bluuebunny/crossref_metadata_2025 huggingface dataset:
A semi‐kinetic model of wave‐ion interaction in the solar corona
Authors: Vocks C. , Marsch E.
Published: February 2004

A semi‐kinetic model of the ions in the solar corona is presented, which includes the effects of wave‐particle interactions and Coulomb collisions. At the coronal base a spectrum of Alfvén waves is assumed. The spatiotemporal evolution of the spectrum in a 2‐D computational domain is determined, taking the absorption of the waves by the particles into account. Integration over the velocity components perpendicular to the magnetic field yields “reduced” velocity distribution functions, for which a Vlasov equation is derived and solved numerically. The results obtained for a model of the heating process in a coronal funnel and in the lower corona are presented. The calculated velocity distribution functions show distinct kinetic effects and reflect key properties of the corona as observed by SOHO.
The value of grain‐size hydraulic conductivity estimates: Comparison with high resolution in‐situ field hydraulic conductivity
Authors: Eggleston J. , Rojstaczer S.
Published: February 2004

Hydraulic conductivity is commonly estimated from grain size characteristics of unconsolidated sediments. We present an extensive set of air‐permeability and grain size measurements from a heterogeneous aquifer. These data are unique in that the hydraulic conductivity measurements are made on undisturbed in‐situ sediment and, like the grain size measurements, have a small sample volume size (<1200 cm³). Hydraulic conductivity estimated from grain‐size has a higher mean and a lower variance than direct measurements. While grain size estimates of hydraulic conductivity do not correspond well to field measured hydraulic conductivity, spatial correlation of the two data sets is quite similar.
Enhanced sensitivity of persistent events to weak forcing in dynamical and stochastic systems: Implications for climate change
Authors: Khatiwala Samar , Shaw Bruce E. , Cane Mark A.
Published: February 2004

Low‐dimensional models can give insight into the climate system, in particular its response to externally imposed forcing such as the anthropogenic emission of green‐house gases. Here, we use the Lorenz system, a chaotic dynamical system characterized by two “regimes”, to examine the effect of a weak imposed forcing. We show that the probability density functions (PDF's) of time‐spent in the two regimes are exponential, and that the most dramatic response to forcing is a change in the frequency of occurrence of extremely persistent events, rather than the weaker change in the mean persistence time. This enhanced sensitivity of the “tails” of the PDF's to forcing is quantitatively explained by changes in the stability of the regimes. We demonstrate similar behavior in a stochastically forced double well system. Our results suggest that the most significant effect of anthropogenic forcing may be to change the frequency of occurrence of persistent climate events, such as droughts, rather than the mean.
Long‐term monitoring of creep rate along the Hayward Fault and evidence for a lasting creep response to 1989 Loma Prieta Earthquake
Authors: Lienkaemper James J. , Galehouse Jon S. , Simpson Robert W.
Published: February 2004

We present results from over 30 yr of precise surveys of creep along the Hayward fault. Along most of the fault, spatial variability in long‐term creep rates is well determined by these data and can help constrain 3D‐models of the depth of the creeping zone. However, creep at the south end of the fault stopped completely for more than 6 years after the M7 1989 Loma Prieta Earthquake (LPEQ), perhaps delayed by stress drop imposed by this event. With a decade of detailed data before LPEQ and a decade after it, we report that creep response to that event does indeed indicate the expected deficit in creep.
Patterns of coherent decadal and interdecadal climate signals in the Pacific Basin during the 20th century
Authors: Tourre Yves M. , Rajagopalan Balaji , Kushnir Yochanan , Barlow Mathew , White Warren B.
Published: February 2004

Two distinct low‐frequency fluctuations are suggested from a joint frequency domain analysis of the Pacific Ocean (30°S‐60°N) sea surface temperature (SST) and sea level pressure (SLP). The lowest frequency signal reveals a spatially coherent interdecadal evolution, In‐phase SST and SLP anomalies are found along the subarctic frontal zone (SAFZ). It is symmetric about the equator, with tropical SST anomalies peaking near 15° latitudes in the eastern Pacific. The other low‐frequency signal reveals a spatially coherent decadal evolution. It is primarily a low‐latitude phenomenon. Tropical SST anomalies peak in the central equatorial ocean with evidence of atmospheric teleconnections. These interdecadal and decadal signals join the ENSO and quasi‐biennial signals in determining dominant patterns of Pacific Ocean natural climate variability. Relative phasing and location of the SST and SLP anomalies for the decadal, ENSO, and the quasi‐biennial signals, are similar to one another but significantly different from that of the interdecadal signal.
Scale dependence of tracer microstructure: PDFs, intermittency and the dissipation scale
Authors: Sparling L. C. , Bacmeister J. T.
Published: February 2004

