bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical remanent magnetization during γFeOOH phase transformations
Authors: Özdemir Özden , Dunlop David J.
Published: February 2008

We have studied chemical remanent magnetization (CRM) accompanying the phase transformations lepidocrocite (γFeOOH) → maghemite (γFe2O3) → hematite (αFe2O3) in a field of 50 μT at a series of 13 temperatures from 200 to 650°C. CRM intensity peaked after the 250°C run as paramagnetic lepidocrocite transformed to ferrimagnetic maghemite of single‐domain size. After the 275, 300 and 350°C runs, CRM intensity decreased about an order of magnitude, probably because of the formation of antiphase domains with oppositely coupled magnetizations in the maghemite. A second CRM peak after the 400°C run is thought to reflect the growth of hematite on antiphase boundaries, breaking the negative coupling and allowing maghemite antiphase domains to align individually with the field. After runs at 450°C and above, CRM intensities were very low and the CRM consisted of two vectors, sometimes reversed in direction to each other and always at a large angle to the field. The explanation seems to lie in perpendicular coupling of the magnetic sublattices of maghemite and hematite.
Triggering of volcanic pulses in the Campanian Area, south Italy, by periodic deep magma influx
Authors: Paterne Martine , Guichard Francois
Published: February 2008

Geochemical fluctuations through time of the eruptions originating from Ischia island and from the Phlegrean Fields in the Campanian area have been analyzed in order to determine the relationships between the mechanisms responsible for compositional variations of the magmas and the fluctuations in eruptions. Geochemical fluctuations are derived from the detailed chronological and geochemical record of marine tephra in deep‐sea sediment cores from the central Mediterranean Sea. Such fluctuations are marked by the repetition of similar compositional gradients during each eruptive cycle, as defined by a period of intense activity followed by a quiet eruptive period. Geochemical changes through time in both areas may be explained by magma evolution in a continuously crystallizing, tapped, and periodically refilled stratified magma chamber. Each eruptive and geochemical cycle starts by an influx of magma from depth into the shallow reservoirs, which leads to an increase in the frequency of the eruptions and maintains the activity in a particular volcanic area. This inflow may be controlled by changes of glacio‐eustatic pressure, as suggested by its periodic occurrence every 24 kyr.
Interactions between mantle‐derived magmas and mafic crust, Henry Mountains, Utah
Authors: Nelson Stephen T. , Davidson Jon P.
Published: February 2008

The Henry Mountains represent one of three major igneous centers of the Colorado Plateau interior during mid‐Tertiary (25–30 Ma) time. These intrusions occur ∼1000 km from the paleotrench and are unrelated to subduction in a classical sense, despite many compositional similarities to orogenic magmatic rocks. Most of the intrusive volume (95%) consists of plagioclase‐hornblende porphyry of intermediate composition, with minor syenite making up the remainder, and both suites appear to have cooled rapidly as evidenced by the fine‐grained texture of their groundmass. Although many elemental variations can be explained by fractional crystallization, Sr, Nd, Pb, and O isotope systematics require open system interaction of mantle derived magmas and Proterozoic amphibolite crust. The plagioclase‐hornblende porphyry evolved via assimilation fractional crystallization (AFC) in deep‐crustal magma chambers. Syenite porphyry evolved in two distinct stages. First, AFC produced a shallow, zoned, subintrusive magma chamber containing Ne‐normative syenite magma. Second, as magma was withdrawn, batches of Ne‐normative magma mixed with tonalitic melts produced by fusion of amphibolite country rock to produce Q‐normative syenite porphyry. Trace element abundance patterns suggest that both plagioclase‐homblende and syenite porphyry were derived from the same mantle source. A higher degree of partial melting may explain the silica‐oversaturated character and lower trace element abundances in the plagioclase‐hornblende porphyry relative to the syenite porphyry. Trace‐element systematics also indicate that high large ion lithophile (LILE) to high field strength element (HFSE) ratios, typical of subduction‐derived magmas, are characteristic of the source and have not been imposed solely by crustal contamination. Therefore, the porphyries of the Henry Mountains appear to be related to contemporaneous voluminous regional magmatism of the western United States as part of a large‐scale igneous system with arc‐like affinities. The relatively minor volume of igneous rocks implies that the Colorado Plateau acted as a structural barrier to the ascent of magma to high crustal levels.
Origin of the sedimentary magnetic record at Ocean Drilling Program Sites on the Owen Ridge, western Arabian Sea
Authors: Bloemendal J. , King J. W. , Hunt A. , Demenocal P. B. , Hayashida A.
Published: February 2008

