bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluctuation analysis of elastic waves in random media via phase screen simulations
Authors: Fisk M. D. , McCartor G. D.
Published: February 2008

Phase screen simulations of vector wave propagation in elastic random media are applied to two independent studies of seismic wave scattering. In the first study, transmission fluctuations are analyzed for incident P waves, propagated through a two‐dimensional version of the two‐layer model of Flatté and Wu (1988) of the crust and upper mantle beneath NORSAR. Numerically simulated coherence functions for vector elastic waves are compared to analytic and simulated results based on the parabolic approximation to the scalar wave equation. The usefulness of phase screen simulations for analyzing fluctuations for moderately strong scattering (∼ 2%–5% perturbations), and for three‐component data, is demonstrated. In the second study, effects of multi‐scale random structure on the variances of the initial P wave peak‐to‐peak amplitude and the rms amplitude of the transverse component in the velocity window between the average P and S wave speeds are investigated. A large number of synthetic seismograms are generated for eight random media models. This study shows that the relative variance of the rms scattered phase is far less dependent on the model than that of the direct phase. The variance of the forward scattered P wave depends greatly on the strength of the large‐scale heterogeneities.
Paleoseismology along the 1980 surface rupture of the Irpinia Fault: Implications for earthquake recurrence in the southern Apennines, Italy
Authors: Pantosti Daniela , Schwartz David P. , Valensise Gianluca
Published: February 2008

The Irpinia fault was the source of the Ms 6.9 1980 Irpinia earthquake and produced the first unequivocal historical surface faulting in Italy. Trenching of the 1980 fault scarp at Piano di Pecore, a flat intermontane basin about 5 km south of the 1980 instrumental epicenter, provides the first data on earthquake recurrence intervals, slip per event, and slip rate on a major normal fault in the Southern Apennines fault zone. The trenches exposed evidence of four pre‐1980 paleoearthquakes that occurred during the past 8600 years. A best estimate average recurrence interval is 2150 years, although the time interval between individual events varies by as much as a factor of 2. Each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation, which suggests that the 1980 event is characteristic for the Irpinia fault. Slip per event values average 61 cm. The net vertical displacement of 2.12–2.36 m since 8600 cal year B.P. observed in the trenches gives a vertical slip rate of 0.25–0.35 mm/yr, a dip slip rate of 0.29–0.40 mm/yr, and an extension rate of 0.14–0.20 mm/yr. Although fault behavior data are only available for the Irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern Apennines. They suggest that (1) fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude (M7) earthquakes and (2) the Holocene rate of extension across the Apennines is ≤1 mm/yr. The 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern Apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation.
Induced magnetization of the oceanic lithosphere and ocean‐continent magnetization contrast inferred from Magsat anomalies
Authors: Toft Paul B. , Arkani‐Hamed Jafar
Published: February 2008

A petromagnetic forward model of induced magnetization of Iceland Plateau is presented. Magnetic susceptibilities of Icelandic rocks from the literature are placed in a vertical stratigraphic context according to a model of igneous and metamorphic layering of the Icelandic crust that is compatible with seismic and other geophysical data. Curie isotherm depths for the magnetic forward model are based on the geothermal gradients measured in drill holes and those derived from magnetotelluric surveys. The petromagnetic model is used to predict the satellite altitude (400 km) magnetic field of Iceland Plateau. Comparison of calculated and Magsat anomalies shows that the total induced magnetization (the vertically integrated induced magnetization) in the surrounding oceanic lithosphere is ≈2300 A. This value is very similar to independent estimates based on rock magnetic models of the oceanic crust Rockall Plateau is a Precambrian microcontinent largely surrounded by oceanic crust a few hundred kilometers south and east of Iceland. The size, age, and location of this plateau are ideally suited to characterize the ocean‐continent magnetization contrast The total induced magnetization contrast between the plateau and its surroundings estimated from the Magsat anomaly of Rockall Plateau is ≈14,000 A.
Numerical simulation of Earth's core formation
Authors: Honda Rie , Mizutani Hitoshi , Yamamoto Tetsuo
Published: February 2008

Formation process of Earth's core is studied by numerical simulations of flow field in a self‐gravitating fluid sphere. The proto‐Earth was assumed to have gravitationally unstable three‐layered structure initially, which consists of the uppermost silicate melt layer, the middle iron layer, and the central undifferentiated silicate‐rich protocore. This structure of the heavy iron layer overlying the light protocore leads to Rayleigh‐Taylor instability. The numerical simulation supports that the fastest mode of the instability is the translation (l= 1 mode in spherical harmonics) of the protocore relative to the iron layer, as pointed out by previous studies. Furthermore, the present calculation shows that the translational mode is followed by growth of a gigantic iron drop and intrusion of the iron drop into the protocore. This late stage process is found to be the controlling process of Earth's core formation. In order to complete the core formation within 109years required by various geophysical and geochemical constraints, the protocore viscosity η1should be less than 1026Pas, which is larger than the present mantle viscosity but much smaller than the viscosity estimated for the protocore under low temperature and high pressure. Heating of the protocore by gravitational energy released during the core formation, change of flow mechanisms from diffusional flow to either power law creep or plastic flow and effect of volatiles contained in the protocore on the effective viscosity may assist acceleration of the core formation process.
Estimation of seasonal primary production from moored optical sensors in the Sargasso Sea
Authors: Marra John , Dickey T. , Chamberlin W. S. , Ho C. , Granata T. , Kiefer D. A. , Langdon C. , Smith R. , Baker K. , Bidigare R. , Hamilton M.
Published: February 2008