The statistics of tracer variability on small scales (<200 km) is investigated using high resolution aircraft measurements of ozone in the northern winter middle/high latitudes. Conditioning based on potential temperature is used to isolate the statistics of filamentation on isentropic surfaces from spurious variability due to cross‐isentropic motion of the platform. The distribution of isentropic increments Δrχ in the tracer field χ across a horizontal scale r have non‐Gaussian tails and are consistent with stretched exponential functions of the form P(Δrχ) ∼ exp(−a|Δrχ|p), where a is a scale‐dependent parameter and the exponent p increases overall with r. A scale break in the second order structure function suggests a dissipation scale rd ∼ 20 km during northern winter 91–92, but the scale break is closer to 100 km during northern winter 88–89. Possible reasons for this are discussed.
Spectroscopic measurements of tropospheric iodine oxide at Neumayer Station, Antarctica
Authors: Frieß U. , Wagner T. , Pundt I. , Pfeilsticker K. , Platt U.
Published: February 2004

First measurements of iodine oxide (IO) in the Antarctic troposphere are reported. Since March 1999, a newly developed dual channel spectrograph has been continuously performing Differential Optical Absorption Spectroscopy (DOAS) measurements of zenith scattered sunlight at Neumayer‐Station, Antarctica (70°39′S, 8°15′W). The spectral signature of IO was clearly detected by observing five vibrational absorption bands located in the wavelength region between 415 and 461 nm. The observed diurnal variation of IO is characterized by a rapid decrease in the differential slant column density (DSCD) with increasing solar zenith angle (SZA) during twilight. This observation points to a fast conversion of reactive iodine into its nighttime reservoir species. It also strongly indicates that the detected IO is located in the troposphere. The decrease of the IO DSCD of up to 1 · 1014 molec/cm² between 80° and 95° SZA is unexpectedly large. Under the assumption that IO is located in the marine boundary layer (MBL) (below 2 km), IO mixing ratios may reach up to ∼10 ppt. The seasonal variation shows higher IO amounts during summer than during winter. This finding is possibly caused by the smaller distance to the open sea, where the iodocarbons are emitted, and by the more efficient photodissociation of the organic iodine precursors.
Stability of KAlSi3O8 Hollandite‐type structure in the Earth's lower mantle conditions
Authors: Tutti Faramarz , Dubrovinsky Leonid S. , Saxena Surendra K. , Carlson Stefan
Published: February 2004

The stability of KAlSi3O8 hollandite‐type structure is investigated in a series of synthesis experiments between 20 and 95 GPa at 650(±50) °C and between 1200 to 2300 °C, using electrically‐ and laser‐heated diamond anvil cells, respectively. Potassium feldspar transformed to a hollandite‐type structure in the experimental pressure range from 20 to 95 GPa at temperatures between 1200 to 2300 °C, whereas it remained amorphous at 65(±5) GPa and 650(±50) °C. KAlSi3O8 hollandite is, therefore, a stable phase under the pressure and temperature conditions of at least 2200 km deep in the lower mantle and could be an important host for potassium in that region.
Empirical ozone isopleths as a tool to identify ozone production regimes
Authors: Thielmann Axel , Prévôt André S. H. , Grüebler Franca C. , Staehelin Johannes
Published: February 2004

Ozone isopleths plotted with measured values of ozone, hydrocarbons and total reactive nitrogen (NOy) are proposed to visualize ozone production regimes at distinct locations. The applicability of the concept is confirmed with measurements at two distinct sites in the Italian Po Basin. Empirical ozone isopleths at the urban site clearly show ROG‐sensitive ozone production, with ozone increasing with increasing ROG concentrations and decreasing with increasing ambient levels of NOx. At the rural site NOx‐sensitive ozone production prevails, in accordance with previous results.
The impacts of the model assimilated wind stress data in the initialization of an intermediate ocean and the ENSO predictability
Authors: Kug Jong‐Seong , Kang In‐Sik , Zebiak Stephen E.
Published: February 2004

A historical wind stress data is obtained based on the 925hPa winds of NCEP reanalysis data and is compared to the FSU wind stress. A particular difference between the two wind stress data appears in the eastern Pacific. The prediction experiments with an intermediate ocean and statistical atmosphere model indicate that the prediction skill of the tropical Pacific SST with the NCEP wind stress data is better than that of FSU wind stress for the period of 1980–1999. In particular, the prediction skill is considerably improved for the recent years of 1992–1999, when the NCEP wind stress is used for the ocean initialization.
Micro‐seismicity around the seaward updip limit of the 1946 Nankai Earthquake dislocation area
Authors: Obana Koichiro , Kodaira Shuichi , Mochizuki Kimihiro , Shinohara Masanao
Published: February 2004

We observed micro‐seismicity around the seaward updip limit of the 1946 Nankai earthquake dislocation area by using pop‐up type ocean bottom seismographs deployed for three months in 1998. At the subduction zone, we must consider strong lateral heterogeneity of the seismic velocity structure to locate earthquakes precisely. We constructed 3‐D P‐ and S‐wave velocity structure models based on the results of airgun‐OBS seismic surveys along the Nankai Trough. We located 83 earthquakes using these 3‐D models. These hypocenters show two groups of seismicity. One seismicity group locates in oceanic layer 3 within a subducting seamount. These earthquakes may reflect the deformation of the seamount caused by its subduction. The other seismicity group locates near the updip limit of the thermally modeled locked zone. These hypocenters are determined to be near the boundary between the accretionary prism and the subducting oceanic crust.
Particle Hall‐MHD simulation of collisionless reconnection: Ion gyro‐radius correction
Authors: Yin L. , Winske D. , Gary S. P. , Birn J.
Published: February 2004

