bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical model of frazil ice and suspended sediment concentrations and formation of sediment laden ice in the Kara Sea
Authors: Sherwood C. R.
Published: September 2002

A one‐dimensional (vertical) numerical model of currents, mixing, frazil ice concentration, and suspended sediment concentration has been developed and applied in the shallow southeastern Kara Sea. The objective of the calculations is to determine whether conditions suitable for turbid ice formation can occur during times of rapid cooling and wind‐ and wave‐induced sediment resuspension. Although the model uses a simplistic approach to ice particles and neglects ice‐sediment interactions, the results for low‐stratification, shallow (∼20‐m) freeze‐up conditions indicate that the coconcentrations of frazil ice and suspended sediment in the water column are similar to observed concentrations of sediment in turbid ice. This suggests that wave‐induced sediment resuspension is a viable mechanism for turbid ice formation, and enrichment mechanisms proposed to explain the high concentrations of sediment in turbid ice relative to sediment concentrations in underlying water may not be necessary in energetic conditions. However, salinity stratification found near the Ob' and Yenisey Rivers damps mixing between ice‐laden surface water and sediment‐laden bottom water and probably limits incorporation of resuspended sediment into turbid ice until prolonged or repeated wind events mix away the stratification. Sensitivity analyses indicate that shallow (≤20 m), unstratified waters with fine bottom sediment (settling speeds of ∼1 mm s−1 or less) and long open water fetches (>25 km) are ideal conditions for resuspension.
Identification of intense, intermittent coherent motions under shoaling and breaking waves
Authors: Cox Daniel T. , Kobayashi Nobuhisa
Published: September 2002

Laboratory measurements of the instantaneous horizontal and vertical turbulent velocities,u′ andw′, induced by regular waves spilling on a rough, impermeable slope were analyzed to elucidate the nature of turbulence generated in the bottom boundary layer and by wave breaking. The analyses focused on the instantaneous turbulent events such as the absolute shear stress |τ′| = |−u′w′| and turbulent kinetic energyk′ = (u′2+w′2)/2. Below trough level inside the surf zone, the horizontal and vertical velocity records showed intense, intermittent turbulent events that did not occur with the passing of each wave, and the instantaneous quantities of |τ′| andk′ could not be explained in terms of the phase‐averaged quantities. The intermittent turbulent events extended into the bottom boundary layer inside the surf zone, and in this region the infrequent but intense turbulence generated by wave breaking was an order of magnitude larger than the turbulence generated locally at the boundary. Two techniques were used to analyze the turbulent motions. A quadrant analysis technique showed that although the large turbulent motions did not occur with each wave, they were phase‐dependent near trough level and less so near the bottom. A conditional sampling technique quantified the intensity and duration of the turbulent motions, indicating that coherent events (intense events) occurred for ∼10% (2%) of the record and accounted for approximately 50% (20%) of the turbulent motion. It is likely that the exactitude of these statistics will differ depending on breaker type. Nevertheless, these statistics indicated that large turbulent motions are infrequent but contribute significantly to the turbulence intensity and possibly the suspension of bottom sediments.
Internal hydraulics and mixing in a highly stratified estuary
Authors: Chant Robert J. , Wilson Robert E.
Published: September 2002

Shipboard acoustic Doppler current profiler and conductivity‐temperature‐depth data obtained during highly stratified conditions in the Hudson River estuary along a section of variable width and breadth are presented. The observations emphasize tidal period asymmetries in the vertical structure of current and salinity. However, these asymmetries exhibit significant along‐channel structure which is determined by channel morphology. During the ebb the flow is linearly sheared, and steep halocline slopes in the vicinity of channel contractions are maintained by momentum advection. A minimum in vertical shear across the pycnocline occurs in channel contractions. During food the pycnocline sharpens and flattens with a middepth velocity maximum embedded in the pycnocline which separates a stratified surface layer from a bottom mixed layer. The along‐channel structure in vertical shear is consistent with a lateral vorticity equation. Estimates of Richardson numbers suggest that vertical mixing across the pycnocline is enhanced downstream of channel contractions.
Influence of extremely large solar proton events in a changing stratosphere
Authors: Jackman Charles H. , Fleming Eric L. , Vitt Francis M.
Published: September 2002

