bluuebunny/crossref_metadata_2025 huggingface dataset:
Permeability, permselectivity, and penetrant‐induced plasticization in fluorinated polyimides studied by positron lifetime measurements
Authors: Shimazu Akira , Miyazaki Tukasa , Katayama Shigeru , Ito Yasuo
Published: December 2002

The ortho‐positronium (o‐Ps) lifetime τ3 and its intensity I3 in various fluorinated polyimides were determined by the positron annihilation technique and were studied with the spin–lattice relaxation time T1 and the propylene permeability, solubility, diffusivity, and permselectivity for propylene/propane in them. τ3, I3, and the distribution of τ3 changed when the bulky moieties in the polyimides were changed. The polyimides, having both large τ3 and I3 values, exhibited a short T1 and a high permeability with a low permselectivity. The propylene permeability and diffusivity were exponentially correlated with the product of I3 and the average free‐volume hole size estimated from τ3. In highly plasticized states induced by the sorption of propylene, the permeability increased with the propylene pressure in excellent agreement with the change in the free‐volume hole properties probed by o‐Ps. The large and broad distribution of the free‐volume holes and increased local chain mobility for the 2,2‐bis(3,4‐decarboxyphenyl) hexafluoropropane dianhydride‐based polyimides are thought to be important physical properties for promoting penetrant‐induced plasticization. These results suggest that o‐Ps is a powerful probe of not only the free‐volume holes but also the corresponding permeation mechanism and penetrant‐induced plasticization phenomenon. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 308–318, 2003
Structure and properties of the mesophase of syndiotactic polystyrene. II. Effect of stepwise extraction on the preparation of the mesophase
Authors: Rani D. Amutha , Yamamoto Y. , Mohri S. , Sivakumar M. , Tsujita Y. , Yoshimizu H.
Published: December 2002

In this study, a novel stepwise extraction method has been examined. The guest molecules housed between the helices of the clathrate δ form of syndiotactic polystyrene can be removed completely with this method. A systematic study of the preparation of a solvent‐free mesophase (emptied clathrate) membrane, its helical and residual solvent contents, and its structural transformations has been performed. In this first attempt, an enhancement in the TTGG helical content has been observed in the extracted membrane, and a conceptual mechanism is proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 269–273, 2003
Crystallization and melting of narrow composition distribution polyethylenes: Investigation and implications of crystal thickening
Authors: Mirabella Francis M.
Published: December 2002

The crystal structure produced during the isothermal crystallization of polyethylene (PE) copolymers with a broad range of comonomer concentrations was determined by the measurement of the melting endotherms directly after crystallization. PE copolymers with higher concentrations of short‐chain branches (≥10 branches per 1000 total carbon atoms) exhibited strong resistance to crystal thickening during isothermal crystallization. Negligible thickening, estimated to be only about 0.1 nm in 10 min of isothermal crystallization, was observed. The side‐chain branches apparently acted as limiting points of chain incorporation into the crystals, which exhibited great resistance to the modification of their position, that is, crystal thickening. Even with long periods (up to 8 h) of isothermal storage, crystal thickening was very small or negligible, about 0.3 nm. The crystal thickness was calculated from differential scanning calorimetry data. The behavior of copolymers with lower branching concentrations and the unbranched PE homopolymer was quite different from that of the copolymers with higher branching. Polymers with low or no branching exhibited the initial crystallization of a thinner crystal population, which thickened substantially with increasing time. The thickening observed for these lower or unbranched polymers was an order of magnitude larger, that is, 1.6–2.0 nm in 10 min of isothermal crystallization. Copolymers with higher concentrations of branching had relatively short sequence lengths of ethylene units between branch points, and this resulted in strong control over the crystal thickness by the branch points and great resistance to crystal thickening, even with long times of isothermal crystallization. Copolymers with low concentrations of branching had relatively long sequence lengths of ethylene units between branch points, and this resulted in little control over the crystal thickness by the branch points and rapid crystal thickening upon isothermal crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 235–246, 2003
Reconfiguration issues of future mobile software radio platforms
Authors: Laddomada Massimiliano
Published: December 2002

The evolution in the semiconductor industry leading to constant increase in the clock speeds and increase in device density and the advent of reconfigurable hardware have paved the way to the creation of a new generation of very agile reprogrammable communication systems generically known as Software Radio (SR) platforms. SR uses a mix of fixed and programmable and/or reconfigurable hardware entities for the implementation of radio transceiver functions capable of commanding the RF (Radio Frequency) spectrum used for wireless communications. While many articles have addressed the general architecture and features of such platforms, the literature on the mechanics of reconfiguring such units is rather scarce. This paper addresses the issue of reconfigurability for the SR platform. We look at a variety of techniques that may be employed to reconfigure the SR platform, list advantages and disadvantages of each, and address some of the questions that arise in the context of standardization efforts that may be needed to render such units truly universal in scope and in application. Copyright © 2002 John Wiley & Sons, Ltd.
Channel‐adaptive techniques in wireless communications: an overview
Authors: Cianca Ernestina , De Luise Aldo , Ruggieri Marina , Prasad Ramjee
Published: December 2002

