bluuebunny/crossref_metadata_2025 huggingface dataset:
Exact mathematical derivation of a two‐term infiltration equation
Authors: Swartzendruber D.
Published: February 2004

Previous mathematical developments of so‐called two‐term infiltration equations have been approximate instead of exact and thus provide little insight to account for any experimental shortcomings of such equations. In contrast, it is here shown that one form of two‐term infiltration equations is obtainable by integration from an exact solution of the one‐dimensional downward Richards equation subject to the customary initial and boundary conditions. The mathematical trial solution is a variables‐separable combination of product and additive forms. Intrinsic to the solution are two further stipulations: (1) the unsaturated hydraulic conductivity function must be linear with the water content, and (2) the ponded‐water head on the soil surface must increase as the square root of time from initial water application. The absence of both these stipulations in customary ponded‐infiltration measurements thus accounts for why the two‐term equation frequently fails experimentally. Nonetheless, the mathematical simplicity of the new solution makes it useful pedagogically.
On the spatial variability of parameters of the unsaturated hydraulic conductivity
Authors: Russo David , Russo Itay , Laufer Asher
Published: February 2004

In situ measurements of discharge‐head (steady state rate of water discharge and water head (Q, H)) pairs were used to estimate the saturated conductivity Ks and the parameter α of the Gardner [1958] model of local unsaturated hydraulic conductivity at N=152 observation points in a field plot (40 m long, 6 m wide, and 2 m deep) by means of the Geulph permeameter method. Twenty‐two of these (Q, H) pairs yielded negative Ks and were excluded from the subsequent analyses. Estimates of ln Ks and ln α from N=130 observation points were used to estimate parameters of the spatial covariance and the drift functions of the two soil properties, using the restricted maximum likelihood estimation procedure and the weighted least squares procedure, respectively. For both soil properties a deterministic drift was identified, suggesting that in the site under investigation, approximately 59% and 37% of the total variability of ƒ=ln Ks and a=ln α, respectively, may stem from large‐scale variations. Results of the analyses of the uncorrelated residuals of ln Ks and ln α suggest that both follow an approximately normal distribution and are moderately cross‐correlated (ρƒa=0.68). The fitted covariance models of both soil properties exhibited slight statistical anisotropy in the horizontal plane and a significant anisotropy in the vertical planes. Components of the range of ln Ks were 2.5 m and 0.6 m in the horizontal and the vertical directions, respectively, while those of ln α were 1.4 m and 0.4 m, respectively. Estimates of the correlation scales of both properties in the horizontal plane were associated with relatively large uncertainty, which probably stemmed from the less than optimum selection of the number and locations of the observation points in this plane. The results of the model validation test, however, indicated that the fitted covariance and drift models were theoretically consistent with the observations of both soil properties.
The cooling pond of the Chernobyl Nuclear Power Plant: A groundwater remediation case history
Authors: Bugai Dmitri A. , Waters Robert D. , Dzhepo Sergei P. , Skalsk'ij Alexander S.
Published: February 2004

The cooling pond of the Chernobyl nuclear power plant was heavily contaminated as a result of the reactor accident in April 1986. From 1989 to 1993 the cooling pond represented one of the major sources of 90Sr migration from the Chernobyl site to the Dnieper River. Several attempts have been made to contain radioactive contamination within the pond. Overestimation of releases via groundwater pathway and design mistakes led to unsuccessful remedial actions in 1986 and in later periods. In addition, remediation criteria based solely on comparison of contaminant concentrations in groundwater with drinking water standards were not effective from the health risk perspective, because the public was not directly exposed to contaminated groundwater; the exclusion zone surrounding the site acted as an institutional control to prevent public access. In light of recent estimates of low risks due to radionuclide transport outside the exclusion zone, a “no action” approach may represent the most reasonable strategy for the near‐term management of the cooling pond.
Modeling and experimental validation of volatile organic contaminant diffusion through an unsaturated soil
Authors: Arands R. , Lam T. , Massry I. , Berler D. H. , Muzzio F. J. , Kosson D. S.
Published: February 2004

Modeling the diffusive transport of volatile organic contaminants (VOCs) has been previously described by lumping together vastly different soil regions (e.g., interaggregate and intra‐aggregate regions) and assuming local equilibrium with linear contaminant phase distributions. This approach has sometimes failed to adequately describe diffusive transport. In this work, a diffusive transport model was developed that separately considered diffusion in the intra‐aggregate and interaggregate regions and utilized nonlinear contaminant distributions among the phases. Input parameters were determined from independent sources, calculations, and measurements. No adjustments were made to input parameters. The model was compared to breakthrough and desorption data for two VOCs (toluene and trichloroethylene) on a silt loam soil at moistures from 1.6 to 14%. Breakthrough predictions were significantly better than those from a commonly used model. Desorption predictions were excellent over the first 3 orders of magnitude in contaminant flux after which they deviated by a factor of ∼3.
Ground‐penetrating‐radar‐assisted saturation and permeability estimation in bimodal systems
Authors: Hubbard Susan S. , Rubin Yoram , Majer Ernie
Published: February 2004

