bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for soil relative hydraulic conductivity based on the water retention characteristic curve
Authors: Assouline S.
Published: February 2004

A simple model is proposed which predicts the relative unsaturated hydraulic conductivity function of nonswelling soils by using the first two moments of their water retention curve (WRC). The model is based on the statistical approach but uses less restrictive assumptions concerning the pore configuration than Mualem's [1976] model. The result is that the relative hydraulic conductivity (RHC) is a power function of the relative contribution of the pores filled with water. It is shown that the power value is related to the coefficient of variation characterizing the retention curve, expressed in terms of the WRC model of Assouline et al. [1998]. Therefore a relationship is established between the RHC and the soil structure and texture, as reflected by the measured WRC. The model is calibrated on data from eight soils and tested on data from five soils, representing a wide range of soil textures, from sand to silt. The performances of the model are compared to those resulting from the application of Mualem's [1976] model to the analytical expressions of Brooks and Corey [1964] and Assouline et al. [1998] for the WRC. In most of the cases the proposed model improves the fit of the predicted RHC to the measured data, although its performance in the case of loam soils seems to be weak.
Prediction of plume migration in heterogeneous media using artificial neural networks
Authors: Hassan Ahmed E. , Hamed Khaled H.
Published: February 2004

Because of the many uncertainties associated with most flow and transport parameters, studies often implement the numerical simulations within a Monte Carlo frame of work. The large numbers of realizations needed to achieve convergence for the statistics of concern make the Monte Carlo approach computationally demanding. In this study we attempt to develop an empirical‐numerical approach to generate Lagrangian particle trajectories in two‐dimensional domain given a certain input heterogeneity model without repeatedly solving the flow equation for each realization, with the purpose of evaluating ensemble plume statistics. Artificial neural networks are used to map the relationship between the particle trajectories and the physical properties of the formation. This is achieved by training the neural network through a set of sufficient examples derived from a few realizations using an exponential log‐K covariance. The trained network is then used to predict particle trajectories in heterogeneity characterized by an exponential, a Gaussian, a hole‐type, or a fractal covariance model. The network that is trained using the exponential model successfully predicts transport results for other models with high accuracy and low computational effort. Accuracy of prediction of the trajectory (percent of explained variance) reaches 94% in the direction of mean flow and 91% normal to it. For the cases studied here, the speed of calculation is ∼7.8 times faster than the traditional approach for 1000 realizations. As the number of realizations increases, the speed factor approaches 12.8. Solute mass flux (mean and standard deviation) and mean‐plume concentrations predicted by the trained network agree very closely with the target result obtained via traditional Monte Carlo simulations.
Modeling of two‐phase flow in membranes and porous media in microgravity as applied to plant irrigation in space
Authors: Scovazzo Paul , Illangasekare Tissa H. , Hoehn Alex , Todd Paul
Published: February 2004

In traditional applications in soil physics it is convention to scale porous media properties, such as hydraulic conductivity, soil water diffusivity, and capillary head, with the gravitational acceleration. In addition, the Richards equation for water flux in partially saturated porous media also contains a gravity term. With the plans to develop plant habitats in space, such as in the International Space Station, it becomes necessary to evaluate these properties and this equation under conditions of microgravitational acceleration. This article develops models for microgravity steady state two‐phase flow, as found in irrigation systems, that addresses critical design issues. Conventional dimensionless groups in two‐phase mathematical models are scaled with gravity, which must be assigned a value of zero for microgravity modeling. The use of these conventional solutions in microgravity, therefore, is not possible. This article therefore introduces new dimensionless groups for two‐phase models. The microgravity models introduced here determined that in addition to porous media properties, important design factors for microgravity systems include applied water potential and the ratio of inner to outer radii for cylindrical and spherical porous media systems.
Measurement of aperture distribution, capillary pressure, relative permeability, and in situ saturation in a rock fracture using computed tomography scanning
Authors: Bertels Stephanie P. , DiCarlo David A. , Blunt Martin J.
Published: February 2004

We develop an experimental technique that uses computed tomography (CT) scanning to provide high‐resolution measurements of aperture distribution and in situ saturation along with capillary pressure and relative permeability for the same rough‐walled fracture. We apply this technique to an induced fracture in a cylindrical basalt core undergoing water drainage. We find that the sum of the water and gas relative permeabilities is much <1 at intermediate saturations and the water relative permeability shows a sharp change over a narrow range of average water saturation. In situ saturation maps show channeling of gas and significant retention of the wetting phase (water). The capillary pressure initially increased and then decreased with decreasing water saturation. Although this type of capillary behavior is atypical for unsaturated flows, we find that the sizes of the gas‐filled apertures are consistent with the measured capillary pressure.
Hierarchical modeling of flow in naturally fractured formations with multiple length scales
Authors: Lee S. H. , Lough M. F. , Jensen C. L.
Published: February 2004