The concept of adaptivity is of paramount importance in the design of future communication systems, in which a careful exploitation of the limited available resources (bandwidth, power, etc.) is required. The potential of channel‐adaptive transmission has been already recognized 30 years ago, but it did not receive much interest at that time. In the last decade, the advent of feasible software radio systems, and hence, the availability of fast flexible and reconfigurable transceivers has renewed interest in adaptive techniques, which include adaptive modulation and coding, adaptive antennas and adaptive equalization techniques. This paper focuses on adaptive modulation and adaptive error control mechanisms. Basic concepts are highlighted and an overview on the achieved results and new trends in this research area are presented. Some results from information theory are also presented, which show the limitations of these techniques and motivate further research on the practical and design issues that have to be addressed to enable performance to reach close to the theoretical limit. Copyright © 2002 John Wiley & Sons, Ltd.
Formation of electrostatic solitary waves in space plasmas: Particle simulations with open boundary conditions
Authors: Umeda T. , Omura Y. , Matsumoto H. , Usui H.
Published: December 2002

We study formation process of electrostatic solitary waves (ESW) observed by recent spacecraft via one‐ and two‐dimensional electrostatic particle simulations with open boundaries. The previous simulations have demonstrated that ESW correspond to Bernstein‐Greene‐Kruskal electron holes formed by electron beam instabilities. However, since the previous simulations were performed in uniform periodic systems, wave‐particle interaction of an electron beam instability was taking place uniformly in the systems. In the present study, we inject a weak electron beam from an open boundary into the background plasma to study spatial and temporal development of a bump‐on‐tail instability from a localized source. In the open system, spatial structures of electron holes vary depending on the distance from the source of the electron beam. In an early phase of the simulation run, electron holes that are initially uniform in the direction perpendicular to the magnetic field become twisted through modulation by oblique electron beam modes. As the electron holes propagate along the magnetic field, they are aligned in the perpendicular direction through coalescence. Spatial structures of electron holes in a distant region from the source become one‐dimensional. In a long‐time evolution of the instability, ion dynamics becomes important in determining spatial structures of electron holes. A lower hybrid mode is excited locally in the region close to the source of the electron beam through coupling with electron holes at the same parallel phase velocity. The lower hybrid mode modulates electron holes excited in later phases, resulting in formation of modulated one‐dimensional potentials. Since the perpendicular electric fields of electron holes are carried by the electron holes at the drift velocity of the electron holes, they can be observed even at a distant place from the source.
Excitation of a Hall‐current generator by field‐aligned current closure, via an ionospheric, divergent Hall‐current, during the transient phase of magnetosphere–ionosphere coupling
Authors: Yoshikawa Akimasa
Published: December 2002

To clarify the process by which an ionospheric current system is formed by field‐aligned current (FAC) closure in the ionosphere, an inclusive formulation of magnetosphere–ionosphere (MI) coupling is undertaken. The “Hall‐current generator”, which is excited during the transient phase of MI coupling, plays a crucial role in the formation of the ionospheric rotational‐current system. It extracts energy from the FAC system through the divergent Hall‐current and pumps it into the rotational Hall‐current. The energy of the rotational current accumulates as an evanescent poloidal magnetic field, associated with the ionospheric surface wave. This accumulated energy is also fed back to the FAC system through the change in energy flow of the Hall‐current generator. It is found that there is a typical timescale for the rotational‐current system to accumulate or extract the poloidal magnetic energy of ionospheric surface waves. This depends on the inductance of the rotational‐current system and the effective conductivity of the ionospheric rotational current. This characteristic timescale becomes the cause of an ionospheric inductive effect, such as a time delay or phase lag between the source electromagnetic field of the FAC and the corresponding poloidal magnetic field on the ground. The latter causes an inductive shielding effect on the amplitude of the geomagnetic disturbance. Numerical simulation has been able to explain the details of the physical process that occurs when the incident FAC is developing and decaying, and how the energy and current are redistributed into the other elements during the transient MI‐coupling process.
Denudational history along a transect across the Drakensberg Escarpment of southern Africa derived from apatite fission track thermochronology
Authors: Brown Roderick W. , Summerfield Michael A. , Gleadow Andrew J. W.
Published: December 2002

