bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of the Madden‐Julian Oscillation on upper tropospheric humidity
Authors: Sassi Fabrizio , Salby Murry , Pumphrey Hugh C. , Read William G.
Published: December 2002

Variations of upper tropospheric humidity (UTH) and cold cloud in the tropics reveal coherent changes that propagate eastward from the Indian Ocean into the Pacific. The coherence of UTH is high at periods of 30–90 days along the equator. Near the tropical tropopause, UTH coherence is large over the equator in the Indian Ocean and over the subtropics in the central Pacific. A composite life cycle shows that enhanced UTH in the subtropics coincides with two anticyclonic gyres. They straddle anomalous cold cloud over the equator as it propagates eastward. The enhancement of UTH is consistent with anomalously‐cold temperature, which mirrors the anticyclonic gyres. Those temperature anomalies cancel the height anomaly of the gyres overhead. The complex of anomalous UTH, cold cloud, and anticyclonic motion marches eastward across the western Pacific, until reaching the dateline. Anomalous convection over the equator then collapses, along with the subtropical gyres and enhanced UTH accompanying it.
Rapid changes in ozone mixing ratios at Cerro Tololo (30°10′S, 70°48′W, 2200 m) in connection with cutoff lows and deep troughs
Authors: Rondanelli R. , Gallardo L. , Garreaud R. D.
Published: December 2002

Rapid changes in surface ozone mixing ratios (up to 15 ppbv over 12 hours) at the Cerro Tololo (CT) station (70°W, 30°S, 2200 m) over the period 1996–2000 are analyzed. These changes explain most of the wintertime variability of the ozone data at CT. Since the wintertime data show no significant diurnal cycle, local circulations and in situ photochemical processes cannot account for the sudden changes in ozone. Rather, a synoptic‐scale forcing of the changes is described on the basis of composite analyses of reanalysis data. Synoptic configurations associated with sudden changes in ozone were separated into W and D cases, according to the local evolution of humidity. W cases are characterized by humidity values in excess of 4 hPa lasting one or two days before the ozone maximum. D cases show no significant change in humidity before the ozone maximum. Both in W and in D cases, stratospheric air appears to be advected from polar latitudes to midlatitudes and subtropical latitudes during the development of cutoff lows and deep troughs. The origin of the air arriving at CT during a W and a D case study was assessed using mesoscale numerical simulations. The model calculations for the W case show a deep stratospheric intrusion reaching the lower troposphere in subtropical latitudes during the amplification and mature stages of a cutoff low. For D cases the stratospheric intrusion is mostly confined to midlatitudes and to an oceanic area far from the site of study. Analyses of trajectories show that in both cases, high ozone mixing ratios at the surface occur in connection with air parcels having a stratospheric history.
Global and zonal total ozone variations estimated from ground‐based and satellite measurements: 1964–2000
Authors: Fioletov V. E. , Bodeker G. E. , Miller A. J. , McPeters R. D. , Stolarski R.
Published: December 2002

Six data sets of monthly average zonal total ozone were intercompared and then used to estimate latitudinal and global total ozone temporal variations and trends. The data sets were prepared by different groups and are based on TOMS, SBUV‐SBUV/2, GOME, and ground‐based measurements. Different approaches have been used to homogenize the records over the period 1979–2000. Systematic differences of up to 3% were found between different data sets for zonal and global total ozone area weighted average values. However, when these systematic differences were removed by deseasonalizing the data, the residuals agreed to within ±0.5% of the long‐term mean ozone values. All data sets show changes in the rate of the total ozone decline in recent years. While global ozone was fairly constant during the 1990s, the average values of the 1990s are about 2–3% lower than those of the late 1970s. About 38% of the global ozone is located between 25°S and 25°N where the data show no decline. The strongest decline and the largest variability occur over the 35°N–60°N zone during the winter‐spring season with the largest negative deviations occurring in 1993 and 1995. The decline in autumn is much smaller at these latitudes. Over the 35°S–60°S zone the ozone decline shows less seasonal dependence, and the largest deviations there were observed in 1985 and 1997. Sliding 11‐year trends were calculated to estimate ozone changes over different time intervals. The first interval was from 1964 to 1974, and the last interval was from 1990 to 2000. The steepest year‐round trends, of up to −5% per decade, occurred in the 11‐year periods ending between 1992 and 1997 over the 35°–60°N zone and between 1985 and 1993 over the 35°–55°S zone. More recent 11‐year trends have smaller declines.
An updated parameterization for infrared emission and absorption by water vapor in the National Center for Atmospheric Research Community Atmosphere Model
Authors: Collins William D. , Hackney Jeremy K. , Edwards David P.
Published: December 2002