This paper describes hierarchical approach to modeling flow in a naturally fractured formation. Our model is based on calculating the effective permeability of a fractured formation, as a function of grid block size, and using the results in a conventional finite difference flow simulator. On the basis of their length (lf) relative to the finite difference grid size (lg), fractures are classified as belonging to one of three groups: (1) short fractures (lf≪ lg), (2) medium‐length fractures (lf ∼ lg), and (3) long fractures (lf≫ lg). The effects of the fractures belonging to each class are computed in a hierarchical manner. The permeability contribution from short fractures is derived in an analytical expression and used as an enhanced matrix permeability for the next‐scale (medium‐length) calculation. The effective matrix permeability associated with medium‐length fractures is numerically solved using a boundary element method. The long fractures are modeled explicitly as major fluid conduits. As numerical examples, tracer transport in fractured formations was illustrated. The numerical results clearly indicated that effective tensor permeability well represented directional, enhanced permeability in fractured formations. The fluid‐conduit formulation captured the efficient fluid transport by long fractures.
Meander migration and the lateral tilting of floodplains
Authors: Sun Tao , Meakin Paul , Jøssang Torstein
Published: February 2004

The response of a meandering river to lateral tilting of the floodplain that supports the river is studied theoretically. Here, the lateral direction is perpendicular to the overall direction of the river on the floodplain. It is found that for most natural meandering rivers, the river will migrate down the tilt direction to a lower elevation. However, the theory also shows that under some conditions, especially when the Froude number of the flow in the channel is very small, the river migration is biased in the opposite direction. Two major contributions to the response of the river to lateral tilting, identified in the theory, are (1) perturbation of the cross‐section‐averaged flow velocities and (2) modification of the near‐bank flow velocity variation along the channel. The theory is one of the first attempts to develop a quantitative understanding of the effects of floodplain tilting on meandering rivers, and it provides a framework for future studies.
Regionalization of rainfall‐runoff model parameters using Markov Chain Monte Carlo samples
Authors: Campbell Edward P. , Bates Bryson C.
Published: February 2004

A general approach to the regionalization of rainfall‐runoff model parameters is developed that uses posterior calibration samples derived by Markov Chain Monte Carlo methods. For each watershed the posterior calibration samples are used to define the second‐order properties of the posterior distribution of the model parameters. Regionalization of the model parameters is accomplished for all parameters simultaneously via a regional link function that links the posterior means to watershed characteristics. A linear model is a particular case of our general approach, and we examine its performance in some detail. We indicate nonlinear and nonparametric extensions that may also be accommodated. A case study involving a quasi‐distributed, nonlinear flood event model and 39 watersheds in southwestern Australia is presented. We find that the regional model has substantial predictive ability.
Practical identifiability analysis of large environmental simulation models
Authors: Brun Roland , Reichert Peter , Künsch Hans R.
Published: February 2004

Large environmental simulation models are usually overparameterized with respect to given sets of observations. This results in poorly identifiable or nonidentifiable model parameters. For small models, plots of sensitivity functions have proven to be useful for the analysis of parameter identifiability. For models with many parameters, however, near‐linear dependence of sensitivity functions can no longer be assessed graphically. In this paper a systematic approach for tackling the parameter identifiability problem of large models based on local sensitivity analysis is presented. The calculation of two identifiability measures that are easy to handle and interpret is suggested. The first accounts for the sensitivity of model results to single parameters, and the second accounts for the degree of near‐linear dependence of sensitivity functions of parameter subsets. It is shown how these measures provide identifiability diagnosis for parameter subsets, how they are able to guide the selection of identifiable parameter subsets for parameter estimation, and how they facilitate the interpretation of the correlation matrix of the parameter estimate with respect to parameter identifiability. In addition, we show how potential bias of the parameter estimates, due to a priori fixing of some of the parameters, can be analyzed. Finally, two case studies are presented in order to illustrate the suggested approach.
Separation of year and site effects by generalized linear models in regionalization of annual floods
Authors: Clarke Robin T.
Published: February 2004

This paper explores the utility of generalized linear models (GLMs) and generalized linear mixed models (GLMMs) for regionalization and record augmentation of flood data. Because both models allow the separation of site and year effects, each gives an estimate of a site's mean annual flood free from effects of the particular years in which floods were recorded. In addition, a GLM gives a measure of the extent to which a short flood record can be regarded as representative of a longer period. The way in which GLMs and GLMMs are formulated effectively unites two problems that have largely been regarded as separate: namely, regional regression and data augmentation. GLMM model structure implicitly includes correlation between flood records at neighboring sites, and both models contain facilities for testing whether time trends exist in flood data, for testing whether such trends are regionally uniform when they are found to exist, and for testing which climatological and/or physiographic variables are useful for information transfer. By appropriate selection of probability distribution and link function, biases are avoided that arise where log transformed data are back transformed to the scale on which flood flows are recorded. The paper illustrates GLMs and GLMMs by fitting them to flood records of variable length from 12 sites in the Ibicui drainage basin in southern Brazil. While the models are extremely flexible and are easily fitted for gamma‐distributed data (and to data following certain other probability distributions), the assumptions on which they are based are arguably more complex than in more familiar methods, requiring careful checking procedures.
A study of solute transport mechanisms using rare earth element tracers and artificial rainstorms on snow
Authors: Feng Xiahong , Kirchner James W. , Renshaw Carl E. , Osterhuber Randall S. , Klaue Björn , Taylor Susan
Published: February 2004