Near‐surface investigations often require characterization of vadose zone hydraulic parameters. Conventional sampling or borehole techniques for estimating these parameters are costly, time consuming, and invasive, all of which limit collection of hydrogeological data at a spacing needed for detailed site characterization. Incorporation of two‐ or three‐dimensional densely sampled geophysical data with conventional hydrological data increases the amount of data available for the characterization and thus has the potential to significantly improve the hydraulic parameter estimates over those obtained from borehole data alone. The hydraulic estimation procedure can be greatly improved by incorporating dielectric information potentially available from ground penetrating radar (GPR), a noninvasive, high‐resolution geophysical method. The procedures for collecting and processing GPR data in the format needed for the proposed estimation technique are relatively new and still a topic of research; our method requires as a starting point the ability to estimate dielectric constants from GPR data. Numerical experiments were performed to investigate the general utility of the GPR‐assisted estimation technique under a range of conditions. Three bimodal systems were investigated, each system being composed of a sand facies together with another facies with a larger clay volume fraction; each facies was defined using characteristic values of clay content, porosity, and permeability. Using dielectric information and petrophysical relations, degree of saturation and intrinsic permeability values at each location within the three systems were identified. For bimodal systems, a dielectric constant measurement corresponds to two possible values of saturation and intrinsic permeability at each location; single values of saturation and intrinsic permeability were estimated from these values using the principle of maximum likelihood. Results from case studies demonstrate that a combination of GPR data with conventional borehole data significantly improves the estimates of saturation and has the potential to improve the estimates of permeability over those obtained from well bore data alone. The proposed method should be especially advantageous for vadose zone characterization in areas favorable for GPR data acquisition, where detailed hydraulic parameter information is required but the drilling of numerous boreholes is prohibited.
Photolytic regulation of dissolved organic carbon in northern lakes
Authors: Molot Lewis A. , Dillon Peter J.
Published: February 2004

We examined the extent to which photolytic and nonphotolytic decomposition rates of dissolved organic carbon (DOC) could account for the annual retention or loss of DOC inputs in lakes (retention is equal to stream inputs plus atmospheric inputs minus stream discharge which is equivalent to storage in sediments plus degassed to atmosphere). Losses of DOC inputs to sediments and the atmosphere were large, averaging 38 to 70% of total inputs in seven study lakes between 1980 and 1992. Up to 50% of stream DOC was lost as inorganic C when exposed to solar radiation during 6 to 11 day surface exposures in bottles whereas lake DOC concentration was unaffected by solar radiation. Stream DOC loss was significantly less in the dark suggesting a low microbial consumption rate. Photodecay constants, extrapolated to each of the study lakes after correction for in situ mixing conditions and extinction of UVA and UVB, were similar to corresponding mass balance rate constants representing sediment storage and losses to the atmosphere. This suggests that photodecay is potentially large enough in situ to account for all of the DOC losses to the atmosphere and sediments in the low DOC lakes (<4 mg L−1) but cannot account for all of the DOC lost in the high DOC lakes (>4 mg L−1). The mass balance and photodecay approaches employed in the study of carbon budgets show that UV degradation is probably an important mechanism in transfer of stream DOC to the sediment particulate C pool and to the atmosphere.
Sensitivity of global vegetation models to present‐day climate simulated by global climate models
Authors: Ciret C. , Henderson‐Sellers A.
Published: February 2004

This paper provides an evaluation of the reliability of several climate models with respect to ecosystem modeling and describes a methodology for undertaking such an evaluation. The global climate models (GCM) and global vegetation models used in this study are linked in a one way mode (i.e., no feedback from the vegetation model to the GCM is allowed), and the vegetation distributions are compared with those obtained using observed climatology. The aim of this study is to identify which simulated ecosystems are sensitive to the biases of the climate simulations. Two global static vegetation models, BIOME‐1 and a version of the Holdridge scheme, are used in conjunction with several present‐day climate simulations. The climate simulations employed come from the GCMs participating in the Model Evaluation Consortium for Climate Assessment project. The results indicate that the overall performance of coarse resolution GCMs with respect to vegetation prediction is poor. The discrepancies between vegetation distributions computed from observed and simulated climatologies represent more than 50% of land area. The comparison of vegetation distributions shows that there are some common tendencies amongst these GCMs to induce the overprediction or underprediction of certain biomes. For example, the biomes belonging to dry climate regions are underpredicted, and the woodlands and temperate/cold forests are overpredicted. The climatic variables responsible for the discrepancies between vegetation predictions are identified, and it appears that the differences in vegetation predictions are overall due to the overestimation of the soil moisture index and precipitation, to the overestimation of growing degree days, and to the underestimation of the annual minimum temperatures. In summary, this research has shown that the prediction of biomes using simulated climatologies is not yet fully satisfactory; however, it is possible to increase our level of confidence in the prediction of vegetation by carefully evaluating the performance of the vegetation models driven by simulated climatologies and by identifying the causes of the biases.
Effects of land use, climate variation, and N deposition on N cycling and C storage in northern hardwood forests
Authors: Aber John D. , Driscoll Charles T.
Published: February 2004

