authors: Chapelle Francis H. , McMahon Peter B. , Dubrovsky Neil M. , Fujii Roger F. , Oaksford Edward T. , Vroblesky Don A.
Published: February 2004

The distribution of microbially mediated terminal electron‐accepting processes (TEAPs( was investigated in four hydrologically diverse groundwater systems by considering patterns of electron acceptor (nitrate, sulfate) consumption, intermediate product (hydrogen (H2)) concentrations, and final product (ferrous iron, sulfide, and methane) production. In each hydrologic system a determination of predominant TEAPs could be arrived at, but the level of confidence appropriate for each determination differed. In a portion of the lacustrine aquifer of the San Joaquin Valley, for example, all three indicators (sulfate concentrations decreasing, H2 concentrations in the 1–2 nmol range, and sulfide concentrations increasing along flow paths identified sulfate reduction as the predominant TEAP, leading to a high degree of confidence in the determination. In portions of the Floridan aquifer and a petroleum hydrocarbon‐contaminated aquifer, sulfate reduction and methanogenesis are indicated by production of sulfide and methane, and hydrogen oncentrations in the 1–4 nmol and 5–14 nmol range, respectively. However, because electron acceptor consumption could not be documented in these systems, less confidence is warranted in the TEAP determination. In the Black Creek aquifer, no pattern of sulfate consumption and sulfide production were observed, but H2 concentrations indicated sulfate reduction as the predominant TEAP. In this case, where just a single line of evidence is available, the least confidence in the TEAP diagnosis is justified. Because this methodology is based on measurable water chemistry parameters and upon the physiology of microbial electron transfer processes, it provides a better description of predominant redox processes in groundwater systems than more traditional Eh‐based methods.
Chlorofluorocarbons as Tracers of Groundwater Transport Processes in a Shallow, Silty Sand Aquifer
Authors: Cook P. G. , Solomon D. K. , Plummer L. N. , Busenberg E. , Schiff S. L.
Published: February 2004

Detailed depth profiles of Chlorofluorocarbons CFC‐11(CFCl3(, CFC‐12 (CF2Cl2) and CFC‐113 (C2F3Cl3) have been obtained from a well‐characterized field site in central Ontario. Aquifer materials comprise predominantly silty sands, with a mean organic carbon content of 0.03%. Nearly one‐dimensional flow exists at this site, and the vertical migration of a well‐defined 3H peak has been tracked through time. Detailed vertical sampling has allowed CFC tracer velocities to be estimated to within 10%. Comparison with 3H profiles enables estimation of chlorofluorocarbon transport parameters. CFC‐12 appears to be the most conservative of the CFCs measured. Sorption at this site is low (Kd < 0.03), and degradation does not appear to be important. CFC‐ 113 is retarded both with respect to CFC‐12 and with respect to 3H (Kd = 0.09−0.14). CFC‐11 appears to be degraded both in the highly organic unsaturated zone and below 3.5 m depth in the aquifer, where dissolved oxygen concentrations decrease to below 0.5 mg L−1. The half‐life for CFC‐11 degradation below 3.5 m depth is less than 2 years. While apparent CFC‐12 ages match hydraulic ages to within 20% (up to 30 years), apparent CFC‐11 and CFC‐113 ages significantly overestimate hydraulic ages at our field site.
Pore‐Scale Modeling of Nonwetting‐Phase Residual in Porous Media
Authors: Lowry Michael I. , Miller Cass T.
Published: February 2004

The complexity of the formation and removal of nonwetting‐phase residual in porous media provides a significant challenge to the development of theories needed to advance more effective aquifer remediation schemes. Constitutive theories are required to overcome theoretical deficiencies in describing behaviors such as the formation, dissolution, and mobilization of residual. Current macroscopic approaches are often unable to capture adequately the complexity of such processes; in these cases, pore‐scale research should provide the necessary conceptual and quantitative basis for constitutive theory development. This work contributes to these efforts by investigating factors affecting residual in strongly wet, capillary‐dominated systems. Porous media are modeled as three‐dimensional networks of pore bodies and throats. A random network approach is developed, which provides stochastic geometry and variable connectivity. Immiscible displacement occurs via piston‐displacement and film‐flow mechanisms. Residual ganglia become trapped during imbibition. Wetting‐phase continuity is maintained during drainage, disallowing the entrapment of wetting‐phase ganglia. Results show that pore‐ scale geometric parameters significantly affect residual saturation, the ganglia‐volume distribution, and nonwetting‐wetting‐phase interfacial area. Thus constitutive theory of residual should incorporate pore‐scale knowledge to be valid for a wide range of conditions and porous media types. Comparison with quantitative experimental ganglia‐ volume distributions yields good agreement with the data trends but poor quantitative agreement. These results indicate the need for further experimental comparisons with theoretical results, linking macroscale and microscale theory and experiment.
A Noniterative Technique for the Direct Implementation of Well Bore Boundary Conditions in Three‐Dimensional Heterogeneous Formations
Authors: Sudicky E. A. , Unger A. J. A. , Lacombe S.
Published: February 2004

