bluuebunny/crossref_metadata_2025 huggingface dataset:
Metal‐coated printed circuit board time domain reflectometry probes for measuring water and solute transport in soil
Authors: Nissen Henrik H. , Ferré Paul A. , Moldrup Per
Published: July 2003

Metal‐coated printed circuit board (PCB) time domain reflectometry (TDR) probes for simultaneous small‐scale measurements of soil water content (θ) and bulk soil electrical conductivity (ECb) was developed and tested. To resolve corrosion problems associated with previous PCB probe designs, a nickel and gold plated copper waveguide is positioned at the surface of the PCB circuit laminate. In a numerical analysis the effect of PCB probe design parameters on the sample area distribution transverse to wave propagation was examined. The conclusions of the analysis were (1) two‐rod PCB probes have a larger sample area compared to three‐rod probes, (2) the probe rod separation has a larger effect on the sample area compared to rod width, and (3) if the thickness of the circuit laminate is 2/3 of the rod separation, a two‐rod PCB probe is insensitive to the medium below the laminate. On the basis of the numerical analysis, 20 different metal‐coated PCB probes were produced: five probe types on four laminates. Correct determination of PCB probe measured θ and ECb was ensured by calibration. The performance of the PCB probes in soil was tested in a water and solute transport experiment. Twenty PCB probes and five conventional two‐rod TDR probes were positioned in five horizontal transects. Tap water was added to an air‐dry soil and exchanged with a KCl solution. All probes registered changes in θ and ECb, but in some cases the level measured by the PCB probes differed significantly from the conventional probes. An ECb(θ) data analysis revealed that the differences was caused by local variations in θ rather than erroneous calibrations of the PCB probes. The metal‐coated PCB probe is a useful new TDR probe design for simultaneous small‐scale measurements of water and solute transport.
Dissolved aluminum and silica in the Changjiang (Yangtze River): Impact of weathering in subcontinental scale
Authors: Zhang J. , Ren J. L. , Liu S. M. , Zhang Z. F. , Wu Y. , Xiong H. , Chen H. T.
Published: July 2003

Water samples were collected from the river mouth upstream over a distance of 3500–4000 km in the Changjiang (Yangtze River) and its 15 tributaries in April–May 1997, and at Nantong close to the river mouth in 1996–1998. Dissolved Al and Si were determined and compared with data of water flow, total suspended matter, and alkalinity. Data obtained show that dissolved Al in the Changjiang illustrates a rather uniform distribution from upstream toward river mouth, and a broad increasing of Al with higher Si to alkalinity ratio in tributaries has been found. The plots of Al:Si ratio against specific runoff and sediment yields reveal different weathering mechanisms involved in tributaries. At Nantong, concentration of Al is considerably stable relative to important seasonal variations of water discharge and sediment load. Acidification of bulk riverine samples releases an amount of Al that can be one order of magnitude higher than the natural concentrations, indicating potential remobilization by acid deposition and/or waste drainage. Compared with other world rivers, concentration of Al in the Changjiang is low and similar to those from primitive systems.
Numerical models of lithospheric deformation forming the Southern Alps of New Zealand
Authors: Gerbault Muriel , Henrys Stuart , Davey Fred
Published: July 2003

Compression of the entire continental lithosphere is considered using two‐dimensional numerical models to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages. The numerical scheme incorporates brittle‐elastic‐ductile rheology, heat transfer, surface processes, and fault localization. Models are based on the central section of the New Zealand Southern Alps, where continental collision has occurred along the Alpine Fault since about 7 Ma. The results are compared to the surface relief, the GPS convergence velocity, the measured electrical conductivity, and the geometry of the crustal root imaged from seismic velocity measurements. The crustal deformation is characterized by localized uplift at the plate boundary (Alpine Fault) and by two secondary zones of faulting. One is located ∼60–80 km east of the Alpine Fault, at the start of upper crust bending (or tilting), and the other is located ∼100–130 km east of the Alpine Fault as a result of shear strain propagating to the surface through the ductile lower crust. The observed asymmetric shape of the crustal root is best reproduced for mantle lithosphere strength of the order of 200 MPa and an intermediate rate of strain softening. A lower strength of the mantle lithosphere can produce symmetric thickening, but the amplitude of the crustal root is too small when compared to observations. The observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced. This offset is most likely due to the three dimensionality of oblique collision in the Southern Alps.
Possible dynamic and thermal causes for the recent decrease in sea ice in the Arctic Basin
Authors: Makshtas Alexander P. , Shoutilin Sergey V. , Andreas Edgar L.
Published: July 2003