A bio‐optical mooring, which was deployed for 240 days during 1987 in the western Sargasso Sea (∼35°N, 70°W), provided among other things a detailed record of the seasonal distribution of chlorophyll a fluorescence and the scalar irradiance of photosynthetically available radiation. This data base was augmented by shipboard measurements of chlorophyll a concentration, chlorophyll a fluorescence, scalar irradiance, and net photosynthetic rate during four seasonal cruises to the mooring. The combined data base clearly shows a spring bloom in the surface mixed layer associated with initial stabilization of the water column, formation of a subsurface chlorophyll maximum caused by enhanced stratification of the water column, and disappearance of this feature in the winter caused by deepening of the surface mixed layer. The moored and shipboard data were applied to a detailed description of the seasonal variability in the vertical distribution of chlorophyll a and photosynthetic rate. Specifically, photosynthetic rate at a given depth was calculated as the product of scalar irradiance, chlorophyll concentration, the specific absorption coefficient of the phytoplankton crop, and the quantum yield of photosynthesis. Values of scalar irradiance and chlorophyll a concentration were obtained directly from the moored sensors, values for the specific absorption coefficient of the phytoplankton were obtained by linear interpolation of shipboard measurements, and the quantum yield of photosynthesis was calculated as a function of scalar irradiance. Comparisons of photosynthetic rate calculated from such a formulation with measured rates of carbon assimilation indicated good agreement, with no apparent or easily explained biases in the predictions. Surprisingly, daily values in both the crop of phytoplankton and gross photosynthetic rate varied by only a factor of 2 during the year. The annual rate of gross photosynthetic production at the mooring was 12 mol C m−2. An analysis of errors suggests that the precision of this estimate is about ±20%.
Light scattering by marine heterotrophic bacteria
Authors: Ulloa Osvaldo , Sathyendranath Shubha , Platt Trevor , Quiñones Renato A.
Published: February 2008

Mie theory is applied to estimate scattering by polydispersions of marine heterotrophic bacteria, and a simple expression is derived for the bacterial scattering coefficient. The error incurred in deriving bacterial optical properties by use of the van de Hulst approximations is computed. The scattering properties of natural bacterial assemblages in three marine environments, Georges Bank, Northeast Channel, and Sargasso Sea, are assessed by applying Mie theory to field data on bacterial size and abundance. Results are used to examine the potential contribution of bacteria to the scattering properties of seawater. The utility of using pigment data to predict the magnitude of scattering by bacteria is discussed.
The coupling of a submarine slide and the surface waves which it generates
Authors: Jiang L. , LeBlond P. H.
Published: February 2008

Underwater landslides are a common source of small‐scale tsunamis in coastal areas. They have been the object of only a few theoretical and experimental studies. A numerical model is developed to study the coupling of a submarine landslide and the surface waves which it generates. A formulation of the dynamics of the problem is presented, where the landslide is treated as the laminar flow of an incompressible viscous fluid and the water motion is assumed irrotational. Long‐wave approximation is adopted for both water waves and the mudslide. The resulting differential equations are solved by a finite‐difference method. We present the numerical results which contrast the behavior of the mud flow under a fixed surface, in the presence of one‐way coupling (bottom deformations affect the free surface), and with full coupling (surface pressure gradients react on the mud flow). It is found that three main waves are generated by a landslide starting from rest on a gentle uniform slope. The first wave is a crest which propagates away from the mudslide site into deeper water; this crest is followed by a trough in the form of a forced wave which propagates with the speed of the mudslide front. The third wave is a relatively small trough which propagates shoreward. Two major parameters dominate the interaction between the slide and the waves it produces: the density of sliding material and the depth of water at the mudslide site. The two‐way interactions are significant for the cases of a smaller mud density and shallower waters. For the cases of larger mud density and shallower waters, the two‐way interactions are small, but the waves generated are very large. For larger initiation depth of the mudslide, the interactions are weak and the waves generated are small. We also examined the possibility of a resonance between the slide and the waves. Our numerical results indicate that a resonance is not expected in many practical cases.
Recent observations of variability in the western Bering Sea current system
Authors: Verkhunov A. V. , Tkachenko Y. Y.
Published: February 2008

