bluuebunny/crossref_metadata_2025 huggingface dataset:
Controls on the distribution of major ions in pore waters of a thick surficial aquitard
Authors: Hendry M. J. , Wassenaar L. I.
Published: September 2002

Transport and geochemical processes controlling the chemical and isotopic composition of pore waters in the upper 45 m of a thick, surficial, clay‐rich till (Battleford Formation) were studied. The upper 3 m of the till is oxidized and fractured. The remaining 77 m of the till (the aquitard) is nonfractured and unoxidized. Concentrations of total dissolved solids, SO42−, Na+, Mg2+, and K+ decrease with depth through the unoxidized zone to a depth of about 15 m below ground, below which the concentrations remain constant. A similar trend was observed for alkalinity; however, the decrease in concentration occurs over 20 m. In contrast to these ions, Ca2+ concentrations increase with depth through the upper 20‐m unoxidized zone, below which the concentrations remain relatively constant. The distribution of dissolved ions shows the presence of two end‐members: elevated solute concentrations in the oxidized zone attributed to geochemical weathering that occurred since the start of the Holocene and connate Pleistocene‐age water at depths between 20 and 45 m. Abiotic and biotic geochemical reaction rates in the aquitard were shown to be extremely slow or not occurring.
Snow patch and glacier size distributions
Authors: Bahr David B. , Meier Mark F.
Published: September 2002

A simple theoretical model demonstrates that some amount of randomness in snow and ice mass balance is sufficient to reproduce empirically observed power law and exponential distributions of snow patch and glacier sizes. No other assumptions about the Underlying topography or snow accumulation and ablation processes are necessary to extract this important spatial property. The inclusion of additional geometrical and physical processes can alter the specific scaling constants of the size distributions, but the fundamental behavior remains unchanged. Specifically, for snow patch and glacier sizes less than some correlation length the size distribution is a decreasing power law, and for sizes larger than the correlation length the distribution decreases rapidly as an exponential. The solution is based on a mapping to a relatively well explored class of problems in percolation theory.
Statistical approach to sediment sampling accuracy
Authors: Petrie John , Diplas Panayiotis
Published: September 2002

A variety of hydraulic and ecological processes in rivers depend in part on the grain size distribution of the riverbed. For this reason it is important to accurately sample the bed to determine the grain size distribution. To describe the accuracy of riverbed sediment samples, techniques based on the multinomial distribution are presented. These techniques allow confidence intervals to be calculated for grid, areal, and volume samples. Additionally, sample sizes can be estimated to achieve a desired accuracy for the three sample types. A transformation is necessary when considering areal and volume samples. This is because neither of these two sample types is well conditioned for statistical analysis. The transformation is based on the results of a previously performed grid sample. The transformation then estimates either an areal or volume sample size that is necessary to match the accuracy of the grid sample. This procedure is useful when obtaining a hybrid distribution from grid and areal samples. Having areal and grid sample results of known accuracy allows confidence intervals to be placed on the final hybrid distribution. A series of laboratory experiments is used to illustrate the various techniques. Comparisons are also made with techniques presented in the literature.
Generalized maximum‐likelihood generalized extreme‐value quantile estimators for hydrologic data
Authors: Martins Eduardo S. , Stedinger Jery R.
Published: September 2002

The three‐parameter generalized extreme‐value (GEV) distribution has found wide application for describing annual floods, rainfall, wind speeds, wave heights, snow depths, and other maxima. Previous studies show that small‐sample maximum‐likelihood estimators (MLE) of parameters are unstable and recommendLmoment estimators. More recent research shows that method of moments quantile estimators have for −0.25 < κ < 0.30 smaller root‐mean‐square error thanLmoments and MLEs. Examination of the behavior of MLEs in small samples demonstrates that absurd values of the GEV‐shape parameter κ can be generated. Use of a Bayesian prior distribution to restrict κ values to a statistically/physically reasonable range in a generalized maximum likelihood (GML) analysis eliminates this problem. In our examples the GML estimator did substantially better than moment andLmoment quantile estimators for − 0.4 ≤ κ ≤ 0.
Stochastic analysis of transport in unsaturated heterogeneous soils uder transient flow regimes
Authors: Foussereau X. , Graham W. D. , Akpoji G. Ashie , Destouni G. , Rao P. S. C.
Published: September 2002

Approximate analytic solutions are developed to predict the transport of inert solutes in variably saturated, randomly heterogeneous porous media subject to a random boundary flux condition. The mean and variance of solute mass flux past a series of vadose zone control planes are determined using a Lagrangian approach. The method requires the specification of an advective travel time probability density function (pdf) between the soil surface and the control planes and an impulse response function which quantifies the specific flow and transport processes that affect the solute along an individual streamline in the soil. In this study an advective‐dispersive impulse response function for inert solute is assumed, and an advective travel time pdf is derived from the statistics of the boundary flux and soil properties using a one‐dimensional transient vadose zone flow model [Foussereau et al., this issue]. Monte Carlo simulations are conducted using random boundary flux sequences and spatially correlated random hydraulic conductivity fields in a multidimensional variably saturated flow and transport code to test the analytic model assumptions. The analytic model accurately predicts the ensemble mean solute flux breakthrough curves for a variety of soil‐climate combinations. The general behavior of the coefficient of variation of solute flux is also accurately reproduced; however, the minimum coefficient of variation at the mean center of mass was slightly underpredicted in most cases. Results show that for shallow soils in humid climates, boundary flux variability dominates soil variability in determining the uncertainty of solute transport predictions in the vadose zone.
Transformation approaches for simulating flow in variably saturated porous media
Authors: Williams Glenn A. , Miller Cass T. , Kelley C. T.
Published: September 2002

