bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification and separation of Mount Pinatubo and El Niño‐Southern Oscillation land surface temperature anomalies
Authors: Yang Fanglin , Schlesinger Michael E.
Published: February 2004

Empirical data analyses are performed to detect and separate the signals of the volcanic eruption and El Niño events in the observed surface air temperature (SAT) anomalies over land for the 2 years following the Pinatubo eruption. Composite analyses of surface temperatures for the 1950–1997 period, excluding the 2 years following each of the three major volcanic eruptions, Agung, El Chichón, and Pinatubo, show that the distribution of SAT anomalies over most land areas for the El Niño composite is opposite to that for the La Niña composite. The pattern of SAT anomalies changes from season to season. This feature is more prominent over North America than over the other continents. Singular value decomposition analyses show that following the Pinatubo eruption, El Niño signals were weak over Eurasia but relatively strong over the other continents. Over North America, the 1991–1992 El Niño event contributed more than 50% to the observed surface cooling of about −1.0°C in June, July, and August 1992. Globally averaged, the maximum cooling over land with the El Niño‐Southern Oscillation signals removed is −0.5°C in September, October, and November 1992 and September, October, and November 1993.
Ice‐age variability from the Vostok deuterium and deuterium excess records
Authors: Yiou P. , Vimeux F. , Jouzel J.
Published: February 2004

The four climatic cycles obtained from the Vostok ice core offer a unique opportunity to study the high‐frequency paleoclimatic variability, i.e., on time scales smaller than 15 kyr. We focused our study on the deuterium (δD) and deuterium excess (d) records, which are proxies for local temperature and remote evaporation source temperature and hence give us access to spatial climatic variations. Spectral analyses of paleoclimatic records have shown that the last glacial period (110–15 kyr before present) is characterized by large and fast temperature oscillations. Examples of such variations in the Northern Hemisphere are the so‐called Dansgaard‐Oeschger oscillations and Heinrich events. At Vostok, these oscillations are also imprinted in the ice, with broadband periodicities between 5 and 8 kyr and between 1.4 and 1.8 kyr. Scenarios for this behavior have recently been developed and argued, as well as connections with the Northern Hemisphere. We now can investigate the stability of the fast spectral features in the previous ice ages recovered in the Vostok isotopic records. This allows us to document the different types of climatic behavior under glacial conditions and hence connect the fast variation statistics with the slower ones controlled by insolation and sea level change. Our results show that δD and d do have a distinct spectral behavior. We discuss the implications on the ocean circulation from such a difference.
GPS measurements of vertical crustal motion in Greenland
Authors: Wahr John , van Dam Tonie , Larson Kristine , Francis Olivier
Published: February 2004

We have analyzed 5 years of continuous Global Positioning System (GPS) measurements taken at Kellyville, just off the western margin of the ice sheet in southern Greenland. A fit to the vertical component gives a negative secular uplift rate of −5.8±1.0 mm/yr. A negative rate (i.e., a subsidence) is consistent with archeological and historical evidence that the surrounding region has been subsiding over the last 3 kyr. However, it is inconsistent with estimates of the Earth's continuing viscoelastic response to melting ice prior to 4 ka years ago, which predict that Kellyville should be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting −7.8±3.6 mm/yr discrepancy is too large to be the result of loading from present‐day changes in nearby ice. We show, instead, that it is consistent with independent suggestions that the western ice sheet margin in this region of Greenland may have advanced by ≈50 km during the past 3–4 kyr.
Trend surface analysis of Greenland accumulation
Authors: van der Veen C. J. , Bromwich D. H. , Csatho B. M. , Kim C.
Published: February 2004

Multivariate regression methods are applied to measurements of accumulation covering much of the interior of the Greenland ice sheet to evaluate the important factors that describe the current distribution of accumulation. Predictor variables considered in the regressions are geographical coordinates and three independent factors describing the geometry of the ice sheet. The results indicate that most of the variance in the data is explained by the combined effect of large‐scale atmospheric circulation and ice sheet topography. This finding implies that climate change scenarios in which changes in accumulation are mostly associated with changes in temperature or some other parameter may only be correct if the pattern of atmospheric circulation remains unaltered. Comparison with values predicted with a precipitation retrieval model is favorable, suggesting that the model captures the most important features of Greenland precipitation.
Surface variability effects on the remote sensing of thin cirrus optical and microphysical properties
Authors: Rolland P. , Liou K. N.
Published: February 2004

