bluuebunny/crossref_metadata_2025 huggingface dataset:
Tests of a space‐time model of daily rainfall in southwestern Australia based on nonhomogeneous random cascades
Authors: Jothityangkoon Chatchai , Sivapalan Murugesu , Viney Neil R.
Published: September 2002

A space‐time rainfall model is constructed to generate synthetic fields of space‐time rainfall, with a daily time step. The model has two components: a temporal model based on a first‐order, four‐state discrete Markov chain which generates a daily time series of the regionally averaged rainfall and a spatial model based on a nonhomogeneous random cascade process which disaggregates the above regionally averaged rainfall amounts to produce spatial patterns of daily rainfall across the region. The cascade generators used to disaggregate the rainfall spatially are a product of stochastic and deterministic factors; the latter enable the model to capture systematic spatial gradients exhibited by measured data. The model is applied to a 400 km × 400 km region encompassing the Swan‐Avon River Basin in the southwest of Western Australia. The model parameters are estimated on the basis of 11 years of observed daily rainfall data from 490 rain gauges located in the region. A detailed testing of the model is undertaken on the basis of a comparison of the statistical characteristics of the spatial and temporal variability of rainfall between the rainfall fields obtained from the rain gauge network and those generated by the simulation model. It was found that the spatial and temporal characteristics of the simulated rainfall field are in very good agreement with those of observed rainfall. In particular, the model preserves the observed systematic spatial variations of daily, monthly, and annual rainfall across the region and the observed seasonal variations. Statistical characteristics of storm durations and interstorm periods at individual gauge locations are not reproduced that well, which suggests generalizations need to be made to include space‐time correlations.
Reactive solute transport in streams: A surface complexation approach for trace metal sorption
Authors: Runkel Robert L. , Kimball Briant A. , McKnight Diane M. , Bencala Kenneth E.
Published: September 2002

A model for trace metals that considers in‐stream transport, metal oxide precipitation‐dissolution, and pH‐dependent sorption is presented. Linkage between a surface complexation submodel and the stream transport equations provides a framework for modeling sorption onto static and/or dynamic surfaces. A static surface (e.g., an iron‐ oxide‐coated streambed) is defined as a surface with a temporally constant solid concentration. Limited contact between solutes in the water column and the static surface is considered using a pseudokinetic approach. A dynamic surface (e.g., freshly precipitated metal oxides) has a temporally variable solid concentration and is in equilibrium with the water column. Transport and deposition of solute mass sorbed to the dynamic surface is represented in the stream transport equations that include precipitate settling. The model is applied to a pH‐modification experiment in an acid mine drainage stream. Dissolved copper concentrations were depressed for a 3 hour period in response to the experimentally elevated pH. After passage of the pH front, copper was desorbed, and dissolved concentrations returned to ambient levels. Copper sorption is modeled by considering sorption to aged hydrous ferric oxide (HFO) on the streambed (static surface) and freshly precipitated HFO in the water column (dynamic surface). Comparison of parameter estimates with reported values suggests that naturally formed iron oxides may be more effective in removing trace metals than synthetic oxides used in laboratory studies. The model's ability to simulate pH, metal oxide precipitation‐dissolution, and pH‐dependent sorption provides a means of evaluating the complex interactions between trace metal chemistry and hydrologic transport at the field scale.
Chloride and chlorine isotopes (36Cl and δ37Cl) as tracers of solute migration in a thick, clay‐rich aquitard system
Authors: Hendry M. J. , Wassenaar L. I. , Kotzer T.
Published: September 2002

Detailed vertical profiles of dissolved Cl− and its isotopes (36Cl and δ37Cl) provided new information on the origin and systematics of this conservative tracer in pore waters of a thick aquitard system. The aquitard system consists of surficial Quaternary clay‐rich till (80 m thick) deposited 30–20 kyr B.P., overlying Cretaceous marine clay (76 m thick) deposited ∼71 Ma. The distribution of Cl−, δ37Cl, and Br− showed the presence of five distinct end‐members for Cl−: the top of the unoxidized till, a regional aquifer underlying the Cretaceous clay, two localized geological heterogeneities (sand streaks) in the till, and glacial meltwater emplaced with the till and still present at depths of between 36 and 60 m. Numerical simulations of the transport of Cl− from the sand streaks indicated that this geochemical profile has been developing throughout most of the Holocene. The 36Cl measurements showed that the age of the dissolved Cl− in the upper Cretaceous clay is likely between 0.75 and 1.9 Myr. The 36Cl measurements further suggested that the dissolved Cl− in the till was not directly derived from the underlying Cretaceous clay. Finally, it was not possible to quantify the effects of isotopic fractionation of 37Cl relative to 35Cl because of diffusion in this aquitard system.
Multispecies reactive tracer test in an aquifer with spatially variable chemical conditions
Authors: Davis J. A. , Kent D. B. , Coston J. A. , Hess K. M. , Joye J. L.
Published: September 2002