A noniterative algorithm for handling prescribed well bore boundary conditions while pumping or injecting fluid in a three‐dimensional heterogeneous aquifer is described. The algorithm is formulated by superimposing conductive one‐dimensional line elements representing the well screen onto the three‐dimensional matrix elements epresenting the aquifer. Storage in the well casing is also naturally accommodated by the superposition of the line elements. The numerical algorithm is verified by comparison with results obtained from the solution of Papadopulos and Cooper (1967). A large‐scale example problem involving groundwater extraction from a partially penetrating pumping well located in a highly heterogeneous confined aquifer is presented to demonstrate the utility of the approach.
Distribution Flow: A General Process in the Top Layer of Water Repellent Soils
Authors: Ritsema Coen J. , Dekker Louis W.
Published: February 2004

Water and solute input rates on the soil surface may vary considerably from place to place. Distribution flow, i.e., the process of water and solute flow in a lateral direction over and through the very first millimeter or centimeter of the soil profile, is an extremely important process in distributing the rainfall toward places where vertical transport occurs. This study was carried out to quantify the process of distribution flow and its underlying process mechanism. A KBr tracer was applied on two water repellent sandy soils to follow the actual flow paths of water and solutes in the upper part of the profile. On both experimental fields, distribution flow actually displaced the applied bromide laterally through a very thin layer on the top of the soil profile, referred to here as the “distribution layer.” Distribution flow was directed to locations within the 0‐ to 2.5‐cm layer, where the soil was the least water repellent. On these relatively wet areas, the highest concentrations of bromide could be found. There was an acceleration in the vertical transport of water and bromide on these sites. Effects of distribution flow on the local concentration of water and solutes can be expected to be even more pronounced in uneven terrains, where lateral displacements may increase from the millimeter‐centimeter scale to tens of meters. Implications for modeling field‐scale water and solute flow are outlined.
Generalizations of Chain‐Dependent Processes: Application to Hourly Precipitation
Authors: Katz Richard W. , Parlange Marc B.
Published: February 2004

Stochastic models are fitted to time series of hourly precipitation amounts. These models are extensions of a form of chain‐dependent process commonly fit to daily precipitation amounts. The extensions involve allowing hourly intensities to be autocorrelated and allowing the model parameters to possess diurnal cycles. These models are applied to two quite different sets of hourly precipitation data: July at Denver, Colorado, for which diurnal cycles are substantial; and January at Chico, California, for which a relatively high degree of persistence is present. The temporal aggregation properties of the hourly models (e.g., for 12‐hour or daily total precipitation) are examined, and the role of the extensions in improving these properties is quantified. On this basis, it is argued that generalizations of chain‐dependent processes could be competitive with, if not superior to, so‐called conceptual models of the precipitation process.
Three‐dimensional model calculation of ocean uptake of bomb 14C and implications for the global budget of bomb 14C
Authors: Duffy Philip B. , Caldeira Ken
Published: February 2004

Two recent papers have suggested that there is an apparent imbalance in the global bomb 14C budget. These results are based in part on calculations made using simple one‐dimensional ocean models. We recalculate the ocean inventory of bomb 14C using a three‐dimensional ocean general circulation model, and find a significantly smaller inventory after about 1980 than in previous one‐dimensional model estimates. Our new estimate of the ocean inventory implies that the apparent imbalance in the global budget of bomb 14C is smaller than previously thought and is not significantly different from zero. In addition, we use our three‐dimensional model to obtain whole ocean inventories of bomb 14C during the Geochemical Ocean Sections Study (GEOSECS) period; these inventories can be used as benchmarks in calibrating one‐dimensional ocean models.
Latitudinal and seasonal variations in carbon dioxide and oxygen in the northeast Atlantic and the effects on Emiliania huxleyi and other phytoplankton
Authors: Tyrrell Toby , Taylor Arnold H.
Published: February 2004