Rare earth element (REE) tracers and three artificial rain‐on‐snow storms at the Central Sierra Snow Laboratory, California indicate that (1) tracers applied to the snow surface immediately prior to the storm quickly appear at the bottom of the pack, with the tracer traversing the pack faster when the snowpack is wetter; (2) unlike most previous studies in which low solute concentrations were observed at high flow in diurnal cycles, the concentrations of the REE tracers in the outflow are positively related with input water flux; and (3) at a constant input flux the concentrations of all the REE tracers decreased exponentially with time, and the rate of this decrease was greater at high flow than at low flow. These observations can be qualitatively simulated by partitioning liquid water in the snowpack into mobile and immobile phases. Transport of the mobile water phase is governed by the advection‐dispersion equations, while the immobile water only moves by exchanging with the mobile water. The rate of exchange between mobile and immobile waters follows first‐order kinetics. Unlike previous mobile‐immobile models for snow, the exchange rate coefficient is assumed to increase exponentially with the effective water saturation. The model successfully simulates the positive concentration dependency on input water flux. However, it remains unclear how the exchange rate coefficient varies with the nature of the medium and with hydrological conditions. These observations suggest that tracer concentrations in the outflow are largely dominated by solute transport via fast flow channels. This surprising result implies that a spatially averaged flow rate may not be adequate for modeling solute transport properties in unsaturated media.
Spatial characterization of a hydrogeochemically heterogeneous aquifer using partitioning tracers: Optimal estimation of aquifer parameters
Authors: Zhang Yan , Graham Wendy D.
Published: February 2004

In this paper a distributed‐parameter extended Kalman filter is developed to estimate spatially distributed residual saturation of nonaqueous phase liquids (NAPL) and Darcy flux and to predict site‐specific movement of a partitioning tracer plume in a three‐ dimensional heterogeneous aquifer. The NAPL and Darcy flux fields are assumed to be time‐invariant spatial random fields with known spatial correlation structures. The extended Kalman filter consists of two components: (1) a moment propagation algorithm that uses a set of coupled stochastic partial differential equations derived from the advection‐dispersion‐retardation equation to predict how the moments propagate through space between measurement times [Zhang and Graham, this issue] and (2] a moment update algorithm, which updates the moments at measurement times. Because the optimal Kalman filtering algorithm is computationally intensive for three‐dimensional problems, a simplified sequential conditioning algorithm is developed that first conditions the Darcy flux field using only nonpartitioning tracer measurements and then conditions the NAPL field using only partitioning tracer measurements. The extended Kalman filter is applied to a three‐dimensional domain patterned after the interwell partitioning tracer tests conducted at Hill Air Force Base byAnnable et al. [1998]. Two different cross‐correlation scenarios and two different sampling networks are investigated.
Chloride‐bromide‐δ11B systematics of a thick clay‐rich aquitard system
Authors: Vengosh A. , Hendry M. J.
Published: February 2004

Boron, Cl−, and Br− concentrations and boron isotopic compositions (δ11B) were used to provide insight into the sources and water‐sediment interactions of fossil water in a thick nonfractured aquitard system in Saskatchewan, Canada. The aquitard system consists of 80 m of late Pleistocene clay‐rich till disconformably overlying 77 m of late Cretaceous marine clay. The clay is underlain by a regional sand aquifer. Variations in Cl− and Br− confirmed the presence of well‐defined conservative mixing relationships between four types of fluids present in the aquitard system: (1) low‐Cl− water from the base of the fractured and oxidized till zone at 4 m depth; (2) water characterized by an elevated Cl− peak (190 mg/L) at about 12 m; (3) glacial meltwater present between 20 and 60 m depth; and (4) more saline water (Cl− up to 1000 mg/L) from the underlying Cretaceous clay and sand aquifer. Glacial meltwater has low Cl−, δ11B (∼20‰), and Cl−/Br− ratios (50). The low Cl−/Br− ratio of the glacial meltwater is similar to that of modern inland precipitation, which suggests that the Br− budget in the atmosphere 20–30 ka B.P. was similar to that of today and that Br− enrichment in the atmosphere may be a natural process. The relatively low Cl−/Br− ratio (∼150) and δ11B values of the saline water in the sand aquifer (35 and 39‰, respectively) and pore water in the Cretaceous clay (25.6 and 32.6‰, respectively) are not consistent with simple dilution of seawater but suggest dilution and modification of fifteenfold evaporated seawater with a low Cl−/Br− and a high δ11B value (>39‰). This evaporitic seawater may have been entrapped in the Cretaceous clay upon or after its deposition and subsequently flushed from the clay by fresher water. We suggest that the refreshening process modified the original brine into Na‐HCO3 water type and extracted boron with δ11B < 39‰ from the adsorbed sites. In contrast, increase of Cl− in the upper section of the till (the 12 m peak) resulted in apparent depletion of dissolved boron and relative 11B enrichment, which can be explained by adsorption processes. The B‐δ11B relationships can thus be used to trace water‐clay interactions in aquitard systems.
Measurement of fluid contents by light transmission in transient three‐phase oil‐water‐air systems in sand
Authors: Darnault C. J. G. , DiCarlo D. A. , Bauters T. W. J. , Jacobson A. R. , Throop J. A. , Montemagno C. D. , Parlange J.‐Y. , Steenhuis T. S.
Published: February 2004