The 3.2‐m.y. whole core magnetic susceptibility record obtained during Ocean Drilling Program leg 117 from the Owen Ridge in the western Arabian Sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information. The salient features of this record are that (1) it correlates strongly with variations in the concentration and flux of eolian dust; (2) it is driven strongly at Earth orbital periodicities; and (3) there is a significant change in the spectral character at 2.4 m.y., which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas. In view of its potential paleoclimatic significance, we have examined the origin of the rock magnetic signal from the Owen Ridge in more detail. We find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content, there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis. We ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process. The first occurs at about 1.5 m depth and is characterized by a loss of fine‐grained ferrimagnetic material. The second stepshift in magnetic properties occurs at a depth of 6–8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity (magnetite) and high coercivity (hematite and/or goethite) magnetic mineral components. Scanning electron microscope and energy dispersive Xray analyses show that the bulk of the coarse‐grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the Fe:Ti ratio of these grains across the lower diagenetic front. This suggests that titanomagnetite grains with higher Ti contents may be more resistant to the process of reductive diagenesis, and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front.
The physics of the Gulf of Mexico
Authors: Lewis James K.
Published: February 2008

During the last 10 years, a good deal of research has been conducted in the Gulf of Mexico (GOM). Study topics have included the Loop Current, coastal processes, eddies, tides, and the effects of tropical storms. Scientific contributions have been made by the offshore industry, academia, environmental agencies, and naval research groups. A great deal of information has quietly accumulated concerning the gulf, most of which is applicable to other regions in the world.
Characteristics of currents over the continental slope of the eastern Bering Sea
Authors: Schumacher J. D. , Reed R. K.
Published: February 2008

Between September 1989 and 1990, twenty‐six current records were collected by instruments on eight moorings located in Pribilof and Zhemchug canyons, and at a site midway between these features. These records provide the first long‐term Eulerian measurements from the slope and mid‐slope of the eastern Bering Sea. Results from the current records, together with water property observations, permit a characterization of the Bering Slope Current. Moderate flow (∼2 to 18 cm s−1) followed the bathymetry toward the northwest and existed primarily in the upper 300 m. Wind and current energy increased in winter, but vector mean current did not increase at all sites. Wind forcing accounted for only a small fraction of the current fluctuations. At one mid‐slope location in Pribilof Canyon, bathymetry resulted in rectification of the strong daily tidal current. Estimates of heat and salt fluxes indicate some significant shoreward transport; however, this flux did not occur preferentially in the canyons.
Continuous assimilation in an open domain of the northeast Atlantic: 1. Methodology and application to AthenA‐88
Authors: Dombrowsky Eric , De Mey Pierre
Published: February 2008

A scheme to perform suboptimal intermittent assimilation in an open‐ocean, quasi‐geostrophic model is presented and applied to the assimilation of altimeter data in a domain of the northeast Atlantic west of Ireland. Both simulated and real altimeter data are used. Three‐dimensional (3‐D) synoptic observation fields and error variance fields are derived from synoptic dynamic topography anomaly estimates, using an empirical orthogonal mode (EOF) vertical extension technique. The 3‐D estimates are suboptimally combined with the time‐dependent part of the forecast fields. Suboptimality here means that the combination does not directly use spatial correlations of errors; however, the 3‐D synoptic observation estimates used in the combination do contain error spatial statistical information. The resulting fields added to a mean model climatology are used as initial conditions for the model, which is integrated until a new dynamic topography anomaly estimate is available. Locally, the combination is optimal in the sense that it is based on the observational error variances, which reflect the measurement noise and the space‐time distribution of data, and that it minimizes the error variance of the results. A similar combination scheme using past as well as future observations is used to update the boundaries during model integration. Simulated surface topography anomaly maps, typical of the northeast Atlantic, are assimilated every 20 days for a 300‐day period, with different noise characteristics typical of altimetric sampling errors, in a three‐level version of the model. The assimilation fields, especially the vorticity at deeper levels, converge toward the reference fields. The convergence is obtained after O(100 days), regardless of the observational noise levels tested (up to −2 dB). Using a relevant observational error model seems to matter, in particular, the error level should not be underestimated. In the case of an 80‐day gap in the data inflow, the model predictability limits the reliability of the forecast beyond 20 days. The scheme makes the model converge back as soon as new observations are entered, with limited convergence loss due to the gap. The case of an unknown model climatology is also studied. Finally, the scheme is applied to Geosat altimeter data, added to the Robinson, Bauer and Schroeder annual climatology, in the AthenA‐88 cruise area, which is contained in the model box. The assimilation results compare favorably with hydrographical data from the cruise and help the synoptic interpretation of the observed phenomena.
Bottom pressure variability on the Labrador Shelf: Model‐data comparisons
Authors: deYoung Brad , Greatbatch Richard J. , Goulding Allan D. , Venguswamy Kumar
Published: February 2008