Data from two cruises in the western Bering Sea are presented and examined. During spring 1990, conditions were typical, and the Kamchatka Current had a transport of ∼11 Sv. On the other hand, during fall 1990, this flow had weakened to ∼6 Sv, although a strong flow near the Komandorski Islands occurred. Various eddylike features were common during both cruises and were particularly developed in fall. Although some aspects of the flow patterns may be seasonal in nature, the major weakening of transport probably represents an interannual change.
The intrusion of the Kuroshio across the continental shelf northeast of Taiwan
Authors: Hsueh Y. , Wang Joe , Chern Ching‐Sheng
Published: February 2008

Hydrographic observations in an area immediately northeast of Taiwan in April 1989 indicate an on‐shelf intrusion of Kuroshio water across a sharply curved continental shelf break. It appears that a part of the Kuroshio on the cyclonic side overran the shelf break and penetrated northward as a shallow surface current. The remainder of the Kuroshio, presumably affected by the shoaling topography, largely turned and ran along the shelf break to the northeast. Between the two, the flow was weak and disorganized. Conservation of potential vorticity and constancy of the Bernoulli function in an analytical, reduced‐gravity model of the incidence of a baroclinic current upon a step shelf lead to an on‐shelf flow field that is compatible to the observed hydrographic distributions. In particular, the integrated balance of along‐step momentum yields an expression of the angle of intrusion in terms of the incidence angle and of the ratio of step depth to the depth of the upper layer of the ocean. In addition, the transport of the intrusion is shown to be equal to the product of the depth ratio squared and the incident transport. Calculations for the condition of the April survey yield results in agreement with the observation.
An experimental study on the formation and survival of stratified subsurface eddies
Authors: Bormans Myriam
Published: February 2008

We report the results of laboratory experiments on the formation and survival of internally stratified subsurface eddies in a rotating fluid. The eddies were created by injecting a dense turbulent plume at the surface of a linearly stratified environment. The relative vorticity of the lenses was always negative but larger than that of homogeneous lenses created by laminar injection. During the first 100 revolutions, the eddies shed fluid in two symmetric arms. The shedding which is believed to result from shear instabilities always resulted in a stationary axisymmetric eddy. After the eddy had spun down, the remnant fluid persisted for thousands of rotations as a circular feature with internal stratification identical to that of the environment. We created eddies with and without double diffusive convective instabilities and compared the volume of dyed fluid and the evolution of their aspect ratios. Sugar and salt were used as laboratory analogues of salt and heat, respectively. The Burger number of the lenses decreased rapidly within the first 200 revolutions and then much more slowly to reach a value between 0.2 and 0.4, These latter values are larger than those predicted by Gill (1981) for a homogeneous lens due to the internal stratification of the lenses. Radial spreading of the lens due to double diffusive intrusions was found to be larger, but of the same order of magnitude, as that induced by the vertical exchange of momentum in the absence of double diffusive convection. We formed eddies internally stratified in the diffusive sense (stable sugar gradient and unstable salt gradient) or doubly stable (stable sugar and salt gradients) by changing the ratio of the volume flux at the source to the volume flux at the spreading level as described by Bormans and Turner (1990). When the stratification in the eddies was doubly stable, three distinctive regions were observed: a region of convective layers and diffusive density interfaces at the top, a central region with no apparent structure and a bottom region dominated by fingers sheared by rotation. This type of eddy is compared to the particular case of a Meddy.
A theory of the synthetic aperture radar images of time‐dependent scenes
Authors: Milman Andrew S. , Scheffler Albert O. , Bennett John R.
Published: February 2008

Data from synthetic aperture radars (SAR's) can be used to measure the surface height spectrum of the ocean. Traditionally, only the real SAR image is used; we show, both theoretically and with SAR data, that the complex image contains more information about the ocean scene. We derive an expression for the ambiguity function of the complex image, which is an extension of the concept of a power spectrum, but generalized to phenomena with time‐dependent properties. The ambiguity function of the complex image is derived as an integral of the surface height spectrum; this allows us to calculate the statistics of the complex SAR image directly, without having to generate individual realizations of the surface. In the derivation of the ambiguity function, we find that a simple assumption about the complex reflectivity of the surface leads to a formulation of the SAR imaging process that circumvents many traditional problems. By using ambiguity functions, we can remove some of the smearing that is otherwise seen in the images of time‐dependent scenes. One result of this analysis is that we can explain SAR focusing in a simple way, and extend it to range‐traveling waves as well as azimuth‐traveling waves. Also, we can calculate the correlation length and time of the surface directly from the surface height spectrum. While the correlation length seems to play only a minor role in this theory, the correlation time determines the minimum wavelength for azimuth waves that can be imaged. This paper shows how the ambiguity function of the image is related to the surface height spectrum and presents an example using SAR data.
Experiments on the entrainment of sediment into suspension by a dense bottom current
Authors: Garcia Marcelo , Parker Gary
Published: February 2008