Two periods of extremely large solar proton events (SPEs) occurred in the past 30 years, which forced significant long‐term polar stratospheric changes. The August 2–10, 1972, and October 19–27, 1989, SPEs happened in stratospheres that were quite different chemically. The stratospheric chlorine levels were relatively small in 1972 (∼1.2 ppbv) and were fairly substantial in 1989 (∼3 ppbv). Although these SPEs produced both HOx and NOy constituents in the mesosphere and stratosphere, only the NOy constituents had lifetimes long enough to affect ozone for several months to years past the events. Our recently improved two‐dimensional chemistry and transport atmospheric model was used to compute the effects of these gigantic SPEs in a changing stratosphere. Significant upper stratospheric ozone depletions >10% are computed to last for a few months past these SPEs. The long‐lived SPE‐produced NOy constituents were transported to lower levels during winter after these huge SPEs and caused impacts in the middle and lower stratosphere. During periods of high halogen loading, these impacts resulted in interference with the chlorine and bromine loss cycles for ozone destruction. This interference actually led to a predicted total ozone increase that was especially notable in the time period 1992–1994, a few years after the October 1989 SPE. The chemical state of the atmosphere, including the stratospheric sulfate aerosol density, substantially affected the predicted stratospheric influence of these extremely large SPEs.
A comparison of the aerosol thickness derived from ground‐based and airborne measurements
Authors: Kato Seiji , Bergin Michael H. , Ackerman Thomas P. , Charlock Thomas P. , Clothiaux Eugene E. , Ferrare Richard A. , Halthore Rangasayi N. , Laulainen Nels , Mace Gerald G. , Michalsky Joseph , Turner David D.
Published: September 2002

The extinction optical thickness of particles obtained from scattering and absorption coefficients measured by an airborne integrating nephelometer and particle soot absorption photometer, respectively, is compared with the aerosol optical thickness derived from a ground‐based multifilter rotating shadowband radiometer, a Sun photometer, and a Raman lidar for 9 days. These 9 days are selected from intensive operation periods of the Atmospheric Radiation Measurement in April 1997, September 1997, and August 1998 at the southern Great Plains. For April 1997 and September 1997 cases the difference between the extinction optical thickness of particles estimated from vertical profiles and the extinction optical thickness of aerosol derived from the multifilter rotating shadowband radiometer is not significant. For August 1998 cases when the boundary layer relative humidity is higher than April 1997 and September 1997 cases, the extinction optical thickness of particles is 0.03 to 0.07 less than the extinction optical thickness of aerosol. The difference corresponds to 25% to 31% of the extinction optical thickness of aerosol. Based on these comparisons, the upper and lower limits of the single‐scattering albedo of particles present in the lower part of troposphere are 0.97 and 0.84, respectively.
Modeling of dimethyl sulfide ocean mixing, biological production, and sea‐to‐air flux for high latitudes
Authors: Jodwalis Clara M. , Benner Richard L. , Eslinger David L.
Published: September 2002

To explore the extent to which ocean mixed‐layer dynamics influences dimethyl sulfide (DMS) sea‐to‐air flux at high latitudes, a model of DMS ocean mixing, biological production, and sea‐to‐air flux was developed. This biophysical one‐dimensional model is driven by meteorology. The model simulates DMS seawater concentrations and vertical distributions, and DMS sea‐to‐air flux for Prince William Sound and the Gulf of Alaska, from early March through December. Sensitivity analyses revealed that DMS sea‐to‐air flux is most affected by the rates of flagellate production, Zooplankton grazing, photooxidation, and microbial consumption of DMS. Model results show that under conditions of substantial vertical mixing, such as high wind stress or convective mixing, DMS sea‐to‐air flux increases significantly. At high latitudes these events may coincide with wind‐driven mixing or the overturning of surface seawater due to decreasing sea surface temperatures in the autumn. Parameterizations used to estimate emissions of such a highly variable gas as dimethyl sulfide need to include ocean mixed‐layer dynamics. The current model is limited by the small number of DMS loss and production rate measurements available. The measurements that do exist have large ranges and come almost exclusively from low and midlatitude regions, mostly during the summer months under calm conditions. Field measurements are needed from high‐latitude systems to refine this model, making it an effective tool for designing field campaigns, improving the accuracy of DMS sea‐to‐air flux estimations, and assessing the contribution of northern oceans to the atmospheric sulfur budget.
Interannual variations of boundary layer temperature over the African Sahel associated with vegetation and the upper troposphere
Authors: Shinoda Masato , Gamo Minoru
Published: September 2002

The spatial patterns of correlations between the convective boundary layer (CBL) temperature and both vegetation and upper tropospheric temperature over Africa have been detected by a singular value decomposition (SVD) analysis of monthly data for the period 1981–1994. The data used in the present study consist of the National Oceanic and Atmospheric Administration (NOAA)‐derived normalized difference vegetation index (NDVI) and the 850 hPa/200 hPa potential temperatures from the National Centers for Environmental Prediction (NCEP) objective reanalysis. In general, greater‐than‐normal vegetation over the Sahel is related to lower‐than‐normal CBL temperature aloft, whereas less vegetation corresponds to higher CBL temperature aloft. This vegetation‐CBL correlation was extracted as the first SVD mode that has centers of action concentrated over the Sahel. The correlation is strongest during the late dry season (especially during February and April) and is weaker during the rainy season. Typical correlation is observed during April when marked cooling of the CBL occurs in concurrence with positive vegetation anomalies. These anomalies are likely to be produced by an occasional premonsoon rain resulting from a cloud band that propagates from the midlatitudes. During the rainy season the CBL temperature is more strongly associated with upper tropospheric temperatures, which are likely modulated by large‐scale circulation, than with the Sahelian vegetation.
Entrainment due to a thermal impinging on a stratified interface with and without buoyancy reversal
Authors: Zhang Qing , Cotel Aline
Published: September 2002