Bottom pressure variability at synoptic periods (4–12 days) is investigated using observed data from the Labrador shelf and a barotropic numerical model. The model uses realistic bathymetry with 0.25° × 0.25° resolution and is driven by wind stress derived from European Centre for Medium Range Weather Forecasts wind data. In some runs the model is coupled to a coarse‐resolution version applied to the North Atlantic in order to determine the influence of remote oceanic forcing on the shelf. At the northern end of the shelf, the model shows strong coherence with the data. Coupling to the North Atlantic model suggests that remote forcing is important there. At the southern end of the shelf the model shows weaker but still significant coherence except at the outer edge of the shelf. No effect of remote North Atlantic forcing could be detected at the southern end of the shelf. A significant improvement in the fit at all locations and periods could be found by fitting a complex arbitrary constant to the data. This result suggests that some important physical forcing, relevant to the entire shelf, has not been included in this model. A modification of the model to include the effect of stratification did not improve the agreement between model and data.
Response of Kuroshio upwelling to the onset of the northeast monsoon in the sea north of Taiwan: Observations and a numerical simulation
Authors: Liu Kon‐Kee , Gong Gwo‐Ching , Shyu Chung‐Zen , Pai Su‐Cheng , Wei Ching‐Ling , Chao Shenn‐Yu
Published: February 2008

A cold water anomaly, which manifests upwelling of the subsurface Kuroshio Water, has been frequently observed at the shelf break of the East China Sea to the north of Taiwan. Its response to the onset of northeast monsoon was observed during August–October 1990. The wind direction reversed in mid‐September, indicating the onset of northeast monsoon. Shortly thereafter, the sea surface temperature at the center of the cold eddy showed a pulselike sudden drop, and a significant concentration of nitrate (up to 5 μM) appeared in the surface water, suggesting intensification of upwelling. Subsequently, the upper layer of the Kuroshio Water intruded onto the shelf. The general circulation model of the East China Sea previously developed by Chao was used to simulate the overall response of the East China Sea. The numerical simulation reproduced the intensification of upwelling. It also predicted extensive Kuroshio intrusion along the shelf break farther north of Taiwan as well as the temporary intensification of the northeast branch of the Yangtze River outflow. The energy source of this sudden intensification comes from the potential energy released by the Kuroshio as the isopycnals maintaining the Kuroshio rise in response to the wind change.
Estimates of sea ice thickness distribution using observations and theory
Authors: Thorndike A. S.
Published: February 2008

The thickness distribution of sea ice is maintained by a balance of thermal and mechanical processes. Observations now exist that make it possible to quantify this balance and to test models of the individual physical processes. In particular, the observed distributions, used with fairly well established thermodynamic growth rates, give an estimate of the redistribution process that models the formation of pressure ridges. To highlight the main ideas, the evolution of the thickness distribution is recast as a Markov process forced by the mean velocity divergence and a measure of the random short term deformation. This model reproduces features of the observed thickness distribution such as the peak near 3 m, the mean thickness somewhat greater than 3 m, the long tail, and the variable shape of the thin side of the distribution. Analytical expressions for the mean and variance of the ice thickness are given, showing how they depend on the growth rates for thick and thin ice, the equilibrium thickness, the ice deformation, and the rule for building pressure ridges. These are sensitive to the mean divergence and the melt rate for thick ice. With large amounts of data now available for both the ice thickness and the ice motion, further progress is anticipated toward understanding the processes which shape the thickness distribution.
Vertical profiles of suspended sand concentration and size from multifrequency acoustic backscatter
Authors: Hay Alex E. , Sheng Jinyu
Published: February 2008

Vertical profiles of suspended sand concentration and size are obtained from multifrequency acoustic profiling data collected in 1989 during a nearshore experiment at Stanhope Lane Beach, Prince Edward Island. The data were acquired with acoustic sounders operating at 1, 2.25, and 5 MHz. Independent estimates of concentration were made using optical backscatter sensors (OBSs). The algorithm for inversion of the three‐frequency backscatter data to particle size and concentration, based on the ratios of the different signals, was tested in laboratory experiments with an unconfined high Reynolds number suspended sediment jet. Results from the Stanhope Beach experiment are presented for three different surface wave energy regimes, with significant wave orbital velocities ranging from 0.3 to 0.8 m/s and peak wave periods of 4–6 s. The acoustic estimates of mean concentration are shown to be within 10% on average of those determined with the OBS nearest the bottom at 5‐ to 10‐cm height, over time scales ranging from 6.5 min to 4–6 s (one wave period). The acoustic estimates of suspended sediment size near the bottom are within 5–20% of the bottom sediment mean size. The statistical variability of the size estimates is high, with standard deviations in the estimates ranging between 30 and 50% of the mean. The time‐averaged concentration profiles exhibit an exponential decrease with height above an O(10)‐cm‐thick near‐bottom region of nonexpo‐nential decrease. In contrast, the time‐averaged mean size profiles decrease approximately linearly with height, and rather slowly, about 25% in 0.5 m.
Non‐Gaussian effects in the two‐scale model for rough surface scattering
Authors: Nickolaev N. I. , Yordanov O. I. , Michalev M. A.
Published: February 2008