We have developed a methodology for the retrieval of the optical and microphysical properties of thin cirrus clouds with optical depths less than 0.5 using Moderate Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) measurements conducted during the Subsonic Aircraft Cloud and Contrail Special Study (SUCCESS). This methodology involves the use of correlated reflectance at three channels (0.65, 1.6, and 2.2 μm). We demonstrate the necessity of employing accurate values of the upwelling cloud base reflectance fields and present a method for the computation of these fields based on the atmospheric correction approach. For ocean surfaces the anisotropic reflectance is atmospherically corrected using appropriate radiative transfer calculations, along with the retrieved aerosol optical depths based on a simple aerosol microphysical model. For land surfaces a mosaic of ecosystems is used to compute the anisotropic reflectance associated with the surface terrain variability. We show that using these explicit computations of the emerging cloud base reflectance, thin cirrus optical depths and ice crystal size over ocean surfaces can be retrieved accurately. Uncertainties in the retrieved optical depth and ice crystal size are further reduced by 20 and 45%, respectively, over complex land surfaces.
Seasonal variability of the observed vertical wave number spectra of wind and temperature and the effects of prewhitening
Authors: Nastrom G. D. , VanZandt T. E.
Published: February 2004

Variance power spectra of wind and temperature fluctuations as functions of vertical wave number from several hundred balloon ascents over the very flat terrain of central Illinois are analyzed. Special attention is given to the effects of prewhitening the data prior to spectral analysis, and it is shown that prewhitening should be used when spectral slopes are more negative than about −3. The conclusions of a previous study that did not use prewhitening are reexamined using prewhitening with an expanded set of balloon ascents. The present results confirm the past conclusions that the vertical wave number spectra are inconsistent with the major predictions of saturated gravity wave models in several important respects. The expanded data set used here permits the study of changes from summer to winter, and it is shown that spectral slopes are steeper in winter than in summer and that both spectral slope and amplitude have the most significant correlation with parameters of the large‐scale flow during winter.
Evolution of tropospheric ozone under anthropogenic activities and associated radiative forcing of climate
Authors: Hauglustaine D. A. , Brasseur G. P.
Published: February 2004

The budget of ozone and its evolution associated with anthropogenic activities are simulated with the Model for Ozone and Related Chemical Tracers (MOZART) (version 1). We present the changes in tropospheric ozone and its precursors (CH4, NMHCs, CO, NOx) since the preindustrial period. The ozone change at the surface exhibits a maximum increase at midlatitudes in the northern hemisphere reaching more than a factor of 3 over Europe, North America, and Southeast Asia during summer. The calculated preindustrial ozone levels are particularly sensitive to assumptions about natural and biomass burning emissions of precursors. The possible future evolution of ozone to the year 2050 is also simulated, using the Intergovernmental Panel on Climate Change IS92a scenario to estimate the global and geographical changes in surface emissions. The future evolution of ozone stresses the important role played by the tropics and the subtropics. In this case a maximum ozone increase is calculated in the northern subtropical region and is associated with increased emissions in Southeast Asia and Central America. The ozone future evolution also affects the more remote regions of the troposphere, and an increase of 10–20% in background ozone levels is calculated over marine regions in the southern hemisphere. Our best estimate of the global and annual mean radiative forcing associated with tropospheric ozone increase since the preindustrial era is 0.43 W m−2. This value represents about 20% of the forcing associated with well‐mixed greenhouse gases. The normalized tropospheric ozone radiative forcing is 0.048 W m−2 DU−1. An upper estimate on our forcing of 0.77 W m−2 is calculated when a stratospheric tracer is used to approximate background ozone levels. In 2050 an additional ozone forcing of 0.26 W m−2 is calculated, providing a forcing from preindustrial to 2050 of 0.69 W m−2.
Diffusive gas transport through flooded rice systems
Authors: van Bodegom P. M. , Groot T. , van den Hout B. , Leffelaar P. A. , Goudriaan J.
Published: February 2004

A fully mechanistic model based on diffusion equations for gas transport in a flooded rice system is presented. The model has transport descriptions for various compartments in the water‐saturated soil and within the plant. Plant parameters were estimated from published data and experiments independent of the validation experiment. An independent experiment is described in which the diffusion coefficient of sulfurhexafluoride (SF6) in water‐saturated soil was determined. The model was validated by experiments in which transport of SF6 through soil and plant was monitored continuously by photoacoustics. The independent default settings could reasonably predict gas release dynamics in the soil‐plant system. Calculated transmissivities and concentration gradients at the default settings show that transport within the soil was the most limiting step in this system, which explains why most gases are released via plant‐mediated transport. The root‐shoot interface represents the major resistance for gas transport within the plant. A sensitivity analysis of the model showed that gas transport in such a system is highly sensitive to the estimation of the diffusion coefficient of SF6, which helps to understand diel patterns found for greenhouse gas emissions, and to the root distribution with depth. This can be understood from the calculated transmissivities. The model is less sensitive to changes in the resistance at the root‐shoot interface and in the root fraction active in gas exchange. The model thus provides an understanding of limiting steps in gas transport, but quantitative predictions of in situ gas transport rates will be difficult given the plasticity of root distribution.
Synopsis, transport, and physical characteristics of Asian dust in Korea
Authors: Chun Youngsin , Boo Kyung‐On , Kim Jiyoung , Park Soon‐Ung , Lee Meehye
Published: February 2004