The aim of the present paper is to understand the interaction between a rising thermal and an inversion in the atmosphere and to quantify the turbulent entrainment rate due to a thermal impingement on a stratified interface. This problem was simulated in the laboratory using a water tank. The thermal is created by releasing a small volume of buoyant fluid into a stratified environment composed of two layers of different densities. A thin interface separates the lower layer from the lighter upper layer. The entrainment of upper layer fluid into the thermal is investigated using a passive dye flow visualization technique. The entrainment rate is found to obey aRi−3/2power law, as predicted byCotel and Breidenthal[1997]. The effect of simulated evaporative cooling on the entrainment of a thermal impinging on a stratified interface is also investigated experimentally. Evaporative cooling in atmospheric clouds is simulated in the laboratory using alcohol‐water mixtures, so that the mixed fluid is denser than either parent parcel. This is realized in the laboratory by releasing a mixture of ethyl alcohol and ethylene glycol in an aqueous solution. It rises first through a relatively dense lower layer fluid and then impinges on a thin stratified interface, above which is a layer of relatively light fluid. The entrainment rate for values of the buoyancy reversal parameterD*between 0 and 0.5 was found to obey aRi−3/2power law. The entrainment rate is independent ofD*between 0 and 0.5 for a range of Richardson numbersRifrom 3 to 25. This is consistent with the behavior of the buoyancy‐reversing thermal in an unstratified environment observed byJohari[1992].
On the changing abundance of ozone minima at northern midlatitudes
Authors: Reid S. J. , Tuck A. F. , Kiladis G.
Published: September 2002

An analysis of ozonesonde data for stations whose records go back at least a quarter of a century suggests that the number of intrusions of subtropical air into the midlatitude lower stratosphere has recently increased. These air masses, referred to here as ozone minima, contain low ozone concentrations over vertical depths of ∼4 km, sufficient to reduce the total ozone column by 10–15%. The mixing ratios of other trace species, such as nitrous oxide (N2O), carbon monoxide (CO), and water vapor (H2O) recorded in midlatitudes by the NASA ER‐2 aircraft, suggest that these intrusions originate either in the lower tropical stratosphere or in the upper tropical troposphere. Potential vorticity calculated from the European Centre for Medium‐Range Weather Forecasts (ECMWF) and National Center for Environmental Prediction (NCEP) date, have been used in conjunction with TOMS total ozone data to estimate the typical area covered by minima. Their contribution to the midlatitude ozone trend is discussed. TOVS data have also facilitated the identification of a prominent ozone minima source region in the subtropical western Atlantic, where the Atlantic storm track is particularly intense. Minima frequently advect over Europe, explaining the high abundance of these events observed by European ozonesonde stations, relative to ozonesonde data from Canada.
Atmospheric 14CO: A tracer of OH concentration and mixing rates
Authors: Quay Paul , King Stagg , White David , Brockington Melinda , Plotkin Beth , Gammon Richard , Gerst Steven , Stutsman John
Published: September 2002

Time series measurements of the ground level 14CO concentration were made at Olympic Peninsula, Washington (48°N), and Point Barrow, Alaska (71°N), between 1991 and 1997. Measurements of the meridional gradients of the 14CO concentration at sea level were made during five oceanographic cruises in the Pacific Ocean between 55°N and 65°S during 1991–1995. These measurements were combined with earlier time series measurements of atmospheric 14CO at 41°S and 77°S [Brenninkmeijer, 1993] and at 13°N [Mak and Southon, 1998] and meridional transects of 14CO at 6–8 km [Mak et al., 1994]. These 14CO data sets were analyzed using a two‐dimensional atmospheric circulation and chemistry model in order to determine the tropospheric OH concentration that could explain the temporal and spatial trends in 14CO. Additionally, the interannual trend in tropospheric methyl chloroform concentration and the stratospheric time history of bomb 14CO2 were simulated by the model. The results of this analysis indicate that an average tropospheric OH concentration of ∼l0×l05 radicals cm−3 explains both the 14CO and methyl chloroform trends. The model‐predicted 14CO concentrations, however, are sensitive to the rate of stratosphere‐troposphere exchange and horizontal mixing in the troposphere. Model predictions of tropospheric 14CO at high latitudes improved when the stratosphere‐troposphere exchange rate was slowed, based on the results of the stratospheric bomb 14CO2 model simulation. Substantial improvement in the model 14CO simulations occurred with increased horizontal diffusion rates in the troposphere and lower cosmogenic 14CO production rates. Significantly lower 14CO concentrations (∼50%) are observed in the Southern versus Northern Hemisphere. Model simulations indicate that either higher tropospheric horizontal mixing or higher OH concentrations in the Southern Hemisphere can explain the hemispheric asymmetry in 14CO.
Quantification of the isentropic mass transport across the dynamical tropopause
Authors: Dethof Antje , O'Neill Alan , Slingo Julia
Published: September 2002