Fluid‐ion dynamics from a Hall‐MHD calculation and test ions that are advanced in Hall‐MHD fields are compared to particle‐ion dynamics from a hybrid simulation of two‐dimensional reconnection in a thin current sheet. These comparisons demonstrate that off‐diagonal terms of the ion pressure tensor are important to correctly model the ion out‐of‐plane momentum transport from the X point. These effects can be efficiently modeled in Hall‐MHD simulations by advancing test ions in the Hall‐MHD fields, accumulating the pressure tensor onto the spatial grid, and modifying the ion momentum equation. With ion gyro‐radius corrections, the “particle Hall‐MHD” simulation is shown to model accurately the ion out‐of‐plane momentum transport from the X point and the reconnection rate comparison with the hybrid simulation is improved.
Solar illumination as cause of the equinoctial preference for geomagnetic activity
Authors: Lyatsky W. , Newell P. T. , Hamza A.
Published: February 2004

Geomagnetic and auroral activity vary seasonally with maxima at equinoxes, as has been known for more than a century. The cause remains under debate. The angle made by the Earth's dipole axis with the typical direction of the interplanetary magnetic field (IMF) can explain a portion (about 17%) of the effect. To explain the majority of the equinoctial effect, we suggest that geomagnetic activity peaks when the nightside auroral zones of both hemispheres are in darkness, as happens at equinox. Under such conditions, no conducting path exists in the ionosphere to complete the currents required by solar wind‐magnetosphere‐ionosphere coupling, and geomagnetic disturbances maximize. To test this theory, the Universal Time (UT) variation of geomagnetic activity was explored. As our model predicts, geomagnetic activity in December, measured by the Am index, evinces a deep minimum around 0300–0600 UT when the auroral oval of both hemispheres are in darkness and a maximum around 1500–1600 UT when the southern nightside oval is sunlit. In June, complementary effects are predicted and observed.

Previous studies using the AE index have shown more ambiguous results. Here we show that if AE is resolved into the AU and AL components, the discrepancy disappears, with the AL component following the same pattern as does Am. We thus conclude that the intensity of global geomagnetic activity is well ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither.
Probing Europa's hidden ocean from tidal effects on orbital dynamics
Authors: Wu Xiaoping , Bar‐Sever Yoaz E. , Folkner William M. , Williams James G. , Zumberge James F.
Published: February 2004

Recent observations of Europa suggest that the Jovian satellite may have a liquid ocean underneath its icy surface. Geophysical models indicate that the tidal Love number k2 has a strong dependence on the presence or absence of an ocean. The k2 dependence on the ice shell thickness is also significant. Measurements of the static and tidal gravity fields through their dynamic effects on the trajectory of a low Europan orbiter can be essential in the detection of an ocean and inference of other internal structures. Covariance analyses have been carried out to assess accuracies using simulated Doppler tracking data. With 15 days of tracking from 2 Earth stations, the uncertainties for k2, mantle libration amplitude and the epoch radial position of the spacecraft are expected to be 0.0004, 2.8 arcsec and 5.7 m, respectively. These tight constraints will strongly contribute to ocean detection and ice thickness determination when combined with altimeter measurements.
K/Ar dating extended into the last millennium: Application to the youngest effusive episode of the Teide Volcano (Spain)
Authors: Quidelleur X. , Gillot P.‐Y. , Soler V. , Lefèvre J.‐C.
Published: February 2004

Less than one thousand years dating of the youngest volcanic episode of the Teide volcano (Canary Islands, Spain) has been performed using the K/Ar Cassignol technique. Analyses of about 20 g of pure alkali‐feldspar yielded a weighted mean age of 800±300 years. Stratigraphic, historic and archeomagnetic dating validate this age and rule out the hypothesis of the presence of significant excess argon contamination for this flow. Our result shows that the last effusive activity of the Teide volcano took place shortly before European settlement in Tenerife Island. This study confirms that the Cassignol technique can be applied with success for historic dating and demonstrates that it can now even be extended to the last millennium.
Azimuth‐time‐intensity striations of quasiperiodic radar echoes from the midlatitude E region ionosphere
Authors: Haldoupis C. , Hussey G. C. , Bourdillon A. , Delloue J.
Published: February 2004