A dynamic‐thermodynamic sea ice model with 50‐km spatial and 24‐hour temporal resolution was used to investigate the spatial and temporal variability of the sea ice cover and the surface energy exchange in the Arctic Basin. Daily surface level air temperature and pressure data from National Centers for Environmental Prediction for 1958–1997 and climatic data for cloudiness, relative humidity, snow precipitation, and the heat flux from the deep ocean in the Greenland, Barents, and Bering Seas were used as external forcing. The model is integrated using the method of large particles [Belotserkovskii, 1984]. The model satisfactorily reproduces the seasonal and interannual variability of the main characteristics of the sea ice, the sea ice extent and exchange through the main straits, and the surface heat exchange for different parts of the Arctic Basin. In particular, estimates of the year‐to‐year difference between mean sea ice thickness in September show good agreement with the essential thinning of sea ice in the Canadian Basin that Rothrock et al. [1999] found. Most of the decrease in sea ice thickness is caused by a decrease in ridge concentration and an increase in the area covered by undeformed ice. Numerical experiments show that only 20% of the ice decrease can be explained by an increase in surface level air temperature. Model results for the Canadian and Eurasian Basins allow us to determine the main trends in the temporal variability of ice thickness in these regions. Our model results suggest that the recent thinning of the sea ice correlates with a decrease in the concentration of ridged ice. In turn, the concentration of ridged ice and the total sea ice volume in the Canadian Basin correlate with cyclonic and anticyclonic regimes of the wind‐driven ice motion. The sharp decrease in modeled ice volume in the Canadian Basin after 1987 also coincides with a significant increase in the mean atmospheric vorticity index for the central Arctic Ocean.
Effect of biogenic volatile organic compound emissions on tropospheric chemistry during the Atmospheric Pollution Over the Paris Area (ESQUIF) campaign in the Ile‐de‐France region
Authors: Derognat C. , Beekmann M. , Baeumle M. , Martin D. , Schmidt H.
Published: July 2003

This paper investigates the impact of biogenic isoprene and terpene emissions on photochemical species levels in the French Ile‐de‐France region during several photooxidant pollution episodes in summer 1998 and 1999 during the Atmospheric Pollution Over the Paris Area (ESQUIF) project. The effect of biogenic emissions on both ozone produced on a continental scale and advected in Ile‐de‐France and on ozone locally formed are assessed. For this purpose, simulations with and without biogenic emissions are performed with a nested version of the CHIMERE model. This chemistry transport model includes both a continental (western European) domain with 0.5° horizontal resolution and a regional domain (Ile‐de‐France) of 150 × 150 km extension with a horizontal resolution of 6 km. An emissions database for biogenic isoprene and terpene emissions from forests has been set up. These emissions are estimated using emission factors for different tree species recently revised by Simpson et al. [1999] and for different land use data sets, including highly resolved (1 km) satellite measurements for Ile‐de‐France. Good agreement has been found between modeled and measured (by aircraft) isoprene levels (overall bias <10%), which lends confidence to the use of the emissions database for subsequent simulations. The comparison between runs with and without biogenic volatile organic compound (VOC) emissions indicates a significant difference in ozone in Ile‐de‐France, up to 40 ppb for one extreme day. Biogenic VOC emissions from Ile‐de‐France along with those from outside the Ile‐de‐France region have an approximately equal responsibility for the additional ozone buildup. The main reason for this increased ozone formation is the enhancement of radicals due to larger concentrations of carbonyl species and ozone and their subsequent photolysis. Biogenic emissions lead to a shift in the sensitivity to emissions (toward more “NOx‐limited”). However, generally, either with or without biogenic emissions, a “VOC‐limited” regime is simulated over the Ile‐de‐France region.
Application of salinization indicators and initial development of potential global soil salinization scenario under climatic change
Authors: Schofield R. V. , Kirkby M. J.
Published: July 2003

Salt accumulation in soil is a global problem. This paper investigates salt‐affected soils on a global scale through spatial modeling within a Geographical information system (GIS). As salt‐affected soils arise due to a range of processes that can be very localized and complex in nature, three main indicators have been developed to provide insight into areas at risk of salinization at a global scale. At this scale, climate and topography are the main factors influencing solute accumulation. The paper derives salinity indicators from global data sets and compares them to independent maps of salt‐affected areas. The primary indicators are (1) low relief, (2) high two‐way annual moisture flux, and (3) local flow deficit in large catchments. The first two of these are related for conditions where there is potential for a large vertical flux of salts, and where groundwater levels may be high, providing conditions for salt accumulation locally or at the base of slope catenas. The third indicator is related to conditions where there is a risk of accumulation of salts in floodplains and adjacent irrigated areas. These salinity indicators are then estimated for future GCM scenarios of climate for 2079–2099. Comparison of the current and scenario indicator distributions shows that the areas at risk of salinization may increase globally, through both expansion of existing areas and the creation of new ones. Current distributions of risk, and to a greater extent future scenarios, are limited by the quality and resolution of available data, and have the potential for enhancement as these data improve.
On using pigment‐normalized, light‐saturated carbon uptake with satellite‐derived pigment for estimating column photosynthesis
Authors: Banse Karl , Postel James R.
Published: July 2003