Several cruise programs, such as Transient Tracers in the Ocean (TTO), the Biogeochemical Ocean Flux Study (BOFS), and the Joint Global Ocean Flux Study (JGOFS), have measured physical, chemical, and biological variables in the northeast Atlantic, with the aim of understanding the seasonal variation in oceanic components such as carbon dioxide and oxygen and the role of different factors (e.g., the marine biota) in determining these seasonal variations. For this paper, data from the different cruise programs have been collated, and then three‐dimensional (3‐D) interpolated surfaces have been plotted in order to illustrate the seasonal and latitudinal changes in these different components. These data‐generated plots are then compared with plots generated from the results of the simulation model of Taylor et al. (1991). The model‐derived and data‐derived plots are shown to be similar, and the model is subsequently used to explain the interaction of the processes underlying the observed variation in pCO2. It is argued that more data needs to be collected at the time of the spring bloom north of 50°N. The plot of carbon dioxide concentration is discussed in relation to the effects of carbon dioxide concentration on phytoplankton growth, as proposed by Riebesell et al.(1993). In addition, it is shown that the distribution of blooms of Emiliania huxleyi in the NE Atlantic (55°–63°N but not farther south) is not coincident with areas of low [CO2], contrary to the hypothesis that Emiliania huxleyi has evolved to be a successful competitor at low [C02].
Production of the greenhouse gases CH4 and CO2 by hydroelectric reservoirs of the boreal region
Authors: Duchemin E. , Lucotte M. , Canuel R. , Chamberland A.
Published: February 2004

The emission fluxes and the distribution of dissolved methane (CH4) and carbon dioxide (CO2) were determined for 11 sampling stations in two hydroelectric reservoirs (flooded since 1978 and 1993) located in the James Bay territory of northern Québec. The measured benthic fluxes for the two greenhouse gases were found to be either higher or similar to those determined at the water‐air interface during the ice‐free sampling periods. For the 2 year duration of the study, emission fluxes of CH4 to the atmosphere generally varied between 5 and 10 mg m−2 d−1, while those for CO2 ranged from 500 to 1100 mg m−2 d−1. Furthermore, through the use of static chambers at the water‐air interface, we determined that the emission fluxes for the gases are controlled by molecular diffusion. Our calculated fluxes have been separated into two groups: (1) regular emission fluxes and (2) above‐average emission fluxes. The first type comprises the majority of fluxes measured during the sampling periods (i.e., 88% for CH4 and 87% for CO2). The second group reflects unusual sampling conditions (e.g., strong winds, water column depths of less than 1 m, or flooded peatland mats floating at the surface). Although data for this group are limited, our preliminary results suggest that they may be an important component in an atmospheric emissions budget for large reservoirs. Concentration profiles for CH4 and CO2 dissolved in the water column clearly show that oxidation and/or horizontal advection of these gases are controlling factors in their subsequent release to the atmosphere. Most of the CH4 is oxidized within the first 25 cm above the flooded soil‐water interface. Consequently, neither benthic emissions of CH4 and CO2 nor the type of flooded soil appear to control atmospheric emissions of these gases from hydroelectric reservoirs.
A global land primary productivity and phytogeography model
Authors: Woodward F. Ian , Smith Thomas M. , Emanuel William R.
Published: February 2004

A global primary productivity and phytogeography model is described. The model represents the biochemical processes of photosynthesis and the dependence of gas exchange on stomatal conductance, which in turn depends on temperature and soil moisture. Canopy conductance controls soil water loss by evapotranspiration. The assignment of nitrogen uptake to leaf layers is proportional to irradiance, and respiration and maximum assimilation rates depend on nitrogen uptake and temperature. Total nitrogen uptake is derived from soil carbon and nitrogen and depends on temperature. The long‐term average annual carbon and hydrological budgets dictate canopy leaf area. Although observations constrain soil carbon and nitrogen, the distribution of vegetation types is not specified by an underlying map. Variables simulated by the model are compared to experimental results. These comparisons extend from biochemical processes to the whole canopy, and the comparisons are favorable for both current and elevated CO2atmospheres. The model is used to simulate the global distributions of leaf area index and annual net primary productivity. These distributions are sufficiently realistic to demonstrate that the model is useful for analyzing vegetation responses to global environmental change.
Release of entrapped methane from wetland rice fields upon soil drying
Authors: Denier van der Gon H. A. C. , van Breemen N. , Neue H.‐U. , Lantin R. S. , Aduna J. B. , Alberto M. C. R. , Wassmann R.
Published: February 2004