The Valensole high frequency (HF) radar in the south of France is an ionospheric Doppler sounder which can perform E region coherent backscatter measurements over an azimuthal sector of 86°, from 26° E to 58° W, with ∼2° angular resolution. This large azimuthal coverage is taken advantage of in order to study quasiperiodic (QP) echoes in the zonal direction using azimuth‐time‐intensity (ATI) analysis. ATI plots show sequential sloping striations of scatter reminiscent of those detected routinely in the range‐time‐intensity (RTI) plots of midlatitude radars which view the medium at a fixed azimuth about the meridian. It was found that ATI striation periods range from a few minutes to less than 30 min, whereas the striation slopes are systematically negative (motions westward) prior to local midnight, and turn positive (motions eastward) in the post‐midnight hours. The zonal rates, dx/dt, computed from the striation slopes take values between ∼30 and 160 m/s. These are due to real motions of unstable plasma structures, most likely sporadic E patches that drift along with the neutral wind, that have zonal scale lengths of several tens of kilometers. The present observations imply that the mechanism responsible for QP echoes is independent of azimuth and can basically operate effectively in any direction in the horizontal plane.
First GPS signals reflected from the interior of a tropical storm: Preliminary results from Hurricane Michael
Authors: Katzberg Stephen J. , Walker Richard A. , Roles James H. , Lynch Terry , Black Peter G.
Published: February 2004

Using GPS signals reflected from the ocean surface is developing into a simple technique for measuring sea‐state and inferring surface wind speeds. Theoretical models have been developed which are considered valid to approximately 24 meters per second. The GPS reflection technique has an obvious extension to extremely high sea states, cyclones and extra‐tropical storms. In October of 2000 a GPS system mounted in a NOAA Hurricane Hunter research aircraft, was flown into Hurricane Michael off the South Carolina coast. The first acquisition of GPS signals reflected from the sea surface inside tropical cyclones was accomplished. This paper presents some examples of the data sets as well as early wind speed retrieval results using direct extensions of current models. Data from the GPS wind speed retrievals as well as from direct aircraft measurements are compared and discussed.
Solar magnetic flux influences on the dynamics of the winter middle atmosphere
Authors: Arnold Neil F. , Robinson Terry R.
Published: February 2004

A number of observational studies have reported a connection between various lower atmospheric parameters and a range of geomagnetic and solar coronal indices. The bulk of the Sun's magnetised plasma energy is absorbed in the high‐latitude upper atmosphere, far removed from the troposphere and the stratosphere, so a physical mechanism which accounts for these correlations has proved elusive. In this paper, a mechanistic three‐dimensional model of the atmosphere between 10 and 130 km has been developed to demonstrate that high energy particles from the solar wind can perturb the winter stratosphere significantly. Planetary waves provide an effective means of coupling solar‐induced changes in the thermosphere down to the stratosphere. A qualitatively similar response to forcing by increasing solar ultraviolet radiation was obtained even though there was no in situ forcing in the stratosphere.
Anti‐Correlation between stratospheric aerosol extinction and the Ångström parameter from multiple wavelength measurements with SAGE II—A characteristic of the decay period following major volcanic eruptions
Authors: Hayashida Sachiko , Horikawa Mariko
Published: February 2004

Stratospheric Aerosol and Gas Experiment (SAGE) II data at multiple wavelengths were analyzed to show how aerosol extinction decays with time following major volcanic eruptions. Comparisons were made between the lowest background level in 1999 and the past data record of the stratospheric aerosol layer. The time variation of the extinction coefficients was compared with the Ångström parameter, which is a good indicator of particle size. A clear anti‐correlation was found between the extinction and the Ångström parameter after the eruption of Mt. Pinatubo. Comparison of the extinction coefficients and Ångström parameters in 1989 and 1999 made it clear that the aerosol layer was affected by volcanic eruptions in 1989. The distinguishing negative correlation is a characteristic feature of decay periods following volcanic eruptions.
Melt generation at volcanic continental margins: No need for a mantle plume?
Authors: van Wijk J. W. , Huismans R. S. , ter Voorde M. , Cloetingh S. A. P. L.
Published: February 2004

Melt generation in a rifting environment is studied using a dynamic 2‐D finite element model. The lithosphere is extended to large, realistic thinning factors assuming a mantle temperature of 1333°C. The focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations. The timing of melt production (late synrift) and the amounts of melt are consistent with observations at volcanic margins. The dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins.
Measurement of interseismic strain accumulation across the North Anatolian Fault by satellite radar interferometry
Authors: Wright Tim , Parsons Barry , Fielding Eric
Published: February 2004

In recent years, interseismic crustal velocities and strains have been determined for a number of tectonically active areas through repeated measurements using the Global Positioning System. The terrain in such areas is often remote and difficult, and the density of GPS measurements relatively sparse. In principle, satellite radar interferometry can be used to make millimetric‐precision measurements of surface displacement over large surface areas. In practice, the small crustal deformation signal is dominated over short time intervals by errors due to atmospheric, topographic and orbital effects. Here we show that these effects can be over‐come by stacking multiple interferograms, after screening for atmospheric anomalies, effectively creating a new interferogram that covers a longer time interval. In this way, we have isolated a 70 km wide region of crustal deformation across the eastern end of the North Anatolian Fault, Turkey. The distribution of deformation is consistent with slip of 17–32 mm/yr below 5–33 km on the extension of the surface fault at depth. If the GPS determined slip rate of 24±1 mm/yr is accepted, the locking depth is constrained to 18±6 km.
Statistical and physical analysis of soil detachment by raindrop impact: Rain erosivity indices and threshold energy
Authors: Salles Christian , Poesen Jean , Govers Gerard
Published: February 2004