Sharp fronts with rapid changes in fluid saturations over short distance and timescales often exist in multiphase flow in subsurface systems. Such highly nonlinear problems are notoriously difficult to solve, and standard solution approaches are often inefficient and unreliable. We summarize four existing and one new transformation method (IT2) for solving Richards'; equation within a common framework and compare performance for a range of medium properties and simulation conditions. The new IT2 transform is defined as a linear combination of volumetric water fraction of the aqueous phase and integrated hydraulic conductivity terms. We show that transformation methods can significantly improve solution efficiency and robustness compared to standard solution approaches; optimal transformation parameters depend upon auxiliary conditions, medium properties, and spatial and temporal discretization and are difficult to evaluate a priori; and IT2 compares favorably with existing transforms.
Gamma‐ray bursts of atmospheric origin in the MeV energy range
Authors: Brunetti Michele , Cecchini Stefano , Galli Menotti , Giovannini Giovanni , Pagliarin Andrea
Published: September 2002

Downward gamma ray bursts of atmospheric origin in the MeV energy range were observed for the first time. The measurements were carried out on ground (at 2005 m above sea level) with a NaI(T1) 10×20Ø cm scintillation detector. Two kinds of events were observed: slow increases of the radiation below 3 MeV due to rain washout, and minute range bursts of gamma rays up to 10 MeV. The correspondence of these last events with perturbed weather conditions is here analyzed and the correlation with lightning activity is considered.
Formation of a compound slow shock‐rotational discontinuity structure
Authors: Lee L. C. , Wu B. H. , Chao J. K. , Lin C. H. , Li Y.
Published: September 2002

Whang et al. [1996, 1997] reported observations of a new double discontinuity in the interplanetary space and magnetotail. The double discontinuity is a compound structure of a slow shock (SS) followed by an adjoining rotational discontinuity (RD). In a pressure isotropic plasma the SS‐RD compound structure cannot be maintained, because the normal flow speed downstream of an SS is less than the Alfvén speed and the downstream RD will overtake the SS. However, the Alfvén speed in a pressure anisotropic plasma may be highly reduced by an increased value of P‖/P⊥. We suggest that the observed SS‐RD compound structure may be related to this anisotropic effect. In the hybrid simulations of a slow shock, the back streaming ions will result in high P‖ and relatively low P⊥ in the foreshock region. This leads to a region with highly reduced local intermediate wave speed. A rotational discontinuity propagating toward the slow shock from the downstream region would be slowed down in the high P‖ region and attached to the SS transition region to form a structure resembling a double discontinuity. We present hybrid simulations to examine possible results of the interaction between an RD and an SS, and the formation of a compound discontinuity. Our simulations show that there are two different types of interactions: (1) An RD passes through and propagates upstream of an SS, when a weak SS cannot provide enough high anisotropy (P‖/P⊥ > 1); and (2) a compound SS‐RD‐SS structure is formed in which an RD is trapped inside the transition region of a strong SS. In this compound structure, the SS is divided into two portions by the RD. However, this SS‐RD‐SS structure is somewhat different from the observed double discontinuity in which the RD is attached to the downstream part of a slow shock.
An analysis of large‐scale variations in small‐scale mantle heterogeneity using Global Seismographic Network recordings of precursors to PKP
Authors: Hedlin Michael A. H. , Shearer Peter M.
Published: September 2002

High‐frequency precursors to the core phase PKP are caused by scattering off heterogeneities in the lowermost mantle and D″ regions, and they provide a unique window into the small‐scale structure of the deep Earth. We study lower mantle scattering by analyzing 412 high‐quality PKP precursor records at ranges between 120° and 137.5° as obtained from the global seismic networks during the last 10 years. To examine regional variations in scattering strength, we compare individual records with the globally averaged PKP precursor stack of Hedlin et al.. [1997]. We identify strong differences in apparent scattering strength among specific source‐receiver paths. Inversion of these data for scattering source regions is complicated by ambiguity between source‐ and receiver‐side scattering and the sparse and uneven data coverage. Synthetic tests, however, suggest that inversions with applied smoothness constraints can resolve large‐scale differences in scattering strength over significant parts of the lower mantle. We use a conjugate gradient method based on an approximation to Rayleigh‐Born scattering theory to image differences in the average strength of scattering within the lowermost 1000 km of the mantle. Our results indicate particularly strong scattering beneath central Africa, parts of North America, and just north of India, whereas weaker scattering is seen beneath South and Central America, eastern Europe, and Indonesia. Some regions of strong scattering correlate roughly with large‐scale anomalies revealed by seismic tomography including the African plume and the Tethys trench. These correlations are tentative rather than definitive because bootstrap resampling tests show that many details in our model are not reliably resolved and the network data alone do not permit complete resolution of the source‐receiver ambiguity in all areas. Further progress in this area will require integration of available network recordings with data collected by regional networks and arrays and consideration of the phase velocity of the precursors as well as their temporal variations.
Dynamics of dip‐slip faulting: Explorations in two dimensions
Authors: Oglesby David D. , Archuleta Ralph J. , Nielsen Stefan B.
Published: September 2002