An updated parameterization for the absorption and emission of infrared radiation by water vapor has been developed for the Community Atmosphere Model (CAM) from the National Center for Atmospheric Research (NCAR). The CAM is the latest version of the NCAR Community Climate Model (CCM). This updated treatment preserves the formulation of the radiative transfer equations using the absorptivity/emissivity method. However, the components of the absorptivity and emissivity related to water vapor have been replaced with new terms calculated with the General Line‐by‐line Atmospheric Transmittance and Radiance Model (GENLN2). The mean absolute errors in the surface and top‐of‐atmosphere clear‐sky longwave fluxes for standard atmospheres are reduced to less than 1 W/m2. Mean absolute differences between the cooling rates from the original method and GENLN2 are typically 0.2 K/d. These differences are reduced by at least a factor of 3 using the updated parameterization. The updated parameterization increases the longwave cooling at 300 mbar by 0.4 to 0.7 K/d, and it decreases the cooling near 800 mbar by 0.2 to 0.6 K/d. The increased cooling is caused by line absorption and the foreign continuum in the rotation band, and the decreased cooling is caused by the self‐continuum in the rotation band. These changes in the vertical profile of longwave cooling interact strongly with the parameterization of convection. The effects on the fluxes, diabatic cooling rates, and climate state are illustrated using simulations with the new climate model.
Long‐range transport of Asian outflow to the equatorial Pacific
Authors: Martin Brian D. , Fuelberg Henry E. , Blake Nicola J. , Crawford James H. , Logan Jennifer A. , Blake Donald R. , Sachse Glen W.
Published: December 2002

The second Pacific Exploratory Mission to the Tropics (PEM T‐B) was conducted as part of NASA's Global Tropospheric Experiment (GTE) from 25 February through 19 April 1999. Long‐range pollution signatures are examined during four PEM T‐B flights that ranged as far northeast as California and as far southwest as coastal New Guinea. The signatures are studied to determine their ages and chemical evolution during transport. The chemical species examined include nonmethane hydrocarbons, halocarbons, and carbon monoxide. Wind data from the European Centre for Medium Range Weather Forecasts (ECMWF) are used to calculate backward trajectories along the four flight tracks. Results show that some pollutants originating from the Asian continent, and even farther west, are transported across the Pacific by the middle latitude westerly winds and reach the subtropical Pacific anticyclone where they subside and turn southwestward under the influence of the low level trade winds. The parcels ultimately reach the western Pacific near coastal New Guinea after 20–25 days of transit.
Influence of organic compounds on the cloud droplet activation: A model investigation considering the volatility, water solubility, and surface activity of organic matter
Authors: Anttila Tatu , Kerminen Veli‐Matti
Published: December 2002

The influence of organic compounds on the activation of cloud droplets was investigated using a numerical model. The droplet activation process was simulated with an adiabatic air parcel model, coupled with a detailed treatment of aerosol dynamics. To cover the whole range of organic matter present in atmospheric aerosols, four different groups of organic compounds were considered: water‐soluble and water‐insoluble organics that are nonvolatile and slightly soluble and water‐soluble organics that are semivolatile and thereby able to move reversibly between the gas and the particle phase. The modeling results suggest that water‐soluble organics are able to influence the droplet concentrations by increasing the solute mass of the droplets and also by reducing their surface tension. Compared to these substances, slightly soluble compounds clearly have a minor role in the cloud droplet activation. In fact, according to the conducted simulations the clear majority of semivolatile, slightly soluble compounds have practically no influence on the activation process. Concerning nonvolatile organics, it was found that the cloud droplet concentrations are most sensitive to these species when they constitute a dominant fraction of the Aitken mode. Since atmospheric organic compounds are not well characterized at present, the model contained only a relatively simplified treatment of these species.
Influence of wind speed averaging on estimates of dimethylsulfide emission fluxes
Authors: Chapman E. G. , Shaw W. J. , Easter R. C. , Bian X. , Ghan S. J.
Published: December 2002

The effect of various wind speed averaging periods on calculated dimethylsulfide (DMS) emission fluxes is quantitatively assessed. A global climate model and an emission flux module were run in stand‐alone mode for a full year. Twenty‐minute instantaneous surface wind speeds and related variables generated by the climate model were archived, and corresponding 1‐hour, 6‐hour, daily, and monthly averaged quantities were calculated. These various time‐averaged, model‐derived quantities were used as inputs in the emission flux module, and DMS emissions were calculated using two expressions for the mass transfer velocity commonly used in atmospheric models [Liss and Merlivat, 1986; Nightingale et al., 2000]. Results indicate that the time period selected for averaging wind speeds can affect the magnitude of calculated DMS emission fluxes. A number of individual marine cells within the global grid show DMS emissions fluxes that are 10–60% higher when emissions are calculated using 20‐min instantaneous model time step winds rather than monthly averaged wind speeds, and at some locations the differences exceed 200%. Many of these cells are located in the Southern Hemisphere where anthropogenic sulfur emissions are low and changes in oceanic DMS emissions may significantly affect calculated aerosol concentrations and aerosol radiative forcing.
Comparisons of corrected daily integrated erythemal UVR data from the U.S. EPA/UGA network of Brewer spectroradiometers with model and TOMS‐inferred data
Authors: Sabburg J. , Rives J. E. , Meltzer R. S. , Taylor T. , Schmalzle G. , Zheng S. , Huang N. , Wilson A. , Udelhofen P. M.
Published: December 2002