The denudational history of a ∼500 km long transect across the Drakensberg Escarpment on the high‐elevation passive margin of SE Africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples, supplemented by an additional 10 outcrop samples. A minimum of 4.5 km of denudation since formation of the margin ∼130 Myr ago is estimated for the coastal zone, with a marked Early Cretaceous episode of accelerated denudation broadly coincident with continental breakup. Samples from the Swartberg borehole (SW 1/67) located ∼30 km seaward of the present position of the Drakensberg Escarpment indicate a total depth of denudation of 3.1 ± 1.2 km since ∼91 Myr, with a phase of accelerated denudation of 2.1 ± 0.9 km at a mean rate of 95 ± 43 m/Myr between ∼91 and 69 Myr. Samples from the Ladybrand borehole (LA 1/68) west of the Lesotho Highlands indicate 1.7 ± 0.5 km of denudation since ∼78 Myr, with a phase of accelerated denudation at 82 ± 43 m/Myr from ∼78 to 64 Myr. Average denudation rates declined to about 10 m/Myr during much of the Tertiary. Although the apatite fission track data do not provide any direct constraints on the denudational history of the Lesotho Highlands, interpolation between the two boreholes, constrained by geological evidence and extrapolated in situ‐produced cosmogenic 36Cl‐derived denudation rate estimates, suggests a pattern of denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide. These patterns of denudation are incompatible with constant retreat of the Drakensberg Escarpment from an initial position near the present coast. We suggest that the Drakensberg Escarpment formed by rapid post‐breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100–200 m/Myr.
A method of inferring changes in deep ocean currents from satellite measurements of time‐variable gravity
Authors: Wahr John M. , Jayne Steven R. , Bryan Frank O.
Published: December 2002

The NASA/Deutsches Zentrum für Luft‐ und Raumfahrt (DLR) satellite gravity mission Gravity Recovery and Climate Experiment (GRACE), launched in March 2002, will map the Earth's gravity field at scales of a few hundred kilometers and greater every 30 days. We describe a method of using those gravity measurements to estimate temporal variations in deep ocean currents. We examine the probable accuracy of the current estimates by constructing synthetic GRACE data, based in part on output from an ocean general circulation model. We ignore the possible contamination caused by short‐period gravity signals aliasing into the 30‐day solutions. We conclude that in the absence of aliasing, GRACE should be able to recover the 30‐day variability of midlatitude currents at a depth of 2 km with an error of about 6–15% in variance when smoothed with 500–700 km averaging radii.
A scheme for process‐tagged SO4 and BC aerosols in NCAR CCM3: Validation and sensitivity to cloud processes
Authors: Iversen Trond , Seland Øyvind
Published: December 2002

A life cycle scheme for sulfate (SO4) and black carbon (BC) is implemented in an extended version of the National Center for Atmospheric Research (NCAR) Community Climate Model 3 (CCM3). The scheme includes emissions of dimethyl sulfide (DMS), SO2, and sulfate of natural and anthropogenic origins and emissions of BC from biomass burning and fossil fuel combustion. Chemistry and aerosol physics are parameterized based on prescribed oxidant levels and background aerosols of marine, continental, and polar origins. Aqueous chemistry depends on estimated exchange rate of cloudy and clear air. Particulate SO4 and BC are tagged by‐production mechanisms for off‐line reconstruction of aerosol optical and water activity properties. With emissions from International Panel on Climate Change (IPCC), calculations without feedback produce atmospheric turnover times (days) of 1.5 (SO2), 3.5 (SO4), and 4.7 (BC) for the year 2000 and 1.6 (SO2), 4.0 (SO4), and 4.7 (BC) for the year 2100 A2 emission scenario. The modeled SOx compounds compare within a factor 2 with observations at ground level in North America and Europe and for SO4 in the free troposphere. For BC, the ground‐level concentrations are well within a factor 10 from observations over several regions. BC and SO4 are a factor 10 too low in Arctic winter, which can partly be linked to spurious low‐level winter cloudiness. SO4 and BC are a factor 10 too high at ground‐level low latitudes, and upper tropospheric SO2 is largely missing. These major model biases are caused by neglected transport and low scavenging efficiency in cumulus clouds. Cloud processes are discussed by sensitivity tests. SO4 and BC are found very sensitive to the vertical transport and scavenging in convective clouds. More research should aim at improved cloud parameterization schemes that address key processes associated with aerosols to reduce uncertainties associated with climate effects of anthropogenic aerosols.
Scale‐recursive estimation for multisensor Quantitative Precipitation Forecast verification: A preliminary assessment
Authors: Tustison Ben , Foufoula‐Georgiou Efi , Harris Daniel
Published: December 2002

Precipitation is a highly heterogeneous process with considerable natural variability at scales ranging from a few meters to several hundreds of kilometers. This process is monitored with a variety of sensors (e.g., rain gauges, radars, and satellites) which provide direct or indirect measurements of precipitation at different scales. At the same time, physically based models at the storm, regional, continental, and global scales are used to predict precipitation and rely on the observed data for model verification, model initialization, and data assimilation. Because of the tremendous scale‐dependent variability of precipitation fields, merging or comparing observations at different scales, or comparing model outputs at one scale with observations at one or more different scales, is not straightforward. This study explores the use of a recently developed scale‐recursive estimation (SRE) framework for the problem of Quantitative Precipitation Forecast (QPF) verification using observations from multiple sensors. SRE can explicitly account for the multiscale variability of precipitation and the scale‐dependent uncertainty associated with the model output and the multisensor observations. Special emphasis is placed on the specification of the multiscale structure of precipitation under sparse or noisy data. The results demonstrate the potential of SRE as a powerful tool for assessment of QPFs and also for multisensor data fusion and network design studies.
Inverse modeling of carbon monoxide surface emissions using Climate Monitoring and Diagnostics Laboratory network observations
Authors: Pétron Gabrielle , Granier Claire , Khattatov Boris , Lamarque Jean‐Francois , Yudin Valery , Müller Jean‐Francois , Gille John
Published: December 2002