We hypothesized that much of the variability in dissolved inorganic nitrogen (DIN) loss from forested catchments can be explained by land use history and interannual climatic variation, and that these factors determine the degree to which N deposition results in increased storage of C in forests. We used an existing model of C, N, and water balances in forest ecosystems in conjunction with long‐term climate and N leaching loss data from several northern hardwood forest ecosystems to predict the effects of land use, climate variability and N deposition on C storage and N cycling and loss. Six sites from the White Mountains of New Hampshire with very different land use histories and annual stream DIN losses were used. The only model parameter that varied between sites was land use or disturbance history. Each site was simulated using both mean climate data for each year and actual time series climate data. Vegetation removal resulted in a period of increased DIN leaching, followed by losses below those in control stands for both measured and simulated data. One site with an extreme fire event over 170 years ago still showed reduced N losses in both modeled and measured data. Significant interannual variation in DIN loss is evident in the field data. Model predictions using actual climate time series data captured much of this variation. This high interannual variability along with the slow rate of change in DIN loss predicted by PnET‐CN using mean climate throughout the simulations suggests that statistically significant increases in DIN leaching losses due to long‐term increases in N deposition will not be detectable for several decades, given current rates of N deposition. N deposition increased C storage in all simulations, but the quantity stored was about 50% that predicted by another published model. This difference results from differences in the efficiency with which added N is retained in the ecosystem. The previous model used an 80% retention value, while retention was closer to 50% over most of the time period examined here.
Carbon 13 exchanges between the atmosphere and biosphere
Authors: Fung I. , Field C. B. , Berry J. A. , Thompson M. V. , Randerson J. T. , Malmström C. M. , Vitousek P. M. , Collatz G. James , Sellers P. J. , Randall D. A. , Denning A. S. , Badeck F. , John J.
Published: February 2004

We present a detailed investigation of the gross 12C and 13C exchanges between the atmosphere and biosphere and their influence on the δ13C variations in the atmosphere. The photosynthetic discrimination Δ against 13C is derived from a biophysical model coupled to a general circulation model [Sellers et al., 1996a], where stomatal conductance and carbon assimilation are determined simultaneously with the ambient climate. The δ13C of the respired carbon is calculated by a biogeochemical model [Potter et al., 1993; Randerson et al., 1996] as the sum of the contributions from compartments with varying ages. The global flux‐weighted mean photosynthetic discrimination is 12–16‰, which is lower than previous estimates. Factors that lower the discrimination are reduced stomatal conductance and C4 photosynthesis. The decreasing atmospheric δ13C causes an isotopic disequilibrium between the outgoing and incoming fluxes; the disequilibrium is ∼0.33‰ for 1988. The disequilibrium is higher than previous estimates because it accounts for the lifetime of trees and for the ages rather than turnover times of the biospheric pools. The atmospheric δ13C signature resulting from the biospheric fluxes is investigated using a three‐dimensional atmospheric tracer model. The isotopic disequilibrium alone produces a hemispheric difference of ∼0.02‰ in atmospheric δ13C, comparable to the signal from a hypothetical carbon sink of 0.5 Gt C yr−1 into the midlatitude northern hemisphere biosphere. However, the rectifier effect, due to the seasonal covariation of CO2 fluxes and height of the atmospheric boundary layer, yields a background δ13C gradient of the opposite sign. These effects nearly cancel thus favoring a stronger net biospheric uptake than without the background CO2 gradient. Our analysis of the globally averaged carbon budget for the decade of the 1980s indicates that the biospheric uptake of fossil fuel CO2 is likely to be greater than the oceanic uptake; the relative proportions of the sinks cannot be uniquely determined using 12C and 13C alone. The land‐ocean sink partitioning requires, in addition, information about the land use source, isotopic disequilibrium associated with gross oceanic exchanges, as well as the fractions of C3 and C4 vegetation involved in the biospheric uptake.
An ecosystem simulation model for methane production and emission from wetlands
Authors: Potter Christopher S.
Published: February 2004

Previous experimental studies suggest that methane emission from wetlands is influenced by multiple interactive pathways of gas production and transport through soil and sediment layers to the atmosphere. The objective of this study is to evaluate a new simulation model of methane production and emission in wetland soils that was developed initially to help identify key processes that regulate methanogenesis and net flux of CH4 to the air but which is designed ultimately for regional simulation using remotely sensed inputs for land cover characteristics. The foundation for these computer simulations is based on a well‐documented model (Carnegie‐Ames‐Stanford Approach, CASA) of ecosystem production and carbon cycling in the terrestrial biosphere. Modifications to represent flooded wetland soils and anaerobic decomposition include three new submodels for (1) layered soil temperature and water table depth (WTD) as a function of daily climate drivers, (2) CH4 production within the anoxic soil layer as a function of WTD and CO2 production under poorly drained conditions, and (3) CH4 gaseous transport pathways (molecular diffusion, ebullition, and plant vascular transport) as a function of WTD and ecosystem type. The model was applied and tested using climate and ecological data to characterize tundra wetland sites near Fairbanks, Alaska, studied previously by Whalen and Reeburgh [1992]. Comparison of model predictions to measurements of soil temperature and thaw depth, water table depth, and CH4 emissions over a 2‐year period suggest that intersite differences in soil physical conditions and methane fluxes could be reproduced accurately for selected periods. Day‐to‐day comparison of predicted emissions to measured CH4 flux rates reveals good agreement during the early part of the thaw season, but the model tends to underestimate production of CH4 during the months of July and August in both test years. Important seasonal effects, including that of falling WTD during these periods, are apparently overlooked in the model formulation. Nevertheless, reasonably close agreement was achieved between the model's mean daily and seasonal estimates of CH4 flux and observed emission rates for northern wetland ecosystems. Several features of the model are identified as crucial to more accurate prediction of wetland methane emission, including the capacity to incorporate influences of localized topographic and hydrologic features on site‐specific soil temperature and WTD dynamics and mechanistic simulation of methane emission transport pathways from within the soil profile.
Topological constraints on the iron phase diagram
Authors: Seifert F.
Published: February 2004