Historical records in association with Asian dust events were retrieved from ancient Korean literature, which in conjunction with modern observations, indicate that dust events have occurred most frequently in the springtime during the last two centuries. Recent observations through surface network exhibit that Asian dust took place more often in the western part of the Korean peninsula over the last 20 years. In this study, two dust cases, April 1998 and January 1999, were selected to examine detailed conditions most favorable for dust generation, emission, and transport to Korea and to investigate the effect of dust particles on physical and optical properties of aerosols collected in Seoul. Dust transported to the Korean peninsula is closely linked to dust storms generated in upstream regions of the Yellow River or Manzurian plain, which are known as main source regions. Judged from synoptic conditions for both spring and winter dust events, meteorological settings favorable for dust emission are high surface winds and baroclinic instability at 1.5 km level. A strong wind belt, formed at a 5 km level, expedites dust transport, and this is typically much faster in winter. It was confirmed from a backward trajectory analysis that the air carrying mineral dust particles originated from deserts in central Asia and in 1 day passed through the Korean peninsula during wintertime. The effect of mineral dust on aerosol particles is well depicted in the size‐separated number concentrations of aerosols observed in Seoul. In both cases, concentrations of coarse particles larger than 0.82 μm were distinctly enhanced while those of fine particle smaller than 0.5 μm were reduced. The measurements of optical depth also indicate that the atmosphere is more turbid with larger particles during dust events.
Tibetan Plateau summer monsoon northward extent revealed by measurements of water stable isotopes
Authors: Tian L. , Masson‐Delmotte V. , Stievenard M. , Yao T. , Jouzel J.
Published: February 2004

A program of individual precipitation events and river water sampling and of water isotopic measurements (δD, δ18O) was carried out during summer 1996 along a northeast/southwest transect of the Tibetan Plateau. The spatial distribution of both δ18O and deuterium excess (d = δD‐8*δ18O) of the precipitation reveals three distinct regions. Simulations with a simple isotopic model and seasonal isotopic variations measured at two extreme south and north locations support our interpretation in terms of different summer moisture origins: (1) South of the Himalayan mountains, the moisture provided by the Indian monsoon has been recycled over the Indian peninsula. (2) Between the Himalayas and the Tanggula mountains the oceanic moisture is directly transported from the Bay of Bengal along the Brahmaptra River valley. (3) North of the Tanggula mountains, the moisture is not provided by the monsoon anymore but by continental water recycling.
Ice‐stream‐related patterns of ice flow in the interior of northeast Greenland
Authors: Fahnestock M. A. , Joughin I. , Scambos T. A. , Kwok R. , Krabill W. B. , Gogineni S.
Published: February 2004

Ice flow in the interior of the NE quadrant of the Greenland ice sheet is focused on the large ice stream draining the north side of the summit dome. The rapid ice flow in the stream is apparent in the surface features in the stream and at the margins, in the broad scale topography that drives ice flow, and in satellite‐derived ice motion information. The patterns of ice flow in the upper half of the stream are remarkable for their level of organization, simple geometry, and effects on local surface topography. The stream begins less than 100 km from the ice divide as a current 15 km wide and then broadens symmetrically downstream by the addition of ice from the sides to a width of more than 60 km. Elevation data and visible‐band imagery show that the stream has marginal troughs tens of meters deep in its upper reach which are coincident with regions of high shear strain rate. The topography of the margins and undulating surface of the stream is generated by the ice flow; the surface undulations in the stream are fixed in location and shape over the 5 year period from 1994 to 1999. The enhanced flow presents a challenge for researchers trying to understand the history of ice discharge from a significant area in the interior of the ice sheet.
Validation of land surface models using satellite‐derived surface temperature
Authors: Rhoads Joshua , Dubayah Ralph , Lettenmaier Dennis , O'Donnell Greg , Lakshmi Venkat
Published: February 2004

This research examines the feasibility of using remotely sensed surface temperature for validation and updating of land surface hydrologic models. Surface temperature simulated by the Variable Infiltration Capacity (VIC) hydrologic model is compared over the Arkansas‐Red River basin with surface temperature retrievals from TOVS and GOES. The results show that modeled and satellite‐derived surface temperatures agree well when aggregated in space or time. In particular, monthly mean temperatures agree on the pixel scale, and basin mean temperatures agree instantaneously. At the pixel scale, however, surface temperatures from both satellites were found to have higher spatial and temporal variabilities than the modeled temperatures, although the model and satellites display similar patterns of variability through space and time. The largest differences between modeled and remotely sensed surface temperature variability occur at times of maximum net radiation both diurnally and seasonally, i.e., afternoon and summer. Comparison of temporal and spatial patterns of VIC‐predicted surface temperature variability with similar predictions by nine other models involved in the PILPS‐2c experiment show that the VIC patterns are similar to those of the other models. Observed surface temperature and air temperature from FIFE are used to identify possible errors in satellite‐retrieved surface temperatures. The FIFE comparisons show that satellite retrieved surface temperatures likely contain errors that increase variability.
Coordinated observations of optical lightning from space using the FORTE photodiode detector and CCD imager
Authors: Suszcynsky D. M. , Light T. E. , Davis S. , Green J. L. , Guillen J. L. L. , Myre W.
Published: February 2004