Experiments on the entrainment of bed sediment into suspension by density underflows are described. The density underflows were created by allowing saline water to move as a steady, continuous current under a body of essentially stagnant fresh water. The bed slope was set at 0.08. Two grades of fine crushed coal, with geometric mean sizes of 100 μm and 180 μm, were placed in a trough in the bed with a length of 2 m, over which the density currents passed. Measurements of profiles of flow velocity and concentration of suspended sediment allowed for a back calculation of boundary shear stress and the rate of sediment entrainment from the equations of momentum and mass balance. Two techniques for removing the effect of form drag from the boundary shear stress were explored. The resulting data were used to compare sediment entrainment rates due to density underflows with those observed in open channels. It appears possible to make rather modest adjustments in an existing predictive relation for sediment entrainment in open channels, so as to predict the case pertaining to density underflows as well.
Streaks in the vicinity of an oscillating front
Authors: Kirwan A. D. , Liu Juping
Published: February 2008

The fate of particles continuously injected at a fixed location in the vicinity of an oscillating front is studied by numerical integration of the hydrodynamic equations. The analysis is straightforward when the injection is at a location that is always on one side of the front. In this case, the curve traced by these particles, i.e., a streak line, may be complicated and generally does not resemble the path of the initial particle released, but at least is continuous. However, when the front oscillates across the source location, the streak line is broken into a series of discontinuous segments. Moreover, some segments later may recirculate past the source. This can produce a complicated streak line pattern in which old segments intertwine with recent segments.
Satellite altimeter measurements of surface wind
Authors: Glazman Roman E. , Greysukh Alexander
Published: February 2008

Recent analyses of wind speed measurements by the Geosat altimeter showed that the radar cross section is affected by oceanographic factors, particularly by the degree of sea development, which are not directly accounted for in the geophysical model functions (GMF). In the present work, two new GMFs which account for the effects of the actual degree of sea development are proposed. Along with the radar cross section, these models use significant wave height information. One particular version is recommended for applications in oceanographic and climate studies where wind speed (or wind stress) data have to be binned (i.e., averaged over time and/or space intervals). The accuracy of this GMF (overall bias of 0.1 m/s and rms error of about 1.6 m/s) is higher than the accuracy of commonly employed GMFs, while the wave‐age‐related trend is reduced to a geophysically insignificant level. Finally, the wind speed histograms for the collocated data set are derived and compared with the ground truth data as well as with the histograms yielded by presently known GMFs. It is also shown that the accuracy of altimeter measurements could be increased even further if some additional information on the wave field were available from independent sources (e.g., the dominant wavelength from synthetic aperture radar images).
Salt finger signatures in microstructure measurements
Authors: Hamilton J. M. , Oakey N. S. , Kelley D. E.
Published: February 2008

By measuring the “scaled dissipation ratio” Γ, which is the relative magnitude of the dissipation of thermal variance compared with the dissipation of turbulent kinetic energy, one can distinguish between salt fingering and nonfingering regimes. This is illustrated by comparing data from three test cases to predictions from (1) a mixing model which considers turbulence only and (2) a model which describes salt fingers as the sole mixing process. In a turbulent, nondouble‐diffusive surface layer we find that measurements of Γ are very close to the value predicted by the turbulent mixing model. A contrasting case is provided by a thermohaline staircase located at 1200‐ to 1330‐m depth. There the salt finger model provides a better description of the mixing than does the turbulent mixing model. The third case study is of measurements between 150 and 400 m where the water column is characterized by low density ratios and “intermittent steppiness” in temperature and salinity profiles. Here, values of Γ are inconsistent with a model containing only salt finger or turbulent mixing; instead, the observations suggest that both processes are important. Using the observed values of Γ in a combined model suggests that 24% of the observed turbulent kinetic energy dissipation is due to salt fingers. A corresponding estimate of the vertical eddy diffusivity of salt (and nutrients) is 2 times larger than that computed from the turbulence‐only mixing model and 50% larger than the vertical eddy diffusivity for heat as determined by the Osborn‐Cox relation.
Influence of the Orinoco River outflow on distributions of algal pigments in the Caribbean Sea
Authors: Bidigare Robert R. , Ondrusek Michael E. , Brooks James M.
Published: February 2008

Seasonal measurements of photosynthetic pigments were performed in the Caribbean Sea to investigate the potential effects of Orinoco River discharge on the vertical distributions of the major phytoplankton groups. Samples were collected during periods of low (April 1988) and high (September 1988) riverine input. The chlorophyll a concentration of the subsurface chlorophyll maximum (SCM) sampled at inshore and offshore stations displays only minor seasonal variations. However, the depth and pigment composition of the SCM during periods of low and high outflow are markedly different. A “two‐layer” phytoplankton distribution pattern is observed for all stations occupied during the spring and those stations not influenced by Orinoco River discharge in the fall: an upper “light‐adapted” population dominated by prokaryotic phytoplankton (cyanobacteria and possibly prochlorophyte‐like phytoplankton) and a deeper “shade‐adapted” population comprised of chromophytes and “green algae.” The depth of the SCM at these stations averaged 77 ± 12 m. By contrast, stations influenced by Orinoco River discharge during the fall where characterized by near‐surface diatom‐dominated communities and shallower SCM depths (39 ± 16 m). Nutrient loading, elevation of absorption and attenuation coefficients, and transport of coastal “seed” phytoplankton populations are possible mechanisms by which the Orinoco River modifies the composition and distribution of phytoplankton in the Caribbean Sea.
Photochemical free radical production rates in the eastern Caribbean
Authors: Dister Brian , Zafiriou Oliver C.
Published: February 2008