Using five Pacific open‐sea data sets with five subsets, we linearly correlated observed column photosynthesis (Pt, mg C m−2 d−1, from 14C‐uptake) with “satellite pigment” (Csat, mg m−3), simulated from in situ data at the same stations, and modeled Pt by a depth‐integrated approach. The correlation (r2) of Pt with Csat and its accuracy improved after combining Csat with an in situ‐based proxy for the light‐saturated rate of photosynthesis, Poptb (mg C (mg chl d)−1), for the particular stations. Incorporating station Poptb always led to high r2 and accuracy of the regression of modeled on observed Pt. However, Poptb cannot yet be determined from space, so that in practice, geographic or seasonal means of Poptb have to be used. For our 10 sets, such means lead to more imprecise (lower r2 in 9 of 10 cases) and less accurate (for all) results, because Poptb and Pt are often correlated. We expect this error source to be widespread and to afflict all modeling of Pt using a term for light‐saturated photosynthesis, but we cannot offer a solution to the quandary. On the basis of combining of data sets, we do not favor the use of a single algorithm employing light‐saturated photosynthesis for calculating global Pt by a depth‐integrated approach.
Hafnium isotopic variations in volcanic rocks from the Caribbean Large Igneous Province and Galápagos hot spot tracks
Authors: Geldmacher J. , Hanan B. B. , Blichert‐Toft J. , Harpp K. , Hoernle K. , Hauff F. , Werner R. , Kerr A. C.
Published: July 2003

We report Hf isotope compositions of 79 lavas that record the early (∼5–95 Ma) history of the Galápagos plume volcanism. These include lavas from the Caribbean Large Igneous Province (CLIP; ∼95–70 Ma), the accreted Galápagos paleo‐hot spot track terranes (54–65 Ma) of Costa Rica (Quepos, Osa and Burica igneous complexes), and the Galápagos hot spot tracks (<20 Ma) located on the Pacific seafloor (Cocos, Carnegie, Malpelo, and Coiba Ridges and associated seamounts). These samples have previously been well characterized in terms of major and trace elements, Sr‐Nd‐Pb isotopes and Ar/Ar ages. As a result of the relative immobility of the high field strength and rare earth elements during syn‐ and post‐emplacement hydrothermal activity and low‐temperature alteration, combined Lu‐Hf and Sm‐Nd isotope systematics, when used in conjunction with Pb isotopes, provide a particular powerful tool, for evaluating the source compositions of ancient and submarine lavas. The combined Nd‐Hf isotope data suggest that three of the isotopically distinct source components found today in the Galápagos Islands (the Floreana‐like southern component, the Fernandina‐like central component, and the depleted Genovesa‐like eastern component) were present in the CLIP already by 95–70 Ma. The fourth Pinta‐like northern component is first recorded at about 83–85 Ma by volcanism taking place during the transition from the plume head/CLIP to plume tail stage and has then been present in the hot spot track continuously thereafter. The identification of the unique northern and southern Galápagos Plume Hf‐Nd‐Pb isotope source signatures within the CLIP and the oldest hot spot track lavas provides direct evidence that the CLIP represents the plume head stage of the Galápagos hot spot. Hafnium isotopes are consistent with the possibility that two types of sediment components may have contributed to the Hf, Nd and Pb isotope compositions of the Galápagos plume lavas. One component, characterized by Δ207Pb/204Pb ≈ 0 and high positive ΔεHf has an isotope signature indicative of relatively recently recycled pelagic sediment, a signature typical of the southern Galápagos island Floreana. The other component has an EM like isotopic composition resembling modern seafloor sediments with positive Δ207Pb/204Pb and lower ΔεHf, a signature typical of the northern Galápagos island Pinta.
Size‐fractionated iron distributions and iron‐limitation processes in the subarctic NW Pacific
Authors: Nishioka Jun , Takeda Shigenobu , Kudo Isao , Tsumune Daisuke , Yoshimura Takeshi , Kuma Kenshi , Tsuda Atsushi
Published: July 2003

Comparison of vertical profiles of size‐fractionated iron between the western and eastern subarctic North Pacific clearly showed higher labile particulate iron concentrations towards the west and this result strongly supports the higher iron supply in the western region. Additionally, the results of the SEEDS experiment, the first meso‐scale iron enrichment experiment in the subarctic North Pacific, clearly showed that artificially enriched iron in the dissolved fraction (mainly in colloidal fraction) was rapidly transformed to suspended labile particulate iron during phytoplankton growth and was retained in the surface mixed layer. Probably, this same rapid transformation process occurs naturally after sporadic atmospheric iron supply and the labile particulate iron is retained in the western region. Furthermore, this transformation process reduces dissolved concentration of iron and its bioavailability. Therefore, the transformation process is important for understanding how phytoplankton became iron limited and the biogeochemical iron cycle in the western subarctic North Pacific.
A new perspective on magnetotail disturbances in terms of inherent diamagnetic processes
Authors: Nakamizo Aoi , Iijima Takesi
Published: July 2003