Dynamic models of earthquake rupture and slip are a powerful method by which to investigate the physics of earthquakes. Owing to both conceptual and computational constraints, dynamic earthquake models have largely been limited to cases with geometrical symmetry, such as faults in unbounded media or vertical faults. However, there are both observational and theoretical reasons to believe that nonvertical dip‐slip faults behave differently from faults with more symmetrical geometries. Previous observations have shown greater ground motion from thrust/reverse faults than normal faults and higher ground motion on hanging walls than on footwalls. In the present work, two‐dimensional dynamic simulations of thrust/reverse and normal earthquakes show precisely these effects and also elucidate their causes. For typical nonvertical dip‐slip faults the breakdown of symmetry with respect to the free surface allows radiated seismic waves to reflect off the free surface and to hit the fault again, altering the stress field on the fault. This process can lead to time‐dependent normal stress and a feedback between the friction/rupture processes and seismic radiation. This interaction leads to thrust/reverse faults producing much higher fault and ground motion than normal faults with the same geometry and stress magnitudes. The asymmetric geometry also directly leads to higher motion on the hanging walls of such faults than on the footwalls. Simulations show that these effects occur for a variety of dip angles but only for faults that either intersect or closely approach the free surface. The results emphasize the strong effect that the free surface can have on the dynamics of fault rupture and slip.
Glacier surge propagation by thermal evolution at the bed
Authors: Murray Tavi , Stuart Graham W. , Miller Paul J. , Woodward John , Smith Andrew M. , Porter Philip R. , Jiskoot Hester
Published: September 2002

Bakaninbreen, southern Svalbard, began a prolonged surge during 1985. In 1986, an internal reflecting horizon on radio echo sounding data was interpreted to show that the position of the surge front coincided with a transition between areas of warm (unfrozen) and cold (frozen) bed. Ground‐penetrating radar lines run in 1996 and 1998 during early quiescence show that the basal region of the glacier is characterized by a strong reflection, interpreted as the top of a thick layer of sediment‐rich basal ice. Down glacier of the present surge front, features imaged beneath the basal reflection are interpreted as the bottom of the basal ice layer, the base of a permafrost layer, and local ice lenses. This indicates that this region of the bed is cold. Up glacier of the surge front, a scattering zone above the basal reflection is interpreted as warm ice. There is no evidence for this warm zone down glacier of the surge front, nor do we see basal permafrost up glacier of it. Thus, as in early surge phase, the location of the surge front is now at the transition between warm and cold ice at the glacier bed. We suggest that the propagation of the front is associated with this basal thermal transition throughout the surge. Because propagation of the front occurs rapidly and generates only limited heat, basal motion during fast flow must have been restricted to a thin layer at the bed and occurred by sliding or deformation localized at the ice‐bed interface.
Residence time of inertial particles in a vortex
Authors: Fung J. C. H.
Published: September 2002

The residence time of a particle within a spatial domain is the total time it spends within this domain. We study the residence time of small dense particles in a spreading line vortex. Trajectories have been calculated for small dense particles released near a spreading line vortex. The results show that particles released far away from the vortex center will not be trapped by it, but particles released close to it will remain there for a considerable time. This residence time is shown to be a function of the particle inertia τp, its fall velocity VT, and the vortex's Reynolds number Reγ. The results of simulations also show that there is an optimum vortex Reynolds number such that the residence time of the particle is maximized within this simulated spreading line vortex. These findings are expected to be applicable to the coupling between the small‐scale turbulent flow structures and the motion of the suspended particles.
Eddy flow characteristics and mean flow interactions in the North Pacific
Authors: Adamec David
Published: September 2002