A network of 21 Brewer spectroradiometers, owned by the U.S. Environmental Protection Agency (U.S. EPA) and operated by the University of Georgia (UGA), is measuring ultraviolet (UV) spectral irradiances throughout the United States. Corrections to the raw data for 4 of the 21 Brewers have now been implemented. These corrections include (1) the stray light rejection, (2) the cosine errors associated with the full sky diffuser, (3) the temperature dependence of the response of the instruments, and (4) the temporal variation in the instrument response due to changes in the optical characteristics of the instruments. While for many sites the total corrections amount to less than 10%, for certain sites they are much larger, in some cases amounting to more than 25%. It is estimated that application of these corrections brings the uncertainty of the absolute irradiance of individual spectral scans to approximately 6% for all known major sources of error for all solar zenith angles. A comparison is presented of corrected daily integrated erythemal UV doses on clear days to both model and Total Ozone Mapping Spectrometer (TOMS) UV values. The TOMS retrievals show a positive bias with respect to the measured values that falls in the range of 12.5–1.4% with an average value of 5% for the four sites studied.
The annual cycle of water vapor on Mars as observed by the Thermal Emission Spectrometer
Authors: Smith Michael D.
Published: December 2002

Spectra taken by the Mars Global Surveyor Thermal Emission Spectrometer (TES) have been used to monitor the latitude, longitude, and seasonal dependence of water vapor for over one full Martian year (March 1999 to March 2001). A maximum in water vapor abundance is observed at high latitudes during midsummer in both hemispheres, reaching a maximum value of ∼100 pr‐μm in the north and ∼50 pr‐μm in the south. Low water vapor abundance (<5 pr‐μm) is observed at middle and high latitudes in the fall and winter of both hemispheres. There are large differences in the hemispheric (north versus south) and seasonal (perihelion versus aphelion) behavior of water vapor. The latitudinal and seasonal dependence of the decay of the northern summer water vapor maximum implies cross‐equatorial transport of water to the southern hemisphere, while there is little or no corresponding transport during the decay of the southern hemisphere summer maximum. The latitude‐longitude dependence of annually averaged water vapor (corrected for topography) has a significant positive correlation with albedo and significant negative correlations with thermal inertia and surface pressure. Comparison of TES results with those retrieved from the Viking Orbiter Mars Atmospheric Water Detectors (MAWD) experiments [Jakosky and Farmer, 1982] shows some similar features but also many significant differences. The southern hemisphere maximum observed by TES was not observed by MAWD, and the large latitudinal gradient in annually averaged water vapor observed by MAWD does not appear in the TES results.
Plume channeling as one possible mechanism for the origin of multiple coronae on Venus: A case study from the Helen Planitia Quadrangle (V52)
Authors: López Iván
Published: December 2002

Coronae are large volcanotectonic features formed in response to plume/diapir activity that have played an important role in the geodynamical evolution of Venus. Coronae are classified in terms of their morphology; one of the types is the so‐called multiple coronae. Models that account for multiple coronae formation include the following: emplacement of several diapirs, the movement of the lithosphere over a stationary plume, and migration of a plume under a stationary lithosphere. This work presents the study of a multiple corona located in the Helen Planitia Quadrangle (V52) (Oanuava Corona) and another corona (unnamed) formed in the vicinity of Oanuava in order to test the viability of these models. Detailed structural mapping shows a migration of magmatic activity along the structure during corona formation and the development of a set of fractures in response to a local stress field. The latter is used as a time marker for the evolution of Oanuava and the unnamed coronae. We propose that this migration could be explained as the result of a sublithospheric lateral flow and ponding of a mantle plume or diapir into a zone of uplifted and thinned lithosphere (plume channeling) induced by the interaction of both coronae.
Thermal evolution of a growing Mars
Authors: Senshu Hiroki , Kuramoto Kiyoshi , Matsui Takafumi
Published: December 2002

We developed a new numerical model which can simulate the thermal history and metal‐silicate separation of a growing Mars. In this model the thermal disturbance caused by planetesimal impacts is calculated for each impact event by taking into account the effects of shock heating, crater excavation, and isostatic rebound. A metallic blob is assumed to form at the base of a magma pond if an impact site is heated above the melting temperature. Sinking of the metal blobs is traced assuming Stokes' velocity. Their coalescence during sinking is treated by a Monte Carlo approach. A series of simulations is carried out assuming that Mars is formed by the runaway growth from a swarm of planetesimals as was suggested by recent numerical simulations of the planetary accretion process. Our numerical results show that (1) no global magma ocean is formed during accretion, (2) metal‐silicate separation takes place without global scale melting, and (3) instead of a metallic core, a metal‐rich layer is formed at the late stage of accretion.
Gabor‐based narrow‐waisted Gaussian beam algorithm for transmission of aperture‐excited 3D vector fields through arbitrarily shaped 3D dielectric layers
Authors: Maciel John J. , Felsen Leopold B.
Published: December 2002

We have previously developed computationally efficient Gabor‐based, narrow‐waisted (NW), discretized Gaussian beam (GB) algorithms for the determination of the 3D vector fields radiated by 2D arbitrary vector planar aperture distributions. In the present paper, these algorithms are extended to tracking the aperture‐generated GBs through polarization‐sensitive reflection/transmission interactions with arbitrarily shaped 3D dielectric layers. The resulting tracking scheme is applied to predicting the performance of rotationally symmetric hemispherical and ogival missile radomes, which cover 2D plane truncated apertures on gimbaled (rotatable) platforms. Comparisons with measured data show satisfactory agreement with the analytic predictions.
Stochastic theory of waves and localization in two‐dimensional random medium
Authors: Ogura Hisanao
Published: December 2002