Topological analysis of the iron phase diagram in the subsolidus region shows that the slopes of stable univariant equilibria impose constraints for the metastable growth of other phases upon quenching. If the relative slopes of equilibrium univariant curves and the relative positions of invariant points are known or can be estimated, pressure‐temperature regions for the potential growth of metastable phases may be delimited. The results are applied to the proposed stability field of β‐iron.
Ozone measurements over McMurdo Station, Antarctica during spring 1994 and 1995
Authors: Nardi Bruno , Deshler Terry , Hervig Mark E. , Oolman Larry D.
Published: February 2004

Ozone and temperature profiles were measured with balloon‐borne instruments from McMurdo Station, Antarctica (77.85°S, 166.67°E) from late August to late October, on 39 occasions in 1994 and on 35 occasions in 1995. Minimum column ozone values of 138 DU and 139 DU were measured in 1994 and 1995, reduced from initial measurements in late August of 272 DU and 256 DU, respectively. These minima are higher than the record low measurement from McMurdo taken in October 1993 (130 DU), but lower than all other years since 1986 when ozone measurements were initiated at McMurdo. In 1994 and 1995, as in previous years, the onset of ozone recovery began mid‐to‐late October above 20 km. Below 14 km ozone concentrations have returned to pre‐Pinatubo levels. Between 16–22 km they are near, and in 1995 often lower than, the record low levels set in 1993. In late August of 1995, the total and 12–20 km column ozone were also often observed to be near or lower than that for the same period of all previous years since 1986. By early October 1995, almost complete ozone depletion was experienced uniformly over the altitude region 14.5–19.5 km.
First test of stochastic growth theory for Langmuir waves in Earth's foreshock
Authors: Cairns Iver H. , Robinson P. A.
Published: February 2004

This paper presents the first test of whether stochastic growth theory (SGT) can explain the detailed characteristics of Langmuir‐like waves in Earth's foreshock. A period with unusually constant solar wind magnetic field is analyzed. The observed distributions P(log E) of wave fields E for two intervals with relatively constant spacecraft location (DIFF) are shown to agree well with the fundamental prediction of SGT, that P(log E) is Gaussian in log E. This stochastic growth can be accounted for semi‐quantitatively in terms of standard foreshock beam parameters and a model developed for interplanetary type III bursts. Averaged over the entire period with large variations in DIFF, the P(log E) distribution is a power‐law with index ∼ −1; this is interpreted in terms of convolution of intrinsic, spatially varying P(log E) distributions with a probability function describing ISEE's residence time at a given DIFF. Wave data from this interval thus provide good observational evidence that SGT can sometimes explain the clumping, burstiness, persistence, and highly variable fields of the foreshock Langmuir‐like waves.
A two‐layer model of the ionosphere using Global Positioning System data
Authors: Juan J. Miguel , Rius Antonio , Hernández‐Pajares Manuel , Sanz Jaume
Published: February 2004

We present a new approach to model the Ionosphere based on GPS data. Previous authors have used models with an unique shell. In this case we have included a second shell to account for the distribution of the electrons in the outer part of the Ionosphere. We have analyzed the ionospheric electron content of a region above 30 degrees in declination in different conditions of ionospheric activity using the Kalman filter. The data used has been obtained from the International GPS Service for Geodynamics (IGS) network. Simultaneously we have studied the receiver and transmitter differential biases showing the effects of neglecting the outer part of the Ionosphere in the model. It appears a systematic variations for the receivers—depending on its latitude—not for the satellites.
Dayside reconnected field lines in the south‐dusk near‐tail flank during an IMF By > 0 dominated period
Authors: Fujimoto M. , Mukai T. , Matsuoka A. , Nishida A. , Terasawa T. , Seki K. , Hayakawa H. , Yamamoto T. , Kokubun S. , Lepping R. P.
Published: February 2004