The strength, shape, orientation, mean flow interactions, and propagation characteristics of the time‐varying (eddy) flow determined from TOPEX/Poseidon altimeter data in the North Pacific are investigated using equivalent barotropic diagnostic calculations as presented by Hoskins et al. [1983]. Separate calculations for low‐ and high‐pass flow fields, distinguished by periods greater than and less than 6 months, respectively, are performed and revealed a fundamental difference between the eddy characteristics of the two. The areas with largest values of surface eddy kinetic energy occur in the vicinity of the Kuroshio Extension and the subtropical front. Near the Kuroshio Extension the low‐pass surface eddy kinetic energy is greater than the high‐pass energy, but in the subtropical frontal area the high‐pass eddy flow accounts for more energy than the low‐pass flow. For both areas the values of eddy kinetic energy are larger in summer than during winter. Low‐pass eddy flow tends to be aligned zonally, and the high‐pass eddy flow is aligned meridionally. Near the subtropical front the ratio of major to minor axis length is typically ∼1.5∶1, whereas near the Kuroshio Extension that ratio increases to values near 2:1. Along the southern boundary of the subtropical gyre the high‐pass eddy flow tends to propagate eastward relative to the mean flow, tending to decrease the gyre strength. However, the low‐pass eddy flow propagates westward relative to the mean flow and tends to increase the gyre strength. An additional computation of surface heat transport is used to deduce vertical propagation tendencies of the eddy flow and indicates that the low‐pass flow field transports more heat than the high‐pass flow with the largest eddy transports occurring near the coast of Honshu. The sign of the transport indicates a preference for upward energy propagation of the eddy flow there. Many of the characteristics, such as the orientation and propagation tendencies of the eddy flow, are similar to eddy flow in the atmosphere and accentuate a similarity between the general circulations of the atmosphere and ocean.
Impact of a new scheme for optical properties of ice crystals on climates of two GCMs
Authors: Kristjánsson J. E. , Edwards J. M. , Mitchell D. L.
Published: September 2002

A new parameterization scheme for the optical properties of nonspherical ice crystals is implemented in two global climate models. The scheme is based on polynomial fits to the results from a more elaborate scheme recently developed by the third author. The scheme assumes a temperature dependency of ice crystal size and is able to handle five different crystal shapes. We first carry out sensitivity experiments highlighting the direct (“noninteractive”) forcing effect of the new scheme for different assumptions on ice crystal shape and size. It is found that the effects of crystal shape and size are of comparable importance in determining the radiative fluxes at the top of the atmosphere. We have further conducted multiyear simulations with full dynamic coupling. In these experiments we have chosen planar polycrystals as our basic crystal shape. Compared to control integrations with columns in one case and 30 μm spheres in the other, we find a reduction in the cold biases of the models in the upper tropical troposphere. This is shown to be mainly due to enhanced longwave radiative warming there, caused by the presence of smaller ice crystals. The upper tropospheric warming affects tropical convection in such a way that moisture and ice water distributions are modified. Consequently, relative humidity biases are somewhat reduced in both models. In one of the models there is a significant improvement in the longwave radiative budget at the top of the atmosphere, while in the other, the radiative budget remains in fairly close agreement with observations.
Calibration of the Meteosat water vapor channel using collocated NOAA/HIRS 12 measurements
Authors: Bréon Francois‐Marie , Jackson Darren L. , Bates John J.
Published: September 2002

The Meteosat geostationary satellites carry a filtered radiometer channel centered at 6.2 μm for the measurement of upper tropospheric humidity. The operational calibration is derived from radiative transfer calculations applied to radiosonde measurements; large fluctuations in the calibration have been noticed. Here, we apply another method based on collocations with NOAA/High‐Resolution Infrared Radiometer Sounder (HIRS) channel 12. Radiative transfer calculations show that Meteosat measurements can be accurately predicted from HIRS channel 12 brightness temperatures. In addition, the HIRS instrument has onboard calibration of the complete optics, which makes it suitable as a reference for the calibration of other spaceborne radiometers. Application of the method to 3 years of Meteosat‐5 data shows calibration fluctuations significantly smaller than those provided by the operational method. This result indicates that the short‐term stability of the instrument calibration is much better than 5%, the variability given by the operational calibration. The proposed calibration coefficient is smaller by about 10–15%, which results in equivalent brightness temperatures biased by about 3–4 K and 35–50% relative error on the upper tropospheric humidity. Hypotheses of the causes for such large differences between the two calibration procedure results are proposed.
Dynamic and thermodynamic structure of the lower troposphere above rain forest and wet savanna during the EXPRESSO campaign
Authors: Delon Claire , Druilhet Aimé , Delmas Robert , Durand Pierre
Published: September 2002

This paper focuses on the energy budget of atmospheric surface and boundary layers over the wet savanna and the rain forest during the Experiment of Regional Sources and Sinks of Oxidants (EXPRESSO). It assesses dynamic features at the transition between savanna and forest at the beginning of the dry season. The experimental zone was still influenced by the monsoon flow with water vapor mixing ratio of about 16 g/kg. Thermodynamic characteristics of the savanna and forest ecosystems are similar. However, for an equivalent net radiation, evaporation is greater over the forest and leads to lower Bowen ratios (0.2 over the forest, 0.45 over the savanna). Low horizontal winds with no mean direction are observed, due to the proximity of the Intertropical Convergence Zone (ITCZ). However, in spite of low horizontal winds, the momentum flux is high, owing to high drag coefficients and roughness lengths. Some differences appear in the turbulent kinetic energy budget: dynamic and thermal production terms over the forest are nearly half those over the savanna. As a consequence, turbulent exchanges are more efficient over the savanna. Entrainment velocities are deduced from the latent heat flux values at the top of the atmospheric boundary layer (ABL) and from the jump in water vapor mixing ratio between the ABL and the free troposphere. Entrainment velocities are of the same order as subsidence velocities deduced from European Centre Medium‐Range Weather Forecasts statistical analyses. High pollution levels are observed in the forest ABL (CO concentration is around 280 ppb). Pollutants, produced by savanna fires, are transported from the free troposphere into the forest ABL by entrainment. Furthermore, horizontal exchanges between the savanna and the forest, induced by large eddies, may occur, increasing the pollution of the forest ABL. Through this study, the ITCZ appears to be an exchange zone rather than a barrier to the transfer of pollutants.
Accuracy assessments of aerosol optical properties retrieved from Aerosol Robotic Network (AERONET) Sun and sky radiance measurements
Authors: Dubovik O. , Smirnov A. , Holben B. N. , King M. D. , Kaufman Y. J. , Eck T. F. , Slutsker I.
Published: September 2002