Random wave field satisfying two‐dimensional (2D) wave equation with a random medium is studied using the stochastic representation in the polar coordinate system. The random medium is assumed to be a homogeneous and isotropic Gaussian random field, and its polar spectral representation is effectively employed in the method of solution. The isotropy of a random field implies the invariance under stochastic rotational transformation Dϕ, the operator operating on both angular coordinate and probability parameter, and a random wave field with an eigenvalue eiMϕ under Dϕ, M being an integer, represents an angular mode of random cylindrical wave, and M = 0 corresponding to an isotropic mode; the situation being analogous to the case of cylindrical mode in 2D free space. For a concrete approximate solution, the random medium is assumed to be an omnidirectional random grating with an isotropic narrowband spectrum centered at 2k twice as much as the wave number k, which creates the Bragg scattering in the propagation of random cylindrical waves. The asymptotic form of the random cylindrical wave is obtained by means of the stochastic spectral representation of the random wave, as well as the random medium, and is shown to have an exponential factor , where the average exponent coefficient γ is given in terms of the spectral density of random medium. This exponential behavior due to the index α in the amplitude describes the localized nature of an angular mode of random wave field generated by a source or by resonance. In the case of a narrowband grating with the variance σ2 and the spectral bandwidth kΔ, γ is concretely calculated to be proportional to , which should be compared with the one‐dimensional (1D) case where the corresponding exponential coefficient is proportional to the spectral height σ2/Δ.
Scope for further analytical solutions for constant flux infiltration into a semi‐infinite soil profile or redistribution in a finite soil profile
Authors: Barry D. A. , Parlange J.‐Y. , Lisle I. G. , Li L. , Jeng D.‐S. , Stagnitti F. , Sander G. C.
Published: December 2002

We attempt to generate new solutions for the moisture content form of the one‐dimensional Richards' [1931] equation using the Lisle [1992] equivalence mapping. This mapping is used as no more general set of transformations exists for mapping the one‐dimensional Richards' equation into itself. Starting from a given solution, the mapping has the potential to generate an infinite number of new solutions for a series of nonlinear diffusivity and hydraulic conductivity functions. We first seek new analytical solutions satisfying Richards' equation subject to a constant flux surface boundary condition for a semi‐infinite dry soil, starting with the Burgers model. The first iteration produces an existing solution, while subsequent iterations are shown to endlessly reproduce this same solution. Next, we briefly consider the problem of redistribution in a finite‐length soil. In this case, Lisle's equivalence mapping is generalized to account for arbitrary initial conditions. As was the case for infiltration, however, it is found that new analytical solutions are not generated using the equivalence mapping, although existing solutions are recovered.
Equal mobility of gravel in streams: The remains of the day
Authors: Parker Gary , Toro‐Escobar Carlos M.
Published: December 2002

The concept of equal mobility of gravel transport is 2 decades old. The weak form of the hypothesis states that in order to transport the coarse half of the mean annual gravel load through a river reach at a rate equal to the fine half, the coarser material must be overrepresented on the bed surface, giving rise to a mobile‐bed armor. The degree of mobile‐bed armoring necessary to accomplish this diminishes with increasing bed shear stress. The strong form of the hypothesis states that the grain size distribution of the gravel portion of the bed load should be similar to that of the substrate and finer than that of the surface layer. Here the hypothesis is subjected to a rigorous test in the laboratory under controlled conditions which are nevertheless chosen to be as representative as possible of actual field conditions. The grain size distributions used in the experiment included both model “gravel” and model “sand.” At the end of the experiment the entire surface layer and a significant depth of substrate were removed from an entire reach of the channel and sieved. The results of the experiment confirmed the weak form of equal mobility: The surface layer was significantly coarsened compared with the bed load, even when the “sand” was excluded from the size distributions. The strong form of equal mobility was also supported by the experiment, in that the size distribution of the “gravel” substrate was nearly identical to the size distribution of the “gravel” portion of the load. The gravel substrate was, however, slightly but measurably coarser than the gravel load, a tendency that has been observed in the field in cases for which the strong form is not satisfied.
Two‐dimensional modeling of flow and transport in the vadose zone with surfactant‐induced flow
Authors: Henry E. J. , Smith J. E. , Warrick A. W.
Published: December 2002

Surface‐active solutes (surfactants) can induce capillary pressure gradients and water fluxes within the vadose zone. Common numerical models do not account for the effects of surfactant‐induced capillary pressure gradients on unsaturated flow and transport. We modified the variably saturated flow and transport model HYDRUS‐2D [Simunek et al., 1999] to incorporate the effects of surfactant concentration‐dependent changes to surface tension and viscosity on moisture retention and hydraulic conductivity, respectively. The model was tested by simulating the two‐dimensional surfactant infiltration experiment of Henry and Smith [2002]. The model successfully captured the major processes associated with the advance of a surface‐active contaminant plume through the vadose zone, including drainage of the vadose zone and depression of the capillary fringe associated with the solute front. The simulations also provided insight into unusual features of surfactant‐induced flow such as the upward flow of surfactant solution that was associated with capillary fringe drainage.
Water balance dynamics of a boreal forest watershed: White Gull Creek basin, 1994–1996
Authors: Nijssen Bart , Lettenmaier Dennis P.
Published: December 2002

