bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of methane photolysis in the reducing atmospheres of the outer solar system
Authors: Smith Nicholas S. , Raulin François
Published: September 2002

It is important that methane photolysis is accurately treated in models of planetary atmospheres of the outer solar system and of the interstellar medium. Recent measurements of the H atom and H2 yields in the photolysis of methane have been considered and appear to be consistent with quantum yields, at Lyman α (121.6 nm), of (R1) CH3 + H = 0.41, (R2) 1CH2 + H2 = 0.53, (R3) 1CH2 + H + H = 0.0, (R4) 3CH2 + H + H = 0.0, and (R5) CH + H2 + H = 0.06. At shorter wavelengths it is likely that the production of CH will increase due to the decomposition of excited CH3 molecules, although further data are required to be able to accurately model the wavelength dependence of methane photolysis. At the low temperatures of Titan's atmosphere, for example, the possible temperature dependence of CH4 absorption cross sections may need to be considered. By reference to other hydrocarbons, the absorption of methane may increase by 30% or more at 200 K with respect to room temperature values. These dependences, not previously considered in photochemical models of Titan, could significantly change the predicted composition of Titan's atmosphere and the chemical pathways implied.
Atmospheric plumes created by meteoroids impacting the Earth
Authors: Shuvalov V. V.
Published: September 2002

The problem of plume‐forming meteoroid impacts into the Earth's atmosphere is investigated with the use of a simplified analytical model and detailed numerical simulations. The process of plume formation is found to depend strongly on the meteoroid size due to development of Kelvin‐Helmholtz instabilities at the entry column boundary. As a result, the height of plume rising proves to be considerably less than that obtained using simplified model or numerical simulations with low spatial resolution. It is shown that meteoroids several meters or less in size do not create ballistic plumes at all. The results of general consideration are applied to the Šumava bolide (one of the brightest detected by the European Fireball Network). The debris cloud created by this impact rises to an altitude of about 190 km.
An inversion algorithm for nonlinear retrieval problems extending Bayesian optimal estimation
Authors: Rieder Markus J. , Kirchengast Gottfried
Published: September 2002

This paper proposes effective extensions to the well‐known Bayesian optimal estimation, allowing one to cope not only with the ill‐posedness but also with the intrinsic nonlinearity of many geophysical inversion problems. We developed a physical‐statistical retrieval algorithm, which combines nonlinear optimal estimation with further optimization techniques. Profiling of water vapor based on (synthetic) downlooking microwave sounder data as an example for a typical geophysical nonlinear optimization problem is used to demonstrate the skills of the algorithm. Starting with a nonlinear scalar penalty function derived from a Bayesian approach, the sensible guess of a priori information, the selection of useful probability density functions, the advantages of simulated annealing, and the utility of Monte Carlo methods are discussed. These techniques together furnish capability for retrieving state vectors, which depend on the data in a (highly) nonlinear manner. The sensible combination as implemented in the introduced hybrid algorithm can provide solutions to problems that could not be tackled with standard (linearized) inversion methods properly.
Light scattering by a two‐dimensional, rough penetrable medium: A mean‐field theory
Authors: Calvo‐Perez O. , Sentenac A. , Greffet J.‐J.
Published: September 2002

The problem of scattering of electromagnetic waves by two‐dimensional, randomly rough surfaces is solved by means of a mean‐field theory. The results are compared with available numerical simulations and perturbative solutions. It is found that the new method is accurate for surfaces with a dielectric constant lower than 5 and roughness of up to half a wavelength. There are no limitations on the slopes. The explicit form of the result permits us to analyze in simple terms the polarization and angular scattering pattern of the diffuse intensity. It is shown that the angular scattering pattern depends not only on the correlation length but also on the rms height δ, when δ is greater than one tenth of a wavelength. Finally, the theory is used to analyze the scattering pattern produced by a rough film above a mirror.
Basement reactivation in a sub‐Andean foreland flexural bulge: The Pantanal wetland, SW Brazil
Authors: Ussami Naomi , Shiraiwa Shozo , Dominguez José Maria Landim
Published: September 2002

The role of sub‐Andean foreland peripheral bulges in controlling sedimentary sequence deposition and Quaternary geomorphology is discussed in the case of the Pantanal wetland, SW Brazil. This shallow (0.5 km) and wide (>200 km) depression was formed because of uplift and flexural extension in the brittle upper crust which mechanically reactivated the Neoproterozoic Paraguai fold/thrust belt. Depth to the basement and structural information within the Pantanal wetland were obtained from gravity, reflection seismics, drill hole, and digital topography data. The upper part of the basin is filled with Pliocene‐Quaternary unlithified sands. The age of the oldest sediments is unknown, and normal faults are mostly observed on the western border of the basin. A two‐dimensional flexural model composed of a semi‐infinite thin elastic plate for the Brazilian shield lithosphere with an effective elastic thickness of 125–150 km, its western edge at 67.5° W under the Altiplano and loaded on a free end by the Andes topography, predicts the forebulge to be presently located over the Pantanal wetland. Flexural modeling was constrained using gravity data extending from the Pacific Ocean to the east of the Pantanal wetland. The width of the bulge is 700 km, and its amplitude is ∼310 m. The total amount of extension and subsidence observed in the Pantanal basin is interpreted as due to a cumulative strain since late Oligocene when the Brazilian shield started its westward migration, together with compression and uplift of the Central Andes. The main phase of the wetland subsidence is very likely related to the last compressional pulse in the Andes, during the upper Pliocene‐lower Pleistocene (∼2.5 Ma), as inferred from field study of fault kinematics of the Central Andes.
Analytical solutions for solute transfer characteristics at continuous fracture junctions
Authors: Park Young‐Jin , Lee Kang‐Kun
Published: September 2002