Sensitivity studies are conducted regarding aerosol optical property retrieval from radiances measured by ground‐based Sun‐sky scanning radiometers of the Aerosol Robotic Network (AERONET). These studies focus on testing a new inversion concept for simultaneously retrieving aerosol size distribution, complex refractive index, and single‐scattering albedo from spectral measurements of direct and diffuse radiation. The perturbations of the inversion resulting from random errors, instrumental offsets, and known uncertainties in the atmospheric radiation model are analyzed. Sun or sky channel miscalibration, inaccurate azimuth angle pointing during sky radiance measurements, and inaccuracy in accounting for surface reflectance are considered as error sources. The effects of these errors on the characterization of three typical and optically distinct aerosols with bimodal size distributions (weakly absorbing water‐soluble aerosol, absorbing biomass‐burning aerosol, and desert dust) are considered. The aerosol particles are assumed in the retrieval to be polydispersed homogeneous spheres with the same complex refractive index. Therefore we also examined how inversions with such an assumption bias the retrievals in the case of nonspherical dust aerosols and in the case of externally or internally mixed spherical particles with different refractive indices. The analysis shows successful retrieval of all aerosol characteristics (size distribution, complex refractive index, and single‐scattering albedo), provided the inversion includes the data combination of spectral optical depth together with sky radiances in the full solar almucantar (with angular coverage of scattering angles up to 100° or more). The retrieval accuracy is acceptable for most remote sensing applications even in the presence of rather strong systematic or random uncertainties in the measurements. The major limitations relate to the characterization of low optical depth situations for all aerosol types, where high relative errors may occur in the direct radiation measurements of aerosol optical depth. Also, the results of tests indicate that a decrease of angular coverage of scattering (scattering angles of 75° or less) in the sky radiance results in the loss of practical information about refractive index. Accurate azimuth angle pointing is critical for the characterization of dust. Scattering by nonspherical dust particles requires special analysis, whereby approximation of the aerosol by spheres allows us to derive single‐scattering albedo by inverting spectral optical depth together with sky radiances in the full solar almucantar. Inverting sky radiances measured in the first 40° scattering angle only, where nonspherical effects are minor, results in accurate retrievals of aerosol size distributions of nonspherical particles.
Time derivative of the electric field 10, 14, and 30 m from triggered lightning strokes
Authors: Uman M. A. , Rakov V. A. , Schnetzer G. H. , Rambo K. J. , Crawford D. E. , Fisher R. J.
Published: September 2002

We have directly measured the time derivative of the electric field of triggered lightning strokes at distances of 10, 14, and 30 m. The data were taken in 1998 at the International Center for Lightning Research and Testing at Camp Blanding, Florida. We compare our results with those of similar triggered lightning measurements made previously at the Kennedy Space Center at distances of 50 m and 5 km and in France at 50 m. We also compare our electric field derivative waveforms with previous measurements at the Kennedy Space Center of natural lightning strokes over the Atlantic Ocean at distances of the order of tens of kilometers and with overland natural lightning data obtained at 0.7 to 14 km in Germany. Our return stroke electric field derivative peak values normalized (assuming the inverse distance dependence valid for radiation fields) to 100 km are similar to all previous measurements for both natural and triggered lightning at distances from 50 m to about 50 km, all being several tens of volts per meter per microsecond, with the exception of the German overland peak derivative values which are an order of magnitude lower. Our 10‐ to 30‐m field derivative zero‐to‐peak risetimes are typically 50 to 100 ns (minimum of 30 ns and maximum of 180 ns), and widths at half‐peak value are typically 100 to 200 ns. There is essentially no difference between our electric field derivative waveshapes measured simultaneously at 10 m and at 30 m, with the closer waveform being about a factor of 2 greater in amplitude. Fourier analysis of our electric field derivative waveforms indicates that the primary frequency content of the waveforms is below about 20 MHz. Our close return stroke field derivative waveforms differ from those of Leteinturier et al. (1990) recorded 50 m from triggered lightning at the Kennedy Space Center in 1985 in that their derivative waveforms typically decrease rapidly after the peak and exhibit zero crossings and in that their waveforms tend to have multiple peaks, while our derivative waveforms are generally single peaked and decay more gradually to zero after the peak, with no zero crossings. We argue that the differences between their waveforms and ours are related to the relatively large rocket‐launching structure used at the Kennedy Space Center in 1985.
Lévy flights and anomalous diffusion in the stratosphere
Authors: Seo Kyong‐Hwan , Bowman Kenneth P.
Published: September 2002