A method is developed to quantify the quasi‐horizontal, isentropic mass transport across the dynamical tropopause by small‐scale filaments. This method is based on the contour advection technique and is used to calculate isentropic mass fluxes across the tropopause on the 330, 345, and 360 K surfaces from the European Centre for Medium‐Range Weather Forecasts analyses for the years 1997 and 1998. The annually integrated isentropic net mass flux across the extratropical tropopause is directed from the stratosphere into the troposphere. Isentropic stratosphere troposphere exchange is, however, a two‐way process, and the net flux is the residual of troposphere to stratosphere and stratosphere to troposphere transport, which can be of similar magnitude. It is shown that the isentropic mass fluxes are of the same order of magnitude as the upward vertical mass fluxes across the tropical tropopause by the large‐scale circulation. Further, the extratropical lowermost stratosphere consists of a “well‐ventilated” outer zone in which tropospheric and stratospheric air are heavily mixed, enclosing a more isolated inner zone. The mixing in the outer zone is equivalent to a mass exchange of 7 times the mass of the lowermost stratosphere per year in the Northern Hemisphere and 5 times per year in the Southern Hemisphere. The mixing of tropospheric and stratospheric air is strongest on the lower isentropic surfaces, and the stratosphere and the troposphere become more isolated with increasing altitude. There is greater troposphere to stratosphere transport during the spring of 1997 compared with that of 1998. This might partly explain why stratospheric ozone concentrations were lower during the spring of 1997 than during the spring of 1998.
Upper air mechanisms of the Southern Oscillation in the tropical Atlantic sector
Authors: Hastenrath Stefan
Published: September 2002

Anomalies of upper air circulation in January of Pacific warm and cold years are investigated from the 1958–1997 National Center for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) Reanalysis. The focus is on mechanisms conducive to slack meridional pressure gradient on the equatorwardside of the North Atlantic high (NAH) during Pacific warm years, because earlier research has shown how the concomitantly weak North Atlantic tradewinds, through reduced evaporation and wind stirring, in the subsequent months lead to warm anomalies in the tropical North Atlantic and thus anomalously strong interhemispheric gradient of sea surface temperature (SST); this being a major factor for the failure of the March–June rainy season of northeast Brazil. The present analysis revealed for January of Pacific warm years a train of centers with alternatingly positive and negative departures of upper tropospheric topography extending from the equatorial Pacific across North America to the North Atlantic, and these topography anomalies are accompanied by anomalies in the fields of upper tropospheric divergence and divergent flow. In particular, anomalous upper tropospheric convergence and midtropospheric subsidence overly the equatorward flank of the NAH, thus favoring slack meridional gradient of surface pressure. While the origin of the upper tropospheric wave train from the Pacific to the Atlantic merits further study, the present analysis thus identified the critical Atlantic upper air mechanism through which the Southern Oscillation affects surface pressure and SST fields in the tropical Atlantic and rainfall in northeast Brazil.
Oxygen isotopes in tree rings of Abies alba: The climatic significance of interdecadal variations
Authors: Saurer Matthias , Cherubini Paolo , Siegwolf Rolf
Published: September 2002

We determined the δ18O variations in the latewood of tree rings from four silver firs (Abies alba Mill.) for the period 1840–1997 at a mountain site in Switzerland, establishing the longest available tree ring record for δ18O in central Europe. The isotope ratios were determined on whole wood with a rapid continuous flow pyrolysis technique, thus avoiding cellulose extraction. We found significant correlations with δ18O tree ring records from the same region, although these involved different materials (cellulose extracted from whole rings rather than latewood) and different species. This indicates that physical factors are more important than biological influences as a determinant of δ18O in tree rings. The isotope tree ring chronology was highly correlated with the oxygen isotope variations in the June/July precipitation for the period 1972–1992 (r=0.72), and δ18O in whole wood of tree rings is therefore well suited for the reconstruction of δ18O in precipitation. We found a slow, quasi‐periodic variation of the δ18O series with a periodicity of ∼24 years, which is correlated to variations in the July temperature. This could be caused by fluctuations in the large‐scale atmospheric circulation over Europe and the North Atlantic, which may result in a change in source and flow path of atmospheric moisture, affecting the isotope ratio of precipitation in Switzerland. Although a significant correlation with summer temperature was found (p<0.01), the low correlation coefficient (r = 0.31) indicates that the δ18O variations cannot be explained by temperature variations alone. However, even when considering that the factors influencing δ18O in precipitation are not yet fully understood, our study shows the potential of tree rings to provide long records of δ18O in precipitation for continental areas, which will improve our understanding of the causes of natural perturbations of the climate system.
Uncertainty levels in predicted patterns of anthropogenic climate change
Authors: Barnett Tim P. , Hegerl Gabriele , Knutson Tom , Tett Simon
Published: September 2002