A prolonged period of By > 0 dominated IMF conditions was monitored by WIND on Dec., 18–19, 1994. Observations on convection over the southern polar cap in the middle of this period (IMF Bz ∼ 0) were available from AKEBONO. Detection of the well‐known dawn‐to‐dusk flow observing simultaneously energy‐dispersed ions in the cusp strongly indicates that the field lines reconnected at the dayside were azimuthally accelerated. GEOTAIL happened to be in the duskside flank at (XGSM, ZGSM) ∼ (−15 ∼ −23, −1 ∼ −7)RE. The GEOTAIL data not only prove that this near‐tail flank region in the southern hemisphere is filled with azimuthally accelerated flux tubes that are loaded with solar wind plasma, but they also show that O+ ions are transported tailward along these field lines. The latter is a new finding which suggests a route for transporting O+ ions from the ionosphere to the magnetotail.
IMP 8 observations of traveling compression regions in the mid‐tail near substorm expansion phase onset
Authors: Taguchi S. , Slavin J. A. , Lepping R. P.
Published: February 2004

The large data set returned by the IMP 8 magnetic field investigation has been examined to understand the characteristics of traveling compression regions (TCRs). Using 15 years of the AL index, we identified 565 isolated substorm events with well‐developed expansion phase for which IMP 8 was in the mid‐tail lobes and providing magnetic field measurements. From this data set, 17 substorms were found to produce the bipolar BZ TCRs frequently observed farther down the tail. However, another 14 cases have field compressions during which the field tilts north‐then‐south, but the maximum BZ does not reach a positive value, i.e., the BZ variation is not bipolar. These “negative BZ” compression regions (NCRs) are examined in some detail and found to occur at larger |Z| values. The usual bipolar BZ TCRs, in contrast, are found at smaller |Z| values closer to lobe‐plasma sheet interface. We interpret an NCR as a TCR which is observed at large |Z| where the growth phase flaring is too strong for the lobe field draping about the plasmoid to produce a positive BZ between the leading edge and the center of the compression region. Hence, at IMP 8 distances TCRs are often not accompanied by true bipolar BZ signatures; rather only a north‐then‐south tilting of the field relative to flared lobe field is observed.
Tectonic stability of the San Luis Hills, Northern Rio Grande Rift, Colorado: Evidence from paleomagnetic measurements
Authors: Brown Laurie L. , Golombek Matthew P.
Published: February 2004

The San Luis Basin is the largest of four tilted, fault bounded Neogene basins of the northern Rio Grande rift in New Mexico and Colorado. It formed during the second of two episodes of extension beginning in middle Oligocene and extending to the present. The San Luis Hills are an intrarift horst of Oligocene intermediate and basalt rocks preserved in the middle of the San Luis Basin. Previous paleomagnetic and structural studies in the Espanola Basin to the south indicate counterclockwise rotation of that region during the most recent extension. Paleomagnetic samples from the two exposed formations in the San Luis Hills yield a mean direction of inclination=55.6° and declination=347.2° (α95=6.8°, N=23) that is coincident with the expected Oligocene direction for North America. These rocks show no indication of rotation or severe tilting during rift extension.
Physically based inversion of surface snow concentrations of H2O2 to atmospheric concentrations at South Pole
Authors: McConnell Joseph R. , Winterle James R. , Bales Roger C. , Thompson Anne M. , Stewart Richard W.
Published: February 2004

Inversion of chemical records archived in ice cores to atmospheric concentrations requires a detailed understanding of atmosphere‐to‐snow‐to‐ice transfer processes. A unique year‐round series of surface snow samples, collected from November, 1994 through January, 1996 at South Pole and analyzed for H2O2, were used to test a physically based model for the atmosphere‐to‐snow component of the overall transfer function. A comparison of photochemical model estimates of atmospheric H2O2, which are in general agreement with the first measurements of atmospheric H2O2 at South Pole, with the inverted atmospheric record (1) demonstrate that the surface snow acts as an excellent archive of atmospheric H2O2 and (2) suggest that snow temperature is the dominant factor determining atmosphere‐to‐surface snow transfer at South Pole. The estimated annual cycle in atmospheric H2O2 concentration is approximately symmetric about the summer solstice, with a peak value of ∼280 pptv and a minimum around the winter solstice of ∼1 pptv, although some asymmetry results from the springtime stratospheric ozone hole over Antarctica.
Observations of the annual cycle of sea ice temperature and mass balance
Authors: Perovich Donald K. , Elder Bruce C. , Richter‐Menge Jacqueline A.
Published: February 2004

A vertical array of thermistors coupled with an autonomous data‐logging system was used to obtain a 15‐month record of ice temperature profiles in a multiyear floe in the Beaufort Sea. This record was used to monitor atmosphere, ice and ocean temperatures, determine changes in the ice mass balance, and infer estimates of the ocean heat flux and the snow thermal conductivity. Ablation during the summer melt season consisted of approximately 0.3 m of snow melt, 0.67 m of ice surface ablation and 0.25 m of bottom ablation. There was 0.45 m of bottom accretion during the growth season. The annually averaged ocean heat flux was 4 W m−2, with a summertime value of 9 W m−2. Comparing these results to earlier studies conducted in the same region showed considerable interannual variability in summer melting. The thermal conductivity of snow cover was approximately 0.3 W m−1 K−1 during winter.
Depth variation of the mid‐mantle seismic discontinuity
Authors: Niu Fenglin , Kawakatsu Hitoshi
Published: February 2004