Potential photochemical production rates of total (NO‐scavengeable) free radicals were surveyed underway (> 900 points) in the eastern Caribbean and Orinoco delta in spring and fall 1988. These data document seasonal trends and large‐scale (∼ 10–1000 km) variability in the pools of sunlight‐generated reactive transients, which probably mediate a major portion of marine photoredox transformations. Radical production potential was detectable in all waters and was reasonably quantifiable at rates above 0.25 nmol L−1 min−1 sun−1. Radical production rates varied from ∼ 0.1–0.5 nmol L−1 min−1 of full‐sun illumination in “blue water” to > 60 nmol L−1 min−1 in some estuarine waters in the high‐flow season. Qualitatively, spatiotemporal potential rate distributions strikingly resembled that of “chlorophyll” (a riverine‐influence tracer of uncertain specificity) in 1979–1981 CZCS images of the region [Müller‐Karger et al., 1988] at all scales. Basin‐scale occurrence of greatly enhanced rates in fall compared to spring is attributed to terrestrial chromophore inputs, primarily from the Orinoco River, any contributions from Amazon water and nutrient‐stimulus effects could not be resolved. A major part of the functionally photoreactive colored organic matter (COM) involved in radical formation clearly mixes without massive loss out into high‐salinity waters, although humic acids may flocculate in estuaries. A similar conclusion applies over smaller scales for COM as measured optically [Blough et al., this issue]. Furthermore, optical absorption and radical production rates were positively correlated in the estuarine region in fall. These cruises demonstrated that photochemical techniques are now adequate to treat terrestrial photochemical chromophore inputs as an estuarine mixing problem on a large scale, though the ancillary data base does not currently support such an analysis in this region. Eastern Caribbean waters are not markedly more reactive at comparable salinities than waters of the Gulf of Maine and North Atlantic Bight, despite large inputs of colored waters from two large tropical rivers with substantial “black water” tributaries. Other sources of reactive COM, such as grazing, sedimentary diagenesis, and “marine humus” may increase temperate waters' photoreactivity; alternatively, northern waters may be chromophore‐rich because they are light‐poor and photobleaching is a major sink of photoreactive COM.
Influence of the Orinoco River on hydrogen peroxide distribution and production in the eastern Caribbean
Authors: Moore Cynthia A. , Farmer Charles T. , Zika Rod G.
Published: February 2008

Two cruises were used to investigate the effect of the Orinoco River on the photochemistry of the eastern Caribbean in 1988. Hydrogen peroxide distribution was used as an indicator of the extent of modification and persistence of photochemical activity, with comparisons being made between high (fall) and low (spring) river input. During the spring cruise, H2O2 profiles were typical of oligotrophic waters, with accumulation rates and steady state concentrations similar to those reported for open ocean. Results from the fall cruise were characterized by increased surface concentrations, sharper gradients, and shallower penetration of the photochemically generated H2O2. The impact of the river water can be traced in the eastern Caribbean by increased dissolved organic matter fluorescence, lower salinity, and faster hydrogen peroxide accumulation rates in surface samples (up to 20 times greater for some stations). It is apparent that peroxide distribution throughout the eastern Caribbean was significantly affected by the Orinoco river input during the fall.
Gravity wave models for the horizontal wave number spectra of atmospheric velocity and density fluctuations
Authors: Gardner Chester S. , Hostetler Chris A. , Franke Steven J.
Published: February 2008

Gravity wave models for the horizontal wave number spectra of atmospheric velocity and density fluctuations are derived by assuming that both saturated and unsaturated waves obey the polarization and dispersion relations and that the joint vertical wave number (m) and temporal frequency (ω) spectrum is separable. In the wave number region h* = ƒm*/N ≤ h ≤ m*, where h = (k2 + l2)1/2, k is the zonal wave number, l is the meridional wave number, ƒ is the inertial frequency, N is the buoyancy frequency, and m* is vertical wave number of the largest scale saturated wave, the model predicts that the two‐dimensional isotropic spectrum is proportional to h−(p+1) and the one‐dimensional spectra are proportional to k−p and l−p when the temporal spectrum is proportional to ω−p. In the saturation region h, k, l, m > m*, the spectra are proportional to h−(q+1), k−q and l−q when the saturated m spectrum is proportional to m−q. The joint (k, l, m) and (k, l, ω) model spectra are not separable. The models are consistent with existing observations of horizontal wave number spectra in the lower stratosphere and upper mesosphere.
The local diurnal variation of cloud electrification and the global diurnal variation of negative charge on the Earth
Authors: Williams E. R. , Heckman S. J.
Published: February 2008