A field investigation of multispecies reactive transport was conducted in a well‐characterized, sand and gravel aquifer on Cape Cod, Massachusetts. The aquifer is characterized by regions of differing chemical conditions caused by the disposal of secondary sewage effluent. Ten thousand liters of groundwater with added tracers (Br, Cr(VI), and EDTA complexed with Pb, Zn, Cu, and Ni) were injected into the aquifer and distributions of the tracers were monitored for 15 months. Most of the tracers were transported more than 200 m; transport was quantified using spatial moments computed from the results of a series of synoptic samplings. Cr(VI) transport was retarded relative to Br; the retardation factor varied from 1.1 to 2.4 and was dependent on chemical conditions. At 314 days after the injection, dissolved Cr(VI) mass in the tracer cloud had decreased 85%, with the likely cause being reduction to Cr(III) in a suboxic region of the aquifer. Transport of the metal‐EDTA complexes was affected by aqueous complexation, adsorption, and dissolution‐precipitation reactions of Fe oxyhydroxide minerals in the aquifer sediments. Dissolved Pb‐EDTA complexes disappeared from the tracer cloud within 85 days, probably due to metal exchange reactions with Fe and adsorbed Zn (present prior to the injection from contamination by the sewage effluent). About 30% of the Cu‐EDTA complexes remained within the tracer cloud 314 days after injection, even though the thermodynamic stability of the Pb‐EDTA complex is greater than Cu‐EDTA. It is hypothesized that stronger adsorption of Pb2+ to the aquifer sediments causes the Pb‐EDTA complex to disassociate to a greater degree than the Cu‐EDTA complex. The mass of dissolved Zn‐EDTA increased during the first 175 days of the tracer test to 140% of the mass injected, with the increase due to desorption of sewage‐derived Zn. Dissolved Ni‐EDTA mass remained nearly constant throughout the tracer test, apparently only participating in reversible adsorption reactions. The results of the field experiment provide a chemically complex data set that can be used in the testing of reactive transport models of flow coupled with chemical reactions.
Anomalous transport in laboratory‐scale, heterogeneous porous media
Authors: Berkowitz Brian , Scher Harvey , Silliman Stephen E.
Published: September 2002

We analyze measured breakthrough curves in a laboratory model which consists of a uniformly heterogeneous porous medium; these curves were previously shown to be indicative of scale‐dependent transport and therefore inconsistent with the (macroscopic) advection‐dispersion equation [Silliman and Simpson, 1987]. Our analysis is based on an analytical expression for the first‐passage time distribution (FPTD) of migrating contaminants in random media, developed with the use of a continuous time random walk (CTRW) formalism. The general CTRW has been shown to be effective in quantifying anomalous transport patterns frequently observed in fractured and strongly heterogeneous porous media [Berkowitz and Scher, 1997, 1998]. We calculate a family of FPTD curves, usually referred to as “breakthrough curves,” which are a function of an exponent β; this exponent is related to the low‐velocity tail of the velocity distribution. The FPTD curves fit well the measured data, with a single value of the β exponent over the spatial/temporal scale of the experiment. This is in contrast to previous analyses using solutions of the Gaussian‐based advection‐dispersion equation with time‐independent parameters in a uniform flow field. We conclude that the CTRW may allow analysis of transport in porous media subject to complex heterogeneities at large scale, which may not be amenable to analysis using classical advection‐dispersion theory. Hence the CTRW represents a potentially valuable tool in the assessment of dispersive processes in heterogeneous porous media.
Models of flux in porous media with memory
Authors: Caputo Michele
Published: September 2002

Some data on the flow of fluids in rocks exhibit properties which may not be interpreted with the classic theory of propagation of pressure and of fluids in porous media [Bell and Nur, 1978; Roeloffs, 1988] based on Darcy's law, which states that the flux is proportional to the pressure gradient. Concerning the fluids, some may react chemically with the medium enlarging the pores; some carry solid particles, which may obstruct some of the pores; and finally, some may precipitate minerals in the pores diminishing their size or even closing them as in geothermal areas. These phenomena create a spatially variable pattern of mineralization and permeability changes that can be localized. In order to obtain a better representation of the flux and of the pressure of fluids, Darcy's law is modified, introducing general memory formalisms operating on the flow as well as on the pressure gradient, which imply a filtering of the pressure gradient without singularities. We also modify the second constitutive equation of diffusion, which relates the density variations of the fluid to the pressure, by introducing a rheology in the fluid also represented by memory formalisms operating on the pressure as well as on the density variations. The memory formalisms are then specified as derivatives of fractional order. The equations used here for the diffusion of fluids are different from the classic ones; however, the equation governing the diffusion of the pressure is the same as that of the flux, as in the classic case. For technical reasons the majority of the studies on diffusion is devoted to the diffusion of the pressure of the fluid rather than to the flux; in this paper we shall devote our attention to studying the flux and its spectral properties in a practical example seeing that the memory used implies a low‐pass filtering of the flux or a band pass centered in the low‐frequency range. A half space is considered where the boundary values are applied to the plane limiting it, and the problems solved are the computation of the Green function of the flux in the cases when (1) a pressure constant in time is applied on the boundary plane and (2) a periodic pressure is applied to the boundary plane while the half space is initially at zero pressure in both cases. We found closed form formulae for the flux and its spectrum. A discussion follows concerning the mode of determination of the parameters of memory formalisms ruling the diffusion using the observed pressure and/or the flux at several frequencies in problem 2. Concerning the flux, it is tentatively seen that when the medium is oil rock and the fluid is water, for a pressure of 105 Pa at the boundary and derivative of order 0.1, the flux at a distance of 0.1 km from the boundary plane is 1700 kg h−1.
Thermal mediation by littoral wetlands and impact on lake intrusion depth
Authors: Andradóttir Hrund Ó. , Nepf Heidi M.
Published: September 2002