Laboratory experiments have been conducted to study the effects of various simulated rain properties on soil detachment due to raindrop impact. Two soil materials were tested: a fine sand and a silt loam. The mass of detached sediment was measured with the splash cup technique. The splash cups filled with soil material were exposed to simulated rain intensities ranging between 10 and 140 mm h−1. The detached sediment was collected and weighed, whereas rain intensity, drop diameter, and fall velocity of each raindrop were measured with an optical spectropluviometer. A statistical analysis has been made in order to evaluate which rain parameter best predicts the splash detachment rate. Linear fits between the rate of detached sediment and the product of drop size (D) and drop velocity (V), that is, DαVβ with values of α varying between 1 and 6 and values of β varying between 0 and 2, have been computed. The results indicate that values for α ranging between 3 and 6 and for β smaller than or equal to 2 best describe the rate of detached sediment. Although all indices containing the mass of water (i.e., D3) predicted splash detachment rates relatively well, the product of momentum and drop diameter (D4V) was slightly superior in describing splash detachment. The existence of a rain erosivity threshold (critical kinetic energy) to initiate soil detachment is confirmed. This threshold energy equals 5 μJ for the fine sand and 12 μJ for the silt loam and compares relatively well with the threshold energy data for other soils reported in the literature.
Length‐Scale analysis of surface albedo, temperature, and normalized difference vegetation index in desert grassland
Authors: Pelgrum Henk , Schmugge Tom , Rango Al , Ritchie Jerry , Kustas Bill
Published: February 2004

The Jornada Experiment on the Jornada Experimental Range in southern New Mexico aims at the description of the surface energy balance of a desert grassland ecosystem. A large volume of both field and remote sensing data has been collected from 1995 to 1998. Airborne Daedalus scanner data with a spatial resolution of 4 m have been used to infer the following land surface characteristics: surface temperature, albedo, and normalized difference vegetation index (NDVI). These land surface characteristics can be used as input for land surface models. However, land surface models work with very coarse grid cells of at least 50×50 km, in contrast to high‐resolution remote sensing data. Also, land surface models are generally based on nonlinear algorithms. Both restrictions lead to scale problems. One apparent question is how to scale up input remote sensing data to the much coarser resolution of the land surface model. The first step is to derive the length scale of the input land surface characteristics. The length scales of the land surface characteristics have been determined with the following two techniques: autocorrelation and wavelet analysis. Within the Jornada Experimental Range, three different sites with different vegetation characteristics were distinguished: grass, shrub, and a transition site with patches of both grass and shrub. The autocorrelation and wavelet analysis showed similar results for the shrub site. For the grass and transition site the wavelet analysis underestimated the length scale of the surface albedo and temperature. The length scale of the surface albedo was 35, 33, and 10 m for grass, transition, and shrub sites, respectively. The length scale of the surface temperature was 31, 20, and 8 m for grass, transition, and shrub sites, respectively. The length scale of the NDVI was 12, 6, and 5 m for grass, transition, and shrub sites, respectively. These small length scales could hamper the use of low‐resolution remote sensing data for deriving input data for land surface models.
Rainfall downscaling in a space‐time multifractal framework
Authors: Deidda R.
Published: February 2004

A space‐time multifractal analysis on radar rainfall sequences selected from the Global Atmospheric Research Program Atlantic Tropical Experiment database is presented. It is shown that space‐time rainfall can be considered with a good approximation to be a self‐similar multifractal process, so that a multifractal analysis can be carried out assuming Taylor's hypothesis to hold for rainfall over a wide range of spatial and temporal scales. The advection velocity needed to rescale the time dimension is estimated using different tracking techniques. On each selected rainfall sequence, a very good scaling is observed for spatial scales ranging from 4 to 256 km and for timescales from 15 min to 16 hours. A recently developed scale‐covariant multifractal model is then reformulated for numerical simulation of space‐time rainfall fields. The two parameters of the log‐Poisson distribution used as cascade generator within the model are systematically estimated from each selected rainfall sequence, and the dependence of one of these parameters on the large‐scale rain rate is highlighted. The model is then applied to disaggregate large‐scale rainfall, and some comparisons between synthetically downscaled and observed rainfall are discussed.
Simulation of radar reflectivities using a mesoscale weather forecast model
Authors: Haase Günther , Crewell Susanne
Published: February 2004