Both the amplitude and the phase of the ionospheric potential and Carnegie curve of atmospheric electricity are considered to distinguish causes for the negatively charged earth in fair weather. Satellite‐observed longitudinal distributions of electrical activity are convolved with local diurnal variations of cloud‐to‐ground lightning and point discharge current to produce universal diurnal variations which are compared with the Carnegie curve. The amplitude ratio (maximum‐minimum)/mean) for the predicted universal diurnal variation of point discharge shows good agreement with the Carnegie curve, whereas the predicted amplitude ratio for lightning is 2–3 times greater. These comparisons suggest that conduction current other than lightning is the dominant charging agent for the Earth's surface.
An improved chemical amplifier technique for peroxy radical measurements
Authors: Cantrell Christopher A. , Shetter Richard E. , Lind John A. , McDaniel Anthony H. , Calvert Jack G. , Parrish David D. , Fehsenfeld Fred C. , Buhr Martin P. , Trainer Michael
Published: February 2008

The chemical amplifier for atmospheric peroxy radical measurements, first described in the early 1980s has been improved relative to these earlier reports. The details of the instrument and a new radical calibration procedure are discussed as they relate to participation in a field study in the southeastern United States in the summer of 1990. The theoretical behavior of the chemical amplifier is also examined with the use of analytical solutions to the relevant kinetic equations as well as with a numerical model. Several issues of atmospheric relevance are addressed including the response of the instrument to organic peroxy radicals, interferences from PAN and peroxynitric acid, accuracy, precision, and detection limits studied through a number of laboratory and field investigations. Some new findings realized since the summer of 1990 are also included.
Determining torques over the ocean and their role in the planetary momentum budget
Authors: Ponte Rui M. , Rosen Richard D.
Published: February 2008

Knowledge of wind torques at the atmosphere‐ocean interface is important in understanding the global angular momentum balance. A surface wind data set that combines special sensor microwave/imager (SSM/I) and European Centre for Medium Range Weather Forecasts (ECMWF) wind products and covers the period July 1987 to June 1989 is used to estimate zonal torques over the global ocean. Comparison of these fields and fields computed from ECMWF wind analyses alone shows substantial differences in the statistics of both locally and globally integrated torques; the impact of SSM/I data on the torque analysis is largest over the Indian Ocean and eastern Pacific regions. The characteristics of the torques from the combined SSM/I‐ECMWF data set are evaluated. In general, local torques show the largest means and variances over mid‐latitudes (especially in the southern ocean), with fluctuations at periods shorter than 1 month dominating the records. In contrast, variability in the globally integrated ocean torque is largest at seasonal time scales (3 months to 1 year); midsouthern and tropical northern latitudes contribute substantially to this variability. Comparison between the global ocean torque and atmospheric angular momentum M indicates that indirect exchanges of momentum between the atmosphere and solid Earth via oceanic stresses cannot be neglected at seasonal time scales. Intraseasonal fluctuations (1–3 months) in both tropical and global ocean torques are coherent with M, but too weak to account alone for the variability in M. At submonthly periods the ocean torque is apparently not important in the atmospheric momentum balance.
Equilibrium size of atmospheric aerosol sulfates as a function of particle acidity and ambient relative humidity
Authors: Koutrakis Petros , Kelly Brendan P.
Published: February 2008

Acid aerosol events in Uniontown, Pennsylvania, were studied during the summer of 1990. Size‐fractionated acid aerosols were collected using a microorifice cascade impactor. The collected samples were analyzed for hydrogen, sulfate, nitrate, and ammonium ion concentrations. These measurements show evidence of a relationship between the mass median geometric aerodynamic diameter of sulfate particles and both particle acid content (H+/SO4=) and ambient relative humidity. Sulfate particle size increases with both increasing acid content and relative humidity. Considering the uncertainties involved, the growth of atmospheric sulfate particles observed in this study was found to be in good agreement with that predicted on the basis of laboratory‐derived data. To our knowledge, this is the first time that such a relationship has been observed under real atmospheric conditions. Therefore the results of this study suggest that laboratory experiments and thermodynamic calculations can be used in order to understand the chemistry of atmospheric sulfates and their impacts on visibility reduction. In addition, we present evidence to suggest that the chemical composition of acid aerosol particles may be related to the ambient relative humidity.
Remote sensing of atmospheric structure and composition by pressure modulator radiometry from space: The ISAMS experiment on UARS
Authors: Taylor F. W. , Rodgers C. D. , Whitney J. G. , Werrett S. T. , Barnett J. J. , Peskett G. D. , Venters P. , Ballard J. , Palmer C. W. P. , Knight R. J. , Morris P. , Nightingale T. , Dudhia A.
Published: February 2008

The scientific objectives of the improved stratospheric and mesospheric sounder (ISAMS) experiment involve the measurement of global temperature and composition profiles from an instrument on the Upper Atmosphere Research Satellite (UARS). This paper describes the instrument concept, its design, and its performance as calculated and as measured in the laboratory. The data retrieval technique, operating modes, observing strategy, and the error budget are briefly discussed.
The enigmatic object 2201 Oljato: Is it an asteroid or an evolved comet?
Authors: McFadden Lucy A. , Cochran Anita L. , Barker Edwin S. , Cruikshank Dale P. , Hartmann William K.
Published: February 2008