Lake inflow dynamics can be affected by the thermal mediation provided by shallow littoral regions such as wetlands. In this study, wetland thermal mediation is evaluated using a linearized dead‐zone model. Its impact on lake inflow dynamics is then assessed by applying the model sequentially to the river reach, wetland, and lake. Our results suggest that littoral wetlands can dramatically alter the inflow dynamics of reservoirs located in small or forested watersheds, for example, by raising the temperature of the inflow during the summer and creating surface intrusions when a plunging inflow would otherwise exist. Consequently, river‐borne nutrients, contaminants, and pathogens enter directly into the epilimnion, where they enhance eutrophication and the risk of human exposure. The addition of a littoral wetland has less significant effects in larger watersheds, where the water has already equilibrated with the atmosphere upon reaching the wetland and sun shading is less prominent.
Dynamics of soil aggregate coalescence governed by capillary and rheological processes
Authors: Ghezzehei Teamrat A. , Or Dani
Published: September 2002

The desired soil structure following tillage of agricultural soils is often unstable and susceptible to coalescence of aggregates and reduction of interaggregate porosity due to wetting and drying cycles. This process of aggregate rejoining was modeled by equating the rate of work done by liquid‐vapor menisci, to the rate of energy dissipation due to viscous deformation of a pair of spherical aggregates. The nonlinearity of wet soil viscous flow behavior was accounted for by introducing a Bingham rheological model. A natural outcome of the analysis was the formulation of a mathematical condition for the onset and termination of coalescence based on soil strength at specified water content. The condition states that sufficient energy in excess of soil strength (yield stress) must be available for coalescence to proceed. The rate of aggregate coalescence is proportional to available energy and is inversely related to the coefficient of plastic viscosity. Transport of wet soil to the periphery of the interaggregate contact by viscous flow leads to smoothing of the neck, resulting in pore closure, on the one hand, and restricting the minimum matric potential that can be achieved, on the other. The interplay between rheology and geometry prevent coalescence from proceeding indefinitely. Independently determined soil rheological properties were used to illustrate the use of the model. Coalescence under constant water content and during wetting‐drying cycles was calculated. Comparison of data from experiments on one‐dimensional, aggregate bed settlement has shown reasonable agreement with the model predictions.
Self‐organization of river networks to threshold states
Authors: Talling Peter J.
Published: September 2002

Numerical models in which grid‐cells self‐organize, so that the shear stress in each cell equals a predetermined threshold value, are remarkably successful in producing the fundamental structure of river networks. Such models are a reasonable approximation of networks characterized by near‐constant dimensionless shear stress (т* α bed shear stress7sol;bed grain size), with each cell's threshold value being proportional to median‐bed grain size. Previous work has shown that this is the case for channels cut into alluvium, and that the characteristic narrow range of т* is different for gravel‐ and sand‐bed channels. Channels in the Italian Apennines that are slowly incising into weak bedrock, but which are covered by a veneer of gravel for most of the time, are shown to be characterized by the same т* as alluvial gravel‐bed channels. Thus threshold models capture the fundamental behavior of many river networks, even in areas of long‐term bedrock incision.
The importance of a dark ionosphere for ion heating and auroral arc formation
Authors: Hamrin Maria , André Mats , Norqvist Patrik , Rönnmark Kjell
Published: September 2002

We present observations from the Freja Satellite to show that density reductions and ion heating at Freja heights are anticorrelated with solar illumination of the ionosphere. When the ionospheric foot‐point of a flux‐tube is in shadow, the ambient density is lower, transverse ion energization is more common and more intense, and the associated density cavities are deeper. In combination with the suggestion that the electrons must be accelerated to keV energies to carry an imposed current in a low density auroral cavity, these observations may explain the recent observation that auroras are more common when the ionosphere below is in darkness.
Intraseasonal variability and tides in Makassar Strait
Authors: Susanto R. Dwi , Gordon Arnold L. , Sprintall Janet , Herunadi Bambang
Published: September 2002

Intraseasonal variability and tides along the Makassar Strait, the major route of Indonesian throughflow, are investigated using spectral and time‐frequency analyses which are applied to sea level, wind and mooring data. Semidiurnal and diurnal tides are dominant features, with higher (lower) semidiurnal (diurnal) energy in the north compared to the south. Sea levels and mooring data display intraseasonal variability which are probably a response to remotely forced Kelvin waves from the Indian Ocean through Lombok Strait and to Rossby waves from the Pacific Ocean. Sea levels in Tarakan and Balikpapan and Makassar mooring velocities reveal intraseasonal features with periods of 48–62 days associated with Rossby waves from the Sulawesi Sea. Kelvin wave features with periods of 67–100 days are seen in Bali (Lombok Strait), at the mooring sites and in Balikpapan, however, they are not seen in Tarakan, which implies that these waves diminish after passing through the Makassar Strait.
Changes in the rates of North Atlantic major hurricane activity during the 20th century
Authors: Elsner James B. , Jagger Thomas , Niu Xu‐Feng
Published: September 2002