Most three‐phase flow models lack rigorous validation because very few methods exist that can measure transient fluid contents of the order of seconds of whole flow fields. The objective of this study was to develop a method by which fluid content can be measured rapidly in three‐phase systems. The method uses the hue and intensity of light transmitted through a slab chamber to measure fluid contents. The water is colored blue with CuSO4. The light transmitted by high‐frequency light bulbs is recorded with a color video camera in red, green, and blue and then converted to hue, saturation, and intensity. Calibration of hue and intensity with water, oil, and air is made using cells filled with different combinations of the three fluids. The results show that hue and water content are uniquely related over a large range of fluid contents. Total liquid content is a function of both hue and light intensity. The air content is obtained by subtracting the liquid content from the porosity. The method was tested with static and transient experiments. Measurements made with the light transmission method (LTM) and synchrotron X rays of the static experiment agreed well. In the transient experiments, fingers were formed by dripping water on the surface in a two‐dimensional slab chamber with partially oil‐saturated sand. The LTM is able to capture the spatial resolution of the fluid contents and can provide new insights in rapidly changing, three‐phase flow systems.
On the continuum‐scale modeling of gravity‐driven fingers in unsaturated porous media: The inadequacy of the Richards Equation with standard monotonic constitutive relations and hysteretic equations of state
Authors: Eliassi Mehdi , Glass Robert J.
Published: February 2004

We consider the ability of the continuum‐scale Richards equation (RE) with standard monotonic constitutive relations and hysteretic equations of state to model gravity‐driven fingers in unsaturated porous media and, in particular, the underlying nonmonotonicity observed along the vertical extent of fingers. As published in a number of recent papers, finger‐like solutions with nonmonotonic profiles can be simulated numerically using a downwind averaging method. However, we find these fingers to be artifacts, generated by the combined effects of a truncation error induced oscillation or oversaturation at the wetting front, and capillary hysteresis. As the oscillation is removed, either through grid refinement or the use of inherently monotone schemes, the numerically generated fingers disappear, and the solution becomes monotonic. Thus the RE along with standard monotonic hydraulic properties does not contain the critical physics required to model gravity‐driven fingers and must be considered inadequate for unsaturated flow in initially dry, highly nonlinear, and hysteretic media where these fingers occur.
Faulting patterns near 19°30′S on the East Pacific Rise: Fault formation and growth at a superfast spreading center
Authors: Bohnenstiehl DelWayne R. , Carbotte Suzanne M.
Published: February 2004

Structural mapping from high‐resolution sonar and bathymetric data collected near 19°30′S on the southern East Pacific Rise (EPR) reveals multiple packets of small, closely spaced outward‐dipping faults that form antithetic to larger inward‐dipping faults. The mean spacing and height of inward‐dipping scarps are ∼1.6 and ∼1.9 times greater than those of outward‐dipping faults, and the median spacing and height of inward‐dipping scarps are ∼2.0 and ∼1.7 times greater, respectively. Accordingly, inward‐dipping faults are less abundant (35–45%) but accommodate more (50–55%) of the roughly ∼4.0% strain. Although previous studies have shown that EPR fault populations exhibit exponential size‐frequency distributions, one‐dimensional throw and spacing data from the 19°30′S region are not described well by this model. The first major (>40 m scarp height) fault is observed within 1–4 km of the ridge crest and invariably dips inward. Further off‐axis, a transition from greater inward‐dipping to greater outward‐dipping fault abundance constrains the minimum half width of the active plate boundary zone to be ∼5–15 km. Inspection of additional data sets from 16° and 18°30′S indicates a pattern of large inward‐dipping faults with associated hanging wall deformation similar to that observed in the 19°30′S region. This pattern is distinct from the horst and graben morphology observed along the fast spreading northern EPR. The pervasive hanging wall deformation observed on the flanks of the southern EPR suggests master fault geometries that are more sharply curved or kinked at depth, relative to their abyssal hill equivalents on the northern EPR. Boundary element modeling indicates the pattern and geometry of faulting observed along the southern EPR is consistent with fault nucleation above a shallow, sill‐like magma chamber with internal overpressure. Since the influence of the sill on fault growth is strongly dependent on the depth of the sill, the different patterns of faulting observed along the northern and southern EPR may reflect the small difference in axial magma chamber depth between these two regions, with a slightly shallower chamber beneath the southern EPR.
On the strength of interplate coupling and the rate of back arc convergence in the central Andes: An analysis of the interseismic velocity field
Authors: Bevis Michael , Kendrick Eric , Smalley Robert , Brooks Benjamin , Allmendinger Richard , Isacks Bryan
Published: February 2004