Simple analytical solutions for the mixing characteristics and transfer probabilities of solute at the continuous fracture junction are proposed in this study. The proposed analytical solution considers the effects of diffusion, and it is simple enough to be applied to the junctions in discrete fracture network. The analytical solutions are compared with the complete‐mixing and the streamline‐routing models. As the Peclet number increases, the analytical solutions indicate the transition from complete solute mixing to streamline routing at a fracture junction. It is clear from the results that the transfer characteristics at the junctions are a function not only of one dimensionless parameter but of complex combinations of discharge pattern, geometry, and concentration distribution in the related fractures. Comparisons with the existing numerical solutions and possible underestimation of the solute mixing by the analytical solutions are discussed.
Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: A physical model
Authors: Or Dani , Wraith Jon M.
Published: September 2002

Near‐surface measurements of soil water content (θ) using time domain reflectometry (TDR) may exhibit anomalous behavior in the presence of diurnal temperature (T) fluctuations. Experimental results obtained in a companion paper led to the hypothesis that the observed bulk dielectric permittivity (ϵb) is determined by an interplay between two competing phenomena: (1) the reduction in the dielectric permittivity of bulk water with increased T; and (2) the increase in TDR‐measured ϵb with increased T due to release of bound water. In this study we develop a physically based model for the temperature dependency of TDR‐measured soil bulk dielectric permittivity and propose practical correction factors. The model considers the modified properties of water near solid surfaces to define a layer of rotationally hindered water (within the TDR frequency bandwidth) having a temperature dependent thickness. Changes in measured ϵb(T) are thus attributed to variations in the thickness of the rotationally hindered layer which has a lower dielectric permittivity than free water and hence is less “visible” to travel‐time‐based TDR waveform analyses. The model is sensitive to the soil specific surface area and the water content, both of which determine the ratio of bound to bulk soil water. Comparisons with experimental data covering a wide range of soils, water contents, and temperatures showed good agreement. Further studies are needed to evaluate some of the model's critical parameters such as the cutoff frequency below which water is considered bound. A temperature correction approximation is based on analytical expressions for TDR‐measured bulk dielectric permitivity and requires estimates of soil specific surface area and bulk density, which may be estimated from soil texture. The thermodielectric sensitivity of TDR‐measured bulk dielectric permittivity and water content may serve as a basis for estimating soil specific surface area.
Multidimensional linearized nonsteady infiltration with prescribed boundary conditions at the soil surface
Authors: Basha H. A.
Published: September 2002

The present work explores a class of analytical solutions of moisture movement in unsaturated porous media characterized by an exponential dependence of the hydraulic conductivity and the moisture content on water pressure. The Green's function method is used to derive a general analytical model pertaining to multidimensional nonsteady infiltration in a semi‐infinite flow domain with arbitrary initial conditions, boundary conditions, and root‐uptake forcing functions and for various simple source geometry. The general solution is expressed in integral form from which particular analytical solutions pertaining to cases of surface and subsurface irrigation, evaporation, root uptake, and moisture redistribution can be easily deduced from the general analytical model. The model offers the analyst significant flexibility in deriving results and analyzing infiltration phenomena of practical interest. New explicit solutions have been obtained for one‐dimensional infiltration under various prescribed time‐dependent flux boundary conditions and for two‐ and three‐dimensional moisture redistribution. For constant initial or boundary conditions, the multidimensional solution is essentially the product of two or three time‐dependent terms with each term being a function of only one space variable.
Aqueous flow and transport in analog systems of fractures embedded in permeable matrix
Authors: Sonnenborg Torben Obel , Butts Michael B. , Jensen Karsten Høgh
Published: September 2002

Two‐dimensional laboratory investigations of flow and transport in a fractured permeable medium are presented. Matrix blocks of a manufactured consolidated permeable medium were arranged together to create fractures in the spaces between the blocks. Experiments examined flow and transport in four different configurations: (1) matrix only, (2) and (3) matrix blocks containing single fractures of different mean apertures, and (4) a brickwork pattern setup simulating a tortuous multiple fracture network. The observed partitioning of flow and solute concentrations suggested mass exchange between the fractures and the matrix was occurring. An analysis of the experimental results using a discrete fracture model and a range of constant aperture models showed that this approach did not capture the correct flow mechanisms. Subsequent simulations including spatial variations of the fracture aperture were better able to describe the experimental observations and to show that these variations cause mass exchange between the fracture and matrix.
Scales and dimensions of momentum dissipation during preferential flow in soils
Authors: Germann Peter F. , Di Pietro Liliana
Published: September 2002