Methane emissions from Philippine rice paddies, fertilized with either urea or green manure, were monitored for several weeks after harvesting the dry and the wet season crops of 1992. The fields were still flooded during harvest but irrigation was stopped after harvest and the fields were allowed to evaporatively dry while CH4 emissions were monitored with a closed chamber technique. In all plots we observed a sudden, strong increase of CH4 emissions to the atmosphere for 2 to 4 days just after the soil fell dry. As soil drying continued, the soils began to crack and CH4 emissions decreased to nil. The release of CH4 during soil drying was observed for fields on three different soil types and both for urea or organically manured rice fields in both seasons. The absolute amounts of CH4 emitted during soil drying differed greatly depending on fertilizer treatment. However, the ratio between the amount of CH4 released upon soil drying and CH4 emitted during the growing season was quite constant (0.10 ±0.04). This suggests that about 10% of the CH4 emitted during a full rice crop cycle is released during drying of the fields and thus needs to be included in estimates of the total CH4 emission from rice agriculture.
Love numbers for a rotating spheroidal Earth∷ New definitions and numerical values
Authors: Mathews P. M. , Buffett B. A. , Shapiro I. I.
Published: February 2004

For an oceanless, elastic, spheroidal, rotating and nutating Earth, we present new definitions for the latitude‐dependent Love numbers and gravimetric constant which are free of certain drawbacks of the definitions currently in use, and make the computation of quantities of observational interest simpler and more direct. Numerical values, computed for an Earth model characterized by “modified PREM” parameters, are also presented. This model is designed to have the value estimated from VLBI data for the eigenfrequency of the Nearly Diurnal Free Wobble (NDFW), and assures thus an accurate representation of the NDFW resonance in the Earth tides. To achieve current and future accuracy goals, the Love number values used must fully correspond to the definitions employed—a requirement not consistently met in some existing programs for analysis of space‐geodetic data.
O3 → O(¹D) photolysis frequencies determined from spectroradiometric measurements of solar actinic UV‐radiation: Comparison with chemical actinometer measurements
Authors: Müller Martin , Kraus Alexander , Hofzumahaus Andreas
Published: February 2004

The solar actinic flux has been measured directly and spectrally resolved between 280 nm and 330 nm by means of a spectroradiometer. From these measurements the photolysis frequency J(O¹D) of ozone is calculated using ozone absorption cross‐sections and O(¹D) quantum yields from the literature. Different published wavelength dependencies of the O(¹D) quantum yield data are used to compare the spectroradiometric J(O¹D) values to J(O¹D) data measured simultaneously with a chemical actinometer. This field comparison supports laboratory studies that have measured O(¹D) quantum yields of about 0.2 to 0.3 between 315 nm and 320 nm. Implications for tropospheric photochemistry are discussed.
H+ ion acceleration in the mid‐altitude ionosphere
Authors: Dubouloz N. , Berthelier J.‐J. , Jasperse J. R.
Published: February 2004

H+ ion acceleration by electromagnetic ion cyclotron waves in the Earth's mid‐altitude (300 to 2000 km) ionosphere is investigated. Below about 800 km, the ionospheric density is sufficiently high to induce significant collision rates between thermal particles and accelerated H+ ions. The dominant collisions are associated with ion‐ion Coulomb interactions and with charge exchange reactions of the type H+ + A → H + A+. These reactions are able to substantially modify the space and velocity distribution of the accelerated H+ ions. Using Monte‐Carlo simulations, it is shown that the combination of collisions and ion cyclotron acceleration produces energetic H+ ion distributions whose characteristics (flux, average energy and pitch angle) are in quantitative agreement with satellite observations. The distributions of the energetic H atoms which are produced by charge exchange and which might be used for remote sensing of such H+ ion acceleration processes, are also presented.
3D modelling of magnetotelluric and gravity data of Mallorca Island (western Mediterranean)
Authors: Pous Jaume , Ayala Conxi , Ledo Juanjo , Marcuello Alex , Sàbat Francesc
Published: February 2004

A combined interpretation of magnetotelluric and gravity data recorded onland Mallorca is presented. The magnetotelluric data consist of five stations along a NW‐SE profile across the island which periods ranging from 256 Hz to 200 s. An electrical model has been obtained taking into account the 3D sea effect. The gravity data consist of the Bouguer anomaly from which the residual contribution of the sedimentary formations has been obtained. A 3D modelling of this residual anomaly shows a good agreement with the MT model. The results of the combined interpretation of the gravity and magnetotelluric data provide constraints to the crustal structure of the island which is representative of the whole Balearic promontory.
Evidence for enhanced 10Be deposition in Mediterranean sediments 35 kyr BP
Authors: Castagnoli G. Cini , Albrecht A. , Beer J. , Bonino G. , Shen Ch. , Callegari E. , Taricco C. , Dittrich‐Hannen B. , Kubik P. , Suter M. , Zhu G. M.
Published: February 2004