Chaotic transport in the Northern Hemisphere stratosphere is studied with an isentropic Lagrangian transport model. Ensemble statistics of trajectories are computed for 2‐month periods in the winters of 1992 to 1997 by using United Kingdom Meteorological Office assimilated winds. In the midlatitudes, quasi‐stationary anticyclones combine with the jet to produce flying and trapping events of particle trajectories. The flight time probability density functions (PDFs) are described by power laws with a decay exponent of less than 3, which indicates that the trajectories can be characterized as Lévy flights (random walk processes with divergent second moment). These flight and trapping events lead to superdiffusive zonal dispersion. The relationship between the power law decay exponents and the zonal mean and variance exponents is consistent with an analytical derivation. The scaled PDFs of zonal displacement converge to a self‐similar limiting distribution, suggesting the existence of a scale‐invariant regime. The skewness and kurtosis of the PDFs also converge to constant values after about 40 days, which supports the self‐similarity behavior in Lagrangian trajectories.
Influence of stratosphere‐troposphere exchange on tropospheric ozone over the tropical Indian Ocean during the winter monsoon
Authors: Zachariasse M. , van Velthoven P. F. J. , Smit H. G. J. , Lelieveld J. , Mandal T. K. , Kelder H.
Published: September 2002

Ozone (O3) and relative humidity (RH) soundings, launched over the Indian Ocean during the 1998 winter monsoon (February–March), were analyzed. In the marine boundary layer (MBL), O3 mixing ratios were relatively low (10–20 ppbv) except close to the Indian subcontinent (40–50 ppbv) where profiles were strongly influenced by pollution. Sometimes, relatively low O3 levels were observed in the upper troposphere. These were associated with deep convection in regions where MBL O3 levels were also low. In the midtroposphere (300–500 hPa), O3 maxima (60–90 ppbv) were often found with low RH. A remarkable new finding of this study is that in more than a third of the profiles, laminae with very high O3 mixing ratios (up to 120 ppbv) were observed just below the tropical tropopause (between 100 and 200 hPa). Back trajectory analyses showed that these layers originated in the vicinity of the subtropical jet stream (STJ). We hypothesize that stratosphere‐troposphere exchange (STE) near the subtropical jet by either shear‐induced differential advection or clear‐air turbulence (CAT) caused the midtropospheric maxima (STE followed by descent) and the upper tropospheric laminae. Another new finding is that stratospheric intrusions were not only found near the STJ but also deep within the tropics. Given the thickness of the midtroposphere intrusions (typically 3–5 km) and the very high O3 mixing ratios of the upper tropospheric laminae, it seems that STE plays an important role in the tropical tropospheric O3 budget, at least over the Indian Ocean during the winter monsoon.
Simulating convective events using a high‐resolution mesoscale model
Authors: Bernardet Lígia R. , Grasso Lewis D. , Nachamkin Jason E. , Finley Catherine A. , Cotton William R.
Published: September 2002

Four multiscale numerical simulations of convective events are analyzed to determine the essential characteristics of a numerical model which lead to useful simulations of convective events. Although several universities and weather forecasting centers are currently running high‐resolution forecast models, the predictability of convective events, especially in the warm season, is still an issue among researchers and forecasters in the meteorological community. This study shows that explicit simulations of convection depend on the high spatial resolution of physiography (particularly topography and top soil moisture), efficient communication between grids of different scales, and initialization procedures that incorporate mesoscale storm features.
Aerosol physical and optical properties and their relationship to aerosol composition in the free troposphere at Izaña, Tenerife, Canary Islands, during July 1995
Authors: Maring H. , Savoie D. L. , Izaguirre M. A. , McCormick C. , Arimoto R. , Prospero J. M. , Pilinis C.
Published: September 2002

Aerosol physical, optical, and chemical properties were measured at 2360 m above sea level over the Canary Islands during July 1995. Five aerosol size modes were observed. Nucleation aerosols <13 nm (geometric diameter) and a mode at 20 to 30 nm reflected local sources. A mode at 50 to 60 nm (likely from the upper atmosphere), another between 100 to 200 nm (pollutants), and mineral dust reflected distant sources. Green light scattering (adjusted to 1013 mbar and 0°C) ranged from below detection to >160 Mm−1 at relative humidities <30%. The lowest scattering was in air masses from the northwest whose aerosols were mostly pollutants. Air masses with North African mineral dust had the highest scattering. The mass scattering efficiency for dust was 0.50±0.10 m2 g−1 compared to 0.61 m2 g−1 at Barbados. Estimates of monthly mean scattering indicate dust accounts for 42 to 70% of total scattering. Soluble K+ in aerosols was strongly correlated with total scattering during both dusty and nondusty periods regardless of source. The back‐ to total scattering ratio was essentially constant during the dusty and nondusty sampling periods, 0.138±0.013 and 0.144±0.005. Ångström exponents ranged from 1.0 to 2.1 during the nondusty period and −0.12 to 0.72 during the dusty period. The estimated single scattering albedo appeared higher during the dusty period than during the nondusty period, suggesting mineral dust to be more cooling at 550 nm than pollutants.
NLC particle properties from a five‐color lidar observation at 54°N
Authors: Alpers M. , Gerding M. , Höffner J. , von Zahn U.
Published: September 2002