This paper investigates the uncertainties in different model estimates of an expected anthropogenic signal in the near‐surface air temperature field. We first consider nine coupled global climate models (CGCMs) forced by CO2 increasing at the rate of 1%/yr. Averaged over years 71–80 of their integrations, the approximate time of CO2 doubling, the models produce a global mean temperature change that agrees to within about 25% of the nine model average. However, the spatial patterns of change can be rather different. This is likely to be due to different representations of various physical processes in the respective models, especially those associated with land and sea ice processes. We next analyzed 11 different runs from three different CGCMs, each forced by observed/projected greenhouse gases (GHG) and estimated direct sulfate aerosol effects. Concentrating on the patterns of trend of near‐surface air temperature change over the period 1945–1995, we found that the raw individual model simulations often bore little resemblance to each other or to the observations. This was due partially to large magnitude, small‐scale spatial noise that characterized all the model runs, a feature resulting mainly from internal model variability. Heavy spatial smoothing and ensemble averaging improved the intermodel agreement. The existence of substantial differences between different realizations of an ensemble produced by identical forcing almost requires that detection and attribution work be done with ensembles of scenario runs, as single runs can be misleading. Application of recent detection and attribution methods, coupled with ensemble averaging, produced a reasonably consistent match between model predictions of expected patterns of temperature trends due to a combination of GHG and direct sulfate aerosols and those observed. This statement is provisional since the runs studied here did not include other anthropogenic pollutants thought to be important (e.g., indirect sulfate aerosol effects, tropospheric ozone) nor do they include natural forcing mechanisms (volcanoes, solar variability). Our results demonstrate the need to use different estimates of the anthropogenic fingerprint in detection studies. Different models give different estimates of these fingerprints, and we do not currently know which is most correct. Further, the intramodel uncertainty in both the fingerprints and, particularly, the scenario runs can be relatively large. In short, simulation, detection, and attribution of an anthropogenic signal is a job requiring multiple inputs from a diverse set of climate models.
A review of the mesosphere inversion layer phenomenon
Authors: Meriwether John W. , Gardner Chester S.
Published: September 2002

An active topic of current research in aeronomy is the study of the dynamics of the mesosphere and lower thermosphere (MLT) from 60 to 130 km, especially in regard to the influences that govern variability. The physical processes of this region are diverse and complex with strong coupling between the MLT and the adjacent atmospheric regions brought about largely by the propagation and dissipation of atmospheric gravity waves (GWs) from sources above and below. The measurements of MLT winds and temperatures required for such studies represent daunting technical challenges. At low and midlatitudes the mesosphere inversion layer (MIL) phenomenon, a ∼10 km wide region of enhanced temperatures (ΔT ∼ 15–50K), is observed with great regularity in both the upper mesosphere (60–70 km) and the mesopause (90–100 km). Observations are largely based upon Rayleigh and Na temperature lidar systems but coherent radar observations have shown that the MIL phenomenon is linked to layers of turbulence occurring in both the topside and the bottomside regions. GW activity is believed to play an important role in the development of a linkage between the MIL and the tidal structure through GW coupling that results in an amplification of the tidal thermal structure. This linkage is readily evident for the upper MIL but is seen only occasionally for the lower MIL. Further study of MIL properties should emphasize continual 24 hour temperature observations, especially for the lower MIL region, to confirm the linkage of the development of the MIL to the MLT tidal structure.
A note on statistical analysis of the rate coefficient of nonaqueous phase liquid dissolution in porous media
Authors: Zhu Jianting
Published: September 2002

A statistical analysis of the rate coefficient of nonaqueous phase liquid dissolution in porous media is performed based on general functional correlation relationships found in literature. The input variables are treated as random and assumed to be lognormally distributed. The dissolution rate coefficient statistics are derived theoretically, and the influence of various parameters and their uncertainties are examined and discussed. The impact of uncertainties in the mean grain diameter is found to be the greatest on the dissolution rate coefficient uncertainties.
Public marginal willingness to trade off among water quality programs: Estimates of statewide and watershed‐specific budget values
Authors: Blomquist Glenn C. , Newsome Michael A. , Stone D. Brad
Published: September 2002

A budget survey is used to elicit individuals' relative values for various public water quality programs. Because a budget constraint is incorporated explicitly and people allocate across various statewide and watershed‐specific programs, marginal willingness to trade off values is revealed. These values are useful in the decision making of state and federal agencies responsible for water quality programs. We estimate values using the results of a 1997 random sample of Kentucky residents, with oversampling of residents of a small watershed in eastern Kentucky. Results show that people allocate the largest amounts to combat illegal dumping, untreated sewage, and hazardous material disposal. The lowest‐ranked budget category, farming erosion, receives less than half the amount allocated to illegal dumping. We find that in the watershed, while the top two categories are the same as for the state as a whole, mining drainage and logging erosion are more important.
A multiple‐organic‐pollutant simulation/optimization model of industrial and municipal wastewater loading to a riverine environment
Authors: Carmichael Jeffrey J. , Strzepek Kenneth M.
Published: September 2002