Short‐period array seismograms of deep events that occurred in the Indonesia, Japan and Izu‐Bonin arcs are stacked and beam‐formed to identify the near‐source S‐P converted waves that result from the mantle transition discontinuities. Most of the resulting images reveal the existence of a mid‐mantle seismic discontinuity (“920 km discontinuity”) in these regions. Of the 15 events analyzed, three that occurred at the western end of the Indonesia arc show clear S‐P arrivals observable even in individual seismograms. The mid‐mantle discontinuity is characterized by large depth variation (900 ∼ 1080 km) and velocity contrast variation in different subduction zones. Especially, the depth variation of the mid‐mantle discontinuity beneath the Indonesia arc, where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end, appears to be well correlated with the location of the high‐velocity anomalies in recent tomographic models. However, the mid‐mantle discontinuity cannot be simply coincided with the bottom of the high‐velocity anomalies, because a velocity increase at the discontinuity is observed from the waveform analysis.
Linear additivity of climate response for combined albedo and greenhouse perturbations
Authors: Ramaswamy V. , Chen C.‐T.
Published: February 2004

Using an atmospheric general circulation model with fixed cloud amounts and microphysical properties, and coupled to a mixed‐layer static ocean, we perform idealized experiments to inquire into the linear characteristics of the modeled climate system's mean response to simultaneous greenhouse and Northern Hemisphere midlatitude albedo perturbations, two forcings deemed to be important during the present times. The two forcings are chosen to be equal and opposite in the global, annual‐mean such that a linear behavior would be expected to lead to a complete offset of the global, annual‐mean surface temperature change, which is indeed obtained. The monthly and annual zonal‐mean surface temperature, and the annual zonal‐mean precipitation responses to the combined forcings, also are reasonably similar to the sum of the responses to the individual forcings. The albedo forcing case casts a distinct signature on the circulation and precipitation changes in the northern and southern equatorial regions, which is absent for the greenhouse forcing case. The combined simulation yields a result similar to that for the albedo forcing case, one that is consistent with linear additive expectations.
Normal modes and the transient response of the climate system
Authors: MacKay Robert M. , Ko Malcolm K. W.
Published: February 2004

Upwelling diffusion (UD' models are currently used to estimate the transient response of the climate system to perturbations in atmospheric composition. We linearize a global mean vertical upwelling diffusion model of the coupled ocean‐atmosphere system to obtain a solution for the transient response of the climate system to a given perturbation. For an N‐box model, the exact solution for the transient response of the surface air temperature to a given step change in radiative forcing is shown to be related to the equilibrium change in surface temperature (ΔTeq,1' by ΔTeq,1Σka1,k(1‐e−t/τk'. The time constants (τk' and the contribution of each mode (a1,k' can be expressed in terms of the matrix characterizing the energy exchange between the boxes representing the atmosphere and different layers of the ocean. We find that a minimum of 5‐boxes (one atmosphere and four ocean levels' is needed to adequately reproduce the response of a 20‐box UD model. The transient response of the atmospheric temperature to a step change in radiative forcing is dominated by three modes with characteristic time constants of 1.7, 7.2, and 40.2 years and contributions of 22%, 47%, and 27% respectively to the equilibrium response of the system.
Simultaneous observations of polar stratospheric clouds and HNO3 over Scandinavia in January, 1992
Authors: Massie Steven T. , Dye James E. , Baumgardner Darrel , Randel William J. , Wu Fei , Tie XueXi , Pan Liwen , Figarol Francois , Brasseur Guy P. , Santee Michelle L. , Read William G. , Grainger Roy G. , Lambert Alyn , Mergenthaler John L. , Tabazadeh Azadeh
Published: February 2004

Simultaneous observations of Polar Stratospheric Cloud (PSC' aerosol extinction and HNO3 mixing ratios over Scandinavia are examined for January 9–10, 1992. Data measured by the Microwave Limb Sounder (MLS', Cryogenic Limb Array Etalon Spectrometer (CLAES', and Improved Stratospheric and Mesospheric Sounder (ISAMS' experiments on the Upper Atmosphere Research Satellite (UARS' are examined at locations adjacent to parcel trajectory positions. Regression coefficients, obtained from Mie calculations, are used to transform aerosol extinctions into aerosol volume densities. Graphs of volume density versus temperature, and importantly, HNO3 mixing ratio versus temperature, show volume increases and simultaneous loss of HNO3 as temperatures decrease. The data is consistent with initial PSC growth processes which transform sulfate droplets into ternary droplets or nitric acid dihydrate (NAD' particles.
Bulk sound travel times and implications for mantle composition and outer core heterogeneity
Authors: Bina Craig R. , Silver Paul G.
Published: February 2004