In this letter, we report the profiles (∼160 samples) of 10Be, 9Be, Zn in a Mediterranean sediment core spanning the last 60 kyr. We show the existence of a 10Be peak, whose absolute age is estimated to be 34±3 kyr BP, based on its stratigraphic position between two tephra layers originating from two volcanic eruptions (Campanian Ignimbrite and Citara), dated by K‐Ar, 40Ar‐39Ar and 14C methods. This peak is similar to the increase in the 10Be concentration observed in Vostok and Dome C ice cores at the same age. The increase with approximately similar amplitude in both ice and sediment cores at different latitudes and hemispheres cannot be explained by changes in the archives. The present result supports the view of a well defined global enhancement of the 10Be flux, related to an increase of the 10Be production rate and not to a redistribution of the 10Be fallout between different latitudes.
Low altitude signature of the plasma sheet boundary layer: Observations and model
Authors: Onsager T. G. , Mukai T.
Published: February 2004

Low‐altitude spacecraft on magnetotail field lines often detect a distinctive signature in the precipitating ion flux. A velocity‐dispersed ion structure is often observed near the poleward boundary of the auroral oval. At the low‐latitude edge of this structure an absence of precipitating ions is seen, previously referred to as “the gap,” separating the velocity‐dispersed ions at the higher latitudes from the more diffuse, plasma sheet‐like ions at lower latitudes. We present a model of low‐altitude particle precipitation that reproduces these observed features in the ion spectra and provides a quantitative estimate of the downtail plasma sheet properties. The model calculations are compared with observations from the Akebono spacecraft. In this model, the dispersed ion velocity signature maps to a region in the distant plasma sheet where the plasma has a field‐aligned bulk flow. The gap maps to a region in the distant magnetotail where the ion fluxes are below the detection threshold of the instrument, due to the low plasma sheet density and temperature in that region.
SH‐wave propagation in the whole mantle using high‐order finite differences
Authors: Igel Heiner , Weber Michael
Published: February 2004

Finite‐difference approximations to the wave equation in spherical coordinates are used to calculate synthetic seismograms for global Earth models. High‐order finite‐difference (FD) schemes were employed to obtain accurate waveforms and arrival times. Application to SH‐wave propagation in the mantle shows that multiple reflections from the core‐mantle boundary (CMB), with travel times of about one hour, can be modeled successfully. FD techniques, which are applicable in generally heterogeneous media, can be used to determine realistic global earth structure.
The retrieval of total optical depth and effective droplet radius of clouds from solar reflection measurements using the Along Track Scanning Radiometer‐2 (ATSR‐2)
Authors: Evans Simon J. , Haigh Joanna D.
Published: February 2004

A method is presented for the retrieval of liquid phase cloud microphysical properties; effective radius (reff) and total optical depth (τ) using measurements of reflected intensity in visible and near infrared bands produced by satellite radiometry. Previous techniques have been ambiguous for clouds made up of small water droplets i.e. reff ≤ 6µm. A method exploiting the dual look facility of the Along Track Scanning Radiometer ATSR‐2 has been used to remove this problem and determine a unique solution. Explicitly the 0.555µm and the 1.6 µm detectors of ATSR‐2 are used to retrieve reff (≥ 2µm) and τ (up to the semi‐infinite limit).
Atmospheric distributions of HCFC 141b
Authors: Schauffler S. M. , Pollock W. H. , Atlas E. L. , Heidt L. E. , Daniel J. S.
Published: February 2004

In recent years, HCFC 141b (CH3CCl2F) has become commercially available as a replacement for CFC 11 (CCl3F) in closed‐cell foam blowing applications and as a partial replacement for CFC 113 (CCl2FCClF2) in solvent and cleaning applications. Measurements of the atmospheric distribution and rate of growth of HCFC 141b reported here provide initial data to begin evaluation of the atmospheric fate of this replacement compound. HCFC 141b measurements in March 1992 and from various periods in 1993 indicate a strong interhemispheric gradient and rapid growth in tropospheric mixing ratios during this time. Average HCFC 141b mixing ratios in April 1993 in the northern and southern hemispheres were 0.83±0.23 pptv and 0.28±0.07 pptv, respectively. HCFC 141b mixing ratios increased in the mid‐latitude northern hemisphere from 0.26 pptv in late March 1992 to 1.41 pptv in late September 1993. Southern hemisphere mixing ratios increased from approximately 0.28 pptv in April 1993 to 0.45 pptv in October 1993. Emissions estimates, based on the observations, were calculated using a two box model and for 1990–1993 were 0.5, 2.5, 7, and 15 kT, respectively.
Contribution to the droplet effective radius parameterization of warm convective clouds: Clear air entrainment effects
Authors: Pontikis Constantin
Published: February 2004