Momentum dissipation may dominate flow in soils over a considerable distance when input rate and antecedent soil moisture are high enough and when adequate soil structures are present. The concept is derived from momentum balance. It is applied to drainage flow from a column of undisturbed soil and a weighing lysimeter and to water content variations at five depths due to sprinkling. Momentum of input is much lower than momentum during flow in the soil; however, the former is considered important in triggering momentum dissipation within the profile. Drainage flow at a depth of 2.2 m shows flow completely dominated by momentum dissipation, whereas momentum of flow within the soil profile increases with depth, indicating acceleration over a vertical distance from 0.15 to 0.55 m. The Reynolds numbers show laminar flow in all cases.
Limiting pumping from the Edwards Aquifer: An economic investigation of proposals, water markets, and spring flow guarantees
Authors: McCarl Bruce A. , Dillon Carl R. , Keplinger Keith O. , Williams R. Lynn
Published: September 2002

The Edwards Aquifer, near San Antonio, Texas, is an important water source for both pumping and spring flow, which in turn provides water for recreation and habitat for several endangered species. A management authority is charged with aquifer management and is mandated to reduce pumping, facilitate water markets, protect agricultural rights, and protect the species habitat. This paper examines the economic dimensions of authority duties. A combined hydrologic‐economic model is used in the investigation. The results indicate that proposed pumping limits are shown to have large consequences for agricultural usage and to decrease the welfare of current aquifer pumping users. However, the spring flow habitat is found to be protected, and the gains from that protection would have to exceed pumping user losses in order for the protection measures to increase regional economic welfare. Agricultural guarantees are shown to cause use value differences, indicating the opportunity for emergence of an active water market. Fixed quantity pumping limits are found to be an expensive way of insuring adequate spring flow.
On the origin and timing of rapid changes in atmospheric methane during the Last Glacial Period
Authors: Brook Edward J. , Harder Susan , Severinghaus Jeff , Steig Eric J. , Sucher Cara M.
Published: September 2002

We present high resolution records of atmospheric methane from the GISP2 (Greenland Ice Sheet Project 2) ice core for four rapid climate transitions that occurred during the past 50 ka: the end of the Younger Dryas at 11.8 ka, the beginning of the Bølling‐Allerød period at 14.8 ka, the beginning of interstadial 8 at 38.2 ka, and the beginning of interstadial 12 at 45.5 ka. During these events, atmospheric methane concentrations increased by 200–300 ppb over time periods of 100–300 years, significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record. We suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change. We find no evidence for rapid, massive methane emissions that might be associated with large‐scale decomposition of methane hydrates in sediments. With additional results from the Taylor Dome Ice Core (Antarctica) we also reconstruct changes in the interpolar methane gradient (an indicator of the geographical distribution of methane sources) associated with some of the rapid changes in atmospheric methane. The results indicate that the rise in methane at the beginning of the Bølling‐Allerød period and the later rise at the end of the Younger Dryas were driven by increases in both tropical and boreal methane sources. During the Younger Dryas (a 1.3 ka cold period during the last deglaciation) the relative contribution from boreal sources was reduced relative to the early and middle Holocene periods.
Major challenges confronting marine biogeochemical modeling
Authors: Doney Scott C.
Published: September 2002

Substantial improvements are required in the current suite of numerical models if we are to better understand the present ocean biogeochemical state and predict potential future responses to anthropogenic perturbations. At present, major impediments to marine biogeochemical modeling include the inadequate representation of multi‐element cycling and community structure, large‐scale physical circulation, mesoscale space and time variability, and mass exchange between the open ocean and the atmosphere, land, and coastal ocean. Marine biogeochemical modeling is inherently data driven, and significant progress on any of these topics will require close collaboration between the observational and modeling communities. Two main thrusts should be to develop improved, mechanistically based parameterizations of specific biogeochemical processes and to test the overall skill of integrated system models through detailed model‐data comparison of both the mean state and seasonal to interdecadal variability.
Development of the CO2 latitude gradient in recent decades
Authors: Conway Thomas J. , Tans Pieter P.
Published: September 2002

Because 90% of the CO2 from fossil fuel combustion is emitted in the Northern Hemisphere, annual mean atmospheric CO2 mixing ratios are higher at middle and high northern latitudes than in the Southern Hemisphere. The observed CO2 latitude gradient varies interannually and has generally increased as fossil fuel CO2 emissions have increased. Back extrapolation of the measured CO2 latitude gradient to zero fossil fuel emissions gives a latitude gradient with the Northern Hemisphere lower than the Southern. A linear regression of Mauna Loa minus South Pole annual mean differences versus fossil fuel emissions for 1958 through 1996 gives a slope of 0.5 μmol mol−1 (abbreviated as ppm CO2) (Gt C)−1 (σ = 0.03) and an intercept (at zero fossil fuel emissions) of −0.8 ppm (σ = 0.2). Shorter data records yield similar results with larger uncertainties. We argue that this extrapolated gradient does not represent preindustrial conditions but is more correctly viewed as a decadal average gradient due to natural sources and sinks that underlie the anthropogenic perturbation. We interpret the extrapolated gradient as evidence for a contemporary Northern Hemisphere sink that has been proposed on the basis of other measurement and model approaches. The slopes (ppm CO2 per gigaton of C from fossil fuel burning) calculated from sufficiently long records tend to agree with model calculations based on fossil fuel emissions, suggesting that any trend in the Northern Hemisphere sink, during the period of the measurements, has been small relative to the trend in fossil fuel emissions.
Optimal sampling of the atmosphere for purpose of inverse modeling: A model study
Authors: Gloor Manuel , Fan Song‐Miao , Pacala Stephen , Sarmiento Jorge
Published: September 2002