We present a new methodology for constraining mantle composition directly from observed seismic travel times. By measuring travel times of PcP, PcS, ScS, and PKiKP phases generated by the Chinese nuclear explosion of 21 May 1992 and recorded at epicentral distances of ∼10°, we directly determine bulk sound travel times (TΦ' for the mantle and outer core. This eliminates the need to combine independent VP and VS profiles, characterizing different paths and frequencies, to obtain a reference VΦ profile. Our observed PKiKP‐PcP differential travel time indicates that the outer core model of AK135 is slow by 2.1 s (that of IASP91 by 2.3 s' while that of PREM is slow by only 0.7 s and suggests, through comparison with other data, relative lateral homogeneity of the outer core. Furthermore, we find that T0Φ for the crust and mantle in this region exceeds the AK135 prediction by 3.5 s (IASP91 by 3.9 s' ±1.0 s. Since the lower mantle contribution to this delay depends upon how much arises in the upper mantle, we present two end‐member models for such a slower mantle VΦ, in which the delay resides entirely above or below 660 km depth. The latter yields best‐fitting lower mantle compositions which are less enriched in Si than those fit to T0Φ for AK135 (or IASP91' for the former, and neither supports Feenrichment of the lower mantle. Applicability of such localized observations to the larger mantle is suggested by tomographic results indicating that lateral heterogeneity in VΦ is very small relative to that in VS. Our results illustrate the potential utility of T0Φ in constraining mantle composition.
GPS observations of postseismic deformation for the 1995 Hyogo‐Ken Nanbu Earthquake, Japan
Authors: Nakano Takehide , Hirahara Kazuro
Published: February 2004

11‐months of GPS observations, which started within 2 days of the 1995 Hyogo‐ken Nanbu earthquake, detected significant postseismic crustal deformation that reached 20 mm at some stations. A comparison of postseismic and coseismic deformation shows a similar pattern in the direction of displacements. We tried to determine the predominant relaxation mechanism by examining the time dependence of observed postseismic deformation. We considered three models: an exponential, a power‐law, and a logarithmicaly decaying model. However, each of these three time‐dependent models can explain the observed data, equally well. In case of the exponential relaxation model, the relaxation time constant was estimated to be about 50 days. In the power‐law model, the index p was nearly 1.0; this is almost the same as the p value of Omori's decay law of aftershock sequences for this event. This suggests that the mechanism causing postseismic deformation may be related to that producing aftershocks.
Retrieving aerosol optical properties from moments of the particle size distribution
Authors: Yue G. K. , Lu J. , Mohnen V. A. , Wang P.‐H. , Saxena V. K. , Anderson J.
Published: February 2004

The randomized minimization search technique (RMST) is a powerful tool for retrieving aerosol size distribution from a set of aerosol extinction measurements. This technique is now extended to retrieve aerosol optical properties from a known moment sequence. The new usage of the RMST is demonstrated by using particle size distributions obtained from fits to in situ measurements conducted in the troposphere and stratosphere. Good agreements (with differences less than 5%) between some aerosol optical properties calculated from the moments and those calculated directly from the particle size distributions are obtained. Our results illustrate that the RMST can be applied to parameterize particle optical properties from the lower‐order moments of an aerosol size distribution.
Regional ozone from biogenic hydrocarbons deduced from airborne measurements of PAN, PPN, and MPAN
Authors: Williams J. , Roberts J. M. , Fehsenfeld F. C. , Bertman S. B. , Buhr M. P. , Goldan P. D. , Hübler G. , Kuster W. C. , Ryerson T. B. , Trainer M. , Young V.
Published: February 2004

NOx‐catalyzed production of ozone over large regions of North America and Europe is a serious air quality problem that often involves biogenic hydrocarbons, mainly isoprene. Peroxy‐methacrylic nitric anhydride (MPAN, CH2C(CH3)C(O)OONO2) is formed uniquely from isoprene‐NOx photochemistry hence is an indicator of recent ozone production from isoprene. Presented here are the first airborne measurements of MPAN along with PAN (peroxyacetic nitric anhydride, CH3C(O)OONO2), PPN (peroxypropionic nitric anhydride, CH3CH2C(O)OONO2) and ozone measurements. Relationships between these species are used to estimate the contributions of anthropogenic and biogenic hydrocarbons (BHC) to regional tropospheric ozone production, providing direct evidence of ozone production from BHC‐NOx photochemistry.
Molecular dynamics simulation of superionicity in Neighborite, NaMgF3
Authors: Zhou L. X. , Hardy J. R. , Cao H. Z.
Published: February 2004

Superionicity in neighborite, NaMgF3, has been studied by molecular dynamics using parameter‐free Gordon‐Kim potentials. These simulations show that this halide‐based perovskite has a superionic phase. The onset temperature for superionicity is ∼ 50–100 K below the melting point. This is a novel type of superionicity in a halide, occurring as it does in a perovskite rather than a fluorite. Superionicity is demonstrated to occur whether one uses the full ionicity for Mg2+ and F− or employs reduced charges derived from a Quantum Chemistry calculation. The relevance of these findings to predictions of superionicity in the oxide perovskite MgSiO3[Matsui and Price, 1991] is discussed.
3‐D simulation of near‐field strong ground motion: Basin edge effect derived from rupture directivity
Authors: Inoue Tomohiro , Miyatake Takashi
Published: February 2004