The effects of clear air entrainment and mixing on the cloud droplet effective radius are investigated. Approximate expressions of the droplet effective radius corresponding to extreme scenarios of mixing and secondary droplet activation are derived. They depend upon the liquid water path as well as upon the cloud droplet effective radius and liquid water path of the corresponding adiabatic cloud. The validity of these expressions is verified by using field experiment data. A parameterization of the mean droplet effective radius for tropical warm convective clouds is deduced from the above expressions and is further validated with the same field experiment data.
Preliminary interpretation of tracer experiments conducted in a discrete rock fracture under conditions of natural flow
Authors: Novakowski K. S. , Lapcevic P. A. , Voralek J. , Bickerton G.
Published: February 2004

The results and preliminary interpretation of two tracer experiments conducted under natural flow conditions are presented. The experiments were conducted in a horizontal fracture at a depth of 10 m in a shale and limestone formation. The transport of a conservative tracer was followed over a distance of as much as 40 m in a 27‐borehole array. Observation of the pathways of tracer migration and interpretation of the transport process using a one‐dimensional model showed that transverse dispersion and matrix diffusion are likely the predominant processes responsible for spreading and slowing the migration of the solute. Groundwater velocity estimates obtained from the tracer experiments are also compared to estimates obtained using point dilution methods.
Tropical stratospheric water vapor measured by the Microwave Limb Sounder (MLS)
Authors: Carr E. S. , Harwood R. S. , Mote P. W. , Peckham G. E. , Suttie R. A. , Lahoz W. A. , O'Neill A. , Froidevaux L. , Jarnot R. F. , Read W. G. , Waters J. W. , Swinbank R.
Published: February 2004

The lower stratospheric variability of equatorial water vapor, measured by the Microwave Limb Sounder (MLS), follows an annual cycle modulated by the quasi‐biennial oscillation. At levels higher in the stratosphere, water vapor measurements exhibit a semiannual oscillatory signal with the largest amplitudes at 2.2 and 1hPa. Zonal‐mean cross sections of MLS water vapour are consistent with previous satellite measurements from the LIMS and SAGE II instruments in that they show water vapor increasing upwards and polewards from a well defined minimum in the tropics. The minimum values vary in height between the retrieved 46 and 22hPa pressure levels.
Correlation between characteristic parameters of aftershock distributions in time, space and magnitude
Authors: Guo Zhenqi , Ogata Yosihiko
Published: February 2004

Aftershock sequences which occurred in Japan from 1971 through 1993 are investigated by fitting the modified Omori formula and the Gutenberg‐Richter relation to find that p‐ and b‐values have a positive correlation. It is also shown that there are positive correlations between each of these parameters and the fractal dimension of hypocenter distribution of aftershocks.
Generation of ULF electromagnetic emissions by microfracturing
Authors: Molchanov O. A. , Hayakawa M.
Published: February 2004

Microfracturing electrification is discussed as a possible explanation of ULF electromagnetic emissions observed before and after earthquakes. Using a space‐time simple model of microfracture progression, it is possible to compare theoretical results and observed results with special reference to the intensity, spectrum, and temporal development of ULF magnetic field variations.
Towards a realistic simulation of plate margins in mantle convection
Authors: Zhong Shijie , Gurnis Michael
Published: February 2004

We have incorporated faults as plate margins into time‐dependent mantle convection models by using a mixed Eulerian and Lagrangian finite element formulation. Plate margins in our models can migrate dynamically in response to variations in mantle buoyancy, and this enables us to study the dynamics of mantle and plates including changes in plate size. Convection models in a cylindrical geometry with such a faulted converging margin, non‐Newtonian rheology, and continents indicate that the converging margin tends to migrate oceanward. The oceanward trench migration leads to an increase in the size of the overriding continental plate.
Three‐dimensional variable viscosity convection of an infinite Prandtl Number Boussinesq fluid in a spherical shell
Authors: Ratcliff James Todd , Schubert Gerald , Zebib Abdelfattah
Published: February 2004