On June 13/14, 1998, a first simultaneous five‐color lidar measurement of a noctilucent cloud was realized. The experiment was made possible by the fact that at the site of our Institute (54°N), dark summer nights allow lidar observations of overhead noctilucent clouds (NLCs) without expensive spectral filters. We determine the size distribution of the NLC particles assuming spherical ice particles with a monomodal, lognormal size distribution. To this end we introduce a new method for calculation of the NLC particle properties. For the latter the following ranges are found: Particle number density N = 260–610 cm−3, median radius rm = 20.2–27.5 nm, and distribution width σ= 1.5–1.6. Cross checks using different wavelength combinations confirm the robustness of the NLC particle property results. The results are similar to those from recent lidar work on noctilucent clouds, observed at arctic latitudes.
Estimate of the aerosol properties over the ocean with POLDER
Authors: Deuzé J. L. , Goloub P. , Herman M. , Marchand A. , Perry G. , Susana S. , Tanré D.
Published: September 2002

The wide field of view imaging spectroradiometer Polarization and Directionality of the Earth's Reflectance (POLDER) developed by Centre National d'Etudes Spatiales and operated aboard the Japanese heliosynchronous platform Advanced Earth Observation Satellite (ADEOS) from October 30, 1996, to June 30, 1997, provided the first global systematic measurements of the spectral, directional, and polarized characteristics of the solar radiation reflected by the Earth/atmosphere system. These original observational capabilities offer an opportunity to enhance the characterization of several components of the global environment, especially the oceanic and terrestrial vegetal primary production, the aerosol physical and optical properties, and the tridimensional structure and microphysics of clouds. Here we examine the remote sensing of aerosols over the oceans. In a first step the aerosol optical thickness and Ångström exponent are derived from the radiance measurements. In a second step the polarization measurements are used for the retrieval of the aerosol refractive index. The inversion algorithm assumes spherical, nonabsorbing particles with monomodal lognormal size distribution. The adequacy of this modeling is discussed for a representative set of aerosol observations. Successful retrieval is generally achieved in the presence of small aerosols with Ångström exponent larger than ∼1.0. For such particles, polarization may provide information on the particle refractive index. As the Ångstrom exponent of the particle decreases, the data fitting residual errors increase, especially in polarized light, which prevents the retrieval of the aerosol refractive index. The trends of the discrepancies point out two shortcomings of the aerosol modeling. The theoretical results systematically underestimate the contribution of small polarizing particles in the polarization measurements for side‐scattering angles ranging from 80° to 120°. This indicates very probably that aerosol models have to follow bimodal size distribution. On the other hand, the systematic trend of the directional behavior of the upward radiance and the lack of significant rainbow effect in the measurements result probably from nonsphericity of some large aerosols. Confirmation of these points requires improved analysis of the POLDER data.
Interannual variability of the tropical tropopause derived from radiosonde data and NCEP reanalyses
Authors: Randel William J. , Wu Fei , Gaffen Dian J.
Published: September 2002

Interannual variability of the tropical tropopause is studied using long time series of radiosonde data, together with global tropopause analyses from the National Centers for Environmental Prediction (NCEP) reanalyses over 1957–1997. Comparisons for the period 1979–1997 show the NCEP tropopause temperature is too warm by ∼3–5 K and too high in pressure by ∼2–6 mbar. However, these biases are approximately constant in time, so that seasonal and interannual variability is reasonably well captured by the NCEP data. Systematic differences in NCEP tropopause statistics are observed between the presatellite (1957–1978) and postsatellite (1979–1997) periods, precluding the use of the reanalyses for the study of multidecadal variability. Interannual anomalies in tropical average radiosonde and NCEP data show variations of order ±1–2 K over the period 1979–1997, but there can be differences between these two estimates which are of similar magnitude. These differences impact estimates of decadal trends: During 1979–1997, negative trends in tropopause temperature of order −0.5 K/decade are observed in radiosonde data but are not found in NCEP reanalyses. The space‐time patterns of several coherent signals are identified in both sets of tropopause statistics. The volcanic eruption of El Chichón (1982) warmed the tropical tropopause by ∼1–2 K and lowered its altitude by ∼200 m for approximately 1–2 years. Smaller tropopause variations are observed following Mount Pinatubo (1991), particularly in radiosonde data. The signatures of the quasi‐biennial oscillation (QBO) and El‐Nino/Southern Oscillation (ENSO) events are strong in tropopause statistics. QBO variations are primarily zonal mean in character, while ENSO events exhibit dipole patterns over Indonesia and the central Pacific Ocean, with small signals for zonal averages.
Stochastic analysis of transient saturated flow through heterogeneous fractured porous media: A double‐permeability approach
Authors: Zhang Dongxiao , Sun Alexander Y.
Published: September 2002