The 66 stations of the GLOBALVIEW‐CO2 sampling network (GLOBALVIEW‐CO2: Cooperative Atmospheric Data Integration Project ‐ Carbon Dioxide, (1997)) are located primarily remotely from continents where signals of fossil fuel consumption and biospheric exchange are diluted. It is thus not surprising that inversion studies are able to estimate terrestrial sources and sinks only to a very limited extent. The poor constraint on terrestrial fluxes propagates to the oceans and strongly limits estimates of oceanic fluxes as well, at least if no use is made of other information such as isotopic ratios. We analyze here the resolving power of the GLOBALVIEW‐CO2 network, compare the efficiency of different measurement strategies, and determine optimal extensions to the present network. We find the following: (1) GLOBALVIEW‐CO2 is well suited to characterize the meridional distribution of sources and sinks but is poorly suited to separate terrestrial from oceanic sinks at the same latitude. The most poorly constrained regions are South America, Africa, and southern hemispheric oceans. (2) To improve the network, observing stations need to be positioned on the continents near to the largest biospheric signals despite the large diurnal and seasonal fluctuations associated with biological activity and the dynamics of the PBL. The mixing in the atmosphere is too strong to allow positioning of stations remote from large fluxes. Our optimization results prove to be fairly insensitive to the details of model transport and the inversion model with the addition of ∼ 10 optimally positioned stations. (3) The best measurement strategy among surface observations, N‐S airplane transects, and vertical profiles proves to be vertical profiles. (4) Approximately 12 optimally positioned vertical profiles or 30 surface stations in addition to GLOBALVIEW‐CO2 would reduce estimate uncertainties caused by insufficient data coverage from ∼ 1 Pg C yr−1 per region to ∼ 0.2 Pg C yr−1 per region.
Long‐term forest floor carbon dynamics after fire in upland boreal forests of western Canada
Authors: Nalder Ian A. , Wein Ross W.
Published: September 2002

We examined the long‐term dynamics of upland boreal forest floors after disturbance by fire. We selected two important and contrasting upland tree species, Pinus banksiana (jack pine) and Populus tremuloides (trembling aspen), in three distinct climatic zones across the boreal forest of western Canada, and sampled 80 fire‐originated stands divided into six chronosequences with ages ranging from 14 to 149 years. The forest floor was a large component of carbon storage. Averaged across ages and zones, it was 1.31 and 2.78 kg C m−2 for P. banksiana and P. tremuloides, respectively, compared with 4.03 and 5.56 kg C m−2 in aboveground trees. These data exclude decomposing coarse woody debris, which was a significant component of the forest floor (0.18/0.13 kg C m−2 ) and requires further study. The contributions from shrubs (0.035/0.151 kg C m−2), ground vegetation (0.019/0.026 kg C m−2), and moss‐plus‐lichen (0.179/0.004 kg C m−2) were relatively small. An analysis of covariance (ANCOVA) model showed that forest floor carbon was positively related to stand age, as well as being affected by species and climatic zone. Much of the variability was explained by species, and species‐specific regression models showed that for P. tremuloides forest floor carbon was strongly related to stand age, mean annual temperature, and mean annual precipitation, and for P. banksiana, forest floor carbon was strongly related to an index of moss dominance. The regression models suggest that the forest floor carbon pool in upland forests of the western Canadian boreal will be sensitive to climate change, but this sensitivity would need to be tested with process‐based models.
Methane stable isotope distribution at a Carex dominated fen in north central Alberta
Authors: Popp Trevor J. , Chanton Jeffrey P. , Whiting Gary J. , Grant Nick
Published: September 2002