The authors document and explain changes in the rates of North Atlantic major hurricanes over the 20th century. A change‐point analyses identifies two contrasting regimes of activity. The regimes have significantly different occurrence rates that coincide with changes in the climate over the extratropical North Atlantic. In conjunction with the recent Arctic warming and a relaxation of the North Atlantic oscillation, it is speculated that we are beginning a new period of greater major hurricane activity.
Nonlinear prediction of the ionospheric parameter ƒoF2 on hourly, daily, and monthly timescales
Authors: Francis N. M. , Cannon P. S. , Brown A. G. , Broomhead D. S.
Published: September 2002

An application of nonlinear radial basis function (RBF) neural networks (NNs) to improve the accuracy of the prediction of ionospheric parameters is presented. Principal component analysis is also adopted for the purposes of noise and dimension reduction. Hourly, daily, and monthly predictive models have been created for the Slough, England, United Kingdom, foF2 time series. The quality of the model predictions is evaluated by comparison with corresponding predictions from reference persistence or recurrence models. Each RBF NN offers a significant improvement over the performance of the corresponding reference model. The noonday model gives a performance improvement of ∼60% over the baseline persistence model, for a 1 day ahead prediction. For a 1 hour ahead prediction the hourly model offers an improvement of ∼45% over the baseline 24 hour recurrence model. Finally, the monthly median model gives a performance improvement of ∼40% over the baseline persistence model, for a 1 month ahead prediction.
The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere
Authors: Meredith Nigel P. , Horne Richard B. , Johnstone Alan D. , Anderson Roger R.
Published: September 2002

The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere are investigated using data from the CRRES satellite. Equatorial electron distributions and concomitant wave spectra outside the plasmapause on the nightside of the Earth are studied as a function of time since injection determined from the auroral‐electrojet index (AE). The electron cyclotron harmonic (ECH) wave amplitudes are shown to be very sensitive to small modeling errors in the location of the magnetic equator. They are best understood at the ECH equator, defined by the local maximum in the ECH wave activity in the vicinity of the nominal magnetic equator, suggesting that the ECH equator is a better measure of the location of the true equator. Strong ECH and whistler mode wave amplitudes are associated with the injected distributions and at the ECH equator, in the region 6.0 ≤ L < 7.0, exponential fits reveal wave amplitude decay time constants of 6.3±1.2 and 4.6±0.7 hours, respectively. Pancake electron distributions are seen to develop from injected distributions that are nearly isotropic in velocity space and, in this region, are seen to form on a similar timescale of approximately 4 hours suggesting that both wave types are involved in their production. The timescale for pancake production and wave decay is comparable with the average time interval between substorm events so that the wave‐particle interactions are almost continually present in this region leading to a continual supply of electrons to power the diffuse aurora. In the region 3.8 ≤ L < 6.0 the timescale for wave decay at the ECH equator is 2.3 ± 0.6 and 1.1 ± 0.2 hours for ECH waves and whistler mode waves respectively, although the pancakes in this region show no clear evolution as a function of time.
Borehole heat flow along the eastern flank of the Juan de Fuca Ridge, including effects of anisotropy and temperature dependence of sediment thermal conductivity
Authors: Pribnow Dan F. C. , Davis Earl E. , Fisher Andrew T.
Published: September 2002

The thermal conductivities of 15 whole‐round sediment samples collected during Ocean Drilling Program (ODP) Leg 168 between 17 and 440 m below the seafloor on the eastern flank of the Juan de Fuca Ridge were tested to document their anisotropy and temperature dependence using the divided bar technique. Tests over a temperature range of 5°C to 60°C reveal variations in conductivity of up to ±15%. The sign and amplitude of these variations depend on the thermal conductivity at laboratory (room) temperature (λrt): if λrt ≈ 0.8 W m−1 K−1 (high porosity), conductivity increases with temperature; if λrt ≈ 1.2 W m−1 K−1 (moderate porosity), conductivity does not change with temperature; if λrt ≈ 1.6 Wm−1K−1 (low porosity), conductivity decreases with temperature. This behavior results from a positive temperature coefficient for seawater (λ ∝ T) and a negative coefficient for rock matrix (λ ∝ 1/T). A special sampling technique for unconsolidated sediments made it possible to measure horizontal (λHOR) and vertical (λVER) components of thermal conductivity independently and to determine a mean anisotropy value (λHOR/λVER) of 1.2. Corrections, which are <10% for anisotropy and <1% for temperature, were applied using in situ temperatures, shipboard line source thermal conductivities, porosities, and the geometric mean mixing model that accounts for matrix and porewater constituent conductivities. On the basis of these corrected conductivities and a harmonic averaging of values weighted according to the lithologic division of the sediment sections into clay‐rich and sand‐rich units, values of heat flow estimated from borehole temperature measurements are lower than those previously estimated by ∼10% except for one site where the section is dominated by sand. Shallow seafloor heat flow measured with gravity‐driven probes may also need to be corrected for anisotropy, although the degree of anisotropy in the unconsolidated, high‐porosity sediments within a few meters of the seafloor is poorly constrained.
Seismoacoustic recordings of a spreading episode on the Mohns Ridge
Authors: Blackman Donna K. , Nishimura Clyde E. , Orcutt John A.
Published: September 2002