Field measurements from the Boreal Ecosystem‐Atmosphere Study (BOREAS) were combined to calculate the water balance of the White Gull Creek basin for the three year period 1994–1996. Evapotranspiration was mapped from the observations made at the BOREAS flux towers to the basin using a simple evaporation model with a bulk canopy resistance based on tower observations. Runoff ratios were low, and evapotranspiration accounted for most of the precipitation over the area. The accumulated storage change, over the 3 year period, was 47 mm or 3.4% of the total precipitation, but precipitation exceeded the sum of discharge and evapotranspiration by 80 mm or 15% of the precipitation in 1994. Five possible explanations for the discrepancy in the water balance are identified, with the most likely cause an underestimation of the evapotranspiration in 1994, especially during periods when the basin is wet.
Modeling capillary barriers in unsaturated fractured rock
Authors: Wu Yu‐Shu , Zhang W. , Pan Lehua , Hinds Jennifer , Bodvarsson G. S.
Published: December 2002

This work presents a series of numerical modeling studies that investigate the hydrogeologic conditions required to form capillary barriers and the effect that capillary barriers have on fluid flow and tracer transport processes in the unsaturated fractured rock of Yucca Mountain, Nevada, a potential site for storing high‐level radioactive waste. The modeling approach is based on a dual‐continuum formulation of coupled multiphase fluid and tracer transport through fractured porous rock. The numerical modeling results showed that effective capillary barriers can develop where both matrix and fracture capillary gradients tend to move water upward. Under the current hydrogeologic conceptualization of Yucca Mountain, strong capillary barrier effects exist for diverting a significant amount of moisture flow through the relatively shallow Paintbrush nonwelded unit, with major faults observed at the site serving as major downward pathways for laterally diverted percolation fluxes. In addition, we used observed field liquid saturation and goechemical isotopic data to check model results and found consistent agreement.
The role of bedrock topography on subsurface storm flow
Authors: Freer Jim , McDonnell J. J. , Beven K. J. , Peters N. E. , Burns D. A. , Hooper R. P. , Aulenbach B. , Kendall C.
Published: December 2002

We conducted a detailed study of subsurface flow and water table response coupled with digital terrain analysis (DTA) of surface and subsurface features at the hillslope scale in Panola Mountain Research Watershed (PMRW), Georgia. Subsurface storm flow contributions of macropore and matrix flow in different sections along an artificial trench face were highly variable in terms of timing, peak flow, recession characteristics, and total flow volume. The trench flow characteristics showed linkages with the spatial tensiometer response defining water table development upslope. DTA of the ground surface did not capture the observed spatial patterns of trench flow or tensiometric response. However, bedrock surface topographic indices significantly improved the estimation of spatial variation of flow at the trench. Point‐scale tensiometric data were also more highly correlated with the bedrock surface‐based indices. These relationships were further assessed for temporal changes throughout a rainstorm. Linkages between the bedrock indices and the trench flow and spatial water table responses improved during the wetter periods of the rainstorm, when the hillslope became more hydrologically connected. Our results clearly demonstrate that in developing a conceptual framework for understanding the mechanisms of runoff generation, local bedrock topography may be highly significant at the hillslope scale in some catchments where the bedrock surface acts as a relatively impermeable boundary.
Multiscaling analysis and random cascade modeling of dye infiltration
Authors: Olsson Jonas , Persson Magnus , Albergel Jean , Berndtsson Ronny , Zante Patrick , Öhrström Pernilla , Nasri Slah
Published: December 2002

We aimed at investigating whether the spatial variability of infiltration in field soils, as visualized through dye infiltration experiments, is characterized by a multiscaling behavior. Digitized high‐resolution dye images from three sites in an experimental catchment in Tunisia were analyzed using three indicators of scaling: empirical probability distribution functions, power spectra, and raw statistical moments. The two former indicators suggested a general scaling behavior of the data, which through the moments' analysis was found to be of multiscaling type. Random cascade processes are frequently used to model multiscaling processes, and we fitted the “universal multifractal” (UM) model of Schertzer and Lovejoy [1987] to our data. The UM model closely reproduced the empirical K(q) functions, and simulated fields reproduced key features in the observed ones. The results indicate that multiscaling random cascade modeling is useful for statistically describing flow processes and solute transport under field conditions.
Turbulent flow across a natural compound channel
Authors: Carling P. A. , Cao Zhixian , Holland M. J. , Ervine D. A. , Babaeyan‐Koopaei K.
Published: December 2002