A three‐dimensional (3‐D) inverse modeling scheme is used to constrain the direct surface emissions of carbon monoxide CO. A priori estimates of CO emissions are taken from various inventories and are included in the IMAGES model to compute the distribution of CO. The modeled CO mixing ratios are compared with observations at 39 CMDL stations, averaged over the years 1990–1996. The interannual variability of CO sources is therefore ignored. We show that the method used (time‐dependent synthesis inversion) is able to adjust the surface fluxes on a monthly basis in order to improve the agreement between the observed and the modeled CO mixing ratios at the stations. The Earth surface is divided into regions. The spatial distribution of CO sources is fixed inside each of these regions. The inversion scheme optimizes the intensities of the emissions fluxes for the following processes: technological activities, forest and savanna burning, agricultural waste burning and fuelwood use, soil/vegetation emissions, and oceanic emissions. The inversion significantly reduces the uncertainties on the surface sources over Europe, North America and Asia. The most striking result is the increase (almost by a factor of 2) of CO flux from Asia in all a posteriori scenarios. The uncertainties on the Southern Hemisphere emissions remain large after the inversion, because the current observational surface network is too sparse at these latitudes. The inversion, moreover, shifts the peak in biomass burning emissions in the Southern Hemisphere by one month. This temporal shift ensures a better match of the observed and modeled CO seasonal cycle at the Ascension Island station. We also attempted to optimize the annual and global productions of CO due to methane and NMHC. With the current set of data, the scheme was not able to differentiate between these two sources, and hence only the total chemical production of CO can be optimized.
Interannual variability of the Indian winter monsoon circulation and consequences for pollution levels
Authors: de Laat A. T. J. , Lelieveld J.
Published: December 2002

Near‐surface pollution over the northern Indian Ocean during the winter monsoon is subject to large variations between different months and different years. We study this variability using global chemistry‐climate model simulations of carbon monoxide (CO) during February and March of 1996 to 1999. Our results suggest that three processes largely determine the CO variability: the monsoon cycle, the El Niño/Southern Oscillation (ENSO) and the Madden‐Julian Oscillation (MJO). During the winter monsoon, northeasterlies advect polluted air masses in the boundary layer from the Asian continent to the InterTropical Convergence Zone (ITCZ). Variations in the strengths of the Asian continental high pressure and the ITCZ strongly influence pollution levels over the continent and the northern Indian Ocean. Strong continental outflow and a strong ITCZ moderate pollution concentrations through efficient transport. Strong continental outflow combined with a weak ITCZ, on the other hand, prevents efficient removal of pollution from the marine boundary layer, hence CO accumulates. During ENSO the Asian continental high pressure reduces, which weakens advection of polluted air masses, increases CO buildup in the polluted continental boundary layer and decreases CO over the northern Indian Ocean. Variations in the ITCZ strength are associated with active (strong) and passive (weak) phases of the MJO. During active phases the ventilation of pollution from the marine boundary layer reduces CO concentrations.
A new statistically based autoconversion rate parameterization for use in large‐scale models
Authors: Zhang Junhua , Lohmann Ulrike , Lin Bing
Published: December 2002

The autoconversion rate is a key process for the formation of precipitation in warm clouds. In climate models, physical processes such as autoconversion rate, which are calculated from grid mean values, are biased, because they do not take subgrid variability into account. Recently, statistical cloud schemes have been introduced in large‐scale models to account for partially cloud‐covered grid boxes. However, these schemes do not include the in‐cloud variability in their parameterizations. In this paper, a new statistically based autoconversion rate considering the in‐cloud variability is introduced and tested in three cases using the Canadian Single Column Model (SCM) of the global climate model. The results show that the new autoconversion rate improves the model simulation, especially in terms of liquid water path in all three case studies.
Characteristics of impulsive VHF lightning signals observed by the FORTE satellite
Authors: Light T. E. L. , Jacobson A. R.
Published: December 2002

We study very high frequency (VHF) and optical emissions from lightning, observed by the FORTE satellite, differentiating between impulsive (transionospheric pulse pairs (TIPPs)) and nonimpulsive events. TIPPs are seen to constitute 47% of the FORTE VHF data but only 32% of the optically coincident data. The median peak optical irradiance of the optical emission associated with TIPPs is 916 μW/m2 at FORTE and for non‐TIPPs is 195 μW/m2. The median effective pulse width of the optical signal from TIPPs is 658 μs, and it is 548 μs for non‐TIPPs. In the VHF, both event types have similar observed peak powers (0.086 mV2/m2 and 0.089 mV2/m2, for TIPPs and non‐TIPPs, respectively). The optically coincident lightning (of either type) is weaker in peak VHF emission than is the lightning that lacks coincident optical signals, although for non‐TIPPs, the stronger the VHF peak, the more likely the event is to have a coincident optical signal. For TIPPs, however, this is true only for events with peak E2 < 0.1 mV2/m2. Above that threshold, TIPPs are increasingly less likely to show coincident optical emission with increasing VHF peak E2. For both TIPPs and non‐TIPPs, the peak current reported by the U.S. National Lightning Detection Network™ and peak VHF power reported by FORTE are statistically proportional. The nature of the proportionality appears to depend upon the polarity of the discharge but not upon the event type. We also find that only 11% of TIPPs are associated with negative‐polarity discharges, compared to 75% of non‐TIPPs. Finally, we find that TIPPs arise from events with altitudes of 6–15 km, although we see optical coincidence only for those TIPPs occurring above ∼10 km.
Lagrangian simulation of suspended particles in the neutrally stratified surface boundary layer
Authors: Taylor P. A. , Li P. Y. , Wilson J. D.
Published: December 2002