Using the vector magnetic field data and the low‐energy particle data acquired with the Geotail satellite, we have investigated the characteristics of disturbances that occurred in the magnetotail region at a geocentric distance of ∼9 to 30 RE. We focused the investigation on the changes in three kinds of energy density, considering their primary importance, their relationship, and their flux transport. We divided the disturbances into Pi2 oscillatory variation (with a period of 40–150 s) and background disturbances. Characteristics of disturbances determined here include the following: (1) Plasma thermal pressure and magnetic pressure vary exactly in out‐of‐phase. The intensity of energy flux is greater parallel to the magnetic field than transverse to it. The plasma fluid clearly exhibits the diamagnetic properties; (2) Characteristics are seen on almost all time scales relevant to disturbances (both Pi2s and background disturbances) and in almost all the 139 examples. The substance of magnetotail Pi2 is the slow MHD waves, and the nature of background disturbances indicates slow MHD disturbances. We argue that the magnetotail plasma sheet is comprised of two types of flux‐tube plasma exhibiting compound behaviors. One is composed of the plasma that basically shrinks parallel to the magnetic field and simultaneously stretches perpendicular to it. The other is composed of the plasma that basically stretches parallel to the magnetic field and simultaneously shrinks perpendicular to it.
Electron heat flow in the solar corona: Implications of non‐Maxwellian velocity distributions, the solar gravitational field, and Coulomb collisions
Authors: Dorelli John C. , Scudder Jack D.
Published: July 2003

It has long been known that weak electron temperature gradients in fully ionized plamas (satisfying λ ∣∇Te∣/Te ≲ 10−4, where λe is the electron mean free path and Te is the electron temperature) can lead to the development of significant non‐Maxwellian suprathermal tails on electron velocity distributions, invalidating the Spitzer and Härm [1953] perturbation theory [Gray and Kilkenny, 1980; Bell et al., 1981; Scudder and Olbert, 1983]. In this paper we work out the implications of such nonlocal heat flow for electrons in the solar corona, comparing a new analytical theory to numerical solutions of the Fokker‐Planck equation. While electron‐electron Coulomb collisions are strong enough at coronal densities to influence the local temperature, the electron heat flux is determined by the essentially collisionless high‐energy tail. The deceleration of suprathermal electrons in the polarization electric field allows electron heat to flow radially outward against the local temperature gradient, in contrast to the local thermodynamic equilibrium picture, in which heat is constrained to flow down the local temperature gradient. We discuss the implications of this effect for empirical constraints of coronal heating mechanisms.
Cassini plasma spectrometer measurements of Jovian bow shock structure
Authors: Szego Karoly , Young David T. , Barraclough Bruce , Berthelier Jean‐Jacques , Coates Andrew J. , McComas David J. , Crary Frank J. , Dougherty Michele K. , Erdos Geza , Gurnett Donald A. , Kurth William S. , Thomsen Michelle F.
Published: July 2003

The Cassini spacecraft on its way to Saturn flew by Jupiter and crossed its bow shock more than forty times on the dusk‐side of the planet, whereas the early missions targeting Jupiter explored the dawnside. Here we report the first results concerning these bow shock crossings, based on the measurements of the Cassini Plasma Spectrometer (CAPS), the magnetometer, and the radio and plasma wave science (RPWS) instrument. We present data for five bow shock crossings, one at about 1920 local time (LT), the other four between 2100 and 2130 LT, 47.5°–50° beyond terminator. During the flyby the solar activity was high and variable. The measurements confirm that the Jovian bow shock is huge, extending over 700 RJ down the flank; Cassini was the first to observe such distant shock features. The bow shock was turbulent and very dynamic and magnetic fluctuations were superimposed on the shock; the downstream ion distributions exhibited bimodal structure time to time. For all bow shock crossings the onset of ion thermalization was a clear shock signature supported by an electrostatic wave signal; thermalization can be used as a signature of the shock location even in those cases when the field data are rather smeared. The strength of the shock potential weakened toward more distant regions even if the local Mach number did not decrease. Reflected protons were not detected upstream above our current sensitivity limit, but the incoming solar wind fluctuated in the foot region. We argue that the Jovian bow shock is not always in a steady state, and some of the observations might be connected with this fact.
Solitons, oscillitons, and stationary waves in a cold p − α plasma
Authors: Dubinin E. , Sauer K. , McKenzie J. F. , Chanteur G.
Published: July 2003

We investigate the structure of nonlinear stationary waves propagating transverse and obliquely to the magnetic field in a cold plasma consisting of two ion populations, protons and alpha particles. By using the constants of motion which follow from the multifluid equations, the system may be described by a single first‐order differential equation for the transverse case and four coupled first‐order differential equations in the case of oblique propagation. In the transverse case solitons exist if the wave speed lies between the Alfven speed, based on the total mass density, and the Alfven speed modified by the density and charge ratios. At speeds in excess of this latter velocity, periodic solutions exist in which protons and alphas gyrate around each other. An analog of Rankine‐Hugoniot type relations for the amplitude of the solitons and periodic waves is found for structures propagating transverse to the magnetic field. In the case of an oblique stationary wave it is shown that the tip of the proton (alpha ion) flow velocity vector moves on the surface of a sphere whose radius is determined only by the obliquity and the wave speed. Soliton solutions representing both compressions and rarefactions in the ion fluids exist in specific windows in the “Alfven Mach number‐obliquity” space. In other windows, solutions characterized by both oscillating and soliton properties (“oscillitons”) exist. Critical Mach numbers and critical propagation angles limit the size of the windows in which smooth solitons can be constructed.
Solitons, oscillitons, and stationary waves in a warm p − α plasma
Authors: Dubinin E. , Sauer K. , McKenzie J. F. , Chanteur G.
Published: July 2003