The popular two‐scale scattering model is applied to random surfaces whose probability density function of the slopes is described by the Gram‐Charlier type series. The particular form of the roughness spectrum pertains to the sea wave structure. Its angular factor is modified as to agree with the experimental data related to both the upwind and the crosswind rms slopes. A simple empirical factor, suggested earlier, is included in the spectrum to account for the modulation of the small ripples by the large waves. The results related to the azimuthal dependence of the backscattering cross section are shown to agree reasonably well with the airborne radar data acquired at Ku and X band in a circle flight pattern. It is also shown that the experimentally observed effects of upwind‐downwind asymmetry and the shift of the minimum of the backscattered power off the crosswind direction are related to the non‐Gaussian corrections. In addition, the particular coefficients in the slopes distribution responsible for the above two effects are singled out.
Features of the water‐mass front in the northwestern North Pacific
Authors: Zhang Ruo‐Chao , Hanawa Kimio
Published: February 2008

Analysis of the hydrographic data taken in the northern North Pacific has revealed a water‐mass front defined as the abrupt change of mixing rates between the two typical water masses. Typical water masses are situated above the main thermocline in the subtropical and subpolar gyres. This front, termed Water‐Mass Front in the present study, can be regarded as the boundary between the subtropical gyre and the subpolar gyre. The index for detecting the frontal position from the temperature field is given as the 5°C isotherm at the 300‐m level. Comparisons with data from previous studies are also made, and this front appears to correspond to the Subarctic Front in almost every instance.
Contour mapping of Arctic basin ice draft and roughness parameters
Authors: Bourke Robert H. , McLaren Alfred S.
Published: February 2008

A data base of ice draft and roughness parameters has been constructed for selected portions of the Arctic Ocean based upon analysis of under‐ice draft distribution data acquired by inverted echo sounder systems on submarines. From the voyages of 12 submarines which traversed the Alaskan, Canadian, and central Arctic regions of the Arctic Ocean during the summer and winter seasons, a series of mean ice draft and deep‐draft keel statistics was calculated for 50‐km segments along each submarine track. Contour maps of the mean ice draft, its standard deviation, the mean keel draft, and the spatial frequency of ice keels were constructed. They show that the greatest ice drafts, the roughest ice, and the greatest number of deep‐draft keels are found off the north coasts of the Canadian Archipelago and Greenland due to ice convergence on these land barriers.
A nested‐grid ocean model: With application to the simulation of meanders and eddies in the Norwegian Coastal Current
Authors: Oey Lie‐Yauw , Chen Ping
Published: February 2008

Oceanic flow phenomena vary in such large ranges of time and spatial scales that even with the fastest and largest computer to‐date, one cannot, at reasonable costs, compute large‐scale circulation and at the same time resolve mesoscale features like fronts and/or eddies; yet these features are dynamically important, and their inclusion may determine how well we can model the mean flow, including the deep‐ocean circulation. A two‐way interactive, nested‐grid, primitive‐equation model is developed here for coastal ocean applications. Notable features of the model are (1) nesting can be specified on any subregion of a coarse‐grid, large domain, and there can be more than one nest if required; (2) the nested region can be “hot‐started” from earlier calculation results of the coarse‐grid region, that is, the code automatically (by a change of an input flag) generates topography, wind forcing, climatology, currents, density and other prognostic variables in the nest and steps forward in time; (3) a time‐splitting integration, with small timesteps in the nest and large timesteps in the coarse‐grid domain, is used; and (4) nested variables are driven by coarse‐grid solutions around the nest's boundaries, where a flow relaxation scheme may be used, and at the same time drive the coarse‐grid evolution through its averaged action in the overlapped region. To demonstrate its robustness, the model is applied in a February/March 1988 real‐time simulation of meanders and eddies in the Norwegian Coastal Current, initialized from a 585 days' quasi‐equilibrium calculation. The simulation includes meteorological forcings, inflows/outflows across the open boundaries (inflow of the North Atlantic warm water in particular), tides, coastal and Baltic discharges, and wintertime hydrography for depths > 500 m. From March 20 to 31, the development of a meander between the Froya and the Halten Banks is simulated. The timing and location of the meander agree well with observed hydrography.
Sensitivity study of a dynamic thermodynamic sea ice model
Authors: Holland David M. , Mysak Lawrence A. , Manak Davinder K. , Oberhuber Josef M.
Published: February 2008

A numerical simulation of the seasonal sea ice cover in the Arctic Ocean and the Greenland, Iceland, and Norwegian seas is presented. The sea ice model is extracted from Oberhuber's (1990) coupled sea ice‐mixed layer‐isopycnal general circulation model and is written in spherical coordinates. The advantage of such a model over previous sea ice models is that it can be easily coupled to either global atmospheric or ocean general circulation models written in spherical coordinates. In this model, the thermodynamics are a modification of that of Parkinson and Washington (1979), while the dynamics use the full Hibler (1979) viscous‐plastic rheology. Monthly thermodynamic and dynamic forcing fields for the atmosphere and ocean are specified. The simulations of the seasonal cycle of ice thickness, compactness, and velocity, for a control set of parameters, compare favorably with the known seasonal characteristics of these fields. A sensitivity study of the control simulation of the seasonal sea ice cover is presented. The sensitivity runs are carried out under three different themes, namely, numerical conditions, parameter values, and physical processes. This last theme refers to experiments in which physical processes are either newly added or completely removed from the model. Approximately 80 sensitivity runs have been performed in which a change from the control run environment has been implemented. Comparisons have been made between the control run and a particular sensitivity run based on time series of the seasonal cycle of the domain‐averaged ice thickness, compactness, areal coverage, and kinetic energy. In addition, spatially varying fields of ice thickness, compactness, velocity, and surface temperature for each season are presented for selected experiments. A brief description and discussion of the more interesting experiments are presented. The simulation of the seasonal cycle of Arctic sea ice cover is shown to be robust.
Air‐sea interaction measurements in the west Mediterranean Sea during the Tyrrhenian Eddy Multi‐Platform Observations Experiment
Authors: Schiano M. E. , Santoleri R. , Bignami F. , Leonardi R. M. , Marullo S. , Böhm E.
Published: February 2008