The one‐dimensional equation of motion for suspended particles in the neutral atmospheric surface boundary layer is solved for the particle motion assuming that the sequence of vertical fluid velocities at the (moving) particle location can be modeled by a generalized Langevin equation. This inertial particle model is used to produce concentration profiles above a reflecting lower boundary for neutrally buoyant particles with inertia and for heavy particles with gravitational settling. Near the ground, modeled particle number concentration profiles above an infinite plane depart from the standard power law solution predicted by diffusion models. We investigate effective settling velocity and the eddy diffusivity for particles in this turbulent boundary layer flow and find that, near the ground, both are reduced in relation to particle terminal velocity and the assumptions made in boundary layer diffusion models.
Simulation of bioaugmentation involving exogenous bacteria injection
Authors: Wang Sookyun , Corapcioglu M. Yavuz
Published: December 2002

The degradation of organic hydrocarbons through microbial reactions in subsurface environments has been widely studied in the literature. Most studies, however, have focused on biostimulation, which aims to enhance biodegradation by indigenous bacteria. This study presents a mathematical model to simulate the fate and transport of a reactive contaminant degraded through cometabolism during the in situ bioaugmentations involving the injection of nutrients and/or exogenous bacteria. We incorporate hydrogeologic factors affecting the transport of contaminant as well as microbial metabolic reactions into the model. Modified Monod kinetics and a microcolony concept are used to investigate the effects of mobile bacteria in the aqueous phase and bacteria attached on solid surfaces on the transport and biodegradation of an organic contaminant. Permeability reduction due to microbial accumulation in pore spaces and its effect on the biodegradation processes are examined by performing a numerical experiment involving in situ biodegradations by exogenous bacteria. The effect of bacteria as biosorbents is also considered in model formulations to investigate the biocolloid‐facilitated transport of a contaminant. The two‐dimensional governing equations are solved numerically using a fully implicit finite difference method with an alternating direction implicit scheme. For model evaluation purposes, a model comparison is performed against an independently developed biodegradation model and showed remarkably close matches in the concentration profiles. The model is applied to a case study to demonstrate its behavior. The results of simulations show significant effects of bioaugmentative operations on the fate and distribution of all the chemical species and biomass and their interactions. In addition to the enhanced efficiency caused by the operation, operation‐induced limitations such as permeability reduction is also demonstrated. Factors determining the overall biodegradation rate include the bioavailability of the contaminant and the distribution of biomass and relevant chemical species. The overall results implied that the success of in situ bioaugmentation depends on how to control the contact of biodegrading microbes with contaminants and how to supply enough nutrients into the contaminated zone and stimulate microbial activities.
Multifractality of rainfall and scaling of intensity‐duration‐frequency curves
Authors: Veneziano Daniele , Furcolo Pierluigi
Published: December 2002

We derive scaling properties of the intensity‐duration‐frequency (IDF) curves under the condition that temporal rainfall is a stationary multifractal process. We find that under limiting conditions (very short durations D or very long return periods T), the IDF values are simple scaling with a power law dependence on D and T. The exponents of D and T differ in the two limiting cases of very short D and very long T and can be calculated from the moment scaling function K(q) of the rainfall time series. These theoretical findings differ from those of previous studies. They are validated through direct calculation and analysis of an actual rainfall record.
Climate‐induced changes in erosion during the 21st century for eight U.S. locations
Authors: Pruski F. F. , Nearing M. A.
Published: December 2002

Climate in the United States is expected to change during the 21st century, and soil erosion rates may be expected to change in response to changes in climate for a variety of reasons. This study was undertaken to investigate potential impacts of climate change on soil erosion by water. Erosion at eight locations in the United States was modeled using the Water Erosion Prediction Project model modified to account for the effects of atmospheric CO2 concentrations on plant growth. Simulated climate data from the U.K. Meteorological Office's Hadley Centre HadCM3 Global Circulation Model were used. The results indicated a complex set of interactions between the several factors that affect the erosion process. Direct effects of rainfall increases and decreases to runoff and erosion increases and decreases were observed but were often not dominant. One of the key factors of change in the system was the biomass production. Changes in soil moisture, atmospheric CO2 concentration, temperature, and solar radiation each impacted the biomass production at differing levels at the eight different sites. Different types of changes occurring at different periods of the year also complicated the response of the system. Overall, these results suggest that where precipitation increases are significant, erosion can be expected to increase. Where precipitation decreases occur, the results may be more complex due largely to interactions of plant biomass, runoff, and erosion, and either increases or decreases in overall erosion may be expected.
Stochastic modeling of flood peaks using the generalized extreme value distribution
Authors: Morrison Julia E. , Smith James A.
Published: December 2002