This paper generalizes the treatment by Dubinin et al. [2003] on solitons, oscillitons, and stationary waves in a two ion species (protons and alpha particles) cold magnetized plasma to include the effects of plasma thermal pressures. A fully nonlinear approach is used. For waves propagating transverse to the magnetic field the system of equations again reduces to a first‐order differential equation which may be analyzed by the method of phase portraits and the necessary conditions for solitons and periodic waves found. Solutions exist in specific windows of the sonic and Alfvenic Mach numbers. The inclusion of pressure effects leads to the appearance of singularities or critical Mach numbers which lie on certain loci of the speeds of the species. For waves propagating obliquely to the magnetic field, another class of solutions appears, which we call oscillitons, since they exhibit solitary as well as oscillatory wave properties. “Existence windows” in parameter space (speed and propagation angle and ion and electron betas) in which solitons and oscillitons may be constructed are found from the dispersion equation for stationary waves. Possible applications of the results to different space plasma phenomena are discussed.
Tail lobe convection observed by Cluster/EDI
Authors: Noda H. , Baumjohann W. , Nakamura R. , Torkar K. , Paschmann G. , Vaith H. , Puhl‐Quinn P. , Förster M. , Torbert R. , Quinn J. M.
Published: July 2003

By using drift velocity data measured in the near‐Earth tail lobe by the Electron Drift Instrument aboard the Cluster spacecraft, we study the average tail lobe convection of the Earth's magnetosphere. The data confirm the convection pattern toward the neutral sheet in the meridional (XZ) plane. By dividing the data according to the IMF clock angle, it is found that the convection direction in the plane perpendicular to the Sun‐Earth line (YZ plane) are controlled by the IMF By component. It is understood in terms of the solar wind‐induced electric field to the magnetotail lobe via dayside magnetic reconnection. For positive IMF Bz the average convection velocity is very low. For southward IMF, convection becomes strongest and a dawnward component appears.
Diffuse auroral electron scattering by electron cyclotron harmonic and whistler mode waves during an isolated substorm
Authors: Horne R. B. , Thorne R. M. , Meredith N. P. , Anderson R. R.
Published: July 2003

There are two main theories for the origin of diffuse auroral electron precipitation: precipitation by electrostatic ECH waves and precipitation by whistler mode waves. Here we analyze a case event where whistler mode hiss, chorus, and ECH waves are intensified during a weak substorm injection event to identify the source of particle precipitation. Examination of the particle data shows that there are three sources of free energy: a temperature anisotropy, a loss cone, and a pancake distribution. Instability analysis shows that the temperature anisotropy excites whistler mode hiss whereas both the temperature anisotropy and the pancake distribution contribute to the excitation of chorus. ECH waves are driven unstable by the loss cone. Wave propagation studies show that the path integrated gain of hiss and chorus is almost unaffected by changes in the depth of the loss cone, whereas ECH waves are very sensitive. Analysis of the changes in the resonant energy during propagation shows that the hiss resonates with electrons above a few keV while chorus resonates below a few hundred eV. As a result, neither hiss nor chorus are likely to cause significant electron precipitation from a few hundred eV to a few keV for this event. On the other hand, ECH waves resonate with electrons in the energy range between that for chorus and hiss. ECH waves can scatter electrons with pitch angles of up to 80° into the loss cone. We conclude that ECH waves are responsible for the formation of the pancake distribution and are probably the main component of diffuse auroral precipitation during this event. We suggest that substorm‐injected electrons are responsible for the intensification of hiss and ECH waves and that rapid scattering of electrons by ECH waves forms the pancake distribution which then excites chorus. We also suggest that rapid pitch angle scattering by ECH waves could be responsible for double frequency banded chorus emissions.
Small‐scale electric fields in downward auroral current channels
Authors: Streltsov A. V. , Lotko W.
Published: July 2003

The origin and spatiotemporal properties of small‐scale, intense electric fields and currents commonly observed by low‐altitude, polar‐orbiting satellites in the vicinity of discrete auroral arcs are investigated numerically. It is shown that these electromagnetic structures can be generated by the development of ionospheric feedback instability inside the resonant cavity formed by the ionospheric E‐layer and an auroral acceleration region. The major factor regulating feedback unstable dynamics is the downward current channel of a large‐scale, slowly evolving auroral current system interacting with the ionosphere. The downward current lowers the instability threshold by depleting the E‐region plasma density and conductivity, thereby allowing the development of a large perpendicular electric field in the E‐layer. The downward field‐aligned current also produces a collisionless resistive layer in the lower magnetosphere where the parallel drift speed of current‐carrying electrons exceeds a critical threshold for onset of microinstability. This resistive layer provides a highly reflective upper boundary for the resonant cavity confining small‐scale Alfvén waves generated at the ionosphere by the feedback mechanism. The quality of the ionospheric resonator without the turbulent upper boundary is shown to be very poor for resonator modes with transverse wavelengths of the order of 10 km. Application of the simulation results to the interpretation of satellite measurements of small‐scale electromagnetic structures in the auroral zone is demonstrated.
Sedimentation from strong volcanic plumes
Authors: Bonadonna Costanza , Phillips Jeremy C.
Published: July 2003