This paper presents an overview of the coordinated observation of optical lightning from space using the photodiode detector (PDD) and CCD‐based imager known as the Lightning Location System (LLS) aboard the Fast On‐Orbit Recording of Transient Events (FORTE) satellite. PDD/LLS coincidence statistics are presented and show that both the detected energy density and the detected peak irradiance of optical lightning events are proportional to the number of LLS pixels (pixel multiplicity) which are activated during the event. The inference is that LLS pixel multiplicity is more a function of the detected intensity and horizontal extent of the optical event rather than a direct indicator of the degree of scattering. PDD/LLS event coincidence is also used to improve upon traditional recurrence/clustering algorithms that discriminate against false LLS events due to energetic particles and glint. Energy density measurements of coincident events show that about 4% of the optical energy detected by the broadband PDD appears in the narrowband LLS. This is in general agreement with ground‐based measurements and with assumptions incorporated into the design of current and planned CCD‐imaging sensors.
Detection of lightning‐produced NO in the midlatitude upper troposphere during STREAM 1998
Authors: Lange Lutz , Hoor Peter , Helas Günter , Fischer Horst , Brunner Dominik , Scheeren Bert , Williams Jonathan , Wong Stefanie , Wohlfrom Karl‐Heinz , Arnold Frank , Ström Johan , Krejci Radovan , Lelieveld Jos , Andreae Meinrat O.
Published: February 2004

Simultaneous in situ measurements of NO, NOy, HNO3, CO, CO2, O3, and aerosols were performed in the midlatitude upper troposphere (UT) and lower stratosphere during the StratosphereTroposphere Experiment by Aircraft Measurements (STREAM) 1998 summer campaign. The campaign focused on the region around Timmins in the Canadian province of Ontario (79.3°W, 48.2°N), close to the polar jet stream that rapidly transports trace species across the Atlantic Ocean. This paper focuses on the origin of total reactive nitrogen (NOy) in the UT, as our measurements show strong variations, which reflect large local sources. In situ production by lightning, stratospheric downdraft, aircraft emissions, and upward transport of polluted boundary layer air are discussed in two case studies as potential contributors. We use correlations among NO, NOy and CO to distinguish between transport from the boundary layer and in situ formations. Lightning production of NOx is found to be a strong contributor to the budget of NOy during high NOy episodes.
A fast, accurate algorithm to account for non‐Lambertian surface effects on TOA radiance
Authors: Qin Wenhan , Herman Jay R. , Ahmad Ziauddin
Published: February 2004

Surface bidirectional reflectance distribution function (BRDF) influences both the radiance just above the surface and that emerging from the top of the atmosphere (TOA). In this study we propose a new, fast, and accurate algorithm CASBIR (correction for anisotropic surface bidirectional reflection) to account for such influences on TOA radiance. This new algorithm is based on four‐stream theory that separates the radiation field into direct and diffuse components in both upwelling and downwelling directions. Such a separation is important because the direct component accounts for a substantial portion of incident radiation under a clear sky, and the BRDF effect is strongest in the reflection of the incident direct radiation. The model is validated by comparison with a full‐scale, vector radiative transfer model for the atmosphere‐surface system [Ahmad and Fraser, 1982] for wavelengths from UV to near‐IR over three typical but very different surface types. The result demonstrates that CASBIR is accurate for all solar and viewing zenith and azimuth angles considered, with overall relative difference of less than 0.7%. Application of this algorithm includes both accounting for non‐Lambertian surface scattering on the emergent radiation above TOA and developing a more effective approach for surface BRDF retrieval from satellite‐measured radiance. Comparison with the result from the Lambertian model indicates that surface BRDF influence on TOA radiance is both angle and wavelength dependent. It increases as solar zenith angle decreases or wavelength increases and becomes strongest in the view directions where the surface reflection is most anisotropic (such as in the hot spot or Sun glint regions).
Orbital forcing of terrestrial mollusks and climatic changes from the Loess Plateau of China during the past 350 ka
Authors: Wu Naiqin , Rousseau D. D. , Liu Tungsheng , Lu Houyuan , Zhaoyan Gu , Guo Zhengtang , Jiang Wenying
Published: February 2004