In this paper we use a double‐porosity/double‐permeability approach to study transient, saturated flow in heterogeneous, fractured porous media. The spatial variabilities in both the fracture and matrix continua motivate us to treat some of the fracture and matrix medium properties as stochastic processes. Hence flow through heterogeneous, fractured porous media is also amenable to stochastic analysis. In particular, we develop a moment equation‐based stochastic theory for flow in fractured porous media. We derive general equations governing the statistical moments of the pressure heads in such media by perturbation expansions. The solutions of these moment equations are the first two moments of the pressure heads. These may be used to construct the confidence intervals of the fracture and matrix pressure heads, which are measures of uncertainties caused by incomplete knowledge of the fracture and matrix medium heterogeneities. The resulting moment equations of pressure heads are coupled and need to be solved sequentially in an iterative manner. The two‐dimensional version of the equations is implemented numerically and illustrated with some examples.
Modeling embryo survival affected by sediment deposition into salmonid spawning gravels: Application to flushing flow prescriptions
Authors: Wu Fu‐Chun
Published: September 2002

This paper presents a framework for predicting embryo survival in salmonid spawning gravels as a function of sediment deposition. This framework integrates three quantitative relationships modeling the variations of substrate permeability with sediment deposition, apparent velocity with substrate permeability, and embryo survival rate with apparent velocity. The model allows evaluation of the impacts of sediment deposition on embryo survival. The relative sensitivity of embryo survival to three selected environmental factors is investigated. The model results indicate that embryo survival is most sensitive to the composition of fine sediments (or sediment‐gravel size ratio). The maximum influences of the hydraulic pressure head and the length of intragravel flow path are ∼60 and 35% of the value influenced by size ratio. The proposed model is applied to determine the timing of flushing flows. The results suggest that the interval between flushing flows should be reduced when higher levels of embryo survival are prescribed or higher near‐bed sediment concentrations are imposed.
Submarine groundwater discharge into the sea and associated nutrient transport in a Sandy Beach
Authors: Uchiyama Yusuke , Nadaoka Kazuo , Rölke Peter , Adachi Kumiko , Yagi Hiroshi
Published: September 2002

Submarine groundwater discharge (SGWD) and associated nutrient fluxes at Hasaki Beach along the Kashima coast in Japan were investigated through field measurements and numerical simulations. The field data indicate that (1) groundwater has higher concentrations of land‐derived nutrients than seawater and river water; (2) microbial activity near the shoreline is likely to induce mineralization, reduction, and oxidation of nitrogen and phosphorus; and (3) in the portion of the aquifer underlying coastal forest, nitrate is appreciably reduced. A numerical model incorporating effects of water table and tidal fluctuations was developed to evaluate SGWD. Assuming that the aquifer consists of homogeneous sandy soil, the simulation results show that the nutrient flux due to SGWD is less than that of Tone River discharge. The nutrient flux via groundwater seepage is considered to be a minor component of primary productivity in the surf zone.
Application of a fractional advection‐dispersion equation
Authors: Benson David A. , Wheatcraft Stephen W. , Meerschaert Mark M.
Published: September 2002

A transport equation that uses fractional‐order dispersion derivatives has fundamental solutions that are Lévy's α‐stable densities. These densities represent plumes that spread proportional to time 1/α, have heavy tails, and incorporate any degree of skewness. The equation is parsimonious since the dispersion parameter is not a function of time or distance. The scaling behavior of plumes that undergo Lévy motion is accounted for by the fractional derivative. A laboratory tracer test is described by a dispersion term of order 1.55, while the Cape Cod bromide plume is modeled by an equation of order 1.65 to 1.8.
Restricted interval guelph permeameter: Theory and application
Authors: Freifeld Barry M. , Oldenburg Curtis M.
Published: September 2002

A constant head permeameter system has been developed for use in small‐diameter boreholes with any orientation. It is based upon the original Guelph permeameter concept of using a Mariotte siphon reservoir to control the applied head. The new tool, called a Restricted Interval Guelph (RIG) permeameter uses either a single pneumatic packer or straddle packer to restrict the area through which water is allowed to flow so that the borehole wetted area is independent of the applied head. The RIG permeameter has been used at Yucca Mountain, Nevada, in the nonwelded rhyolitic Paintbrush Tuff. Analysis of the acquired data is based upon saturated‐unsaturated flow theory that relies upon the quasi‐linear approximation to estimate field‐saturated hydraulic conductivity (K fs) and the α parameter (sorptive number) of the exponential relative hydraulic conductivity–pressure head relationship. These results are compared with a numerical model based upon the solution of the Richards equation using a van Genuchten capillary pressure‐saturation formulation. The numerical model incorporates laboratory capillary pressure versus saturation functions measured from cores taken from nearby boreholes. Comparison between the analytical and numerical approaches shows that the simple analytic model is valid for analyzing the data collected. Sensitivity analysis performed with the numerical model shows that the RIG permeameter is an effective tool for estimating permeability and sorptive number for the nonwelded Paintbrush Tuff.