The study of the near‐Earth object 2201 Oljato has resulted in many surprises in the decade in which it has been examined. Its orbital properties have been associated with meteor showers, and its modeled orbital evolution is chaotic, a property which might indicate a history related to comets. Telescopic observations of its visible and near‐infrared spectral reflectance, broad‐band visible and near‐infrared photometry, infrared radiometric measurements, and radar echoes are reported here from two apparitions, 1979 and 1983. A look at all available observational data shows that this asteroid has a high radiometric albedo, a property not associated with comet nuclei. In certain wavelength regimes it is classified as an S‐type asteroid, in others, an E‐type, but its overall spectral reflectance is not typical of either taxonomic type, and neither type is thought of as cometlike. Unexpectedly high ultraviolet reflectance at the 1979 apparition was suggested to be the result of residual outgassing as in a comet. The UV photometric data are modeled as fluorescent emission from neutral species found in comets. The resulting calculations indicate a plausible value for OH and CN emission at 0.3085 and 0.38 μm relative to the observed range of active comets. Observations to monitor photometric variations and to search for gaseous activity using observational techniques designed specifically for this purpose were planned and executed in October 1992 to verify or refute the interpretations presented here.
MG‐perovskite/silicate melt and majorite garnet/silicate melt partition coefficients in the SYSTEM CaO ‐ MgO ‐ SiO2at high temperatures and pressures
Authors: Drake Michael J. , McFarlane Elisabeth A. , Gasparik Tibor , Rubie David C.
Published: February 2008

We report the results of a Mg‐perovskite/silicate melt and a majorite garnet/silicate melt elemental partitioning experiment conducted at high pressures and temperatures in the synthetic system CaO ‐ MgO ‐ SiO2. Partition coefficients for some refractory lithophile elements (e.g., Sc, Sm) deviate significantly from unity. If Mg‐perovskite segregated from a terrestrial magma ocean, it would impart nonchondritic refractory lithophile element ratios such as Sc/Sm to the primitive upper mantle of Earth, contrary to inference based on analyses of naturally occurring samples. Either Earth was never substantially molten at the end of accretion, or segregation of minerals from magmas was suppressed by vigorous convection. It is difficult to avoid melting Earth during accretion so the latter alternative seems more plausible.
Mars mesosphere and thermosphere coupling: Semidiurnal tides
Authors: Bougher S. W. , Fesen C. G. , Ridley E. C. , Zurek R. W.
Published: February 2008

The National Center for Atmospheric Research thermosphere ionosphere general circulation model for Earth has been modified to examine the three‐dimensional structure and circulation of the upper atmosphere of Mars (MTGCM) (Bougher et al., 1988b, 1990). Recent examination of Mariner 9 UVS airglow measurements taken during a global dust storm provide evidence of large temperature variations and atomic oxygen distributions uncorrelated with solar activity and the corresponding MTGCM in situ driven winds. We suspect significant forcing of the thermosphere from below as a result of upward propagating gravity waves or tides generated by solar heating of airborne dust (Stewart et al., 1992). The effects of upward propagating tides are introduced into the MTGCM by appropriately specifying its lower boundary condition according to classical tidal theory. We initially adapt the terrestrial scheme used by Fesen et al. (1986) to a Mars model appropriate to Mariner 9 near‐solar‐minimum conditions. Estimates of the amplitude and phase of the likely dominant semidiurnal (2,2) mode at the mesopause (∼100 km) are specified for a range of possible lower atmosphere dust conditions. MTGCM simulations contrasting tidally driven fields with solar‐only forced ones show a dramatic change in the horizontal and vertical wind patterns, whereby the global temperature and oxygen distributions are also modified significantly. This semidiurnal component predominates below 135 km, while the in situ solar‐driven diurnal component is largely dominant above. Constructive interference serves to enhance midafternoon exospheric temperatures toward Mariner 9 observed values. The thermospheric response and the altitude of penetration of these semidiurnal tides is found to be much greater during solar minimum periods when dissipation due to viscosity and thermal conductivity is diminished from that at solar maximum. Finally, the Martian response during dusty periods is predicted to be much larger than that typically observed for Earth. Martian dust‐driven tides, especially during solar minimum time periods, cannot be ignored when addressing the Mars thermospheric structure and dynamics.
Tropospheric gas composition and cloud structure of the Jovian north equatorial belt
Authors: Carlson Barbara E. , Lacis Andrew A. , Rossow William B.
Published: February 2008