High‐resolution analyses of terrestrial mollusks from two loess sequences in the Chinese Loess Plateau provide continuous evidence of paleoecosystem and paleoclimate processes through the recent three glacial‐interglacial cycles. Systematic fluctuations in thermal‐humidiphilous mollusk type (Punctum orphana) indicate that the strength of the SE Asian summer monsoon is sensitive to orbital forcing at the obliquity and precession periods (41 and 23 kyr, respectively). In contrast, the cold‐aridiphilous mollusk (Vallonia tenera and Pupilla aeoli) variability displays a different pattern. The strongest powers are centered at about 100 kyr and 41 kyr periods, which drive the major period of winter monsoon variations in the Chinese loess. These different responses to winter and summer monsoon in the Loess Plateau of China suggest that both of these monsoon systems are tightly coupled in the 41 kyr cycle, but are not coincident relative to the precession (19–23 kyr) and 100 kyr cycles. The relationship between the winter and summer monsoons is not a simple model reflecting a seesaw pattern.
Mechanisms by which extratropical wave forcing in the winter stratosphere induces upwelling in the summer hemisphere
Authors: Tung Ka Kit , Kinnersley Jonathan S.
Published: February 2004

Observational evidence presented here suggests that zonal body force in the middle‐ and high‐latitude winter stratosphere can induce an extensive circulation with upwelling spreading into the summer hemisphere. Numerical model results are used to investigate the nature of the circulation anomaly and the mechanism by which it is produced. It is found that nonlinearity is important in allowing cross‐equatorial flow, which connects the upwelling in the summer hemisphere to the wave forcing in the winter hemisphere. The connection between the summer and winter hemispheres in our model is through the cross‐equatorial flow in the upper stratosphere in a region of low angular momentum gradient. The existence of an extensive circulation and the required cross‐equatorial flow is sensitive to the location of the wave forcing in the winter extratropics. It requires that the wave forcing be present between 40 and 60 km and extends equatorward of 30° in the winter stratosphere. In the real atmosphere, planetary waves presumably break predominantly above 40 km and more equatorward in January and February. We therefore expect that such an extensive circulation exists in the atmosphere during late winter. Indeed, our model tracer, temperature, and zonal wind patterns are very similar to those observed with respect to the response of the nearly global circulation to extratropical wave forcing in winter.
Modeling the evolution of Antarctic ice sheet over the last 420,000 years: Implications for altitude changes in the Vostok region
Authors: Ritz Catherine , Rommelaere Vincent , Dumas Christophe
Published: February 2004

A new thermomechanical three‐dimensional model designed to simulate the evolution of the Antarctic ice sheet over long time periods is presented. This model incorporates the various types of ice flow found in Antarctica: relatively slow inland ice flow that is essentially due to ice deformation, fast ice flow in the regions with ice streams, and ice shelf flow. By coupling these three types of flow, it is possible to predict grounding line migration. Simulations covering four glacial‐interglacial cycles have been conducted by forcing this model with a temperature record from Vostok and a sea level record from marine cores. Our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the Antarctic ice volume. On the other hand, the altitude of the ice sheet surface at Vostok is driven by accumulation rate variations. The amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the Antarctic Plateau.
Mapping Greenland accumulation rates using observations of thermal emission at 4.5‐cm wavelength
Authors: Winebrenner Dale P. , Arthern Robert J. , Shuman Christopher A.
Published: February 2004

Accurate predictions of sea level rise over the coming century will require improved knowledge of the processes controlling accumulation on the great ice sheets. The sparsity of accumulation rate observations, both temporally and spatially, hinder development of this understanding. We introduce a new method to observe accumulation rates (averaged over roughly a decade) using satellite observations of microwave emission at 4.5‐cm wavelength, focusing in this paper on Greenland. At this wavelength, scattering by the grain fabric in firn is unimportant relative to quasi‐reflection from density (and thus dielectric permittivity) stratification. We show observationally a strong link between random firn density stratification, on scales of millimeters to centimeters, and accumulation rate. We then show theoretically how the observed density stratification can produce and is consistent with observations of polarization of 4.5‐cm‐wavelength emission. We employ observations from the Scanning Multichannel Microwave Radiometer (SMMR) and previously published ground observations of accumulation rates in Greenland to complete specification of the relationship between accumulation rate and polarization. The relationship is sufficiently accurate to serve as a basis for mapping accumulation rate fields. We compare our satellite‐derived maps with previously published maps based on syntheses of ground data. We find broad agreement between the two types of maps, though the satellite‐derived map indicates more strongly the importance of topography and prevailing weather patterns in determining detailed accumulation rate patterns. Finally, we discuss possible refinements and the prospects for improved satellite‐derived maps based on a new generation of sensors about to be launched.
Simulation of a reacting pollutant puff using an adaptive grid algorithm
Authors: Srivastava R. K. , McRae D. S. , Odman M. T.
Published: February 2004

A new dynamic solution adaptive grid algorithm, DSAGA‐PPM, has been developed for use in air quality modeling. In this paper, this algorithm is described, and is evaluated with a test problem. Cone‐shaped distributions of various chemical species undergoing chemical reactions are rotated to simulate the transport and chemistry processes that occur in the atmosphere. The results obtained by using DSAGA‐PPM are more accurate than those obtained from a static grid with the same number of nodes. The computational cost associated with a static grid solution with the same level of accuracy is prohibitive. Because of its efficient use of computational resources, DSAGA‐PPM has the potential to improve the accuracy, or efficiency, or a combination of both in air quality models.
Nitrogen oxides and ozone in the tropopause region of the northern hemisphere: Measurements from commercial aircraft in 1995/1996 and 1997
Authors: Brunner Dominik , Staehelin Johannes , Jeker Dominique , Wernli Heini , Schumann Ulrich
Published: February 2004