A simulation/optimization model is developed to demonstrate the need for inclusion of interactive pollutant effects in multiple‐pollutant wastewater management, by comparing single‐ and multiple‐pollutant least cost approaches that maintain downstream ambient quality levels. The model chooses biological oxygen demand (BOD), N, and P abatement levels for point sources that minimize the combined wastewater treatment costs of the pollutants in order to achieve chosen ambient quality standards. Nonlinear pollutant interactions result in a nonlinear Jacobian matrix of marginal water quality impacts, creating a nonlinear constraint set. The Jacobian is iteratively updated by the simulation model using iterative solutions from the optimization model. The open modeling framework allows other types of programming problems to be solved as well, such as mixed integer and mixed integer nonlinear problems. Single‐ and multiple‐pollutant management approaches are applied to a case study of the Nitra River Basin in Slovakia. The regrets resulting from implementing BOD‐only or fixed Jacobian solutions are shown, and the robustness of the solution procedure is demonstrated by varying the initial pollution abatement levels.
Efficient junction sequence method for transport in a directed network
Authors: Barten W. , Niemeyer M. J. , Jakob A.
Published: September 2002

A new junction sequence method for taking into account transport in a directed network of interconnected channels is suggested. It is a slight modification of the pathway and tree methods but is much more efficient. The requirements of the method in terms of computing time increases only linearly with the size of the network and sublinearly in terms of memory. The increased efficiency results from the introduction of a global order for the junctions in the network, which defines a sequence of the junctions such that transport at a junction only depends on preceding junctions but not on following junctions in the sequence. With this method the transport in the entire network can be calculated at “one shot” instead by superimposing a large number of pathways. The capabilities for some nonlinear effects like nonlinear sorption goes beyond the present pathway and tree methods.
Estimating the variance and integral scale of the transmissivity field using head residual increments
Authors: Zheng Li , Silliman Stephen E.
Published: September 2002

A modification of previously published solutions regarding the spatial variation of hydraulic heads is discussed whereby the semivariogram of increments of head residuals (termed head residual increments HRIs) are related to the variance and integral scale of the transmissivity field. A first‐order solution is developed for the case of a transmissivity field which is isotropic and whose second‐order behavior can be characterized by an exponential covariance structure. The estimates of the variance σY2 and the integral scale λ of the log transmissivity field are then obtained via fitting a theoretical semivariogram for the HRI to its sample semivariogram. This approach is applied to head data sampled from a series of two‐dimensional, simulated aquifers with isotropic, exponential covariance structures and varying degrees of heterogeneity (σY2 = 0.25, 0.5, 1.0, 2.0, and 5.0). The results show that this method provided reliable estimates for both λ and σY2 in aquifers with the value of σY2 up to 2.0, but the errors in those estimates were higher for σY2 equal to 5.0. It is also demonstrated through numerical experiments and theoretical arguments that the head residual increments will provide a sample semivariogram with a lower variance than will the use of the head residuals without calculation of increments.
Numerical errors in groundwater and overland flow models
Authors: Lal A. M. Wasantha
Published: September 2002

Numerical error estimates are useful to evaluate the applicability of overland and groundwater flow models and verify the validity of their results. In this paper, methods of estimating numerical errors are developed and then applied to evaluate the numerical accuracy of the South Florida Water Management Model. Analytical expressions for errors generated during the propagation of disturbances due to well pumping, boundary water level changes, and rainfall are obtained for steady and transient conditions using Fourier analysis of the linearized governing equations. Different situations under which truncation errors are introduced into models and their variation with the spatial and temporal discretization are discussed. Numerical experiments are carried out with the three‐dimensional groundwater flow model MODFLOW and a number of implicit and explicit models to verify the results. Dimensionless parameters are used in the expressions so that the results can be used to determine discretization errors in any existing or new finite difference model of regional or local scale.
Dispersion tensor evaluation in heterogeneous media for finite Peclet values
Authors: Salandin P. , Fiorotto V.
Published: September 2002