We develop descriptions of the key processes influencing tephra dispersal from strong volcanic plumes. These are characterized by the development of a subvertical eruption column in the atmosphere that forms a spreading current at a level of neutral buoyancy. We describe the propagation of the spreading current due to both gravity and wind advection using scaling arguments and a simplified geometry to model particle sedimentation in a windy as well as a wind‐free environment. New parameterizations are used to describe the wind field below the spreading current, particle aggregation, and particle density variations. We conducted a broad study to investigate the effects of these processes and made comparisons with field observations. The greatest variations resulted from wind advection below the spreading current, which shifts the plume‐corner mass accumulation and the position of transitions in fallout regimes downwind. Particle aggregation strongly depends on the initial grain‐size distribution and significantly affects deposit thinning, depending on the aggregate size and density, and on the relative amount of aggregating particles. Small aggregates can reproduce secondary maxima of mass accumulation when sedimenting in a wind field. Variation of particle density also affects the resulting thinning trend. The model provides acceptable reproduction of observations of the propagation of the spreading current (tested with the Plinian phase of the 18 May 1980 eruption of Mount St. Helens) and of observed thinning of tephra fallout deposits (tested with deposits from Mount St. Helens, 1980, Askja D 1875, Quizapú 1932, Minoan eruption, 3.6 ka B.P., Novarupta A, 1912 and Hudson, 1991).
Characterization of the time‐dependent strain field at seismogenic depths using first‐motion focal mechanisms: Observations of large‐scale decadal variations in stress along the San Andreas Fault system
Authors: Sipkin Stuart A. , Silver Paul G.
Published: July 2003

We present a method for summing moment tensors derived from first‐motion focal mechanisms to study temporal dependence in features of the subsurface regional strain field. Time‐dependent processes are inferred by comparing mechanisms summed over differing time periods. We apply this methodology to seismogenic zones in central and southern California using focal mechanisms produced by the Northern and Southern California Seismograph Networks for events during 1980–1999. We find a consistent pattern in both the style of deformation (strike‐slip versus compressional) and seismicity rate across the entire region. If these temporal variations are causally related, it suggests a temporal change in the regional‐scale stress field. One change consistent with the observations is a rotation in the regional maximum horizontal compressive stress direction, followed by a reversal to the original direction. Depending upon the dominant style of deformation locally, this change in orientation of the regional stress will tend to either enhance or hinder deformation. The mode of enhanced deformation can range from increased microseismicity and creep to major earthquakes. We hypothesize that these temporal changes in the regional stress field are the result of subtle changes in apparent relative plate motion between the Pacific and North American plates, perhaps due to long‐range postseismic stress diffusion. Others have hypothesized that small changes in plate motion over thousands of years, and/or over decades, are responsible for changes in the style of deformation in southern California. We propose that such changes, over the course of just a few years, also affect the style of deformation.
Modeling of seismic guided waves at the Dead Sea Transform
Authors: Haberland Christian , Agnon Amotz , El‐Kelani Radwan , Maercklin Nils , Qabbani Issam , Rümpker Georg , Ryberg Trond , Scherbaum Frank , Weber Michael
Published: July 2003

On several recordings of linear seismometer arrays crossing the Arava Fault (AF) in the Middle East, we see prominent wave trains emerging from in‐fault explosions which we interpret as waves being guided by a fault zone related low‐velocity layer. The AF is located in the Arava Valley and is considered the principal active fault of the mainly N‐S striking Dead Sea Transform System in this section. Observations of these wave trains are confined to certain segments of the receiver lines and occur only for particular shot locations. They exhibit large amplitudes and are almost monochromatic. We model them by a two‐dimensional (2‐D) analytical solution for the scalar wave field in models with a vertical waveguide embedded in two quarter spaces. A hybrid search scheme combining genetic algorithm and a local random search is employed to explore the multimodal parameter space. Resolution is investigated by synthetic tests. The observations are adequately fit by models with a narrow, only 3–12 m wide waveguide with S wave velocity reduced by 10–60% of the surrounding rock. We relate this vertical low‐velocity layer with the damage zone of the AF since the location of receivers observing and of shots generating the guided waves, respectively, match with the surface trace of the fault. The thickness of the damage zone of the AF, at least at shallow depths, seems to be much smaller than in other major fault zones. This could be due to less total slip on this fault.
Assessment of the reliability of wave observations from voluntary observing ships: Insights from the validation of a global wind wave climatology based on voluntary observing ship data
Authors: Gulev Sergey K. , Grigorieva Vika , Sterl Andreas , Woolf David
Published: July 2003