A three‐dimensional radar simulation model (RSM) using predicted fields of a nonhydrostatic mesoscale weather forecast model, the Lokal‐Modell (LM) with 2.8 km resolution, has been developed. The LM output data include the fields of the simulated hydrometeor types to study realistic cloud and precipitation fields, covering all stages of precipitation development. The RSM is able to simulate radar reflectivity measurements of any kind of radar situated within the model domain and hence allows a quick quality control of the predicted hydrometeor components. We show comparisons of simulated measurements with real measurements of a C‐band radar and an X‐band radar of a squall line at midlatitudes. Another application of the RSM is analytical studies using realistic physical structures of the atmosphere as given by the LM, e.g., the relation between three‐dimensional reflectivity fields and the surface rain rate. The RSM is a useful tool for (1) the validation, and hence improvement, of mesoscale models and (2) the estimation of some of the errors related to the determination of the surface rain rate.
Multisite disaggregation of monthly to daily streamflow
Authors: Kumar D. Nagesh , Lall Upmanu , Petersen Michael R.
Published: February 2004

Streamflow disaggregation is used to preserve statistical attributes of time series across multiple sites and timescales. Several algorithms for spatial disaggregation and for disaggregation of annual to monthly flows are available. However, the disaggregation of monthly to daily or weekly to daily flows remains a challenge. A new algorithm is presented for simultaneously disaggregating monthly flows at a number of sites and daily flows at an index site to daily flows at a number of sites on a drainage network. The continuity of flow in time across months at each site as well as the intersite flow pattern are preserved. The disaggregated daily flows at the multiple sites are conditioned on the spatial (across site) pattern of monthly flows at the respective sites. The probability distribution of the vector of disaggregated flows conditional on the multisite monthly flows is approximated nonparametrically using the k nearest neighbors of the monthly spatial flow pattern. A constrained optimization problem is solved to adaptively estimate the disaggregated flows in space and time for each such neighborhood. An application to data from a tributary of the Colorado River is used to illustrate the modeling process.
Exact closed‐form solution of the two‐dimensional Laplace Equation for Steady Groundwater flow with nonlinearized free‐surface boundary condition
Authors: Schmitz G. H. , Edenhofer J.
Published: February 2004

Steady groundwater flow with steep gradients of the phreatic surface can be described by the Laplace equation. Without employing either the Dupuit assumption nor any other linearization of the nonlinear free‐surface boundary condition, we develop for an aquifer of infinite horizontal extension the exact closed‐form analytical solution of the two‐dimensional Laplace equation with nonsymmetric arbitrary recharge and/or drainage. Conformal mapping, transformation procedures, and inversion of the resulting integral equation represent the essential steps in the development of the solution, providing the location of the phreatic surface throughout the considered domain. The solution is expressed exclusively in algebraic terms without the need for iterative procedures thus offering a new insight and transparency into the solution's structure. The closed‐form solution relates straightforwardly to real‐world problems and can also be used for the evaluation of recharge or drainage in a river‐aquifer system through a relatively simple solution of the inverse problem without numerical inconvenience. The closed‐form solution creates a new standard for investigating both the behavior of numerical solutions and the domain of validity for simplified approaches. The computer code SURF can be downloaded from http://www.tu‐dresden.de/fghhihm/hydrologie.html.
Self‐similarity and multifractality of fluvial erosion topography: 1. Mathematical conditions and physical origin
Authors: Veneziano Daniele , Niemann Jeffrey D.
Published: February 2004

It is suggested that the scaling laws satisfied by fluvial erosion topography and river networks reflect a basic self‐similarity or multifractality property of the topographic surface within river basins. By analyzing the symmetries of fluvial topography, we conclude that this self‐similarity or multifractality condition should be expressed in a particular way in terms of the topographic increments within subbasins. We then analyze whether self‐similar or multifractal topographies can be stationary or transient solutions of dynamic evolution models of the type ∂h/∂t = U − ƒ{β, т}, where U is the uplift rate, ƒ is the fluvial erosion rate, β is a vector of erodibility parameters, and т is hydraulic shear stress. The hydraulic stress on a channel bed is assumed to satisfy т ∝ AmSn, where A is contributing area, S is slope, and m and n are parameters. We allow U to vary randomly in time and β to vary randomly in space and determine conditions on these random functions as well as the parameters m and n under which the topography may remain in a self‐similar or multifractal state. Simulation shows that self‐similar states are attractive also for non‐self‐similar boundary and initial conditions.
Experiments on surface structure and partial sediment transport on a gravel bed
Authors: Hassan Marwan A. , Church Michael
Published: February 2004

Eight flume experiments were conducted to study the development of bed surface texture and structure in the presence of partial bed material transport. The experiments have two phases, a no‐feed degradational phase followed by a feeding phase. A surface structure of irregular, reticulate stone nets and clusters was developed before sediment feeding commenced. Bed load transport equaled or slightly exceeded the fed supply, except at the highest feed rate. The bed structure was maintained, but bed surface texture fined with increasing sediment load. The two phenomena may coexist because the largest grains on the bed moved only very sporadically. The actual sediment transport rates were much less than the expected rate calculated from the ratio of bed surface grain size to transported grain size. The difference reflects the increase in bed stability introduced by the bed structure. Between 17% and 47% of the bed shear stress is estimated to be carried by the structure, <4% being absorbed by the load, while the bed grains carried the balance of the stress. Bed material transport is exceedingly sensitive to bed surface structure and grain size, which raises concerns about the realizable precision of grain size measurements and characterization of the structure.
Gas‐water displacement through fracture networks
Authors: Berger Diego , Braester Carol
Published: February 2004