Measurements of radiative fluxes were carried out in the Tyrrhenian Sea in fall and winter as part of the Tyrrhenian Eddy Multi‐Platform Observations Experiment (TEMPO). These measurements have supplied the first experimental radiation data set over this basin. Seasonal variation of the different components of the budget are investigated. Since data collection was carried out in an area in which a quasi‐permanent eddy is present, the behavior of the radiation parameters across the frontal zone is analyzed. The most interesting result of the air‐sea interaction in proximity of a marine front consists in the covariation of sea surface temperature and downwelling long‐wave radiation. Contemporaneous satellite data show a clear correlation between sea surface structure and horizontal distribution of columnar atmospheric water content. Therefore this inhomogeneity clearly is one of the main factors responsible for the variation of the downwelling radiation across the front. A comparison between experimental data and results of some of the most widely used bulk formulae is carried out for both short‐ and long‐wave radiation. The mean difference between measured and empirical solar radiation values is less than 3%, while in the case of the net long‐wave radiation budget, poor agreement is found. Indeed, a 30 W/m2 bias results from the comparison. This discrepancy is consistent with the imbalance between previous bulk calculations of total heat budget at the surface and corresponding hydrographical observations of heat exchange at Gibraltar.
Postprocessing of satellite altimetry return signals for improved sea surface topography accuracy
Authors: Brenner A. C. , Koblinsky C. J. , Zwally H. J.
Published: February 2008

Large off‐nadir attitude deviations and high surface wave heights cause an alteration in the ocean return signal from a satellite radar altimeter. This leads to an error in the on‐board calculation of the height measurement. The error can be removed by reprocessing the full radar return signal on the ground. In the ground processing, the correct tracking point in the return signal is recomputed through a procedure called retracking. There has been a controversy over whether or not all altimeter data would be retracked. This study analyzes retracked southern ocean data from the first 34 repeat cycles of the Geosat Exact Repeat Mission (ERM), covering November 1986 through April 1988. The final data set consists of over 2.5 million smoothed one‐per‐second measurements of the ocean surface. The significant wave height (SWH) distribution as given on the NOAA geophysical data records (GDRs) for these measurements peaks at around 2.1 m (19% of the measurements) and drops down almost linearly to 2% of the measurements at 5.8 m. There are over 1100 observations with SWH greater than 15 m. The difference between the surface heights calculated from the retracked data and the original on‐board estimates is less than 10 cm for SWH less than 10 m but increases to approximately 1.0 m at a SWH of 18 m. In general, the electromagnetic (EM) bias coefficient calculated using the retracked data is slightly less than that using the unretracked data and does not decrease as much with SWH as do the EM bias coefficients calculated from the unretracked data. A map of the sea surface height variability of the southern ocean created using the retracked data shows differences from variability maps created using the unretracked data in regions of high wave heights. Retracking can be done efficiently on modern UNIX work stations at 0.064 times real‐time acquisition. This study shows that retracking will improve altimeter precision.
Particulate iron formation dynamics in surface waters of the eastern Caribbean
Authors: Waite T. David , Szymczak Ron
Published: February 2008

Both the ability of membrane filters to retain iron added (as both Fe(II) and Fe(III)) to surface waters from various sources, and the ability of near ultraviolet light to induce dissolution of colloidal iron oxides suspended in waters from different locations are examined here. In brief, the presence of naturally occurring organic matter exerts a dominating effect on the tendency of iron oxide particles to interact with each other (both in suspension and on or within the membrane matrix). In coastal waters in the vicinity of the Orinoco River, growth of iron oxide particles initiated on addition of either Fe(II) or Fe(III) to the sample of interest, is significantly retarded in comparison to iron particle growth in open ocean waters. Near‐UV light appears to retard particle growth following both Fe(II) and Fe(III) addition to a coastal sample but does little to induce the dissolution of colloidal oxides.
A nonlinear model for the fluidization of marine mud by waves
Authors: Foda Mostafa A. , Hunt James R. , Chou Hsien‐Ter
Published: February 2008