The methane stable isotope distribution was characterized at a Carex dominated fen in boreal Alberta, Canada, over three growing seasons to examine methane production, oxidation, and transport to the atmosphere; processes which are strongly tied to emergent vegetation and the influence of the rhizosphere (upper 20 cm of peat in this system]. At times when standing floodwater was present, δ13C values of emitted methane averaged −63.6 ± 2.3, −66.3 ± 1.6, and −65.4 ± 1.3‰ for the 1994, 1995, and 1996 seasons, respectively. These emissions were significantly 13C depleted relative to the belowground methane dissolved in rhizospheric pore waters, indicating that gas transport in Carex is dominated by passive diffusion. The rhizosphere was 13CH4 enriched relative to depths below the rhizosphere, consistent with the occurrence of root associated methane oxidation, preferential mobilization of 13CH4, and a relatively greater role of acetate fermentation type methane production. Dual isotope tracers, δ13C and δD, help qualify the role of each of these processes and aid in describing the distribution of production pathways, CO2 reduction, and acetate fermentation. Inverse trends in δ13C‐CH4 and δD‐CH4 depth profiles are consistent with an interpretation suggesting an evolution toward methane production by CO2 reduction with increasing depth. A shift in production mechanisms appears to be the dominate process affecting the stable isotope distribution below 10 cm in the peat column, while oxidation and transport isotope effects are dominant above 10 cm. To test several hypotheses regarding the effects of transport, oxidation, and production on methane isotope distributions, we also present measurements from sites fertilized and sites devegetated (continually clipped) over the 3 year period. Removal of vegetation quickly halted rhizospheric methane oxidation and gas transport while gradually increasing the relative role of CO2 reduction in net methane production as labile substrate was used up. The fertilizer treatment increased above ground biomass and primary productivity but had little effect on the stable isotope distribution. A mass balance calculation indicates that methane emissions are attenuated 0–34% by methane oxidation in the rhizosphere. Results showed little seasonal variability other than during a period when floodwater levels dropped below the peat surface resulting in the13CH4 enrichment of methane emissions.
Modeling 13C discrimination in tree rings
Authors: Berninger Frank , Sonninen Eloni , Aalto Tuula , Lloyd Jon
Published: September 2002

Annual variations from 1877 to 1995 in tree‐ring α‐cellulose 13C/12C isotopic ratios for four subarctic Pinus sylvestris trees were determined, and, in conjunction with a recent record of atmospheric 13CO2/12CO2 ratios, the historical pattern of photosynthetic isotope discrimination, Δ13C, was evaluated. Year‐to‐year variability in Δ13C has been as much as 1.5‰ with the period 1900–1920 showing an extended period of unusually high photosynthetic discriminations. The summers during these years were, on average, unusually cold. Since 1920 a long term trend of increasing Δ13C of ∼0.016‰yr−1 is inferred. We compared measured Δ13C with those predicted on the basis of the theoretical relationship between Δ13C and the ratio of substomatal to ambient CO2 concentration, Ci/Ca using mechanistic equations for chloroplast biochemistry coupled with a stomatal conductance model. Two variations of a nonlinear optimal‐regulation stomatal conductance model were compared. Although both models were based on the assumption that stomata serve to minimize the average transpiration rate for a given average rate of CO2 assimilation, one version of the model incorporated reductions in stomatal conductance in response to recent increases in atmospheric CO2 concentrations and the other did not. The CO2 sensitive stomatal model failed to describe the long‐term increase in 13C discrimination, especially after 1950. The insensitive model gave good agreement, suggesting that an observed increase in subarctic Pinus sylvestris Δ13C since 1920 is attributable to recent increases in atmospheric CO2 concentrations with subsequent increases in the ratio of substomatal to ambient CO2 concentrations. The model was also capable of accounting for high frequency (year‐to‐year) variations in Δ13C, these differences being attributable to year‐to‐year fluctuations in the average leaf‐to‐air vapor pressure difference affecting stomatal conductance and hence Ci/Ca.
The D/H content of methane emitted from biomass burning
Authors: Snover Amy K. , Quay Paul D. , Hao Wei Min
Published: September 2002

The D/H content of methane emitted from biomass burning was measured using samples of smoke collected from large‐scale laboratory combustion experiments and from fires typical of slash burning of primary forest and of pasture burning in the Brazilian Amazon. In laboratory experiments, the mean δD of methane emitted fromPinus ponderosaneedle combustion (δDCH4(bb)) was −233 ± 2‰. In Brazilian fires, δDCH4(bb)was −210 ± 16‰. The measured D/H of the fuel biomass ranged from −59 to −125‰ and indicated a significant hydrogen isotope fractionation (ϵDBURN) during combustion. For the laboratory experiments, ϵDBURN(=(D/H)CH4/(D/H)Biomass‐l)1000 = −124‰ and in the field studies, ϵDBURNranged from −130 to −180‰. These results contrast those for δ13C which showed no fractionation during combustion, consistent with earlier studies. The measurements presented here suggest that a relatively narrow range exists for the δD of CH4emitted from biomass burning on a global scale and that δD is an independent tracer for atmospheric CH4. Combining δDCH4(bb)≈‐210‰ with literature values for the strength and δD of the major CH4sources, we estimate a δD for the global CH4source of ‐283 ± 13‰. This result provides a constraint for the D/H kinetic isotope effect associated with removal of atmospheric CH4by atmospheric hydroxyl radicals.
Short‐term variability in the open ocean cycle of dimethylsulfide
Authors: Simó Rafel , Pedrós‐Alió Carlos
Published: September 2002