This study is focused on gas‐water displacement through fracture networks. First, we investigated the flow patterns in fracture branches following a fracture intersection. Laboratory experiments were conducted to qualitatively identify the gas‐water flow patterns in intersecting fractures. A mathematical model consistent with the actual phenomena observed in the laboratory was then formulated. The mathematical model is represented by a nonlinear system of partial differential equations, which is solved by an iterative numerical method. In the second part of the study, investigations of a vertical cross section of a fracture network were conducted. Two sets of fractures were assumed to intersect orthogonally, as characteristic of some granites. The angles of the fractures with the horizontal direction were either 45° or 135°. Fracture intersection centers, fracture lengths, and fracture openings were generated according to statistical distributions. Simulations were carried out under constant pressure boundary conditions at top and bottom as might be found at offshore radioactive repositories. The present study, which accounts for a gas‐water slug, shows that the flow of water reaches quasi‐steady state with continuous gas release.
Trends in evaporation and Bowen Ratio on urbanizing watersheds in eastern United States
Authors: Dow Charles L. , DeWalle David R.
Published: February 2004

Long‐term trends in annual evaporation and watershed Bowen ratio were studied on 51 eastern United States watersheds which had experienced an estimated 0–58% urban/residential development between 1920 and 1990. Annual watershed evaporation was estimated as the difference between annual streamflow and average watershed precipitation, corrected for watershed elevation effects and gage catch error. Annual watershed Bowen ratios were estimated using spatially and temporally averaged net radiation values and annual evaporation. Significant (α = 0.1) positive trends in annual watershed Bowen ratio and negative trends in evaporation were found for 12 and 10 watersheds, respectively, of 28 watersheds with >2% urban/residential development and only 2 and 3 watersheds, respectively, of 23 watersheds with <2% development. Trend results lead to estimates of the overall change in water and energy balances which were separately related to the percentage change in urban/residential area for all 51 watersheds. These regional relationships, linking watershed hydrology, energy exchange, and land use changes, showed significant decreases in watershed evaporation and significant increases in sensible heating of the atmosphere with increased urban/residential development on watersheds in the eastern United States. At 100% urbanization the regional relationships predicted a decrease in annual evaporation of 22 cm and an increase in the sensible heating of the atmosphere of 13 W/m2.
Modeling the dynamics of soil seal formation: Analysis of the effect of soil and rainfall properties
Authors: Assouline S. , Mualem Y.
Published: February 2004

The dynamic model of soil sealing developed by Assouline and Mualem [1997] is applied to study the effect of the main variables affecting seal formation, namely, the rainfall characteristics, the flow conditions, and the soil properties. The model is further extended to provide for the first time a quantitative estimation of the effect of the soil and water chemical properties on infiltration during soil surface sealing. The predicted effects of the chemical conditions on infiltration seem to be in agreement with experimental observations. Also, the model is used to explain the wide variability in seal thickness, found in the literature, in terms of the resolution of the method applied to measure the soil bulk density.
Examining bacterial transport in intact cores from Oyster, Virginia: Effect of sedimentary facies type on bacterial breakthrough and retention
Authors: Fuller Mark E. , Dong Hailiang , Mailloux Brian J. , Onstott T. C. , DeFlaun Mary F.
Published: February 2004

The effects of the physical7/chemical properties of sediment on the transport of bacteria were examined. An adhesion‐deficient, groundwater isolate was injected into intact cores of three different sedimentary facies. Total effluent recovery of bacteria varied with facies type and appeared to be most influenced by mean grain size and total metal hydroxide content. Bacterial concentrations within the core were highest near the point of injection, but cell distribution exhibited considerable lateral and longitudinal variation. Calculations of the collision efficiency (α) indicated that the bacterial inoculum injected into the cores was composed of a range of subpopulations of cells, each possessing different α values. Although predictions based on previously published research were generally correct, this research highlights the need for further studies of bacterial transport under environmentally relevant conditions and the necessity of examining how the physiological heterogeneity of the injected organisms affects their transport in subsurface sediments.
Pore‐scale measurements of solute breakthrough using microfocus X‐ray computed tomography
Authors: Clausnitzer V. , Hopmans J. W.
Published: February 2004