We interpret the interseismic crustal velocity field of the central Andes using a simple three‐plate model in which the Andean mountain belt is treated as a rigid microplate located between the Nazca and South American (SoAm) plates. We assume that the Euler vectors associated with these plates are strictly coaxial and that the surface velocity field can be decomposed into an elastic loading field (driven by locking of the main plate boundary) and a smaller contribution associated with back arc convergence. We obtain our best fit to the geodetic velocities if the main plate boundary is fully (100%) locked between depths of ∼10–50 km and ∼8.5% of Nazca‐SoAm plate convergence is achieved in the back arc (by underthrusting of the Brazilian Shield beneath the Subandean zone).
Comparison of the radiative impact of aerosols derived from vertically resolved (lidar) and vertically integrated (Sun photometer) measurements: Example of an Indian aerosol plume
Authors: Wagner Frank , Müller Detlef , Ansmann Albert
Published: February 2004

Radiative forcing calculations both at the top of the atmosphere (TOA) and at the surface were done for vertically varying particle properties and their approximation through vertically constant properties. The data sets followed from combined six wavelength aerosol lidar and Sun photometer measurements made on March 25, 1999, in the framework of the Indian Ocean Experiment (INDOEX). On this day a multilayered pollution plume, which was characterized by highly absorbing particles, extended to 3.5 km above ground. The aerosol optical thickness was as high as 0.57 at 530 nm. The uncertainty of the radiative forcing can be as large as 30% owing to the uncertainty of the refractive index and size distribution, which are only given as column‐averaged values if Sun photometer measurements are used alone. The radiative forcing calculated for an ocean albedo of 0.05 varies between −5 and −12 W m−2 at TOA and −55 and −81 W m−2 at the surface, whereas the forcing determined by height dependent aerosol properties is about −10 W m−2 at TOA and −60 W m−2 at the surface. The forcing efficiency is in the range −11 and −24 W m−2 per unit optical thickness at TOA and −101 and −154 W m−2 per unit optical thickness at the surface.
High‐precision dating of volcanic events (A.D. 1301–1995) using ice cores from Law Dome, Antarctica
Authors: Palmer Anne S. , van Ommen Tas D. , Curran Mark A. J. , Morgan Vin , Souney Joe M. , Mayewski Paul A.
Published: February 2004

A record of volcanic activity over the period A.D. 1301–1995 has been extracted from three Law Dome ice cores (East Antarctica). The record dating is unambiguous at the annual level from A.D. 1807 to 1995 and has an uncertainty of ±1 year at A.D. 1301. Signals from 20 eruptions are preserved in the record, including those of two unknown eruptions with acid deposition beginning in A.D. 1810.8 and A.D. 1685.8. The beginning of the ice core signal from the A.D. 1815 Tambora eruption is observed in the austral summer of A.D. 1816/1817. The mean observed stratospheric transport and deposition time to Law Dome from the eruption site is 1.5 years (σ = 0.6 years) from 11 well‐dated eruptions. The largest eruption observed in the Law Dome record has its maximum in A.D. 1460 with volcanic sulfate deposition beginning in the austral winter of A.D. 1459. This event is also observed in other ice core records and is attributed to the volcano Kuwae, with an eruption date in the range A.D. 1455.9–1459.9 if all sources of error are considered. This is at least three years later than the date previously ascribed by dendrochronological and historical studies.
Open‐path Fourier transform infrared spectroscopy of SO2: An empirical error budget analysis, with implications for volcano monitoring
Authors: Horrocks Lisa A. , Oppenheimer Clive , Burton Michael R. , Duffell Hayley J. , Davies Nicholas M. , Martin Nicholas A. , Bell William
Published: February 2004

Fourier transform infrared (FTIR) spectroscopy is increasingly used as a tool for volcano monitoring, allowing measurement of a range of gases including SO2, HCl, and HF. Retrievals are complicated, since the open‐path spectra are typically pressure broadened and contaminated hy atmospheric H2O, CO2, etc., and the field instruments employed have low spectral resolution (∼0.5 cm−1). We present a detailed analysis of 0.5 cm−1 resolution infrared spectra of certified SO2 mixtures in order to assess the sensitivity of such instruments and the retrieval procedures used for field spectra. We investigate the effects on retrievals of the SO2 V1+v3 band (centered at 2499.87 cm−1) of varying the retrieval spectral window and background fit, and of errors in the instrument line shape (ILS), temperature, and pressure. Largest deviations in retrieved amounts result from errors in the ILS (1.5–2.1%) and temperature (2.9–3.0% for a 10 K change). The total error estimate associated with these factors is similar to the uncertainty in line parameter data. Overall, the retrieval accuracy was better than 5%, except for the lowest concentration spectra. Errors calculated in the retrieval algorithm were conservative enough to cover these accuracy limits. We estimate that in field spectra, SO2 concentrations above 2.5 × 1017 molecules cm−2 (100 ppm m at room temperature and pressure) should be measurable with high accuracy. These results are encouraging in the context of deployment of open‐path FTIR spectrometers for surveillance of active volcanoes, and the findings are equally applicable to other open‐path measurements of SO2, for example from anthropogenic sources.
A closer look at United States and global surface temperature change
Authors: Hansen J. , Ruedy R. , Sato M. , Imhoff M. , Lawrence W. , Easterling D. , Peterson T. , Karl T.
Published: February 2004