The marine biogeochemical cycle of dimethylsulfide (DMS), the main natural source of sulfur to the global atmosphere, was studied during a 2‐week Lagrangian experiment in the subpolar North Atlantic, at 60°N 21°W. A bloom of coccolithopores, mostly of the species Emiliania huxleyi, dominated the phytoplankton assemblage over the first week. High surface concentrations of dimethylsulfoniopropionate (DMSP, 37–70 nM) were found along with moderate DMS concentrations (3–9 nM) during the entire experiment. Rates of biological DMSP consumption (8–51 nM d−1) and DMS production (1–14 nM d−1) and consumption (0–6 nM d−1) were measured in short‐term dark incubations of surface seawater. Rates of DMSP biosynthesis (11–31 nM d−1) and DMS photochemical loss (1–10 nM d−1) were estimated by budgeting concentrations and transformation rates between Lagrangian samplings. Air‐sea exchange rates for DMS (0.03–3 nM d−1) were calculated from surface concentrations, seawater temperature, and wind speed. All major processes involved in the DMS cycle showed significant short‐term variability in coupling to the variability of solar radiation, wind speed, and mixing. Biotic and abiotic DMS turnover rates were of similar magnitude and very dynamic, with a prompt response to a rapidly changing physical environment. The rapid impact of meteorological forcing factors on DMS cycling provides the basis for a sulfur‐mediated, short‐term plankton/climate interaction.
Long‐term seismicity and ground deformation at Axial Volcano, Juan de Fuca Ridge
Authors: Dziak Robert P. , Fox Christopher G.
Published: September 2002

Seismicity along the Juan de Fuca Ridge (JdFR) has been monitored since August 1991 using U.S. Navy hydrophones in the northeast Pacific Ocean. During this time, the focus of JdFR seismicity has been Axial Volcano, where a total of 9055 earthquakes have been detected (1669 located). This activity occurred within 48 discrete swarms along the summit caldera and flank rift zones. From August 1991 to May 1996, Axial Volcano exhibited only one swarm with ≥50 earthquakes. From May 1996 to November 1997 swarm activity increased significantly with five swarms of ≥50 earthquakes. Activity culminated at the volcano in January 1998 with an 11‐day, 8247‐event swarm that was the result of a major eruption along the summit and southern flank. Seafloor deformation sensors indicated pre‐eruption swarms in October 1995, May 1996, and June 1996 coincided with subsidence of the caldera floor. The 1996–1997 swarm activity increase also coincided with horizontal extension of the north rift zone, consistent with pre‐eruption deformation of the volcano. Following the 1998 eruption, swarm activity dramatically decreased and no earthquakes have been detected since April 1998. This is the longest aseismic period (∼1 yr) observed at the volcano since acoustic monitoring began, suggesting the January 1998 swarm marked the end of an eruptive cycle at Axial Volcano.
Tethered two‐point measurements of solitary auroral density cavities
Authors: Knudsen D. J. , Wallis D. D. , James H. G.
Published: September 2002

The OEDIPUS‐C sounding rocket observed localized plasma density depletions in the topside auroral ionosphere using two Langmuir probes situated on separate subpayloads several hundred meters apart. One probe was biased in electron saturation (+5 V) and the second in ion saturation (−5 V). Both of these measurements are consistent with density depletions of tens of percent. The depletions have cross‐field dimensions of tens of meters and occur in regions of VLF hiss, suggesting they are likely signatures of lower hybrid solitary structures. Time delays between event detections on the two subpayloads are consistent with spatially localized structures traversed in succession. The two‐point measurements show directly that the cavities extend at least 800 m along the geomagnetic field line, and the apparent electric potential within them is depressed by roughly 0.5 V.
Spatial and temporal dynamics of colored dissolved organic matter in the north water polynya
Authors: Scully Norman M. , Miller William L.
Published: September 2002

The spatial and temporal dynamics of colored dissolved organic matter (CDOM) were examined during the North Water polynya study from April to July 1998. Few data have been collected pertaining to the dynamics of CDOM in Arctic marine ecosystems and none during the progression of an Arctic polynya. Unlike in other marine ecosystems, ultraviolet absorption by CDOM was highest near the surface of the water column and increased in concert with the development of the polynya. Analysis of ice core samples revealed that sea‐ice may be a significant source of CDOM and indicated that Arctic marine ecosystems have different controls of CDOM production and loss than do low latitude, open ocean systems. We hypothesize that the frequent surface CDOM maxima in Arctic ecosystems is the result of sea‐ice providing a substrate for enhanced autochthonous formation of CDOM.
The effect of the Hawaiian plume on the magnetic daily variation
Authors: Simpson Fiona , Steveling Erich , Leven Martin
Published: September 2002

Can electromagnetic techniques be employed to detect the presence of a plume below an ocean approximately 5 km deep? Recent magnetovariational data from the islands of Oahu and Hawaii provide compelling evidence that this question can be answered in the affirmative. A significant diminution of the vertical magnetic field is observed at Hawaii as compared with Oahu, which cannot arise from island/ocean bathymetry effects. The plume paradox is that the effect may not be explained by induction. This could imply distortion and current channelling due to mantle heterogeneities on a scale hitherto unconceded in global induction studies.
Effects of cementation on velocities of siliciclastic sediments
Authors: Jarrard Richard D. , Niessen Frank , Brink Jason D. , Bücker Christian
Published: September 2002

Compressional‐wave velocity in siliciclastic rocks is known to be a function mainly of porosity, and secondarily of mineralogy. We investigate the effect of cementation on velocity, using core‐plug and whole‐core measurements of velocity and porosity. The case study area is a drillhole at Cape Roberts, Antarctica, an environment with a rapid downhole gradient in cementation. After removal of porosity effect from velocity, the change from incipient to intense carbonate cementation is seen to account for up to 50% of total velocity. Increased intergrain coupling, particularly increased frame bulk modulus, is the cause of this velocity increase.
Formation of metastable assemblages and mechanisms of the grain‐size reduction in the Postspinel transformation of Mg2SiO4
Authors: Kubo Tomoaki , Ohtani Eiji , Kato Takumi , Urakawa Satoru , Suzuki Akio , Kanbe Yuichi , Funakoshi Ken‐ichi , Utsumi Wataru , Fujino Kiyoshi
Published: September 2002