The measurements and primary analysis of turbulent flow across a compound channel (River Severn, England) are presented. The velocity was measured using a three‐dimensional acoustic Doppler velocimeter in combination with a directional current meter. The statistical flow structure is examined against existing analytical formulations derived for single channel flows based on laboratory studies. The existence of a vertically two‐layer structure around the interface between the main channel and the floodplain is demonstrated, indicating (1) a vertical shear‐dominated flow zone near the bed; and (2) away from the bed a transverse shear‐dominated flow zone with enhanced turbulent mixing. The temporal spectra clearly reveal the occurrence of anisotropic turbulence both in the main channel and over the floodplain. The present findings necessitate the resolution of both transverse and vertical structures for advanced modeling of compound channel flows. The measured data can be used to assess the performance of mathematical river models.
A random utility model for total maximum daily loads: Estimating the benefits of watershed‐based ambient water quality improvements
Authors: Phaneuf Daniel J.
Published: December 2002

A random utility maximization (RUM) model is presented for the purpose of providing benefits estimates to aid in the implementation of the total maximum daily load (TMDL) process. The specific application is to North Carolina, for which the 1994 Environmental Protection Agency (EPA) National Demand for Water‐Based Recreation Survey is used to characterize the demand for water recreation in the state as a function of ambient water quality. The RUM choice set is defined to include all 8‐digit hydrological units or watersheds in the state, allowing benefits estimates to match the spatial aggregate of interest for the TMDL process. Water quality measures are linked to the choice set via the use of EPA's Index of Watershed Index database. A significant relationship between watershed‐level water quality and recreation trip taking behavior is found. Per trip and aggregate benefits estimates are presented for watershed, river basin, and statewide improvements in water quality.
A lumped‐parameter model of groundwater influx to a mine adit in mountainous terrain
Authors: Desbarats A. J.
Published: December 2002

Acidic and metal‐laden drainage from the portals of abandoned mines is a serious environmental problem in wet, mountainous regions where steep hydraulic gradients ensure that fresh recharge waters are continuously flushed through the workings. Hydrogeological characterization of these sites is hampered by their geological complexity and their frequent inaccessibility. This paper presents a material balance model for the groundwater capture zone associated with influx to a mountainside adit. The model can be used to estimate bulk hydraulic properties of the rock surrounding an adit from hydrograph data commonly recorded at its portal. This application is illustrated using data from the Myra Falls mining camp, Vancouver Island, British Columbia. By simultaneously fitting base flow recession slopes for hydrographs of discharge, water table elevation and head in an underground borehole, the model yields plausible and well‐constrained estimates of hydraulic conductivity, porosity, skin factor, and hydraulic gradient.
Basin average rainfall and its sampling error
Authors: Yoo Chulsang , Ha Eunho
Published: December 2002

The sampling error from the randomly located rain gauge network has a significant meaning, especially for hydrologic applications where the basin average rainfall estimated using point measurements is used as an input for various rainfall‐runoff analyses. In this paper we derived an equation for the sampling error from the randomly located rain gauge network of a basin and compared it with that from the evenly spaced rain gauge network derived by North and Nakamoto [1989], also implemented by Yoo [2000]. To make the analytical derivation of the sampling error equation available, we assumed that the locations of the rain gauges planted through years in a given basin follow the uniform distribution and the rainfall field is weakly statistically homogeneous in space and time. The derived equation is neatly factored with the rainfall spectrum and the design filter, which considers the second‐order statistics of a rainfall field and sampling design characteristics such as the basin size, the basin shape, and the number of rain gauges within the basin. The derived equation is also flexible enough to consider various shapes of the basins such as circles, ellipses, and rectangles as well. We applied the two equations for the sampling errors from evenly spaced and randomly located rain gauge network cases to the GATE data field using the rainfall model of North and Nakamoto [1989] and then compared the results. As a result of the study, we found that the sampling errors from the randomly located and evenly spaced rain gauge networks are more or less the same as the standard deviation of the m gauge averages until some threshold number of rain gauges is reached. With more rain gauges the sampling error from the evenly spaced rain gauge network decreases much faster, while that from the randomly located rain gauge networks stagnates. However, if considering the typical rain gauge density in a natural basin, we may conclude that the sampling error involved in the estimation of basin average rainfall using point measurements is almost the same as the standard deviation of the m gauge averages.
Steady state turnover time of carbon in the Australian terrestrial biosphere
Authors: Barrett Damian J.
Published: December 2002

The turnover time of terrestrial carbon was estimated using a multiobjective parameterization method that combined data sets of plant production, biomass, litter and soil‐C observations in the calibration of a C‐cycle model for the Australian continent (VAST1.1; Vegetation and Soil carbon Transfer). The method employed a genetic algorithm to minimize model‐data deviations and maximize consistency between estimated model parameters and all available data. Based on the parameterization, the turnover time of biosphere C for Australia was estimated to be 78 years which is longer than global C‐turnover estimates (of 26–60 years) due entirely to slower turnover of C in the upper 20 cm of soil. Turnover times of litter and deeper soil‐C were similar to global values. By splitting total C in the upper 20 cm between labile and nonlabile fractions (based on published data) the turnover time of the labile pool was at least 44 years which is still longer than global estimates (9–25 years). Longer C‐turnover in Australian surface soils was attributed to (1) limited soil moisture slowing decomposition more than net primary production, (2) frequent fires leading to a large fraction of nonlabile charcoal C in soil, and (3) strong adsorbing capacity for organic‐C in these highly weathered soils. It was found that >89% of the C flux to the atmosphere from decomposition of organic matter originated from fine litter, coarse woody debris and the upper 20 cm of soil in all biomes.
Seasonal and latitudinal variability of troposphere Δ14CO2: Post bomb contributions from fossil fuels, oceans, the stratosphere, and the terrestrial biosphere
Authors: Randerson J. T. , Enting I. G. , Schuur E. A. G. , Caldeira K. , Fung I. Y.
Published: December 2002