This paper describes development and validation of a global climatology of basic wave parameters based on the voluntary observing ship (VOS) data from the Comprehensive Ocean‐Atmosphere Data Set collection. Climatology covers the period 1958–1997 and presents heights and periods for the wind sea, swell, and significant wave height (SWH) over the global ocean on 2° × 2° spatial resolution. Significant wave height has been derived from separate sea and swell estimates by taking square root of the sum of squares for the seas and swells propagating approximately in the same direction and assuming SWH to be equal to the higher of the two components in all other cases. Special algorithms of corrections were applied to minimize some biases, inherent in visual wave data. Particularly, we corrected overestimation of small seas, corrected underestimation of periods, and analyzed separation between sea and swell. Validation included estimation of random observational errors, observation of sampling errors, and comparison with the alternative wave data. Estimates of random observational errors show that for the majority of locations, observational uncertainties are within 20% of mean values, which allows us to discuss quantitatively the produced climatology. Biases associated with inadequate sampling were quantified using the data from high‐resolution WAM hindcast for the period 1979–1993. The highest sampling biases are observed in the South Ocean, where wave height may be underestimated by 1–1.5 m because of poor sampling, primarily associated with a fair weather bias of ship routing and observation. Comparison to the other VOS‐based products shows in general higher SWH in our climatology, especially in the midlatitudes. However, comparison with the altimeter data shows that even for well‐sampled regions, high waves are still underestimated in VOS, suggesting a ubiquitous fair weather bias. Further ways of improving VOS‐based wave climatologies and possible applications are discussed.
Interannual variability in summer Beaufort Sea ice conditions: Relationship to winter and summer surface and atmospheric variability
Authors: Drobot Sheldon D. , Maslanik James A.
Published: July 2003

Interannual variations in the Beaufort Sea summer ice cover influence climatic, ecological, and socioeconomic conditions. Utilizing an ice severity index based on the distance from Point Barrow, Alaska, to the ice edge and characteristics of the shipping season, variations in summer ice conditions are examined from 1953 through 2000 with respect to variations in winter and summer surface and meteorological conditions. Although there is no significant trend in the ice severity index, ice conditions during the 1990s were less severe than in previous decades. Interannual variations in summer ice conditions are related to dynamical mechanisms in the preceding winter and dynamical and thermodynamical processes during summer. In winters preceding light ice summers, high pressure develops over Siberia, leading to increased ice transport across the Transpolar Drift Stream, while the Beaufort Gyre is diminished, resulting in decreased multiyear ice transport into the Beaufort Sea. Air temperatures are also elevated near the Alaskan North Slope, but there is little difference in winter air temperature over the Beaufort Sea. During light ice summers the Beaufort High is strongly defined over the Canadian Arctic Archipelago, enhancing easterly and northerly winds and advecting sea ice out of the Beaufort Sea. Above‐normal air temperatures also occur over much of the southern Beaufort Sea. Variations in sea level pressure and winds that result in light and heavy ice summers are closely linked to positive and negative winter Arctic Oscillation/North Atlantic Oscillation phases, as well as negative and positive summer Arctic Oscillation/North Atlantic Oscillation phases.
Mass transport between a semienclosed basin and the ocean: Maracaibo System
Authors: Laval B. , Imberger J. , Findikakis A. N.
Published: July 2003

Field data and three‐dimensional numerical (Estuary and Lake Computer Model) simulations are used to investigate exchange between Lake Maracaibo and the Gulf of Venezuela. This exchange occurs along a dredged shipping lane 100 km long and 14 m deep. It is driven by a density difference between the lake and gulf and modulated at tidal and subtidal timescales by water level fluctuations in the gulf propagating to the lake. At tidal frequencies, exchange is modulated by barotropic forcing and nonlinear baroclinic response, leading to a severance of high‐salinity bottom water into seaward and landward segments. Bottom water in the landward segment is moved lakeward along the shipping lane, where it is mixed vertically and flushed out. Subtidal water level fluctuations in the gulf force barotropic currents along the shipping lane, dominating variability of volume and salt flux at timescales of 1–4 weeks. The barotropic component of simulated salt flux decreases from the gulf to the lake, as depth‐averaged salinity decreases, while simulated baroclinic exchange is only 20% of that predicted by quasi‐steady two‐layer hydraulic theory. These reductions in salt flux are due to intense mixing that occurs throughout the shipping lane.
A model of marine ice formation within Antarctic ice shelf rifts
Authors: Khazendar Ala , Jenkins Adrian
Published: July 2003

Large rifts that open in Antarctic ice shelves are known to be filled by snow accumulations, ice shelf fragments, and sea ice. This work demonstrates how these rifts may also be filled from below, through their interaction with ocean water, by marine ice. The model presented here quantifies both the rate of marine ice accumulation at the top of the rift, which results from melt‐driven convection at its sides, and the impact of this process on waters occupying the confined environment of the rift. The results show that such a system could fill the rift with tens of meters of marine ice. In the process the temperature and salinity of the ambient water in the rift evolve through two distinct phases. The first is characterized by rapid change that takes the properties of the ambient water to near‐equilibrium values, followed by a second phase of much slower change. The melting rate at the lower part of the walls of the rift emerges as the principal factor influencing many aspects of model behavior. Testing the model against field observations from an Antarctic rift showed it to be robust and successful in reproducing the main observed features, including the presence of a thick layer of supercooled ocean water inside the rift.
Atmospheric pollution of a remote area of Tianshan Mountain: Ice core record
Authors: Lee Xinqing , Qin Dahe , Jiang Guibin , Duan Keqing , Zhou Hui
Published: July 2003