We investigate a three‐dimensional, spherical‐shell model of mantle convection with strongly temperature‐dependent viscosity. Numerical calculations of convection in an infinite Prandtl number, Boussinesq fluid heated from below at a Rayleigh number of Ra = 105 are carried out for the isoviscous case and for a viscosity contrast across the shell of 1,000. In the isoviscous case, convection is time dependent with quasi‐cylindrical upflow plumes and sheet‐like downflows. When viscosity varies strongly across the shell, convection is also time dependent, but major quasi‐cylindrical downflows with spider‐like extensions occur at both poles and interconnected upflow plumes occur all around the equator. The surface expression of mantle convection in the Earth (downwelling sheets at trenches, upwelling plumes at hot spots, and upwelling sheets at midocean ridges) resembles structures seen in both the isoviscous and variable viscosity models. The dominance of spherical harmonic degree ℓ = 2 in the variable viscosity model agrees with the ℓ = 2 dominance in the Earth's geoid, topography, and seismic tomography. The overall pattern of convection in the variable viscosity case is similar to the distribution of major highlands and volcanic rises on Venus.
Radiation fog and urban climate
Authors: Sachweh M. , Koepke P.
Published: February 2004

Fog data of Southern Germany from the period 1949–1990 indicate a significant urban influence on fog frequency. An increase of the urban building density is connected with a reduction in the average number of fog days, which is interpreted as an effect of the urban heat island and moisture deficit. Feedback mechanisms which intensify the urban‐rural contrast are discussed. The results are transferable to large cities with relatively good air quality. The study is a contribution to the controversial discussion about the urban effect on fog frequency.
H2SO4 photolysis: A source of sulfur dioxide in the upper stratosphere
Authors: Rinsland C. P. , Gunson M. R. , Ko M. K. W. , Weisenstein D. W. , Zander R. , Abrams M. C. , Goldman A. , Sze N. D. , Yue G. K.
Published: February 2004

Numerous absorption lines of stratospheric sulfur dioxide (SO2) have been identified in solar occultation spectra recorded by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer during the Atmospheric Laboratory for Applications and Science (ATLAS)‐1 shuttle mission (March 24‐April 2, 1992). Based on their analysis, a volume mixing ratio profile of SO2 increasing from (13 ± 4) p.p.t.v. (parts per 10−12 by volume) at 16 mbar (∼ 28 km) to 455 ± 90 p.p.t.v. at 0.63 mbar (∼ 52 km) has been measured with no significant profile differences between 20°N and 60°S latitude. The increase in the SO2 mixing ratios with altitude indicates the presence of a source of SO2 in the upper stratosphere. Profiles retrieved from ATMOS spectra recorded during shuttle flights in April‐May 1985 and April 1993 show similar vertical distributions but lower concentrations. Two‐dimensional model calculations with SO2 assumed as the end product of H2SO4 photolysis produce SO2 profiles consistent with the ATMOS measurements to within about a factor of 2.
Supply rates of natural U‐Th series radionuclides from aquifer solids into groundwater
Authors: Hussain N.
Published: February 2004

Supply rates of 224Ra and 212Pb from zircon and monazite grains to the aqueous phase are experimentally determined by dispersing these minerals in 1N nitric acid and PRL (Physical Research Laboratory) groundwater. In nitric acid medium, recoiled 224Ra and 212Pb behave conservatively and are not removed by solid surfaces. However, in groundwater, 212Pb is removed from the aqueous phase. A model relating the actual supply ratio of 212Pb/224Ra, the measured activities of 224Ra and 212Pb in the leachates at different times of contact and the first order removal rate constant of 212Pb is developed. At steady state, the supply ratio of 212Pb/224Ra in nitric acid from zircon and in groundwater from monazite is evaluated to be a unique value 2.3±0.3 which agrees with the model predicted value of 2.67 based on alpha recoil supply of the nuclides and their precursors. The residence time of 212Pb in groundwater, with respect to its first order removal, is estimated to be of the order of 39 minutes.
The relative quantum yields of O2(a¹Δg) from the photolysis of ozone at 227K
Authors: Ball Stephen M. , Hancock Gus
Published: February 2004

The relative quantum yields of O22(a¹Δg) from the photolysis of ozone have been measured at 227(±2)K over the wavelength range 300nm ≥ λ ≥ 322nm, and are compared to those measured previously at 298K [Ball et al., 1993]. The yields at the two temperatures show the same initial wavelength dependence in the fall‐off region, in contrast to literature measurements of the O(¹D) yields which show a shift in the fall‐off curve to shorter wavelengths at low temperature. For λ > 309nm, the yield at 227K falls more rapidly than that at 298K, confirming that internal energy mediated dissociation of ozone provides the major source of O2(a¹Δg) at wavelengths just beyond the 310nm threshold for the spin allowed production of the two singlet products. However, the yields of O2(a¹Δg) at 227K and 298K were found to be approximately equal for photolysis wavelengths λ ≥ 320nm, and substantially larger than O(¹D) yields predicted from modelling currently available for the photolysis of vibrationally excited ozone. These results suggest the existence of a spin forbidden dissociation channel for O2(a¹Δg) with a quantum yield of Φ ≈ 0.1 at wavelengths near to 320nm.
PMSE dependence on long‐period vertical motions
Authors: Cho John Y. N. , Morley Rebecca L.
Published: February 2004