During the mid‐1960s, large seasonal amplitudes were observed in surface measurements of Δ14C in the Northern Hemisphere. These seasonal oscillations were initially caused by stratosphere‐troposphere exchange, with the injection of bomb 14C into the troposphere during winter and spring mixing. Here we show how fossil, ocean, and terrestrial biosphere fluxes modified the stratospheric signal during the 1960s, and the evolution of each of these components in the post bomb era. In our analysis, we used the Goddard Institute for Space Studies (GISS) atmospheric tracer model, gross ocean CO2 fluxes from the Lawrence Livermore National Laboratory (LLNL) ocean model, and terrestrial CO2 fluxes from a biosphere‐atmosphere model driven by normalized difference vegetation index and surface air temperatures. We found that 14C‐depeleted respiration from the terrestrial biosphere partially canceled the 14C‐enriched stratosphere flux in the Northern Hemisphere in the mid and late 1960s. In more recent decades, our analysis suggested that the terrestrial biosphere contribution to the Δ14C seasonal cycle reversed phase, with the terrestrial biosphere currently releasing relatively 14C‐enriched CO2 that mixes with relatively depleted troposphere CO2. The timing of this reversal depended on the residence times of carbon within the footprint of the observation station. Measurements of Δ14C in respiration from tundra and boreal ecosystems in Alaska provide evidence that some boreal forests have undergone this transition, while some tundra ecosystems have not. We predict that over the next century, several features of the latitudinal profile of Δ14C will substantially change because of continued fossil fuel emissions in the Northern Hemisphere, and the partial release of bomb 14C that has accumulated in Southern Hemisphere oceans.
An evaluation of physical and biogeochemical processes regulating the oxygen minimum zone in the water column of the Bay of Bengal
Authors: Sarma V. V. S. S.
Published: December 2002

Monthly oxygen budgets for the intermediate waters (100–1000 m) of the Bay of Bengal were constructed based on Modular Ocean Model (MOM) and oxygen data. The model results reveal that the oxygen levels in the oxygen minimum zone (OMZ) of the Bay of Bengal are controlled by physical and the associated biological processes. It results in maintaining low oxygen levels, with no significant seasonal variability, in the subsurface layer throughout the year. Low oxygen levels in the OMZ are sustained during the period of increased supply of organic matter through river runoff by enhanced supply of oxygen by the physical pump and vice versa. Thus, low oxygen levels in the OMZ are maintained by supply of oxygen by the physical pump to meet the demands of the biological processes. The inconsistency observed among oxygen consumption rates derived based on the present oxygen budget, carbon regeneration rate, and oxygen consumption rates computed based on electron transport system technique could be due to inadequate knowledge of seasonal and spatial variability in oxygen consumption in the latter two estimates. The residence time of intermediate waters (OMZ) of Bay of Bengal was computed to be 12 years.
A model analysis of the effects of nonspecific monoterpenoid storage in leaf tissues on emission kinetics and composition in Mediterranean sclerophyllousQuercusspecies
Authors: Niinemets Ülo , Reichstein Markus
Published: December 2002

Although monoterpenoid‐emittingQuercusspecies lack specific terpene storage structures, they may store monoterpenoids in nonspecific leaf compartments. To determine whether such storage may influence emission responses to diurnal changes in environmental factors, a dynamic emission model including “fast” and “slow” storage pools in parallel was constructed. Existence of two storage pools was inferred from the circumstance that monoterpene efflux from darkened leaves was poorly described by single‐exponential decay relationship, but was well parameterized by double exponentials. Simulations indicated that nonspecific terpenoid storage may significantly alter daily monoterpenoid emission both at leaf and canopy scales. The model also described shifts in fractional monoterpenoid composition after changes in environmental factors that cannot be explained by current algorithms. Time constants for the “fast” pool were negatively associated with monoterpenoid equilibrium gas/water partition coefficient (H), suggesting that the “fast” pool is in leaf liquid phase. The time constants for the “slow” pool were independent ofH, but scaled positively with monoterpenoid octanol/water partition coefficient, indicating that this pool is in lipid phase. Based on tentative pool locations and monoterpenoid physico‐chemical characteristics, time constants of various pools were computed using a flow/conductance model. Although the time constants were correlated, the theoretical estimates were larger than those derived empirically. Nonhomogeneous monoterpenoid distribution, aggregation within leaf liquid phase, and adsorption to apoplast surfaces that all decrease the area for diffusion and decrease the effective diffusion coefficients likely explain this discrepancy. We conclude that physico‐chemical models are needed to parameterize nonspecific storage effects on monoterpene emission dynamics and emission composition.
Effect of climate on interannual variability of terrestrial CO2 fluxes
Authors: Schaefer Kevin , Denning A. Scott , Suits Neil , Kaduk Jörg , Baker Ian , Los Sietse , Prihodko Lara
Published: December 2002