The influence of anthropogenic pollution on the region of Tianshan Mountain, a remote area in arid central Asia, has been debated in the recent years. An ice core, covering the past 43 years, retrieved from Glacier 1 at Urumqi River head in the east Tianshan, northwest China, was analyzed to examine the problem. CH3COO− (acetate) and HCOO− (formate), the chief tropospheric carboxylates that can originate from anthropogenic pollution, average 389.4 ± 336.3 ng g−1 (N = 489) and 61.1 ± 89.0 ng g−1 (N = 541), respectively, with a mean ratio HCOO−/CH3COO− of 0.21 ± 0.23 (N = 489). SO42−, the major inorganic anion in the core, averages 232.9 ± 279.9 ng g−1 (N = 542). The organic and inorganic records have covaried in the past four decades. They originate principally from anthropogenic pollution, coal combustion in particular, of the local and regional atmosphere. The pH values in the record range from 6 to 9 with an average of 6.9 ± 0.5 (N = 541). The general trend of the pH data matches that of HCOO−, CH3COO−, and SO42−, indicating that the anthropogenic pollution has released considerable particulate material along with unsaturated hydrocarbons and SO2. As a result, the pollution has not been acidifying the environment, but making it alkaline.
Estimating European emissions of ozone‐depleting and greenhouse gases using observations and a modeling back‐attribution technique
Authors: Manning A. J. , Ryall D. B. , Derwent R. G. , Simmonds P. G. , O'Doherty S.
Published: July 2003

The Numerical Atmospheric Dispersion Modeling Environment (NAME) dispersion model driven by three‐dimensional (3‐D) synoptic meteorology from the Unified Model has been used to determine the fraction of air arriving at Mace Head, Ireland, from different European regions over a 6‐year period. These data, along with observations of pollutants at Mace Head and a best fit algorithm, have been used to derive emission estimates over Western Europe. The algorithm starts from randomly generated emission maps and iterates toward the best solution. Using an idealized case study, it has been shown to be effective at distinguishing between distinct source regions. The technique has been applied to two ozone‐depleting gases, CFC‐11 and CFC‐12, and two greenhouse gases, methane and nitrous oxide. The emissions derived compare favorably with existing inventories. The technique is able to provide information regarding the emission distribution across Europe and to estimate area and country contributions; information that for some species is not readily available by other means. It is a different methodology to those currently used and so is a useful tool in verifying existing inventories.
Problems in separating aerosol and cloud in the Stratospheric Aerosol and Gas Experiment (SAGE) II data set under conditions of lofted dust: Application to the Asian deserts
Authors: Kent G. S. , Trepte C. R. , Wang P.‐H. , Lucker P. L.
Published: July 2003

Stratospheric Aerosol and Gas Experiment II aerosol data obtained at two wavelengths, 525 nm and 1020 nm, have been used for some time to identify the presence of cloud along the optical path from the Sun to the satellite instrument. Examination of data obtained over desert regions in the Northern Hemisphere, particularly the Taklimakan Desert, indicates that this separation method does not always operate correctly. In regions where there is expected to be a large amount of lofted dust, unexpectedly low values of mean extinction are found, combined with higher than expected amounts of cloud. These anomalous data have been analyzed in detail, and the discrepancy is plausibly shown to be due to faulty identification of lofted dust as cloud. Six Northern Hemisphere desert regions, together with three comparison regions, have been identified for study and the anomalies used to develop a description of the seasonal and altitudinal characteristics of the lofted aerosol over these regions.
An investigation of gravity wave activity in the low‐latitude upper mesosphere: Propagation direction and wind filtering
Authors: Medeiros A. F. , Taylor M. J. , Takahashi H. , Batista P. P. , Gobbi D.
Published: July 2003

An all‐sky charge‐coupled device imager capable of measuring wave structure in the OH, O2, and O I (557.7 nm) airglow emissions was operated at Cachoeira Paulista, Brazil (23°S, 45°W), for 2 years in collaboration with Utah State University, Logan. The dominant quasi‐monochromatic gravity wave components investigated over a ∼1 year period (September 1998 to October 1999) have been extracted, and their seasonal variations have been measured. A total of 283 wave events were measured, exhibiting horizontal wavelengths from 5 to 60 km, observed periods from 5 to 35 min, and horizontal phase speeds of up to ∼80 m s−1. The large‐scale “band” wave patterns (horizontal wavelength between 10 and 60 km) exhibited a clear seasonal dependence on the horizontal propagation direction, propagating toward the southeast during the summer months and toward the northwest during the winter. The direction of propagation was observed to change abruptly around the equinox period in mid March and at the end of September. Using a numerical simulation of gravity wave propagation in a seasonally variable climatological wind field, we have determined that the observed anisotropy in the wave propagation directions can be attributed to a strong filtering of the waves in the middle atmosphere by stratospheric winds.