Rheological properties of marine mud are incorporated into a seawave‐seabed interaction model. The model assumes that the viscoelastic properties of the mud are dependent on the wave‐induced strains inside the stratified bed, and it solves for the depth of fluidization in the bed as a function of the imposed wave height. Below the fluidized layer the mud is assumed to behave elastically with a slow depth stratification. In the fluidized layer the behavior is assumed to be viscoelastic with a complex kinematic viscosity coefficient which is also a function of the imposed wave height.
Topography and grounding in a simple bottom layer model
Authors: Speer Kevin , Tziperman Eli , Feliks Yizhak
Published: February 2008

A reduced‐gravity model for a dense bottom layer under a motionless deep layer is used to investigate bottom layer flow with topography. The two competing effects in the model are topography, which introduces a topographic β effect, and strong layer thickness variation, which allows for the possibility of zero‐layer thickness or grounding. The goal is to arrive at a simple, observationally relevant characterization of the model which can help to distinguish different dynamical mechanisms operating in the ocean. Several regimes are identified, including situations for which the layer depth goes to zero with and without topography. Ways of distinguishing observationally between competing effects are suggested. The study is motivated by the observed shift of Antarctic Bottom Water in the western North Atlantic Ocean from the west to the east; this phenomenon is used to discriminate key elements of the model.
A note on the modulation of Southern Oscillation‐Southern African rainfall associations with the quasi‐biennial oscillation
Authors: Mason S. J. , Lindesay J. A.
Published: February 2008

Since the 1950's, although the association between the Southern Oscillation and South African rainfall variability appears to have been reasonably stable, there is evidence that in certain years the association has broken down completely. The Southern Oscillation‐South African rainfall association is modulated with the phase of the Quasi‐Biennial Oscillation in tropical stratospheric zonal winds such that high phase (low phase) wet (dry) conditions occur over the summer rainfall region in January‐March only when the Quasi‐Biennial Oscillation is in the westerly phase. In contrast, there is no noticeable difference in the Southern Oscillation‐South African rainfall association between easterly and westerly years in October‐December. The Quasi‐Biennial Oscillation modulation appears to occur only when the prevalent synoptic systems over the country are of tropical origin. In the dry winter season (July‐September) the Southern Oscillation‐rainfall association and the Quasi‐Biennial Oscillation modulation are inverse to the relationships in the peak rainfall season of January‐March. Thus, a drier (wetter) than normal dry season occurs after the Southern Oscillation has been in high (low) phase and only when the Quasi‐Biennial Oscillation is in easterly, and not westerly, phase.
Intercomparison of stratospheric water vapor observed by satellite experiments: Stratospheric Aerosol and Gas Experiment II versus Limb Infrared Monitor of the Stratosphere and Atmospheric Trace Molecule Spectroscopy
Authors: Chiou E. W. , McCormick M. P. , McMaster L. R. , Chu W. P. , Larsen J. C. , Rind D. , Oltmans S.
Published: February 2008

This paper presents a comparison of the stratospheric water vapor measurements made by the satellite‐borne sensors the Stratospheric Aerosol and Gas Experiment II (SAGE II), the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS), and the Spacelab 3 Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment. LIMS obtained data for 7 months between November 1978 and May 1979; ATMOS was carried on Shuttle and observed eight profiles from April 30 to May 6, 1985 at approximately 30°N and 50°S; and, SAGE II continues to make measurements since its launch in October 1984. For both 30°N and 50°S in May, the comparisons between SAGE II and ATMOS show agreement within the estimated combined uncertainty of the two experiments. Several important features identified by LIMS observations have been confirmed by SAGE II: a well‐developed hygropause in the lower stratosphere at low‐ to mid‐latitudes, a poleward latitudinal gradient, increasing water vapor mixing ratios with altitude in the tropics, and the transport of dry lower stratospheric water vapor upward and southward in May, and upward and northward in November. A detailed comparative study also indicates that the two previously suggested corrections for LIMS, a correction in tropical lower stratosphere due to a positive temperature bias and the correction above 28 km based on improved emissivities will bring LIMS measurements much closer to those of SAGE II. The only significant difference occurs at high southern latitudes in May below 18 km, where LIMS measurements are 2–3 ppmv greater. It should be noted that LIMS observations are from 16 to 50 km, ATMOS from 14 to 86 km, and SAGE II from mid‐troposphere to 40 km. With multiyear coverage, SAGE II observations should be useful for studying tropospheric‐stratospheric exchange, for stratospheric transport, and for preparing water vapor climatologies for the stratosphere and the upper troposphere.
A jet stream associated heavy dust storm in the western Mediterranean
Authors: Alpert Pinhas , Ganor Eliezer
Published: February 2008

Examination of the dynamical structure of a heavy dust storm event over the western Mediterranean revealed an almost exact coincidence of the western boundary of the dust plume with a cold front and a significant jet stream. A confrontation was found between the dry desert air mass associated with a notable downward flowing jet stream and the moister Mediterranean air mass. Vertical cross sections across and along the dust plume indicated that the high mountain Ahaggar region over the Sahara was probably the source of dust, which was transported northward more than 2400 km and washed out by heavy rainfall.
The radiative budgets of a tropical mesoscale convective system during the EMEX‐STEP‐AMEX experiment: 2. Model results
Authors: Wong Takmeng , Stephens Graeme L. , Stackhouse Paul W. , Valero Francisco P. J.
Published: February 2008