In natural formations the transport process at the local scale is characterized by the spatial heterogeneity of hydraulic conductivity and by the pore‐scale dispersion. Usually, theoretical investigations consider only the effect due to the spatial variation of hydraulic conductivity, because the first prevails over the latter by some order of magnitude. Nevertheless, the pore‐scale dispersion has a noteworthy impact on the concentration variance evaluation and on the ergodicity condition, so that its influence on the overall dispersion processes may be relevant. The note illustrates a theoretical procedure that allows one to define the global dispersion tensor at the local scale by taking into account the pore‐scale effects as evaluated in laboratory columns. To reach this goal, the pore‐scale and the local‐scale velocity fluctuations are coupled via a rigorous analytical procedure in the three‐dimensional (3‐D) domain, and it is demonstrated that the porous media heterogeneity modifies the pore‐scale dispersion effects as measured in laboratory columns. The impact of the hydraulic conductivity heterogeneity on the pore‐scale dispersion at the local scale is due to (1) the path line sinuosity and (2) the module of the local velocity variation. A quantitative evaluation of these effects is made according to a first‐order analysis, i.e., assuming that the spatial fluctuations of the hydraulic conductivity are small, and, in the 2‐D case, a comparison with nonlinear results is made also. The results demonstrate that the only path line sinuosity coupled with the pore‐scale anisotropy enhances the transversal mixing and reduces the longitudinal one, but the global heterogeneity effect, considering also the velocity module fluctuations, gives a generalized increase of the local dispersion tensor components.
A volume law for specification of linear channel storage for estimation of large floods
Authors: Zhang Shangyou , Cordery Ian , Sharma Ashish
Published: September 2002

A method of estimating large floods using a linear storage‐routing approach is presented. The differences between the proposed approach and those traditionally used are (1) that the flood producing properties of basins are represented by a linear system, (2) the storage parameters of the distributed model are determined using a volume law which, unlike other storage‐routing models, accounts for the distribution of storage in natural basins, and (3) the basin outflow hydrograph is determined analytically and expressed in a succinct mathematical form. The single model parameter is estimated from observed data without direct fitting, unlike most traditionally used methods. The model was tested by showing it could reproduce observed large floods on a number of basins. This paper compares the proposed approach with a traditionally used storage routing approach using observed flood data from the Hacking River basin in New South Wales, Australia. Results confirm the usefulness of the proposed approach for estimation of large floods.
Genesis and evolution of unrest episodes at Campi Flegrei caldera: The role of thermal fluid‐dynamical processes in the geothermal system
Authors: Gaeta F. S. , De Natale G. , Peluso F. , Mastrolorenzo G. , Castagnolo D. , Troise C. , Pingue F. , Mita D. G. , Rossano S.
Published: September 2002

We develop a model for describing water flow in a porous medium under the effect of thermal and pressure gradients. The model simulates geothermal systems in calderas. Given the boundary conditions and the fluid‐dynamical properties of the medium, the model allows computation, in fluid‐dynamical stationary states, of parameters characterizing the flow, such as flow velocity and temperature and pressure distributions at depth. The model is applied to investigate the effects of the local geothermal system on the unrest episodes at Campi Flegrei caldera. Using experimentally determined fluid‐dynamical parameters for the caldera rocks, we show that changes of water flow in shallow aquifers under the effect of pressure and/or temperature variations within the geothermal system can be very important in the genesis and evolution of unrest crises. In particular, they can strongly amplify the effect of pressure increase in the magma chamber on ground uplift. They can also explain the timescales of evolution of ground movements in terms of transit times of the water front and of the connected temperature fronts due to advective transport. On such grounds an integrated mechanic‐thermal fluid‐dynamical model was built, allowing us to give a semiquantitative, global explanation to the genesis and evolution of unrest phenomena. Results obtained here can be generalized to other similar calderas.
Galileo direction finding of Jovian radio emissions
Authors: Menietti J. D. , Gurnett D. A. , Kurth W. S. , Groene J. B. , Granroth L. J.
Published: September 2002

The Galileo spacecraft, in orbit about Jupiter, has observed distinct spin modulation of plasma wave emissions near the Ganymede (G1 and G2) encounters in the frequency range from about 100 kHz to approximately 6 MHz. Assuming circularly polarized, transverse electromagnetic radiation, we have used the spin modulation of the sweep‐frequency receivers of the electric dipole antenna over many spins to estimate the source location in the spin plane of the spacecraft. Hectometric (HOM) and decametric (DAM) emission is observed by Galileo as a general and continuous background with frequent bursts that last tens of minutes and can be separated by minutes or hours. We have analyzed HOM and DAM emissions observed near Jupiter just after the G1 and G2 encounters, including two HOM/DAM “arc” signatures observed after the G2 encounter. These latter appear to be low‐frequency extensions of DAM arcs, with source regions along either the Io or the Ganymede flux tube. While the uncertainties associated with the data analysis do not allow a precise source location, the HOM/DAM emission observed near the G1 and G2 encounters is consistent with a gyroresonant source region, but it is necessary to require refraction due to the Io torus to understand the results. To explain emission from apparent source regions above a gyroresonant source region, wave refraction from asymmetries in the Io plasma torus that extend along magnetic field lines is postulated. Alternatively, if such torus density asymmetries do not exist, emission with sources above a gyroresonant source region would require another free‐energy source such as energetic plasma beams in the presence of density gradients or temperature anisotropies.
The middle ultraviolet‐visible spectrum of H2 excited by electron impact
Authors: James Geoffrey K. , Ajello Joseph M. , Pryor Wayne R.
Published: September 2002