Measurements of nitrogen oxides (NO and NO2) and ozone (O3) were performed from a Swissair B‐747 passenger aircraft in two extended time periods (May 1995 to May 1996, August to November 1997) in the framework of the Swiss NOXAR and the European POLINAT 2 project. The measurements were obtained on a total of 623 flights between Europe and destinations in the United States and the Far East. NO2 measurements were obtained only after December 1995 and were less precise than the NO measurements. Therefore daytime NO2 values were derived from measured NO and O3 concentrations assuming photostationary equilibrium. The completed NOx data set (measured NO, measured NO2 during night, and calculated NO2 during day) includes a complete annual cycle and is the most extensive and representative data set currently available for the upper troposphere (UT) and the lower stratosphere (LS) covering a significant proportion of the northern hemisphere between 15°N and 65°N. NOx concentrations in midlatitudes (30°–60°N) showed a marked seasonal variation both in the UT and the LS with a maximum in summer (median/mean values of 159/264 pptv in UT, 199/237 pptv in LS) and a minimum in winter (51/99 pptv in UT, 67/91 pptv in LS). Mean NOx concentrations were generally much higher than the respective median values, in particular in the UT, which reflects the important contribution from comparatively few very high concentrations observed in large‐scale convection/lightning and small‐scale aircraft plumes. Seasonal mean NOx concentrations in the UT were up to 3–4 times higher over continental regions than over the North Atlantic during summer. Lightning production of NO and convective vertical transport from the polluted boundary layer thus appear to have dominated the upper tropospheric NOx budget over these continental regions, particularly during summer. Ozone concentrations at aircraft cruising levels typically varied by an order of magnitude due to the strong vertical gradient in the LS. Seasonal mean values were dominated by large‐scale dynamical processes controlling the altitude of the tropopause and the O3 abundance in the LS. O3 in the UT in midlatitudes showed a broad maximum between June and August, typical of observations in the free troposphere.
Influence of the source formulation on modeling the atmospheric global distribution of sea salt aerosol
Authors: Guelle Walter , Schulz Michael , Balkanski Yves , Dentener Frank
Published: February 2004

Three different sea salt generation functions are investigated for use in global three‐dimensional atmospheric models. Complementary observational data are used to validate an annual simulation of the whole size range (film, jet, and spume droplet derived particles). Aerosol concentrations are corrected for humidity growth and sampler inlet characteristics. Data from the North American deposition network are corrected for mineral dust to derive sea salt wet fluxes. We find that sea salt transport to inner continental areas requires substantial mass in the jet droplet range, which is best reproduced with the source of Monahan et al. [1986]. The results from this source formulation also shows the best agreement with aerosol concentration seasonality and sea salt size distributions below 4 μm dry radius. Measured wind speed dependence of coarse particle occurrence suggests that above 4 μ the source from Smith and Harrison [1998] is most appropriate. Such sea salt simulations are relevant for assessing heterogeneous chemistry and radiative effects. Sea salt aerosol provides on an annual average, in marine regions, an aggregate surface area equal to 1–10% of the area of the underlying Earth's surface. Together with mineral dust, sulfate, and carbonaceous aerosol the total aerosol surface area globally amounts to 13% of that of the Earth's surface. On the basis of atmospheric column burdens, sea salt represents 21% of the total global aerosol surface area. Equal partitioning of the aerosol surface area among the four components suggests that one has to consider all of them if the global aerosol impact is to be fully determined.
Polar cap convection relationships with solar wind
Authors: MacDougall J. W. , Jayachandran P. T.
Published: February 2004

Relationships between the speed of polar cap antisunward convection and the southward component of the interplanetary magnetic field (IMF Bz negative), and also the induced solar wind electric field (VswBz) are derived from polar cap convection measurements using Canadian Advanced Digital Ionosondes at Eureka (CGM latitude 88.67°N) and Resolute Bay (CGM latitude. 83.55°N). The relationships are similar for Eureka and Resolute Bay, which are separated by ∼600 km, showing that as a first approximation, one can assume uniform polar cap convection. Resolute Bay, however, appeared to show some saturation at IMF Bz values less than approximately −10 nT. The average relationships are as follows: Vconv (m/s) ≈ 294 – 66Bz (nT), and Vconv (m/s) ≈ 293 – 166VswBz (mV/m). The convection speed did not show a significant dependence on the IMF By and Bx components. The amplification factor between the induced solar wind electric field and ionospheric electric field is ∼8.
Reactive transport modeling of an in situ reactive barrier for the treatment of hexavalent chromium and trichloroethylene in groundwater
Authors: Mayer K. Ulrich , Blowes David W. , Frind Emil O.
Published: February 2004