A period of very active seismicity near 72.7°N, 4°E marks an episode of seafloor spreading on the Mohns Ridge. The earthquakes were recorded from November 1995 to January 1996 by onshore seismic stations and by U.S. Navy hydrophone arrays in the North Atlantic. Both the temporal and spatial histories of the activity suggest that volcanism accompanied the tectonic events. The hydrophone arrays recorded 2–3 orders of magnitude more events than the onshore seismic arrays with up to 1000 events per day observed during the most intense phase of activity. A level of 50–200 events per day was sustained throughout the episode. Initial locations of the events were obtained from the seismic bulletin. Further refinement of the epicenters was possible using P, S (converted to an acoustic phase at the seafloor), and T waves in the hydrophone data. Analysis of arrival time differences between these phases indicates that one main area and two subsidiary areas along the rift were active during the swarm. A few events occurred at a more distant location. The activity tends to concentrate in one area or another for short periods (a few days), but at times it is clear that events occur simultaneously at more than one location. We have not found evidence of steady migration of activity, such as might accompany propagation of a magma‐filled dike. We thus infer that despite the 50–70 km length of ridge involved in the spreading episode, rupture and magmatic eruption at the seafloor probably only occurred in a few discrete areas.
Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States
Authors: Savage M. K. , Sheehan A. F.
Published: September 2002

Shear wave splitting and P, SKS, and S travel time residuals are calculated for teleseismic arrivals recorded on the Colorado Plateau‐Great Basin Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) portable broadband seismic deployment and for permanent stations in the western United States. Little shear wave splitting is observed for broadband recordings in the northern Colorado Plateau, the Rocky Mountains, or the central Great Basin. The transition between the Colorado Plateau and the Great Basin is marked by moderate shear wave splitting (1.0 s) and unusually late teleseismic phase arrivals. This suggests material with a higher content of mantle melt or volatiles than regions to either side. Splitting in the transition between the Colorado Plateau and Great Basin is part of a pattern of fast polarizations that align in a semicircle, surrounding a central Great Basin region of null (no splitting) measurements. Away from the California plate boundary, splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the North American plate. No simple spatial relation of splitting with geological and geophysical features such as mountain ranges, velocity anomalies, gravity, magnetics, or heat flow is evident in most of the western United States. However, splitting in the Great Basin is compatible with asthenospheric flow. The smallest shear wave splitting delay times coincide with the Eureka Low in heat flow, also having low S velocity at 300 km depth. We suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the Great Basin is caused by mantle flow, by the interaction of mantle up welling and the absolute motion of the North American plate.
Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography
Authors: Calvert Alexander , Sandvol Eric , Seber Dogan , Barazangi Muawia , Roecker Steven , Mourabit Taoufik , Vidal Francisco , Alguacil Gerardo , Jabour Nacer
Published: September 2002

A number of different geodynamic models have been proposed to explain the extension that occurred during the Miocene in the Alboran Sea region of the western Mediterranean despite the continued convergence and shortening of northern Africa and southern Iberia. In an effort to provide additional geophysical constraints on these models, we performed a local, regional, and teleseismic tomographic travel time inversion for the lithospheric and upper mantle velocity structure and earthquake locations beneath the Alboran region in an area of 800×800 km2. We picked P and S arrival times from digital and analog seismograms recorded by 96 seismic stations in Morocco and Spain between 1989 and 1996 and combined them with arrivals carefully selected from local and global catalogs (1964–1998) to generate a starting data set containing over 100,000 arrival times. Our results indicate that a N‐S line of intermediate‐depth earthquakes extending from crustal depths significantly inland from the southern Iberian coast to depths of over 100 km beneath the center of the Alboran Sea coincides with a W to E transition from high to low velocities imaged in the uppermost mantle. A high‐velocity body, striking approximately NE‐SW, is imaged to dip southeastwards from lithospheric depths beneath the low‐velocity region to depths of ∼350 km. Between 350 and 500 km the imaged velocity anomalies become more diffuse. However, pronounced high‐velocity anomalies are again imaged at 600 km near an isolated cluster of deep earthquakes. In addition to standard tomographic methods of error assessment, the effects of systematic and random errors were assessed using block shifting and bootstrap resampling techniques, respectively. We interpret the upper mantle high‐velocity anomalies as regions of colder mantle that originate from lithospheric depths. These observations, when combined with results from other studies, suggest that delamination of a continental lithosphere played an important role in the Neogene and Quaternary evolution of the region.
Exhumation of ultrahigh‐pressure continental crust in east central China: Late Triassic‐Early Jurassic tectonic unroofing
Authors: Hacker Bradley R. , Ratschbacher Lothar , Webb Laura , McWilliams Michael O. , Ireland Trevor , Calvert Andrew , Dong Shuwen , Wenk Hans‐Rudolf , Chateigner Daniel
Published: September 2002