This paper describes calculations of the spatial and temporal variation of the radiation budget of a tropical mesoscale convective system (MCS). A combination of cloud model simulations, radiation model simulations, and analyses of observations obtained during the Equatorial Mesoscale Experiment (EMEX), the Stratosphere‐Troposphere Exchange Program (STEP), and the Australian Monsoon Experiment (AMEX) are used to obtain these heating rates. The two‐dimensional version of the Colorado State University regional atmospheric modeling system is used to simulate a tropical MCS that occurred during EMEX mission 9 on February 2, 1987. The simulation is shown to broadly agree with the observations reported in a related paper. The spatial radiative heating distributions derived from a two‐stream radiative transfer model corresponding to the mature stage of the simulated cloud system indicate that significant horizontal inhomogeneities exist. According to the model results the effects of the MCS are to (1) increase in the infrared emission to the surface and to decrease in the net infrared energy loss from the atmosphere relative to the clear sky emission and (2) change the transmission of solar flux to the surface, the shortwave albedo of the atmosphere, and the solar absorption in the atmosphere. The results show how the MCS significantly reduces the solar flux to the surface relative to the clear sky values and that the largest reduction occurs under the convective portions of the mature MCS. (3) The MCS creates a total (solar plus infrared) radiative warming in the atmosphere relative to the surrounding clear sky. The value of this total heating is governed by both infrared and solar absorption. Vertical profiles of this heating show the dominance of infrared cooling near cloud top and infrared heating inside and near cloud base. The shortwave heating rate can also be as large as the infrared cooling near the cloud top region of the tropical MCS, especially at a local noon. (4) The temporal changes in radiation profiles also demonstrate how the MCS modulates the radiation budget of the atmosphere. Specifically, the total radiation energy loss of the entire two‐dimensional domain of the model atmosphere decreases and eventually becomes positive as the cloud system decays, becomes a stratiform in nature, and fills the domain. This change in the column divergence of flux translates into a total column radiative heating rate of approximately 1.7 K/d (relative to the clear sky radiative cooling rate). The solar component of this domain heating tends to be concentrated in the upper troposphere, whereas the infrared component of the heating is spread over the lower and middle troposphere. These results also show how tropical mesoscale cloud system provides an effective radiative heat source for the tropical atmosphere.
Ultraviolet absorption spectrum of HOCl
Authors: Burkholder James B.
Published: February 2008

The room temperature UV absorption spectrum of HOCl was measured over the wavelength range 200 to 380 nm with a diode array spectrometer. The absorption spectrum was identified from UV absorption spectra recorded following UV photolysis of equilibrium mixtures of Cl2O/H2O/HOCl. The HOCl spectrum is continuous with a maximum at 242 nm and a secondary peak at 304 nm. The measured absorption cross section at 242 nm was (2.1 ± 0.3) × 10−19 cm2 ( 2 σ error limits). These results are in excellent agreement with the work of Knauth et al. (1979) but in poor agreement with the more recent measurements of Mishalanie et al. (1986) and Permien et al. (1988]). An HOCl ν2 infrared band intensity of 230 ± 35 cm−2 atm−1 was determined based on this UV absorption cross section. The present results are compared with these previous measurements and the discrepancies are discussed.
A receptor‐oriented methodology for determining source regions of particulate sulfate observed at Dorset, Ontario
Authors: Cheng Meng‐Dawn , Hopke Philip K. , Zeng Yousheng
Published: February 2008

A statistical receptor‐oriented model was developed for long‐range transport of atmospheric sulfate to Dorset (elevation 320 m, latitude 4513′26″ N and longitude of 7855′52″ W), Ontario. This model computes the potential for sources within 1 latitude by 1 longitude grid cells across North America that contribute to the airborne concentrations measured at the ground station at Dorset. Airborne concentration data and air parcel backward trajectories were incorporated explicitly in the model calculation to identify the geographical areas of potential contributing sources. The present model is qualitative in nature; however, it provides a reasonable receptor‐oriented approach to examine the long‐range transport of atmospheric species. In order to fully understand the methodology and in a hope to optimize it, several aspects of the PSCF methodology have been examined in detailed in this study. Results of this study are presented that suggest interpolation of trajectory endpoints to increase the counting statistics for the potential source contribution function (PSCF) values is not reliable. The average concentration provides a reasonable criterion value; however, using the fiftieth percentile value as the criterion point provides an opportunity for identifying source areas that cannot be previously found by using the average concentration. The fiftieth percentile value may be a better choice for the particulate sulfate data in this case since Dorset is a relatively clean background site. Using the seventy‐fifth percentile, which is generally larger than the average, may not be suitable because it reduces the number of degrees of freedom. This could render the model to behave like a regular trajectory analysis model that has been used commonly for analyzing episodic pollution events. Separation of data into summer and winter periods is useful to illustrate the effects of photochemistry and meteorology on the PSCF results. Invoking the total probability concept and examining the trajectory arrival at different heights directly above the sampling site, the total PSCF was computed. This resultant function thus provides a time‐integrated geographical map useful for identifying sources of airborne particulate sulfate in a receptor‐oriented manner.
Soil texture and granulometry at the surface of Mars
Authors: Dollfus Audouin , Deschamps Marc , Zimbelman James R.
Published: February 2008