We compare the United States and global surface air temperature changes of the past century using the current Goddard Institute for Space Studies (GISS) analysis and the U.S. Historical Climatology Network (USHCN) record [Karl et al., 1990]. Changes in the GISS analysis subsequent to the documentation by Hansen et al. [1999] are as follows: (1) incorporation of corrections for time‐of‐observation bias and station history adjustments in the United States based on Easterling et al. [1996a], (2) reclassification of rural, small‐town, and urban stations in the United States, southern Canada, and northern Mexico based on satellite measurements of night light intensity [Imhoff et al., 1997], and (3) a more flexible urban adjustment than that employed by Hansen et al. [1999], including reliance on only unlit stations in the United States and rural stations in the rest of the world for determining long‐term trends. We find evidence of local human effects (“urban warming”) even in suburban and small‐town surface air temperature records, but the effect is modest in magnitude and conceivably could be an artifact of inhomogeneities in the station records. We suggest further studies, including more complete satellite night light analyses, which may clarify the potential urban effect. There are inherent uncertainties in the long‐term temperature change at least of the order of 0.1°C for both the U.S. mean and the global mean. Nevertheless, it is clear that the post‐1930s cooling was much larger in the United States than in the global mean. The U.S. mean temperature has now reached a level comparable to that of the 1930s, while the global temperature is now far above the levels earlier in the century. The successive periods of global warming (1900–1940), cooling (1940–1965), and warming (1965–2000) in the 20th century show distinctive patterns of temperature change suggestive of roles for both climate forcings and dynamical variability. The U.S. was warm in 2000 but cooler than the warmest years in the 1930s and 1990s. Global temperature was moderately high in 2000 despite a lingering La Niña in the Pacific Ocean.
Changes in cloud cover associated with Forbush decreases of galactic cosmic rays
Authors: Todd Martin C. , Kniveton Dominic R.
Published: February 2004

The results of a study to quantify the relationship between cloud cover and short‐term Forbush decreases (FD) of galactic cosmic ray flux are presented. Using an extensive record of global satellite‐derived cloud products from the International Satellite Cloud Climatology Project (ISCCP) D1 data series, epoch superposition analysis of a sample of FD events is conducted. This analysis is conducted at a range of spatial scales from global, through 5° geomagnetic latitude bands to a global grid with 2.5° resolution. Resulting cloud anomalies are tested for significance using a randomized Monte Carlo experiment. The results indicate a small but significant (at 0.001 probability level) decline in the global proportion of cloud cover (of up to 1.4%) immediately prior to and following FD events. Analysis of data averaged over geomagnetic latitude (φ) bands reveals that significant cloud anomalies are concentrated in the high latitudes. A substantial (small) decline in cloud cover occurs at Southern (Northern) Hemisphere polar latitudes and is accompanied by a small but significant increase near φ = 30°N. The high‐latitude anomalies occur largely in the high‐level cloud and are particularly pronounced (up to −30%) in the uppermost cloud (occurring at 10–180 mbar) over Antarctica. In contrast, analysis using a sample of FD events associated with solar proton events shows no statistically significant cloud anomalies. A discussion of possible explanations of the results is provided.
The 1.27‐μm O2 continuum absorption in O2/CO2 mixtures
Authors: Fraser G. T. , Lafferty W. J.
Published: February 2004

The collision‐induced, near‐infrared O2 continuum band overlapping the weak a1Δg−X3Σg−, v = 0−0, 1.27 μm discrete band of O2 has been investigated in O2/CO2 mixtures at room temperature (T = 296 K) for total densities from 1.8 to 9.3 times that of an ideal gas under standard conditions (T = 273.15 K and P = 101.325 kPa), i.e., from 1.8 to 9.3 amagats. Absorption spectra were recorded at 0.5 cm−1 resolution using a Fourier transform spectrometer and an 84‐m pathlength. A least squares analysis of the integrated band strength, Stotal = SO2ρO2 + SO2 − O2 ρO22 + SO2 − CO2ρO2ρCO2, as a function of the carbon dioxide density, ρCO2, and the oxygen density, ρ02, yields SO2‐Co2 = 2.953(32) × 10−43 cm−2 (molecule/cm3)−2 (i.e., 2.132(23) × 10−4 cm−2 amagat−2). The SO2‐CO2 coefficient is ∼3 times greater than the corresponding SO2‐N2 coefficient determined from studies of O2/N2 mixtures, illustrating the efficiency of large electric multipolar moments in inducing continuum absorption in the 1.27‐μm band of O2. The results support the calculations by Brown and Tipping [2000], which demonstrate the importance of water, with its large electric dipole moment, in enhancing the collision‐induced absorption bands of O2 and N2 in the atmosphere. We suggest that the apparent inability of radiative transfer models to accurately account for the increased atmospheric absorption present when water vapor levels increase may be due in part to the neglect of the intensity enhancement of a number of continuum bands and of the far wings of discrete bands by water molecule collisions.
Introduction to special section: BOREAS III
Authors: Hall Forrest G.
Published: February 2004