We evaluated how climate influences interannual variability in the terrestrial Net Ecosystem Exchange (NEE) of CO2 using the Simple Biosphere Model, Version 2 (SiB2) for 1983 to 1993 on a global, 1° by 1° latitude/longitude grid with a 10‐min time step. We quantified climate influences on NEE, explained regional differences, and related NEE variability to the Arctic Oscillation (AO) and the El Niño‐Southern Oscillation (ENSO). The simulated NEE reproduces the salient features and magnitude of the measured global CO2 growth rate. The Northern Hemisphere shows a pattern of alternating positive and negative NEE anomalies that cancel such that the tropics dominate the global simulated NEE interannual variability. Climate influences have strong regional differences with precipitation dominating in the tropics and temperature in the extratropics. In tropical regions with drier soils, precipitation control of photosynthesis (i.e., drought stress) dominates; in nearly saturated soils, precipitation control of respiration dominates. Because of cancellation and competing effects, no single climate variable controls global or regional NEE interannual variability. Globally, precipitation accounts for 44% of NEE variability; followed by Leaf Area Index (23%), soil carbon (12%), and temperature (16%). The influence of ENSO on NEE variability is consistent with that expected for shifting precipitation patterns in the tropics. Except in northern Europe, temperature advection by the AO does not significantly influence NEE variability. Neither the AO nor ENSO fully explain the temperature influence on respiration or the simulated NEE anomaly pattern in the Northern Hemisphere.
Carbon dioxide and methane dynamics and annual carbon balance in tundra wetland in NE Europe, Russia
Authors: Heikkinen Juha E. P. , Elsakov Vladimir , Martikainen Pertti J.
Published: December 2002

Methane (CH4) and carbon dioxide (CO2) fluxes on east European tundra wetland ecosystems (67°23′N, 63°22′E) were measured by static chamber techniques in 1999. The 29 microsites were divided into wet flarks (WF), intermediate flarks (IF), wet lawns (WL), intermediate lawns (IL), and hummocks (HCK) in accordance to the water table (WT) and vegetation. The seasonal net ecosystem exchange (NEE) varied from −104 mg emission to 249 mg CO2–C m−2 h−1 uptake. Regression models were constructed to estimate the seasonal gross photosynthesis and respiration on each of the microsites. The cumulative gross photosynthesis (and respiration) was 68 (37), 131 (77), 176 (126), 164 (129), and 189 (187) g C m−2 in a 100 day summer period in WF, IF, WL, IL, and HCK, respectively. The losses of carbon in winter accounted for 23.7%, 5.0%, 3.1%, 2.6%, and 2.3% of the net ecosystem production. CH4 emissions in summer (and winter) were 7.7 (7.8), 6.6 (0.8), 8.2 (0.4), 0.7 (0.3), and 0.1 (0.2) g C m−2 for WF, IF, WL, IL, and HCK, respectively. The modeled annual carbon balance for grouped microsites shows that dry HCKs were carbon sources −7.5 g C yr−1 while flarks and lawns were carbon sinks ranging from 2.2 to 35.9 g C yr−1. The results show the importance of WT and vegetation composition on annual flux dynamics in a tundra wetland. CH4 emissions from arctic peatlands seem to be sensitive to changes in the WT.
Measurements of the fate of gas hydrates during transit through the ocean water column
Authors: Brewer Peter G. , Paull Charles , Peltzer Edward T. , Ussler William , Rehder Gregor , Friederich Gernot
Published: December 2002

We report on controlled experiments to document the fate of naturally occurring methane hydrate released from the sea floor (780 m, 4.3°C) by remotely operated vehicle (ROV) disturbance. Images of buoyant sediment‐coated solids rising (∼0.24 m/s) from the debris cloud, soon revealed clear crystals of methane hydrate as surficial material sloughed off. Decomposition and visible degassing began close to the predicted phase boundary, yet pieces initially of ∼0.10 m size easily survived transit to the surface ocean. Smaller pieces dissolved or dissociated before reaching the surface ocean, yet effectively transferred gas to depths where atmospheric ventilation times are short relative to methane oxidation rates.
Tropical pathways, equatorial undercurrent variability and the 1998 La Niña
Authors: Izumo Takeshi , Picaut Joel , Blanke Bruno
Published: December 2002

In the equatorial Pacific Ocean, the rapid transition from the 1997 El Niño to the 1998 La Niña was associated with an outcrop of cold water appearing at the surface around (0°, 125°W) end of May 1998. In a validated ocean general circulation model, the cold water of the outcrop appears to come from two pathways (one in each hemisphere) that connect the subtropics to the equatorial upwelling region through the western boundary currents and the Equatorial Undercurrent (EUC). The EUC and subtropics may have an influence on the rapid turn into La Niña in 1998. Large variations in transport (up to 60 × 106 m3 s−1) and in transport‐weighted temperature (up to 3°C) of the EUC are observed in TAO moored data and in the model. The rapid variability of EUC transport and transport‐weighted temperature may interact with the long‐term variability of the thermocline circulation linking the subtropics to the equator.