The physical behavior of the Martian surface soil has been characterized remotely by both photopolarimetry and radiometry. The degree of linear polarization defines a coefficient b which is related to the top surface soil texture and is calibrated in terms of grain size, or as a fraction of the area exhibiting uncovered clean rocks. This coefficient b was recorded with the instrument VPM (Visual Polarimeter Mars) on board Soviet orbiter MARS 5 in 1974. The radiometric thermal inertia coefficient I is essentially a measurement of the soil compaction, or an effective average particle size in the soil texture, through the few decimeters below the top surface sensed by polarimetry. The instrument IRTM (Infra Red Thermal Mapper) was used on board the Viking spacecraft between 1976 and 1982. The polarimetric scans raked a strip covering two contrasting regions, the dark‐hued Mare Erythraeum and the light‐hued Thaumasia. Over these wide areas, several smaller typical terrains were characterized by the three parameters A (albedo), b (related to top surface grain size) and I (underlaying compaction or block size). The large dark region Erythraeum is characterized everywhere by a uniform polarization response, despite the large geomorphological diversity of the surface. The values of A and b indicate a ubiquitous coating or mantling with small dark grains of albedo 12.7%, with a radius of 10 to 20 μm. Thermal inertia coefficient I indicates that the sub‐surface is divided in pieces around 300 to 600 μm in size. A simple model consisting of sand‐size particles completely coated with 15 μm black grains is compatible with both measurements. Conversely, the brighter terrain Thaumasia discloses a large variety of soil properties. A typical location with albedo 16.3% has a surface covered with orange grains, probably very dispersed in size, for which the largest grains are 20 to 40 μm. The subsurface is divided into pieces 180–300 μm or smaller, if cemented. On the basis of terrestrial analogs of the Martian soil (Morris et al., 1990), it is surmised that the near‐surface soil on the dark areas could be tachylite sand‐size grains surficially coated by cohesive black particles of titanomagnetite. The bright orange grains in the Thaumasia‐like terrains could be made of the weathered (palagonitized) basalt glass particles of sideromelane, as found in terrestrial analogs (Singer, 1982), Thaumasia is known to be a source area for dust storm production. The observed soil texture provides the large grains needed for saltation to occur, causing the intermixed small grains to be ejected from the surface and carried by wind.
The production and escape of nitrogen atoms on Mars
Authors: Fox J. L.
Published: February 2008

We have computed the production rates and densities of odd nitrogen species in the Martian atmosphere using updated rate coefficients and a revised ionosphere‐thermosphere model. We find that the computed densities of NO are somewhat smaller than those measured by Viking 1, but reasonable agreement can be obtained by assuming that the rate coefficient for loss of odd nitrogen in the reaction of N with NO is smaller at temperatures that prevail in the lower Martian thermosphere (about 130–160 K) than the standard value, which applies to temperatures of 200–400 K. We have also modeled the escape fluxes of N from the Martian atmosphere due to photodissociation, photodissociative ionization, and electron impact dissociative ionization of N2, ion‐molecule reactions, and dissociative recombination of N2+. The magnitudes of the latter two sources depend strongly on the N2+ density profile, for which there are no in situ measurements. The revised ionospheric model, which incorporates the larger electron temperatures recently derived from Viking measurements (Hanson and Mantes, 1988) and a loss process for ions at high altitudes (Shinagawa and Cravens, 1989), differs substantially from those of previous models (Fox and Dalgarno, 1979; 1983; Fox, 1989). In dissociative recombination of N2+ sufficient energy for escape is available only when the products are N(4S) + N(2D), and the results are therefore sensitive to assumptions made about the product yields. Unfortunately, the experimental values for the yields of excited states of N in dissociative recombination (Queffelec et al., 1985) differ significantly from those obtained in recent ab initio calculations (Guberman, 1991). When the theoretical data are employed, the computed isotope ratio enhancement and the initial column density obtained are about 2.6 and 1.0 × 1023 cm−2, respectively. The existence of a dense early atmosphere is consistent with this model. If the initial CO2 pressure is assumed to be 2 bars, exponential loss of the atmosphere with a time constant of about 2.2 × 1016 s (7 ×108 years) reproduces the measured isotope ratio.
Electron impact ionization and attachment cross sections for H2S
Authors: Rao M. V. V. S. , Srivastava S. K.
Published: February 2008

First measurements of the absolute values of cross sections for ionization and dissociative ionization of H2S by electron impact for the energy range of 0 to 1000 eV are reported. Cross sections for the formation of negative ions (HS− and S−) resulting from the electron attachment to H2S have also been obtained and are compared with previous measurements.