The generalized extreme value (GEV) distribution is a standard tool for modeling flood peaks, both in annual maximum series (AMS) and in partial duration series (PDS). In this paper, combined maximum likelihood estimation (MLE) and L moment (LMOM) procedures are developed for estimating location, shape, and scale parameters of the GEV distribution. Particular attention is given to estimation of the shape parameter, which determines the “thickness” of the upper tail of the flood frequency distribution. Mixed MLE–LMOM methods avoid problems with both MLE (estimator variance) and LMOM (estimator bias) estimators of the shape parameter. The mixed MLE–LMOM procedure is extended to use the two largest flood peaks in a year. This extension is developed in a PDS framework. Estimation procedures are applied to flood peak observations from 104 central Appalachian basins. The estimated values of the shape parameter for the central Appalachian basins are more negative than has been considered physically reasonable, independent of the estimation procedure that is used. Twenty‐eight percent of mixed method estimators of the shape parameter have values less than −0.5, implying that the moments of order 2 and above are infinite. The estimated shape parameters for the central Appalachian basins do not depend on basin morphological parameters (such as drainage area) or land cover properties (such as percent urban, forest, or agricultural land use). Estimated values of the location and scale parameters for the central Appalachian watersheds correspond well with GEV‐based simple scaling theory. Estimated values of the shape parameter for central Appalachian watersheds are shown to differ markedly from those of southern Appalachian watersheds and the difference is shown to be linked to contrasting properties of extreme floods. To conclude the paper, analyses of mixture distribution models are presented to address the question of whether flood peaks really have extreme “heavy tail” behavior or whether the GEV distribution is not the appropriate model for flood peaks.
Land data assimilation and estimation of soil moisture using measurements from the Southern Great Plains 1997 Field Experiment
Authors: Margulis Steven A. , McLaughlin Dennis , Entekhabi Dara , Dunne Susan
Published: December 2002

Remotely sensed microwave measurements provide useful but indirect observations of surface soil moisture. Ground‐based measurements are more direct but are very localized and limited in coverage. Model predictions provide a more regional perspective but rely on many simplifications and approximations and depend on inputs that are difficult to obtain over extensive areas. The only effective way to achieve soil moisture estimates with the accuracy and coverage required for hydrologic and meteorological applications is to merge information from satellites, ground‐based stations, and models. In this paper we describe a convenient data merging (or data assimilation) procedure based on an ensemble Kalman filter. This procedure is illustrated with an application to the Southern Great Plains 1997 (SGP97) field experiment. It uses land surface and radiative transfer models to derive soil moisture estimates from airborne L band microwave observations and ground‐based measurements of micrometeorological variables, soil texture, and vegetation type. The ensemble filter approach is appealing because (1) it can accommodate a wide range of models, (2) it can account for input and measurement uncertainties, (3) it provides information on the accuracy of its estimates, and (4) it is relatively efficient, making large‐scale applications feasible. Results from our SGP97 application of the ensemble Kalman filter include large‐scale maps (∼10,000 km2) of soil moisture estimates and estimation error standard deviations for the entire month long experiment and comparisons of filter soil moisture and latent heat estimates to ground truth measurements (gravimetric and flux tower observations). The ground truth comparisons show that the filter is able to track soil moisture fluctuations. The filter estimates are significantly better than those from an “open loop” simulation that includes the same ground‐based data but does not incorporate radio brightness measurements. Overall, the results from this field test indicate that the ensemble Kalman filter is an accurate, efficient, and flexible data assimilation procedure that is able to extract useful information from remote sensing measurements.
Energy‐constrained open‐system magmatic processes 3. Energy‐Constrained Recharge, Assimilation, and Fractional Crystallization (EC‐RAFC)
Authors: Spera Frank J. , Bohrson Wendy A.
Published: December 2002