Multicomponent reactive transport modeling was conducted for the permeable reactive barrier at the Coast Guard Support Center near Elizabeth City, North Carolina. The zero‐valent iron barrier was installed to treat groundwater contaminated by hexavalent chromium and chlorinated solvents. The simulations were performed using the reactive transport model MIN3P, applied to an existing site‐specific conceptual model. Reaction processes controlling the geochemical evolution within and down gradient of the barrier were considered. Within the barrier, the treatment of the contaminants, the reduction of other electron acceptors present in the ambient groundwater, microbially mediated sulfate reduction, the precipitation of secondary minerals, and degassing of hydrogen gas were included. Down gradient of the barrier, water‐rock interactions between the highly alkaline and reducing pore water emanating from the barrier and the aquifer material were considered. The model results illustrate removal of Cr(VI) and the chlorinated solvents by the reactive barrier and highlight that reactions other than the remediation reactions most significantly affect the water chemistry in the barrier. In particular, sulfate reduction and iron corrosion by water control the evolution of the pore water while passing through the treatment system. The simulation results indicate that secondary mineral formation has the potential to decrease the porosity in the barrier over the long term and illustrate that the precipitation of minerals is concentrated in the upgradient portion of the barrier. Two‐dimensional simulations demonstrate how preferential flow can affect the reduction of electron acceptors, the consumption of the treatment material, and the formation of secondary minerals. In addition, the model results indicate that deprotonation and the adsorption of cations down gradient of the barrier can potentially explain the observed pH buffering.
A physically based filter for separating base flow from streamflow time series
Authors: Furey Peter R. , Gupta Vijay K.
Published: February 2004

A filter for separating base flow from streamflow in a river basin is derived from a mass balance equation for base flow from a hillside. Unlike other existing filters, this filter is physically based both during streamflow recession and rise, and each of the parameters in it has a well‐defined physical meaning. Unlike other approaches to base flow separation, the filter is applied without any calibration or constraints. Yet, using two criteria as measures of filter performance, it performs as well as or better than two existing filters that require calibration. Using the same two criteria, weaknesses in the filter are identified and attributed mainly to a ratio of two parameters that are related to groundwater recharge and overland flow. This ratio is estimated using precipitation and streamflow data, and its estimate is conjectured to be inaccurate because of the poor spatial resolution of the precipitation data. Moreover, this ratio is assumed to be constant in time, yet, on physical grounds, it should be treated as a time‐dependent quantity.
Approximate boundaries between different flow regimes in fractured rocks
Authors: Tokunaga Tetsu K. , Wan Jiamin
Published: February 2004

Recent studies have shown how water films at near‐zero matric potentials on fracture surfaces could permit fast flow in unsaturated fractures. The present work is aimed at delineating the ranges of conditions necessary to permit film flow. The approximate matric potential needed to permit thickening of water films on fracture surfaces was estimated through correlating air‐entry matric potentials and permeabilities for a wide range of porous media. An upper matric potential limit, associated with saturating the fracture, was related to the fracture aperture. These two limits, one dependent on matrix permeability and the other on fracture aperture, delineate the matric potential range within which thick water films develop on fracture surfaces. Rocks with matrix permeabilities less than ∼10−14 m2, and fractures with apertures greater than ∼30 μm permit some matric potential range over which thick water films can form. Highly transmissive films can develop on fracture surfaces only under near‐zero matric and pressure potentials.
Two‐region flow and decreased sorption of uranium (VI) during transport in hanford groundwater and unsaturated sands
Authors: Gamerdinger A. P. , Kaplan D. I. , Wellman D. M. , Serne R. J.
Published: February 2004

Uranium, U(VI), sorption and transport in unsaturated coarse‐ and fine‐ textured sands were evaluated using a centrifuge method; batch incubation and saturated column experiments were conducted to isolate the effect of flow from that of water content. At higher water contents (≥66% saturation), decreases in U(VI) sorption were due to rate limitations. These breakthrough curves (BTCs) were well characterized with a two‐site model for sorption (linear distribution coefficient Kd and first‐order kinetic term); BTC characteristics were not captured with an equilibrium sorption model. At lower water contents (<30% saturation), two‐region flow (mobile‐immobile water) was apparent and had a significant impact on U(VI) sorption and transport. Uranium BTCs were characterized with two‐region mass transfer parameters that were determined in experiments with conservative tracers. Decreases in uranium sorption at 22% and 13% saturation were greater than could be explained by velocity or rate‐limited effects. Conservative tracer experiments indicated that solutes were excluded from a fraction of the pore volume; however, the decrease in uranium sorption was greater than predicted from this fraction. The results suggest that a greater propensity to sorb uranium was associated with sediment particles in contact with the excluded immobile water domain.
On non‐Newtonian fluid flow in rough fractures
Authors: Federico Vittorio Di
Published: February 2004