The largest tract of ultrahigh‐pressure rocks, the Dabie‐Hong'an area of China, was exhumed from 125 km depth by a combination of normal‐sense shear from beneath the hanging wall Sino‐Korean craton, southeastward thrusting onto the footwall Yangtze craton, and orogen‐parallel eastward extrusion. Prior to exhumation the UHP slab extended into the mantle a downdip distance of 125–200 km at its eastern end, whereas it was subducted perhaps only 20–30 km at its far western end ∼200 km away. Structural reconstructions imply that the slab was >10 km thick. U/Pb zircon and 40Ar/39Ar geochronology indicate that exhumation up to crustal depths occurred diachronously between 240 and ∼225–210 Ma, reflecting a vertical exhumation rate of >2 mm/yr. The upper boundary of the slab is the Huwan shear zone, a normal‐sense detachment that reactivated the plate suture. The lower boundary is represented by the Lower Yangtze fold‐thrust belt. NW‐trending stretching lineations, NE‐vergent, WNW‐ESE trending  folds, dominant top‐NW shear, and conjugate, but overall asymmetric, shear band fabrics, document that exhumation was accomplished by updip and orogen‐parallel extrusion accompanied by layer‐parallel thinning. The orientation and shape of the folds, and a change from SE to SW flow directions, imply that the slab rotated clockwise about a western pivot during exhumation; this rotation was likely caused by the eastward increasing depth of subduction mentioned above, combined with a possible marginal basin and a weak eastern plate boundary. Exhumation of the slab produced considerable shortening in the Lower Yangtze fold‐thrust belt, perhaps producing the foreland orocline.
Seismic wide‐angle constraints on the crust of the southern Urals
Authors: Carbonell R. , Gallart J. , Pérez‐Estaún A. , Diaz J. , Kashubin S. , Mechie J. , Wenzel F. , Knapp J.
Published: September 2002

A wide‐angle seismic reflection/refraction data set was acquired during spring 1995 across the southern Urals to characterize the lithosphere beneath this Paleozoic orogen. The wide‐angle reflectivity features a strong frequency dependence. While the lower crustal reflectivity is in the range of 6–15 Hz, the PmP is characterized by frequencies below 6 Hz. After detailed frequency filtering, the seismic phases constrain a new average P wave velocity crustal model that consists of an upper layer of 5.0–6.0 km/s, which correlates with the surface geology; 5–7 km depths at which the velocities increase to 6.2–6.3 km/s; 10–30 km depths at which, on average, the crust is characterized by velocities of 6.6 km/s; and finally, the lower crust, from 30–35 km down to the Moho, which has velocities ranging from 6.8 to 7.4 km/s. Two different S wave velocity models, one for the N‐S and one for the E‐W, were derived from the analysis of the horizontal component recordings. Crustal sections of Poisson's ratio and anisotropy were calculated from the velocity models. The Poisson's ratio increases in the lower crust at both sides of the root zone. A localized 2–3% anisotropy zone is imaged within the lower crust beneath the terranes east of the root. This feature is supported by time differences in the SmS phase and by the particle motion diagrams, which reveal two polarized directions of motion. Velocities are higher in the central part of the orogen than for the Siberian and eastern plates. These seismic recordings support a 50–56 km crustal thickness beneath the central part of the orogen in contrast to Moho depths of ≈ 45 km documented at the edges of the transect. The lateral variation of the PmP phase in frequency content and in waveform can be taken as evidence of different genetic origins of the Moho in the southern Urals.
Exploratory analysis of the distribution of condensation nuclei in the northern hemisphere upper troposphere and lower stratosphere during the late 1970s
Authors: Detwiler Andrew G. , Johnson L. Ronald , Schauer Allison G.
Published: September 2002

Data obtained from the Global Atmospheric Sampling Program (GASP) during the late 1970s are used to evaluate the hypothesis that particulate emissions by aircraft are a significant source for the upper tropospheric/lower stratospheric fine and ultrafine aerosol. Measurements of condensation nuclei (CN) were acquired above 6 km mean sea level as part of GASP from October 1977 through June 1979, using automated packages on up to four commercial airliners simultaneously flying their normal daily routes. These routes spanned much of the midlatitude Northern Hemisphere, with the notable exceptions of the former Soviet Union and China. The GASP measurements are analyzed to determine the vertical, geographical, and seasonal distribution of upper tropospheric/lower stratospheric CN in clear air, spanning an altitude range from several kilometers above to several kilometers below the tropopause. The general lack of vertical gradients in median CN mixing ratio at altitudes in the troposphere suggests that CN in the clear troposphere are generally well mixed. CN mixing ratios decrease with distance above the tropopause, consistent with a tropospheric source for stratospheric CN. Median CN mixing ratios for selected regions were compared to the Northern Hemisphere median, by altitude and season. Regional median CN mixing ratios can differ from the hemispheric median by more than a factor of 2 but never an order of magnitude. The lack of vertical variation of CN concentrations in the troposphere is not at all consistent with the vertical variation of fuel consumed by aircraft, which peaks at altitudes near the tropopause. The observations of CN show some regional medians exceeding Northern Hemisphere medians in some seasons and not others, but the regions with the highest medians are not always those with the highest aircraft fuel consumption. We conclude that aircraft emissions are not a major component of the global clear air background CN measured during GASP.
Is there a cosmic ray signal in recent variations in global cloudiness and cloud radiative forcing?
Authors: Kristjánsson Jón Egill , Kristiansen Jørn
Published: September 2002