The heavily damaged area in a narrow belt in Kobe was a very characteristic feature of the 1995 Hyogoken‐nambu (Kobe) earthquake. The processes responsible for the amplification of the strong ground motion there have not yet been clarified. We model the basin edge effect derived from rupture directivity by simulating dynamic rupture for an event with M=7 on a strike‐slip fault buried near a basin. Ground velocity seismograms up to 1 Hz are computed using a 3‐D finite difference method. Patterns of the peak ground velocities show that they are amplified in a very narrow belt along the basin edge, a few kilometers in width, when the fault runs parallel to the edge. This amplification effect also strongly depends on the fault location. However, the large amplitudes are not concentrated in a narrow belt when the whole fault ruptures at once. This suggests that an intense ground shaking in a narrow belt along a basin edge occurs only when rupture propagates parallel to the basin edge with a finite velocity. This appears to have been the case in the Kobe area in which the basin edge effect derived from rupture directivity may have been the cause of the “heavily damaged belt.”
Convection‐driven reconnection and the stability of the near‐Earth plasma sheet
Authors: Pritchett P. L. , Coroniti F. V. , Pellat R.
Published: February 2004

Driven convection in the near‐Earth plasma sheet is investigated using 3‐D, full particle simulations. The inductive response to an externally imposed, uniform convection electric field leads to the effective tailward propagation of magnetic flux, the erosion of the equatorial magnetic field, the development of an embedded thin current sheet, and the eventual formation of a neutral line at the inward edge of the plasma sheet. In addition to creating a magnetic island, the neutral line permits the growth of a kink‐like mode with a characteristic scale of 1–2 RE in the east‐west (y) direction. The combined plasma flows from reconnection and the kink mode are in the range of 200–400 km/s and exhibit a reversal between earthward and tailward flow as a function of y. These modifications of the plasma sheet resemble the substorm breakup changes which are observed in the current disruption region.
Carbonates in Martian meteorite ALH84001: A planetary perspective on formation temperature
Authors: Hutchins Kevin S. , Jakosky Bruce M.
Published: February 2004

We have re‐examined the formation temperature of carbonates in ALH84001, accounting for the prior evolution of the stable isotopes used in the analysis. Oxygen and carbon in the atmosphere likely have become isotopically fractionated due to various nonthermal loss mechanisms operating throughout geologic time. Taking this previous evolution into account, we derive a formation temperature from oxygen isotope thermometry of 40–250 °C. This range of temperatures represents a lower limit; an increase in groundwater salinity could further raise the δ18O of the groundwater, as seen in terrestrial hydrothermal systems, allowing for an even higher temperature of formation derived from the isotopic data.
Accuracy of temperatures from UKMO analyses of 1994/95 in the Arctic winter stratosphere
Authors: Pullen S. , Jones R. L.
Published: February 2004

Chemical transport models rely on meteorological analyses to provide temperatures which control model chemistry. An accurate representation of stratospheric temperatures becomes particularly important when temperatures at which polar stratospheric clouds (PSCs) are thought to form and chlorine activation begin are approached. This study examines the errors in United Kingdom Meteorological Office (UKMO) lower stratospheric temperatures during the winter 1994/95 by comparing them with temperatures obtained independently by ozonesondes during the same winter. The study shows that there is a consistently positive bias in the analysed temperatures which leads to a mean overestimation of lower stratospheric temperatures of up to 1.7 K at the NAT point. There is also a significant random scatter which increases with altitude. If corrections in analysis temperature are made for both bias and scatter, considerably larger areas of the Northern Hemisphere are exposed to temperatures below the NAT point and frost point and hence chemical activation. These discrepancies in the UKMO temperature fields have important implications for modeling studies carried out using these analyses.
Geometry of the plasma sheet in the midnight‐to‐dawn sector of the Jovian Magnetosphere: Plasma observations with the Galileo spacecraft
Authors: Vasyliūnas Vytenis M. , Frank Louis A. , Ackerson Kent L. , Paterson William R.
Published: February 2004

From August 10 to October 22, 1996, the Galileo spacecraft made observations in the midnight‐to‐dawn sector of the magnetosphere of Jupiter out to a distance of 124RJ, encompasing the region previously explored by the outbound passes of the Pioneer 10, Voyager 1, and Voyager 2 spacecraft. We use the count rates of electrons with energy between 1 and 4 keV, measured by the PLS experiment, to identify the plasma sheet and to study its configuration. We find a pronounced dependence of the count rates on the System III longitude of the spacecraft, with one or two maxima per Jovian rotation, consistent with the generally accepted assumption of a thin plasma sheet in the magnetic equatorial region, which propagates outward as a surface wave generated by the rotation of the tilted magnetic dipole of Jupiter. The inferred effective propagation speed agrees approximately with previous estimates. With Galileo located essentially in the equatorial plane, the plasma sheet can be traced unambiguosly to distances of 100RJ and beyond. A new result, made possible by the long duration of the Galileo observations, is that the observed rotational variation of the count rates is subject to a longer‐term, ≈ 5‐7 day modulation, indicating changes in the mean location, or the surface‐wave amplitude, or the thickness of the plasma sheet, or a combination of these. In at least one instance, only a change of mean location can account for what is observed. The cause of this modulation has not been yet definitively identified. Solar wind effects, dynamics of plasma supply from the Io torus to the plasma sheet, variability of the plasma source due to volcanic activity of Io, and drifting longitudinal asymmetry of the hinging distance for the plasma‐sheet surface wave are among the possibilities.