X‐ray computed tomography (CT) offers distinct advantages to study fundamental physical processes of water movement and contaminant transport in porous media. Tomography provides nondestructive and noninvasive cross‐sectional or three‐dimensional representations of porous media and has the potential to measure phase distribution and species concentration at the pore scale. Sources of error are discussed for the application of industrial microfocus CT to quantitative studies of flow and transport. Specifically, effective resolution and measurement uncertainties due to photon randomness are considered for a miscible displacement experiment. A calibration method for the measurement of solute concentration is proposed that accounts for the effect of beam hardening, which is characteristic for polychromatic industrial X‐ray sources. The results of an X‐ray microfocus CT experiment are presented, emphasizing the need to correct for beam hardening and describing the inherent spatial variability of solute breakthrough through a glass‐bead porous medium with an effective spatial resolution of approximately 85 μm.
Transient film flow on rough fracture surfaces
Authors: Tokunaga Tetsu K. , Wan Jiamin , Sutton Stephen R.
Published: February 2004

Transient film flow in unsaturated fractures was investigated conceptually and experimentally. By considering films on fracture surfaces as analogs to water in partially saturated porous media, the film hydraulic diffusivity and equation for transient film flow are obtained from their porous medium counterparts, the hydraulic diffusivity and the Richards equation. Experiments on roughened glass show that the average film thickness dependence on matric potential can be approximated as a power function. It is also shown that the film hydraulic diffusivity increases with increased film thickness (and with increased matric potential). Fast film flow (average velocities greater than 3×10−7 m s−1 under unit gradient conditions) was observed for average film thicknesses greater than 2μm and matric potentials greater than −1 kPa.
A full‐Bayesian approach to the groundwater inverse problem for steady state flow
Authors: Woodbury Allan D. , Ulrych Tadeusz J.
Published: February 2004

A full‐Bayesian approach to the estimation of transmissivity from hydraulic head and transmissivity measurements is developed for two‐dimensional steady state groundwater flow. The approach combines both Bayesian and maximum entropy viewpoints of probability. In the first phase, log transmissivity measurements are incorporated into Bayes' theorem, and the prior probability density function is updated, yielding posterior estimates of the mean value of the log transmissivity field and covariance. The two central moments are generated assuming that the prior mean, variance, and integral scales are “hyperparameters”; that is, they are treated as random variables in themselves which is contrary to classical statistical approaches. The probability density functions (pdfs) of these hyperparameters are, in turn, determined from maximum entropy considerations. In other words, pdfs are chosen for each of the hyperparameters that are maximally uncommitted with respect to unknown information. This methodology is quite general and provides an alternative to kriging for spatial interpolation. The final step consists of updating the conditioned natural logarithm transmissivity (ln(T)) field with hydraulic head measurements, utilizing a linearized aquifer equation. It is assumed that the statistical properties of the noise in the hydraulic head measurements are also uncertain. At each step, uncertainties in all pertinent hyperparameters are removed by marginalization. Finally, what is produced is a ln(T) field conditioned on measurements of both hydraulic heads and log transmissivity and covariances of the ln(T) field. In addition, we can also produce resolution matrices, confidence (credibility) limits, and the like for the ln(T) field. It is shown that the application of the methodology yields good estimates of transmissivities, even when hydraulic head measurements are noisy and little or no information is specified on mean values of ln(T), variance of ln(T), and integral scales.
An adsorption isotherm for bromide
Authors: Korom Scott F.
Published: February 2004

Bromide is often used as a conservative tracer in soils and sediments; however, it may be adsorbed onto variably charged sediments such as exist at the Savannah River Site (SRS) in South Carolina. Using a methodology simulating concentration gradients observed during field‐scale injection of anionic tracers, a series of column experiments were used to determine the nature of bromide adsorption onto and desorption from a sediment sample from SRS. Bromide concentrations ranged from <0.1 to 700 mg/L. Adsorption and desorption were reversible and repeatable; however, desorption showed tailing. Adsorption followed an apparent Freundlich isotherm (95% confidence interval) having an exponent of 0.906 lpar;±0.033). A second‐order polynomial fit provided a more accurate but less conventional model of the adsorption.
Modeling particle transport and aggregation in a quiescent aqueous environment using the residence‐time scheme
Authors: Perigault J. Gabriel , Leckie James O. , Kitanidis Peter K.
Published: February 2004

Suspended particles are a ubiquitous component of aqueous environments and are found over broad ranges of size and density. Particle transport and fate have an important role in the regulation of contaminants and nutrients in natural settings. The mechanisms that control the transport and size of particulate material in solution also play a fundamental role in the successful operation of engineered systems, such as sedimentation ponds and flocculation tanks, as well as flotation and filtering reactors. Adequate modeling of particle transport and aggregation is required for better understanding and prediction of the effects of particulate material in natural aqueous systems, as well as for designing efficient physicochemical processes to deal with suspended solids. In this paper we illustrate how numerical diffusion produced by the use of first‐order finite difference schemes can introduce significant errors in the modeling of particle settling in quiescent systems and how this error is compounded when aggregation is considered. To model settling without introducing numerical diffusion, while preserving numerical efficiency, we propose the residence‐time scheme, a simple numerical scheme based on the residence time of each size fraction in the elements of the spatial discretization. For the solution of the settling‐aggregation equation the alternating‐operator‐splitting technique (AOST) is used. The inherent modularity and simplicity of AOST allows smooth incorporation of additional particle transport mechanisms such as mixing, advection, etc.