In order to evaluate a recent hypothesis of a coupling between galactic cosmic rays, clouds, and climate we have investigated temporal variations in global cloudiness and radiative fluxes at the top of the atmosphere. For this purpose we have used the best available global data sets, i.e., those of the International Satellite Cloud Climatology Project (ISCCP) and the Earth Radiation Budget Experiment (ERBE), respectively. Both globally and over midlatitude oceans only, we find a decrease in total cloud coverage between 1986 and 1990 of 2%, while between 1990 and 1993 there is a slightly smaller decrease. When the results are related to temporal variations in cosmic ray activity, we do not find support for a coupling between cosmic rays, total cloudiness, and radiative forcing of climate. A possible exception is low marine clouds at midlatitudes, characterized by few cloud condensation nuclei and a large net cooling effect, but no physical mechanism is known which might explain a connection between cosmic rays and low clouds. The net radiative effect of clouds during the period 1985–1989 shows an enhanced cooling effect despite a reduction in both total and low cloud cover. This contradicts the simple relationship between cloud cover and radiation assumed in the cosmic‐ray‐cloud‐climate hypothesis. The interpretation of the results is rendered difficult by the short time series of ISCCP and ERBE data and by uncertainties concerning instrument calibrations and changes of satellites. When a 43‐year time series of synoptic observations over sea is related to cosmic rays over the same period, a weak, negative correlation is found.
Radiative decay of idealized stratospheric filaments in the troposphere
Authors: Forster Caroline , Wirth Volkmar
Published: September 2002

The radiative decay of idealized stratospheric filaments embedded in a reference midlatitude troposphere is investigated. The filaments are characterized by typical lower stratospheric values of potential vorticity (PV), ozone, and humidity. The evolution of the filaments is obtained numerically in the framework of a plane symmetric balanced model. Diabatic heating due to radiation is accounted for by using a realistic parameterization scheme. The control parameter for the numerical experiments is the aspect ratio of the initial PV anomaly, which determines the partitioning into a thermal and a dynamical anomaly. For shallow filaments, both the temperature and the humidity anomaly are radiatively relevant. Initially, the decay of the filament is independent on whether or not the moisture anomaly is accounted for. At later times the effect of the moisture anomaly slows down the evolution significantly. For intermediate and tall filaments the humidity anomaly dominates the evolution from the beginning, leading to an enhancement rather than a decay of the PV anomaly. In addition, for tall filaments the advection by the induced Sawyer‐Eliassen circulation has a significant impact on the evolution of the anomaly. In all experiments, ozone plays a minor role. The characteristic timescale of the evolution scales somewhat less than linearly with the filament thickness and shows only weak sensitivity to the aspect ratio.
Characterization of atmospheric ammonia emissions from swine waste storage and treatment lagoons
Authors: Aneja Viney P. , Chauhan J. P. , Walker J. T.
Published: September 2002

Fluxes of atmospheric ammonia‐nitrogen (NH3‐N, where NH3‐N = (14/17)NH3) from an anaerobic ∼2.5 ha (1 ha = 10,000 m2) commercial hog waste storage lagoon were measured during the summer of 1997 through the spring of 1998 in order to study the seasonal variability in emissions of NH3‐N and its relationship to lagoon physicochemical properties. Ammonia‐nitrogen fluxes were measured during each season (summer, fall, winter, and spring) using a dynamic flow through chamber system. Measured lagoon physicochemical parameters included surface lagoon temperature (Tℓ°C, ∼15 cm below surface), lagoon pH, and Total Kjeldahl Nitrogen (TKN). The pH and TKN of the surface lagoon water ranged from 7 to 8 pH units, and 500 to 750 mg N L−1, respectively. The largest fluxes were observed during the summer (August 1997) (mean NH3‐N flux = 4017 ± 987 μg N m−2 min−1). Fluxes decreased through the fall (December 1997) months (844 ± 401 μg N m−2 min−1) to a minimum flux during the winter (February 1998) months (305 ± 154 μg N m−2 min−1). Emission rates increased during spring (May 1998) (1706 ± 552 μg N m−2 min−1), but did not reach the magnitude of fluxes observed during the summer. Lagoon emissions in eastern North Carolina were estimated to constitute ∼33% of total NH3‐N emissions from commercial hog operations in North Carolina based on current inventories for NH3‐N emissions published by the North Carolina Division of Air Quality, North Carolina Department of Environment and Natural Resources. The ammonia flux may be predicted by an observational model log10 (NH3‐N flux) = 0.048 Tℓ + 2.1.
NOx‐catalyzed ozone destruction and NOx activation at midlatitudes to high latitudes as the main cause of the spring to fall ozone decline in the northern hemisphere
Authors: Brühl Christoph , Crutzen Paul J.
Published: September 2002