An in situ X‐ray observation of the postspinel transformation kinetics was made using intense synchrotron radiation. We confirmed that Mg2SiO4 spinel transforms into fine lamellae of SiO2 stishovite and periclase, and/or MgSiO3 ilmenite and periclase as an intermediate step in the postspinel transformation. These metastable assemblages eventually disappear and form the stable assemblages of MgSiO3 perovskite and periclase. Initial grain size just after the postspinel transformation drastically changes with overpressure. Viscosity of the subducting slab into the lower mantle, which is thought to be deformed by grain‐size‐sensitive creep, would depend on overpressure needed for the postspinel transformation at geological time scale.
Mixing and phytoplankton bloom in the wake of the Marquesas Islands
Authors: Signorini Sergio R. , McClain Charles R. , Dandonneau Yves
Published: September 2002

A persistent phytoplankton bloom was observed during August–December 1998 around the Marquesas Islands in the South Pacific using a spaceborne ocean color sensor. The enhancement of the phytoplankton production is attributed to the island mass effect. The effect consists of a combination of turbulent mixing and advection from the south equatorial current flowing through and around the islands, and iron‐enriched waters originating from land drainage and hydrothermal fluxes through old volcanic formations. The enhanced phytoplankton production effects are noticeable a large distance downstream from the island (500 to 1000 km). This island mass effect is an important contributor to the productivity of the region and therefore potentially important for fisheries in the area.
The impact of the SSM/I antenna gain function on land surface parameter retrieval
Authors: Drusch Matthias , Wood Eric F. , Lindau Ralf
Published: September 2002

In order to investigate the effect of the antenna gain on land surface parameter retrievals with SSM/I, a Gauss‐function is fitted to the 19 GHz antenna gain functions. Soil moisture and surface temperature distributions for the Red‐Arkansas River basins are calculated from a soil‐vegetation‐atmosphere model. A passive microwave emission model is applied to derive polarized 19 GHz brightness temperatures. Those parameters are aggregated up to 19 GHz footprint size taking into account different spatial weighting according to the Gauss‐function. These reference averages represent the full radiometer resolution, which is approximately three times the −3dB resolution. The comparisons between the reference values and simple linear averages for several spatial resolutions lead to differences varying from 2% to 17% volumetric soil moisture.
Diurnal cycle of rainfall and surface salinity in the Western Pacific Warm Pool
Authors: Cronin Meghan F. , McPhaden Michael J.
Published: September 2002

The diurnal cycles of rainfall and surface salinity in the western equatorial Pacific were computed for the period August 1991–December 1994 using hourly data from the Tropical Atmosphere Ocean (TAO) mooring array enhanced for the Coupled Ocean Atmosphere Response Experiment (COARE). The analysis shows preferential rainfall during predawn hours. Nighttime mixing typically caused the predawn rainfall to mix downward into higher salinity subsurface waters. In contrast, afternoon rainfall, although weaker, generally produced a stable shallow layer of very low salinity. Consequently, sea surface salinity (SSS) exhibited a weak diurnal cycle, with anomalously low salinity in the late afternoon and anomalously high salinity at night. At 1 m depth, the SSS diurnal cycle anomalies were ∼0.005 psu. Although nearly two orders of magnitude less than the SSS standard deviation for this region, the anomalies were significant at the 95% confidence level.
On the role of charged dust in mesospheric electric fields
Authors: Zadorozhny Alexander M.
Published: September 2002

A strong interaction of charged dust with electrons and ions similar to that leading to formation of polar mesosphere summer echoes (PMSE) assumes to take also place in the lower mesosphere. This assumption predicts extremely low effective conductivity of the region where both positive and negative charges carried by the dust exceed the charges carried by ions and electrons. The low effective conductivity of the mesospheric dusty plasma can explain the existence of V/m vertical electric fields observed in the lower mesosphere and in the vicinity of noctilucent clouds and PMSE. Gravitational sedimentation of charged dust as well as currents of the global atmospheric electric circuit and precipitating high‐energy electrons and protons can produce the V/m fields under these conditions. The proposed theory allows understanding the observed behavior of the mesospheric vertical electric fields: their large variability, universal diurnal variation, increasing during solar proton events and geomagnetic disturbances.
Discovery of chlorine in the Io torus
Authors: Küppers Michael , Schneider Nicholas M.
Published: September 2002