We analyze the temporal relationship between PMSE (polar mesosphere summer echoes) and long‐period vertical motions using the Poker Flat, Alaska radar data. The results show that the vertical velocity leads PMSE by 90° to 180° with a possible upward trend in phase with increasing frequency. We show that this is consistent with the current PMSE theories which depend primarily upon the presence of charged ice aerosols for the enhancement of radar scatter.
Latitudinal structure of a coronal mass ejection inferred from Ulysses and Geotail observations
Authors: Hammond C. M. , Crawford G. K. , Gosling J. T. , Kojima H. , Phillips J. L. , Matsumoto H. , Balogh A. , Frank L. A. , Kokubun S. , Yamamoto T.
Published: February 2004

We present the first observations of a CME by two spacecraft separated substantially in heliographic latitude. Ulysses and Geotail both see similar features in the plasma and magnetic field parameters during an interval in which Geotail is located in the deep magnetosheath (>150 RE) and Ulysses is located in the solar wind at 5 AU, ∼20° S of Geotail, and ∼51° W (in the direction of solar rotation) of Geotail. Based on the similarity in plasma and magnetic field parameters and similar inferred ejection times from the Sun for both features we argue that the same CME is observed by both spacecraft. The portion of the CME observed by Ulysses is traveling much faster than the portion observed by Geotail. Thus the CME has significant latitudinal structure since at any given time the high latitude portion of the CME extends much further out in radial distance. Furthermore, this implies that a simple calculation of the arrival time of a CME at the Earth may not be done if the observing spacecraft is located substantially away from the ecliptic plane.
Ozone and NO2 air‐mass factors for zenith‐sky spectrometers: Intercomparison of calculations with different radiative transfer models
Authors: Sarkissian A. , Roscoe H. K. , Fish D. , Van Roozendael M. , Gil M. , Chen H. B. , Wang P. , Pommereau J.‐P. , Lenoble J.
Published: February 2004

Calculations of air‐mass factors (AMFs) for ground‐based zenith‐sky UV‐visible spectrometers are now well developed in laboratories where stratospheric constituents are measured with this technique. An intercomparison between results from the different radiative transfer models used to calculate AMFs at twilight is presented here. The comparison was made for ozone AMFs at 510 nm and for NO2 AMFs at 440 nm. Vertical profiles were specified. Results are presented firstly for calculations in a pure Rayleigh atmosphere, then including background aerosols. Relative differences between calculated AMFs from different models cause relative errors in vertical columns of ozone and NO2 measured by zenith‐sky spectrometers. For commonly used averages over solar zenith angles, these relative errors are ±2.3% in the vertical column of ozone and ±1.1% in the vertical column of NO2. Refinements to the calculations, suggested by the intercomparison, should reduce these errors to ±1.0% for ozone and ±0.5% for NO2.
Flow properties of the solar wind derived from a two‐fluid model with constraints from white light and in situ interplanetary observations
Authors: Rifai Habbal Shadia , Esser Ruth , Guhathakurta Madhulika , Fisher Richard R.
Published: February 2004

We derive the flow properties of the solar wind in coronal holes using a two‐fluid model constrained by density profiles inferred from simultaneous space‐based SPARTAN 201–01 and ground‐based Mauna Loa White Light coronagraph observations, and by in situ interplanetary measurements. Also used as a guide is the hydrostatic temperature profile derived from the density gradient. Density profiles are inferred between 1.16 and 5.5 Rs, for two different density structures observed along the line of sight in a polar coronal hole. The model computations that fit remarkably well the empirical constraints yield a supersonic flow at 2.3 Rs for the less dense ambient coronal hole, and at 3.4 Rs for the denser structures. The novel result that emerges from these fits is a proton temperature twice as large as the electron temperature in the inner corona, reaching a peak of 2 × 106 K at 2 Rs.
Laboratory characterization of fluid flow parameters in a porous rock containing a discrete fracture
Authors: Rasmussen Todd C.
Published: February 2004

A porous block of volcanic tuff with an embedded fracture was used to perform laboratory‐scale water and gas flow experiments for the purpose of obtaining unsaturated fractured rock characterization parameters. Rock matrix properties included porosity, hydraulic diffusivity and the gas‐phase diffusion coefficient. Fracture properties included apertures, transmissivity, hydraulic conductivity, and air‐entry pressure. Gas tracer experiments using argon and helium are presented for estimating fracture hydraulic conductivity, fracture apertures and rock matrix porosities.