Flow of non‐Newtonian fluids between rough walls is of interest in several geophysical and industrial applications. In this work (mainly geared toward fractured media) a governing equation for creeping flow of a purely viscous power law fluid of flow behavior index n in a rough‐walled fracture is obtained, generalizing past results for a Newtonian fluid. An equivalent fracture aperture is defined, in analogy to the well‐known hydraulic aperture valid for n = 1. Tortuosity is introduced as a vectorial quantity, thereby distinguishing between true and apparent fracture aperture. Examples are provided to illustrate the utility of the proposed approach. It is demonstrated that tortuosity effects significantly decrease the equivalent fracture permeability. Depending on the specific geometry considered, the flow behavior index may or may not have a significant impact on the equivalent fracture permeability. When it does, the reduction effect due to tortuosity is enhanced as the flow behavior index decreases.
Gaining and losing sections of horizontal wells
Authors: Steward David R. , Jin Wei
Published: February 2004

The flux along a horizontal well in uniform flow is examined using an analytic three‐dimensional, steady model. Wells with uniform head and low pumping rates have gaining sections along which water enters the well and losing sections along which water exits. Such a well may provide a conduit for contaminated groundwater to be drawn into the well, conveyed a large distance, and injected into an uncontaminated region of an aquifer. Dimensionless ratios of the well's length L and radius R, aquifer thickness H, and uniform flow rate U are developed to quantify the minimum pumping rate Qmin at which no losing section occurs. The ratio Qmin/(ULH) is presented as a nomograph usingR/H and L/H for placement parallel to flow and is 6πR/H for placement perpendicular to flow, and a parabolic relationship between these limiting cases is developed for placement oblique to flow. Capture zone geometry is quantified using Qmin/(ULH).
A coupled inversion of pressure and surface displacement
Authors: Vasco D. W. , Karasaki Kenzi , Kishida Kiyoshi
Published: February 2004

A coupled inversion of transient pressure observations and surface displacement measurements provides an efficient technique for estimating subsurface permeability variations. The methodology has the advantage of utilizing surface observations, which are typically much less expensive than measurements requiring boreholes. Furthermore, unlike many other geophysical observables, the relationship between surface deformation and reservoir pore fluid volume changes is relatively well understood. Our treatment enables us to partition the estimation problem into a sequence of three linear subproblems. An application of the approach to a set of tilt and borehole pressure data from the Raymond field site in California illustrates its efficiency and utility. The observations are associated with a well test in which fluid is withdrawn from a shallow fracture zone. During the test, 13 tiltmeters recorded the movement of the ground surface. Simultaneously, nine transducers measured pressure changes in boreholes intersecting the fracture system. We are able to image a high permeability, north trending channel located within the fracture zone. The existence and orientation of this high‐permeability feature is substantiated by a semiquantitative analysis of some 4000 transient pressure curves.
Numerical analysis of flow and transport in variably saturated bimodal heterogeneous porous media
Authors: Russo David , Zaidel Jacob , Laufer Asher
Published: February 2004

Numerical simulations of flow and transport of a tracer solute were used to investigate solute spreading and breakthrough in three‐dimensional, heterogeneous, partially saturated porous media. Two different media were considered, the properties of which were modeled as a bimodal and as an “equivalent” (same mean and variance but not the same two‐point covariance), unimodal spatially correlated random functions. The bimodal medium consisted of a spatially distributed background soil and randomly dispersed low‐permeability clay lenses which occupied 10% of the media volume. Both time‐invariant and periodic influx (rain/irrigation) rates at the soil surface were considered in this investigation. Under steady state flow conditions originating from a time‐invariant influx rate at the soil surface, when the soil is relatively wet, the difference between the hydraulic responses of the bimodal medium and the unimodal medium increased with increasing influx rate. For a given influx regime at the soil surface the embedded clay lenses associated with the bimodal medium are shown to enhance both solute spreading and the skewing of solute breakthrough curves considerably. Transient flow, originating from a periodic influx at the soil surface, which, in turn, is characterized by substantial redistribution periods with diminishing water saturation, may considerably decrease the difference between the responses of the two media. The latter result stems from the fact that in the bimodal medium considered in this investigation, for the range of water saturations pertinent to the redistribution periods, the difference between the conductivities of the background soil and the embedded clay lenses diminishes substantially.
Long‐Term Snow Database, Reynolds Creek Experimental Watershed, Idaho, United States
Authors: Marks Danny , Cooley Keith R. , Robertson David C. , Winstral Adam
Published: February 2004

Snow is the dominant form of precipitation in the Reynolds Creek Experimental Watershed (RCEW). Water from snowmelt is critical to the ecosystems and resources in RCEW because the water stored in the seasonal snow cover is the primary source of spring and summer soil moisture and streamflow. Snow water equivalent (SWE) has been measured at eight locations in RCEW every 2 weeks throughout the snow season (December 1 to June 1) for 35 water years (1962–1996). SWE was continuously monitored at one reference site for 14 water years (1983–1996). The measurement sites are described, the methods used are presented and discussed, these data are summarized, and examples of how they have been used are presented.