We report the discovery of singly ionized chlorine in the Io plasma torus through the detection of emission at 857.9 nm. The 40 Rayleigh peak intensity at 857.9 nm corresponds to a Cl+ abundance of 2±0.5% relative to all ions. Since Cl+ probably originates at Io, Io's atmosphere must have the highest proportion of chlorine of any solar system body. While Cl+ plays only a minor role in torus physics, the presence of chlorine in Io's atmosphere, surface, and interior has more profound implications. Salt as a solid or gas is probably an important source of both Na and Cl in Io's vicinity.
Sensitivity of retrieved POLDER directional cloud optical thickness to various ice particle models
Authors: Doutriaux‐Boucher Marie , Buriez Jean‐Claude , Brogniez Gérard , C.‐Labonnote Laurent , Baran Anthony J.
Published: September 2002

The ”directional” values of cloud optical thickness (or cloud spherical albedo) retrieved from ADEOS‐POLDER data constitute a strong constraint on the microphysical model used in the retrieval algorithm. In this paper, we focus on ice clouds. We quantify the departure of the directional values of spherical albedo from their averaged value. By so doing, we can assess the suitability of different ice particle scattering models (i.e., spheres, fractal polycrystal, inhomogeneous hexagonal monocrystal, or a synthesized phase function). The liquid water droplet and the ice fractal polycrystal models appear to be inappropriate since they induce a notable dependence of the ice cloud spherical albedo on scattering angle. On the contrary, much better agreement is achieved by using a synthesized phase function or an inhomegeneous hexagonal monocrystal model. Using these models, the retrieved ice cloud optical thickness is then found to be 40% smaller than using the droplet model.
An alternative mechanism explaining the hygropause formation in tropical regions
Authors: Teitelbaum H. , Moustaoui M. , Basdevant C. , Holton J. R.
Published: September 2002

Radiosonde data obtained during CEPEX and TOGA‐COARE experiments show that a very simple mechanism can explain the existence, in tropical regions, of a local hygropause with a very low water vapor mixing ratio, less than 2 ppmv. This mechanism explains also why this hygropause is generally located well above the average tropopause in the lower stratosphere, and why it can temporarily coincide with the local tropopause. This interpretation is based on the existence, during periods of intense convective phenomena, of an uplift of the tropopause inducing a cooling of its temperature. We indeed find in these circumstances, when temperature and water vapor measurements are simultaneously available, very low water vapor values at the same level as the temperature minimum. After the end of the intense convection episode, the minimum temperature level is reestablished at a lower altitude while the hygropause remains positioned in the lower stratosphere but with a low value for the water vapor mixing ratio.
effects of driving mechanisms in geodynamo models
Authors: Kutzner Carsten , Christensen Ulrich
Published: September 2002

We compare the influence of different mechanisms for driving convection in the Earth's core on the structure of the magnetic and velocity fields using a 3D numerical dynamo model. We find dynamos with a dipolar magnetic field in cases of chemical convection or convection driven by an imposed temperature contrast. With purely internal heating we obtain only dynamos with a quadrupolar or more complex field. The relative strength of convection and magnetic field generation in the regions close to the poles depends on whether a condition of fixed composition or of fixed chemical flux is specified on the inner core boundary. If applicable to the geodynamo, our results favor the dominance of chemical convection during the past 3 Gyr.
On the teleconnectivity of the “Arctic Oscillation”
Authors: Deser Clara
Published: September 2002

The term “Arctic Oscillation” (AO) has recently been introduced to describe the leading structure of SLP variability over the Northern Hemisphere. A key feature of the AO is its zonally symmetric appearance, with a primary center of action over the Arctic and opposing anomalies in midlatitudes. Does the AO's annular appearance result from significant temporal correlations between SLP anomalies at distant longitudes? The results presented indicate that the temporal coherence between the Arctic and midlatitudes is strongest over the Atlantic sector, with weak correlations between the Atlantic and Pacific midlatitudes, both on intraseasonal and interannual time scales during the past 50 yrs. Hence, the “annular” character of the AO is more a reflection of the dominance of its Arctic center of action than any coordinated behavior of the Atlantic and Pacific centers of action in the SLP field. The AO is nearly indistinguishable from the leading structure of variability in the Atlantic sector (e.g., the North Atlantic Oscillation): their temporal correlation is 0.95 for monthly data.
Atlantic hurricanes and NW Pacific typhoons: ENSO spatial impacts on occurrence and landfall
Authors: Saunders M. A. , Chandler R. E. , Merchant C. J. , Roberts F. P.
Published: September 2002

Hurricanes are the United States' costliest natural disaster. Typhoons rank as the most expensive and deadly natural catastrophe affecting much of southeast Asia. A significant contributor to the year‐to‐year variability in intense tropical cyclone numbers in the north Atlantic and northwest Pacific is ENSO — the strongest interannual climate signal on the planet. We establish for the first time: (1) the spatial (0.5 degree grid) impacts of ENSO on the basin‐wide occurrence and landfall strike incidence of hurricanes and typhoons; (2) the spatial (7.5 degree grid or US state level) statistical significance behind the different incidence rates in warm and cold ENSO episodes; and (3) the effect of strengthening ENSO on regional strike rates and significances (hurricanes only). Our data comprise 98 years (1900–97) for the Atlantic and 33 years (1965–97) for the NW Pacific. At the US state level, we find several regions where the difference in landfalling incidence rate between warm and cold ENSO regimes is significant at the 90% level or higher. Our findings offer promise of useful long‐range predictability to seasonal forecasts of landfalling tropical cyclones.