The electron‐impact‐induced emission spectrum of H2 has been measured in the extended wavelength region 175–530 nm at a spectral resolution of 1.7 nm (full width at half maximum). The laboratory spectra observed in the middle ultraviolet (MUV) and visible spectral region are characterized by underlying H2 (
) continuum emission, together with many strong lines assigned to the radiative decay of the gerade singlet states of H2, and to members of the H Balmer series resulting from dissociative excitation of H2. Our calibrated MUV spectral data, obtained at 14, 19, and 100 eV electron‐impact energies, provide absolute emission cross sections of these H2 lines and will assist in the interpretation of planned Galileo ultraviolet spectrometer observations of Jupiter's aurora in this wavelength region.
Stratospheric trends of CFC‐12 over the past two decades: Recent observational evidence of declining growth rates
Authors: Engel Andreas , Schmidt Ulrich , McKenna Daniel
Published: September 2002

The long term temporal trend of CCl2F2 (CFC‐12) in the lower stratosphere has been derived from a series of balloon‐borne observations of long‐lived trace gases made since 1978. In order to remove the effect of dynamical processes in the individual profile observations, nitrous oxide (N2O) is used as a vertical coordinate. The calculated trend in the lowermost stratosphere follows the observed tropospheric increase very closely, however, the observed mixing ratios in the lowermost stratosphere appear to lag the global mean tropospheric trend by about one year. The most recent observations indicate that the slowing down of the tropospheric increase has propagated into the lowermost stratosphere. While CFC‐12 increased at an average rate of 18.5±1.8 ppt per year between 1978 and 1990, the growth rate decreased to 11.9±4.2 ppt per year between 1990 and 1997.
On dynamic recrystallization during solid state flow: Effects of stress and temperature
Authors: De Bresser J. H. P. , Peach C. J. , Reijs J. P. J. , Spiers C. J.
Published: September 2002

A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (σ) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D=101.12exp[29.3/RT]σ−1.23 and demonstrated that D (µm) decreases with increasing σ (MPa) and increasing temperature (T) in a manner which is in agreement with the field boundary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical D‐σ relations, used in paleo‐piezometry, will need to be modified to account for temperature effects.
Ground based millimeter‐wave observations of Arctic chlorine activation during winter and spring 1996/97
Authors: Raffalski U. , Klein U. , Franke B. , Langer J. , Sinnhuber B.‐M. , Trentmann J. , Künzi K. F. , Schrems O.
Published: September 2002

We have performed measurements of stratospheric chlorine monoxide (ClO) in Ny‐Ålesund, Spitsbergen (78.9°N, 11.9°E), using a ground‐based mm‐wave radiometer. In this paper we describe the observed degree of chlorine activation inside the polar vortex in the winter 1996/97. We obtained daily averaged ClO mixing ratios for the lower stratospheric layer on 20 days covering the period from mid‐February until the beginning of April. The volume mixing ratio of the lower ClO peak at an altitude of approximately 21 km reached a maximum of 1.6 ppbv. These measurements support the strong chemically induced ozone depletion which was found until April 5 by simultaneous ozone measurements reported in a companion paper [Sinnhuber et al., 1998]. They observed an unusual ozone loss in late April, which corresponds to the observed ClO abundance on April 18–22. On several days we observed diurnal ClO cycles which were compared with model calculations.
Ulysses and WIND particle observations of the November 1997 solar events
Authors: Lario D. , Marsden R. G. , Sanderson T. R. , Maksimovic M. , Balogh A. , Forsyth R. J. , Lin R. P. , Gosling J. T.
Published: September 2002

The episode of intense solar activity on 4 and 6 November 1997 resulted in large and extended particle events observed by the Ulysses and WIND spacecraft. Ulysses was at a heliocentric distance of 5.34 AU, very close to the solar equator and 100° west in heliolongitude from earth. WIND was near the earth at 0.99 AU from the Sun and in the ecliptic plane. Both spacecraft detected particle events and magnetic field structures related to this intense activity. A likely scenario for these particle events is proposed. The differences in the particle intensity profiles observed by both spacecraft are explained in terms of their location with respect to the solar activity site, the existence of interplanetary structures traveling towards the spacecraft, and the nature of particle transport from the source up to the two spacecraft.
Seismic events discrimination by neuro‐fuzzy‐based data merging
Authors: Muller S. , Legrand J. ‐F. , Muller J.‐D. , Cansi Y. , Crusem R. , Garda P.
Published: September 2002

This article involves an original method to classify low magnitude seismic events recorded in France by a network of seismometers. This method is based on the merging of high‐level data with possibly incomplete low‐level data extracted from seismic signals. The merging is performed by a multi‐layer neural network. A fuzzy coding is applied to the neural network's inputs to process efficiently incomplete data. The results reveal that the fuzzy coding coupled with the data merging increases the correct classification rate to more than 90% even when the database contains missing values.