Geochemical data for igneous rock suites provide conclusive evidence for the occurrence of open‐system processes within thermally and compositionally evolving magma bodies. The most significant processes include magma Recharge (with possible enclave formation and magma mixing), Assimilation of anatectic melt derived from wallrock partial melting and formation of cumulates by Fractional Crystallization (RAFC). In this study, we extend the Energetically Constrained Assimilation and Fractional Crystallization (EC‐AFC) model [Spera and Bohrson, 2001; Bohrson and Spera, 2001] to include the addition of compositionally and thermally distinct recharge melt during simultaneous assimilation and fractional crystallization. Energy‐Constrained Recharge, Assimilation, and Fractional Crystallization (EC‐RAFC) tracks the trace element and isotopic composition of melt, cumulates and enclaves during simultaneous recharge, assimilation and fractional crystallization. EC‐RAFC is formulated as a set of 3 + t + i + s coupled nonlinear differential equations, where the number of trace elements and radiogenic and stable isotope ratios modeled are t, i, and s, respectively. Solution of the EC‐RAFC equations provides values for the average wallrock temperature (Ta), mass of melt within the magma body (Mm), mass of cumulates (Mct) and enclaves (Men), mass of wallrock involved in the thermal interaction (Mao), mass of anatectic melt assimilated (M*a), concentration of t trace elements and i + s isotopic ratios in melt (Cm), cumulates (Cct), enclaves (Cen), and anatectic melt (Ca) as a function of magma temperature (Tm). Input parameters include the equilibration temperature (Teq), the initial temperature and composition of pristine melt (Tmo, Cmo, εmo), recharge melt (Tro, Cro, εro), and wallrock (Tao, Cao, εao), temperature‐dependent trace element distribution coefficients (Dm, Dr, Da), heats of transition for wallrock (Δha), pristine melt (Δhm), and recharge melt (Δhr), and the isobaric specific heat capacity of assimilant (Cp,a), pristine melt (Cp,m), and recharge melt (Cp,r). The magma recharge mass function, Mr(Tm), is specified a priori and defines how recharge magma is added to standing magma. The present EC‐RAFC simulator incorporates a weak coupling to major element mass balance and phase relations based on laboratory experiments or Gibbs Energy minimization [e.g., Ghiorso, 1997]. EC‐RAFC can be applied to a variety of magmatic systems including volcanic suites that show evidence of magma mixing, layered mafic intrusions, and granitoid plutons. Predictions for masses, as well as compositions of magmatic products, are part of the EC‐RAFC solution. The “systems” approach provides an opportunity to quantitatively assess the roles of assimilation, fractional crystallization, and magma recharge in magma evolution using trace element and isotopic constraints together with energy conservation.
Microbial mediation of authigenic clays during hydrothermal alteration of basaltic tephra, Kilauea Volcano
Authors: Konhauser Kurt O. , Schiffman Peter , Fisher Quentin J.
Published: December 2002

Highly altered, glassy tephras within the active steam vents at Kilauea Volcano, Hawaii, contain subsurface bacteria characterized by small (<500 nm in diameter), epicellular grains attached directly to the cell walls. Compositionally, the grains were dominated by Si, Al, Fe, and K, in a stoichiometry similar to a dioctahedral smectite. The initial dissolution of glass, which may in part have been microbiologically mediated, served as the source for many of the elements sequestered into the biomineralized clays. Overlying the tephras are white crusts (silica and calcite) and green‐colored biofilms. The biofilms comprise a filamentous, likely cyanobacterial, community coated with spherical (<100 nm in diameter) grains of amorphous silica directly attached to the sheaths. Individual precipitates can easily be resolved, but quite often they coalesce, forming a dense mineral matrix of amorphous silica. For both the clays and silica, the microbial surfaces are clearly sites for mineral nucleation and growth. These observations imply that microbial biomineralization may be a significant process in the overall alteration of primary basaltic glass in active steam vent systems.
Oceanic molybdenum isotope fractionation: Diagenesis and hydrothermal ridge‐flank alteration
Authors: McManus James , Nägler Thomas F. , Siebert Christopher , Wheat C. Geoffrey , Hammond Douglas E.
Published: December 2002

Isotopic analyses of dissolved molybdenum are presented for sediment pore waters from a reducing sedimentary basin and for fluids from a low‐temperature ridge flank hydrothermal system. δ98/95Mo in these fluids range from 0.8 to 3.5‰ (relative to a laboratory standard), demonstrating that marine sedimentary reactions significantly fractionate Mo isotopes. Within the upper 3 cm of sediment, manganese oxide dissolution produces an isotopically light fluid relative to seawater (mean of four analyses = 2.1 ± 0.1‰ versus seawater = 2.3 ± 0.1‰). Below 6 cm depth, authigenic Mo uptake results in an isotopically heavier fluid (up to 3.5‰) indicating that reducing sediments are likely to be a net sink for isotopically light dissolved Mo. In contrast, fluid circulation within a low‐temperature ridge‐flank hydrothermal system is a source of isotopically light Mo to the ocean having an end‐member fluid of ∼0.8‰.
The 2001 Bhuj earthquake: Tomographic evidence for fluids at the hypocenter and its implications for rupture nucleation
Authors: Kayal J. R. , Zhao Dapeng , Mishra O. P. , De Reena , Singh O. P.
Published: December 2002