The goal of the Boreal Ecosystem‐Atmosphere Study (BOREAS) was to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. This introduction briefly describes the papers within this BOREAS special section, the third in a series in the Journal of Geophysical Research‐Atmospheres, and how they relate to the overall BOREAS Guest Investigator program, the final phase of the BOREAS experiment. The major contributions of BOREAS are also summarized, and future directions are identified.
Extinction of UV radiation in Arctic snow at Alert, Canada (82°N)
Authors: King Martin D. , Simpson William R.
Published: February 2004

Extinction of ultraviolet (UV) and visible radiation (300–548 nm) in snowpack is reported for surface snow used in snow‐atmosphere exchange experiments during the Polar Sunrise Experiment ALERT 2000 (Canadian Forces Station Alert, Canada). The UV penetration distance is reported as the e‐folding depth (the depth over which the monochromatic light irradiance decreases by a factor of e) for 2 nm wavelength intervals. Values in the range 5–6 cm are obtained for a uniform snowpack in the wavelength region between the ozone absorption edge in the UV and green light in the visible. Experimental evidence shows at most a weak dependence of the asymptotic e‐folding depth on the solar zenith angle. Comparison of these data to previous studies indicates that the e‐folding depths vary greatly between sites, most probably due to variations in impurities at each of the sites. These data imply that approximately 85% of a photochemical reaction occurs in the top 10 cm of snowpack at this site.
Four‐dimensional variational chemical assimilation of CRISTA stratospheric measurements
Authors: Errera Quentin , Fonteyn Dominique
Published: February 2004

This paper presents a chemical assimilation system based on the variational method. It has been applied to an off‐line stratospheric three‐dimensional chemical transport model. This technique determines the best model initial conditions that minimize the model errors when compared to a set of observations for a predefined time window. The system is applied to the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) measurements for the November 5–11, 1994, mission. Observed species are O3, CH4, HNO3, ClONO2, N2O5, N2O, and CFC‐11. For an assimilation period of 12 hours, CRISTA observations are well estimated by the assimilation system. These results vary according to the species considered and depend on the CRISTA's observational error. In particular, assimilated ozone differ from CRISTA by less than 5%. Analyses have been compared with independent observations in order to validate the assimilation system. It was found that observations by the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment and the Halogen Occultation Experiment (HALOE) agree well with the analyses. These comparisons suggested systematic differences between CRISTA and HALOE or ATMOS. Unconstrained constituents like NOx and HCl are influenced through chemical coupling and show small discrepancies with ATMOS and HALOE data.
Formaldehyde over the central Pacific during PEM‐Tropics B
Authors: Heikes Brian , Snow Julie , Egli Peter , O'Sullivan Daniel , Crawford James , Olson Jennifer , Chen Gao , Davis Douglas , Blake Nicola , Blake Donald
Published: February 2004

Formaldehyde, CH2O, mixing ratios are reported for the central Pacific troposphere from a series of 41 flights, which took place in March‐April 1999 as part of the NASA Pacific Exploratory Mission (PEM) ‐Tropics B mission. Ambient CH2O was collected in aqueous media and quantified using an enzyme‐derivatization fluorescence technique. Primary calibration was performed using aqueous standards and known flow rates. Occasionally, CH2O gas standard additions to ambient air were performed as a secondary calibration. Analytical blanks were determined by replacing ambient air with pure air. The estimated precision was ±30 pptv and the estimated accuracy was the sum of ±30 parts per trillion by volume (pptv) ±15% of the measured value. Approximately 25% of the observations were less than the instrumental detection limit of 50 pptv, and 85% of these occurred above 6 km. CH2O mixing ratios decreased with altitude; for example, near the equator the median value in the lowest 2 km was 275 pptv, decreased to 150 pptv by 6 km and was below 100 pptv above 8 km. Between 130 and 170 W and below 1 km, a small variation of CH2O mixing ratio with latitude was noted as near‐surface median mixing ratios decreased near the equator (275 pptv) and were greater on either side (375 pptv). A marked decrease in near‐surface CH2O (200 pptv) was noted south of 23°S on two flights. Between 3° and 23°S, median CH2O mixing ratios were lower in the eastern tropical Pacific than in the western or central Pacific; nominal differences were >100 pptv near the surface to ∼100 pptv at midaltitude to ∼50 pptv at high altitude. Off the coast of Central America and Mexico, mixing ratios as high as 1200 pptv were observed in plumes that originated to the east over land. CH2O observations were consistently higher than the results from a point model constrained by other photochemical species and meteorological parameters. Regardless of latitude or longitude, agreement was best at altitudes above 4 km where the difference between measured and modeled CH2O medians was less than 50 pptv. Below 2 km the model median was approximately 150 pptv less than the measured median.
Recalibration of the NOAA microwave sounding unit
Authors: Mo Tsan , Goldberg Mitchell D. , Crosby David S. , Cheng Zhaohui
Published: February 2004