High spatial resolution Voyager infrared interferometer spectrometer spectra of the North Equatorial Belt (NEB) reveal longitudinal variability of 5‐μm brightness temperatures of order 100°C. These observations are used to investigate spatial variations in the gas composition and cloud structure of the NEB. We use an anisotropic multiple scattering radiative transfer model to calculate synthetic spectra for comparison with the IRIS observations. The spectral dependence of cloud extinction from 180 to 2300 cm−1 is modeled using Mie theory. The entire spectral range of the IRIS observations (180–2300 cm−1) is used to constrain the cloud properties and vertical structure of the NEB. Within the model, cloud base locations vary with assumed gas abundances according to thermochemical equilibrium. We find that spatial variations in the abundance profiles of the condensible species, para hydrogen profiles and cloud optical depths can be used as tracers of the local and large‐scale dynamics. Based on the spectral dependence of NH3 cloud extinction that is required to fit the IRIS observations, we conclude that the bulk of the NH3 cloud extinction is provided by large particles, effective radii ≈100 μm; however, a small particle mode may also be present. We find that the observed 5‐μm brightness temperature structure can be reproduced by spatial variations in cloud opacity and water relative humidity. NEB hot spots, due to their low cloud opacity, provide a unique opportunity to study the deep cloud structure in the Jovian atmosphere. Cloud opacity is required at P > 4 bars (coincident with the location of the thermochemically predicted H2O cloud) to reproduce the observed continuum level near 2130 cm−1, as well as to model the overall shape of the continuum between 2100 and 2300 cm−1. Water relative humidity is found to vary spatially above the base of the water cloud increasing from ≈15% in hot spots to 100% in colder spectral ensembles. The variation of relative humidity is strongly correlated with the variation of cloud opacity, suggesting dynamic depletion of water vapor above the cloud forming level as the most plausible model to explain the spatial variation in the water profile within the NEB.
Surface water temperature changes in the high latitudes of the southern hemisphere over the Last Glacial‐Interglacial Cycle
Authors: Pichon Jean‐Jacques , Labeyrie Laurent D. , Bareille Gilles , Labracherie Monique , Duprat Josette , Jouzel Jean
Published: February 2008

A set of numerical equations is developed to estimate past sea surface temperatures (SST) from fossil Antarctic diatoms. These equations take into account both the biogeographic distribution and experimentally derived silica dissolution. The data represent a revision and expansion of a floral data base used previously and includes samples resulting from progressive opal dissolution experiments. Factor analysis of 166 samples (124 Holocene core top and 42 artificial samples) resolved four factors. Three of these factors depend on the water mass distribution (one Subantarctic and two Antarctic assemblages); factor 4 corresponds to a “dissolution assemblage”. Inclusion of this factor in the data analysis minimizes the effect of opal dissolution on the assemblages and gives accurate estimates of SST over a wide range of biosiliceous dissolution. A transfer function (DTF 166/34/4) is derived from the distribution of these factors versus summer SST. Its standard error is ± 1°C in the −1 to +10 °C summer temperature range. This transfer function is used to estimate SST changes in two southern ocean cores (43°S and 55°S) which cover the last climatic cycle. The time scale is derived from the changes in foraminiferal oxygen and carbon isotopic ratios. The reconstructed SST records present strong analogies with the air temperature record over Antarctica at the Vostok site, derived from changes in the isotopic ratio of the ice. This similarity may be used to compare the oceanic isotope stratigraphy and the Vostok time scale derived from ice flow model. The oceanic time scale, if taken at face value, would indicate that large changes in ice accumulation rates occurred between warm and cold periods.
Southwest Monsoon Circulation and Environments of Recent Planktonic Foraminifera in the Northwestern Arabian Sea
Authors: Brock John C. , McClain Charles R. , Anderson David M. , Prell Warren L. , Hay William W.
Published: February 2008

Digital hydrographic data combined with satellite thermal infrared and visible band remote sensing provide a synoptic climatological view of the shallow planktonic environment. This paper uses wind, hydrographic, and ocean remote sensing data to examine southwest monsoon controls on the foraminiferal faunal composition of Recent seafloor sediments of the northwestern Arabian Sea. Ekman pumping resulting in open‐ocean upwelling and coastal upwelling create two distinctly different mixed layer plankton environments in the northwestern Arabian Sea during the summer monsoon. Open‐sea upwelling to the northwest of the mean July position of the Findlater Jet axis yields a mixed layer environment with temperatures of less than 25°C to about 26.5°C, phytoplankton pigment concentrations between 1.5 and 5.0 mg/m³, and mixed layer depths less than 50 m. Convergence in the Ekman layer in the central Arabian Sea drives the formation of a mixed layer that is greater than 50 m thick, warmer than about 26.5°C, and has phytoplankton pigment concentrations generally below 2.0 mg/m³. Coastal upwelling creates an extremely eutrophic plankton environment that persists over and immediately adjacent to the Omani shelf and undergoes significant offshore transport only within topographically induced coastal squirts. The foraminiferal faunal composition of upper Pleistocene deep‐sea sediments of the northwestern Arabian Sea are mainly controlled by vertical nutrient fluxes caused by Ekman pumping, not coastal upwelling. Transfer functions for late Pleistocene mixed layer depth, temperature, and chlorophyll have been obtained through factor analysis and nonlinear multiple regression between late summer mixed layer environment and Recent sediment faunal observations.