The January 26, 2001 Bhuj earthquake (Mw 7.6) is one of the most catastrophic Indian earthquakes. We have investigated the 3‐D seismic velocity and Poisson's ratio structures of the Bhuj source area to understand the probable cause of triggering the earthquake. We used 1948 P and 1865 S‐wave high‐quality arrival times from 331 aftershocks recorded at a temporary seismic network. Significant variations up to 5% in velocity and 10% in Poisson's ratio are revealed in the aftershock area. The mainshock is located in a distinctive zone characterized by high‐Vp, low‐Vs and high Poisson's ratio (σ) in the depth range of 20 to 30 km and extending 15 to 30 km laterally. This feature is very similar to that of the 1995 Kobe earthquake [Zhao et al., 1996]. The anomaly may be due to a fluid‐filled, fractured rock matrix, which might have contributed to the initiation of the Bhuj earthquake.
The evolution of extreme temperatures in the Central England temperature record
Authors: Brabson B. B. , Palutikof J. P.
Published: December 2002

The Central England daily mean temperature record (CET) extends from 1772 to present. An extreme value analysis based on the Generalized Pareto distributions shows that both cold winter and hot summer extremes have evolved differently from their means. The reasons for the trends in extremes occurrence are related to changes in the underlying atmospheric circulation. Of particular interest are the winter extreme cold days. In recent years, around 80% of these occurred on anticyclonic or easterly type days, compared to only 50% at the beginning of the 20th century. This change is related to cooling temperatures on these type days since the 1930s. Such a trend has placed an effective cap on any upward trend in winter cold extremes related to global warming so far.
C‐vector for identification of oceanic secondary circulations across Arctic Fronts in Fram Strait
Authors: Chu Peter C.
Published: December 2002

Secondary circulation, referring to the motion relative to a basic flow (geostrophic and hydrostatic balanced), occurs often in the ocean such as deep convection and circulations driven by fronts and eddies. It affects the general circulation and the mass, heat, salt, and energy balance. The oceanic secondary circulation is difficult to measure directly, but is easy to be identified by pseudovorticity using routine observations. A C‐vector method, commonly used in atmospheric mesoscale moist frontogenesis, is applied to oceanography for identifying frontal secondary circulation in Fram Strait using Conductivity‐Temperature‐Depth data collected during a large‐scale hydrographic survey on R/V Valdivia cruise‐54 of the eastern Greenland Sea and Fram Strait from 16 March to 5 April 1987. Possible application of this method to large‐scale motion is also discussed.
Tetragonal structure of CaSiO3 perovskite above 20 GPa
Authors: Shim Sang‐Heon , Jeanloz Raymond , Duffy Thomas S.
Published: December 2002

High‐resolution synchrotron X‐ray diffraction measurements were performed at 20–46 GPa on CaSiO3 perovskite (Ca‐pv) synthesized in a laser heated diamond cell. After heating, we observed peak splittings for the cubic 200 and 211 diffraction lines, attributable to a tetragonal distortion of Ca‐pv. The bulk modulus of the tetragonal phase is 255 ± 5 GPa for K0T′ = 4. The observed intensities and d‐spacings of the split lines indicate that the c‐axis is shorter than the a‐axes (c/a = 0.993 − 0.996). This is compatible with either SiO6 octahedron rotation around the a‐axes or SiO6 octahedron distortion (shorter Si‐O bonds along the c‐axis than the a‐axes), provided that the observed phase has a tetragonal unit cell. These structural distortions in Ca‐pv can affect its physical properties and capacity for large cations.
A biomass burning source of C1–C4alkyl nitrates
Authors: Simpson Isobel J. , Meinardi Simone , Blake Donald R. , Blake Nicola J. , Rowland F. Sherwood , Atlas Elliot , Flocke Frank
Published: December 2002

We report the first observations of the emission of five C1–C4alkyl nitrates (methyl‐, ethyl‐,n‐propyl‐,i‐propyl‐, and 2‐butyl nitrate) from savanna burning. Average alkyl nitrate mixing ratios in the immediate vicinity of three bushfires in Northern Australia were 47–122 times higher than local background mixing ratios. These are the highest alkyl nitrate mixing ratios we have ever detected, with maximum mixing ratios exceeding 3 ppbv for methyl nitrate. Methyl nitrate dominated the alkyl nitrate emissions during the flaming stage of savanna burning, whereas C2–C4alkyl nitrates were mostly emitted during the smoldering stage. To explain the formation of alkyl nitrates from biomass burning, we propose a reaction mechanism involving the combination of reactive radicals at high temperature. Bearing in mind the uncertainties associated with extrapolating small data sets to much larger scales, alkyl nitrate emissions from global savanna burning are estimated to be on the order of 8 Gg/yr.
Improved interplanetary magnetic field propagation timing by correction of the phase front orientation using two spacecraft
Authors: Blanchard G. T. , Bankston D.
Published: December 2002

We present a method of calculating the interplanetary magnetic field propagation delay in the solar wind that that uses observations from two spacecraft to partially correct the phase front orientation. A third spacecraft is used to test the accuracy of the method. We show that the two‐spacecraft method reduces the overall propagation error by 23%. The reduction in the error is greater for larger values of the perpendicular spacecraft separation; for perpendicular separation less than 13 RE, the error reduction is only 7%, while for perpendicular separation greater than 13 RE, the error reduction is 26%. In addition, the two‐spacecraft technique allows the calculation of the uncertainty in the propagation delay in individual cases, which allows the user to identify cases in which the propagation is more precise than on average.