The microwave sounding unit (MSU) prelaunch thermal‐vacuum chamber test data for eight MSU flight models that flew on TIROS‐N, NOAA 7 through NOAA 12, and NOAA 14 were reanalyzed using an improved calibration algorithm, originally designed for the advanced microwave sounding unit‐A (AMSU‐A) operations. The new calibration algorithm can automatically adjust for any channel gain shift in operation. Adoption of this calibration algorithm as the MSU calibration procedure in the recalibration project will make the data sets from the MSU and the AMSU‐A more consistent. This will be useful for future blending of the climate trends generated from the MSU and AMSU‐A data. A single nonlinearity parameter u, which appears in the new calibration algorithm, was obtained for each channel from analysis of the prelaunch calibration test data. A software package for implementing this new calibration algorithm was developed and applied to calculate the MSU time series for improvement of the accuracy of the climate record. Sample calculations of MSU antenna temperatures with the new calibration algorithm were made for two satellites and are compared with similar results obtained from the old MSU calibration algorithm. Significant differences are observed, and strong evidence indicates that the new calibration procedure will provide a more accurate quantification of climate trends.
The stratospheric quasi‐biennial oscillation in the NCEP reanalyses: Climatological structures
Authors: Huesmann Amihan S. , Hitchman Matthew H.
Published: February 2004

Global quasi‐biennial variation in the lower stratosphere and tropopause region is studied using 41 years (1958–1998) of reanalyses from the National Centers for Environmental Prediction (NCEP). Horizontal wind, temperature, geopotential height, tropopause temperature and pressure fields are used. A new quasi‐biennial oscillation (QBO) indexing method is presented, which is based on the zonal mean zonal wind shear anomaly at the equator and is compared to the Singapore index. A phase difference compositing technique provides “snapshots” of the QBO meridional‐vertical structure as it descends, and “composite phases” provide a look at its time progression. Via binning large amounts of data, the first observation‐based estimate of the QBO meridional circulation is obtained. High‐latitude QBO variability supports previous studies that invoke planetary wave‐mean flow interaction as an explanation. The meridional distribution of the range in QBO zonal wind is compared with the stratospheric annual cycle, with the annual cycle dominating poleward of ∼12° latitude but still being significant in the deep tropics. The issues of temporal shear zone asymmetries and phase locking with the annual cycle are critically examined. Subtracting the time mean and annual cycle removes ∼2/3 of the asymmetry in wind (and wind shear) zone descent rate. The NCEP data validate previous findings that both the easterly and westerly QBO anomalous wind regimes in the lower stratosphere change sign preferentially during northern summer. It is noteworthy that the NCEP QBO amplitude and the relationships among the reanalyzed zonal wind, temperature, and meridional circulation undergo a substantial change around 1978.
Greenland ice sheet response to external forcing
Authors: van der Veen C. J.
Published: February 2004

Kinematic wave modeling is used to evaluate possible responses of the Greenland ice sheet to changes in its surface mass balance. In the approach followed here the reference state is defined based on measured velocity and discharge flux along the central flow line of Petermann Glacier in the northwest, and perturbations on this state are considered. The results indicate that significant rates of thickness change can occur immediately after the prescribed change in surface mass balance but adjustments in flow rapidly diminish these rates to a few centimeters per year at most. Full adjustment of the ice sheet requires times of the order of 1000 years. The instability mechanism known as the Jakobshavn Effect is discussed and, based on observational evidence as well as results from prior modeling studies, it is concluded that this is an unlikely mechanism for destabilizing major drainage basins of the Greenland ice sheet.
An assessment of irrigation needs and crop yield for the United States under potential climate changes
Authors: Brumbelow Kelly , Georgakakos Aris
Published: February 2004

Past assessments of climate change on U.S. agriculture have mostly focused on changes in crop yield. Few studies have included the entire conterminous United States, and few studies have assessed changing irrigation requirements. None have included the effects of changing soil moisture characteristics as determined by changing climatic forcing. This study assesses changes in irrigation requirements and crop yields for five crops in the areas of the United States where they have traditionally been grown. Physiologically based crop models are used to incorporate inputs of climate, soils, agricultural management, and drought stress tolerance. Soil moisture values from a macroscale hydrologic model run under a future climate scenario are used to initialize soil moisture content at the beginning of each growing season. Historical crop yield data are used to calibrate model parameters and to determine locally acceptable drought stress as a management parameter. Changes in irrigation demand and crop yield are assessed for both means and extremes by comparing results for atmospheric forcing close to the present climate with those for a future climate scenario. Assessments using the Canadian Center for Climate Modeling and Analysis General Circulation Model (CGCM1) indicate greater irrigation demands in the southern United States and decreased irrigation demands in the northern and western United States. Crop yields typically increase, except for winter wheat in the southern United States and corn. Variability in both irrigation demands and crop yields increases in most cases. Assessment results for the CGCM1 climate scenario are compared to those for the Hadley Centre for Climate Prediction and Research GCM (HadCM2) scenario for southwestern Georgia. The comparison shows significant differences in irrigation and yield trends, both in magnitude and in direction. The differences reflect the high forecast uncertainty of current GCMs. Nonetheless, both GCMs indicate higher variability in future climatic forcing and, consequently, in the response of agricultural systems.