Recent redeterminations of the rate coefficients for the reaction O + NO2 → NO + O2, the rate‐limiting step in the NOx catalytic ozone destruction cycle, and two rate constants dominating the NOx/NOy ratio, OH + NO2 (+M) → HNO3 (+M) and OH + HNO3 → H2O + NO3, have the consequence that the decline of total ozone between spring and fall, at midlatitudes to high latitudes, is even more determined by NOx‐catalyzed ozone destruction than previously calculated. Their use also leads to consistent new conclusions regarding the ozone budget of the stratosphere (and ozone transport to the troposphere). Using UARS Halogen Occultation Experiment satellite data as constraints, we derive model estimates of the ozone budget of the Northern Hemisphere lower stratosphere between spring and fall. Although net chemical ozone production occurs in the latitude region between the subtropical barrier (25°–30°N) and 50°N, total ozone declines in this region because of transport to higher latitudes and to the troposphere. North of 50°N, strong chemical ozone loss leads to total ozone loss as well. The tropical region appears not to play a significant role in seasonal total ozone variation.
An experimental determination of the scale length of N2O in the soil of a grassland
Authors: Neftel Albrecht , Blatter Andreas , Schmid Martin , Lehmann Bernhard , Tarakanov Sergei V.
Published: September 2002

Concentration profiles of N2O in a grassland soil and dynamic response curves to disturbance of the soil concentration (relaxation curves) were measured with a new membrane tube technique. Diffusive properties of the soil were derived from 222Rn measurements. The mathematical analysis of the relaxation curves yielded N2O uptake rates U soil diffusivities Ds, scale lengths z*, and production rates P at different levels under the surface. The following ranges were found during 2 days of measurements: Ds = (0.4–5) × 10−7 m2 s−1, U = (1–20) × 10−4 s−1, z* = 0.7–2.8 cm, and P = 0.02–4.4 ppb s−1. These values were used to reproduce the measured N2O concentration profiles with a one‐dimensional diffusive transport model of N2O in the soil air‐filled pore space and to deduce flux profiles. Bidirectional fluxes occurred with small deposition fluxes up to a few ppt ms−1 during intensive growing phases of the grass. Uptake rates were high enough that N2O produced at greater depth did not reach the atmosphere.
Comparisons of aircraft, ship, and buoy radiation and SST measurements from TOGA COARE
Authors: Burns Sean P. , Khelif Djamal , Friehe Carl A. , Hignett Phil , Williams Alastair G. , Grant Alan L. M. , Hacker Jörg M. , Hagan Denise E. , Serra Yolande L. , Rogers David P. , Bradley E. Frank , Weller Robert A. , Fairall Chris W. , Anderson Steven P. , Paulson Clayton A. , Coppin Peter A.
Published: September 2002

Mean radiative fluxes and sea surface temperature measured by the five Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) boundary layer research aircraft were compared with each other and with surface measurements from moored buoys and ships. The basic data‐processing techniques for radiative flux and sea surface temperature (SST) measurements from an aircraft were reviewed, and an empirical optimization method to calibrate an Eppley pyrgeometer was introduced. On the basis of aircraft wingtip‐to‐wingtip comparison periods, the processed aircraft downwelling shortwave and longwave irradiance and SST measurements were found to agree to 28±18 W m−2, 9±4 W m−2, and 0.7±0.4°C, respectively. By using the same comparison periods, empirical corrections that removed systematic errors in the aircraft data were determined. Application of these corrections improved the wingtip comparison accuracy to 3±16 W m−2, 1±4 W m−2, and 0.1±0.3°C, respectively. Comparisons between the (fully corrected) aircraft and the surface platform measurements revealed the aircraft data to be slightly greater for all three parameters. The agreement was around 3±37 W m−2, 3±6 W m−2, and 0.3±0.5°C for shortwave irradiance, longwave irradiance, and SST, respectively. (Detailed comparison results were provided for each individual ship and buoy.) After applying the aircraft empirical corrections the level of accuracy was near the COARE objectives.
Direct measurements of episodic snow accumulation on the Antarctic polar plateau
Authors: Braaten David A.
Published: September 2002

During a 1‐year field experiment at a remote location on the Antarctic polar plateau (85.67°S, 46.38°W) influenced by moderate magnitude katabatic winds, snow accumulation was characterized at three different spatial and temporal scales using snow stakes, tracer material dispersed periodically on the snow surface, and an acoustic depth gauge. The spatial variability of snow accumulation was found to be large, on both annual and intra‐annual timescales, and is attributed to the high frequency of moderate to strong winds at the site. Accumulation throughout the year was observed to be episodic in nature, with a small number of snow accumulation events producing the majority of the annual total accumulation for the site, averaging 0.174 m. In the intervals between observed accumulation events (up to several months), negative changes to snow surface height caused by sublimation and densification of the firn were quantified using an acoustic depth gauge. The rate of decrease in snow surface elevation was largest during the austral summer, as expected, and the overall change in snow surface elevation due to sublimation/densification during the year was estimated to be about −0.10 m. Using the precise timing of accumulation events provided by the acoustic depth gauge, meteorological surface observations, numerical model analyses, and satellite imagery were used to gain insights into whether the event was associated with precipitation or related exclusively to blowing snow and to diagnose the meteorological conditions producing the event. Meteorological conditions during the accumulation events were found to strongly support an association with precipitation events caused by mesoscale or synoptic‐scale cyclones along the coastal margin. Dating of the accumulation profile using the dispersed tracer technique identified several other accumulation events that were not measured within the target area of the acoustic depth gauge, suggesting that snow accumulation data from a single acoustic depth gauge cannot be extrapolated over a broad area.