bluuebunny/crossref_metadata_2025 huggingface dataset:
Satellite‐derived phytoplankton pigment structures in the Portuguese upwelling area
Authors: Sousa Fátima M. , Bricaud Annick
Published: February 2008

A series of 25 Nimbus 7 coastal zone color scanner images obtained from July 1981 to September 1983 in Portuguese coastal waters was analyzed in order to study the space‐time distribution of phytoplankton pigment. In the images from winter and spring, the phytoplankton distributions showed no significant spatial patterns. During the upwelling season, from late June to October, several recurrent patterns were observed. Filaments of high pigment concentration extended zonally 200 km off the west coast of Portugal, north of Lisbon. Pigment‐rich plumes were observed south of capes along the west coast, south of Lisbon, and parallel to the south coast of Portugal. The location of the filaments and plumes coincided with topographic features such as submarine ridges. South of Lisbon the location of plumes was also related to coastal morphology. The temporal variability of the pigment patterns was compared with the wind‐induced offshore Ekman transport calculated from measurements made at two meteorological stations, located at Lisbon and Faro, on the south coast. A significant relationship was found: well‐developed phytoplankton structures were generally related to moderate or intense offshore transport, whereas the absence of plumes corresponded to either weak offshore transport or coastal convergence. For each of the three coastal areas in this study, scatter plots showed linear relationships between digital counts of phytoplankton concentration and sea surface temperature. The spatial variation of the temperature range decreased in time after the onset of an upwelling event. One cause of this variation could be the north‐south change in temperature of the upwelled Eastern North Atlantic Central Water along the Portuguese coast.
Low‐frequency variability of the tropical Atlantic surface topography: Altimetry and model comparison
Authors: Arnault S. , Morliére A. , Merle J. , Ménard Y.
Published: February 2008

Altimetric data, climatological hydrological data, and numerical model results are compared over the tropical Atlantic Ocean between November 1986 and November 1988. All reproduce the seasonal cycle of the dynamic topography rather well, and the agreement is particularly good between altimetry and the primitive equation model. The study of the 1986–1988 period reveals interannual events evidenced by both the altimetry and the models, especially during spring 1988 in the Gulf of Guinea.
A numerical investigation of circulation in the Arabian Gulf
Authors: Chao Shenn‐Yu , Kao Timothy W. , Al‐Hajri Khalid R.
Published: February 2008

A three‐dimensional hydrodynamic model is developed to study the circulation in the Arabian Gulf. The model contains realistic basin geometries and bathymetries of the Arabian Gulf and a good portion of the Gulf of Oman and is driven by monthly climatological winds, evaporation, and net ocean heat gain in both gulfs and the Shatt‐al‐Arab discharge. It is found that the cyclonic circulation in the southern portion of the gulf is primarily driven by the evaporation‐induced freshening from the Strait of Hormuz. In the northwestern corner of the gulf, the Shatt‐al‐Arab discharge maintains the cyclonic circulation, which would otherwise be anticyclonic. The northwestward intrusion of fresher water along the Iranian coast is weakened by northwesterly winds in winter but strengthens and extends almost to the head of the gulf in summer owing to the warming of the Gulf of Oman waters, the development of the summer thermocline in the Arabian Gulf, and diminishing winds. The southward coastal current along the Arabian coast is most prominent between the head of the gulf and Qatar. The model also predicts a strong southward coastal jet east of Qatar which is primarily wind driven. No similar coastal jet can be developed in the Gulf of Salwa, west of Qatar.
Sea spray and the turbulent air‐sea heat fluxes
Authors: Andreas Edgar L
Published: February 2008

Heat and moisture carried by sea spray have long been suspected of contributing to the air‐sea fluxes of sensible and latent heat. Using time scales that parameterize how long sea spray droplets reside in the air and how quickly they exchange heat and moisture with their environment, I estimate sea spray contributions to the air‐sea heat fluxes. To make these estimates, I first develop a new sea spray generation function that predicts more realistic spume production than earlier models. Spray droplets with initial radii between 10 and 300 μm contribute most to the heat fluxes; the vast majority of these are spume droplets. The modeling not only demonstrates how spray droplets participate in the air‐sea heat exchange but also confirms earlier predictions that the heat carried by sea spray (especially the latent heat) is an important component of the air‐sea heat balance. In my examples, the maximum magnitude of the spray latent heat flux for a 20‐m/s wind is 170 W/m2; the maximum spray sensible heat flux is 33 W/m2. For winds over 10 m/s, the spray latent heat flux is usually a substantial fraction of the interfacial (or turbulent) latent heat flux (estimated from the bulk‐aerodynamic equations) and will thus confound measurements of the air‐sea transfer coefficient for latent heat.
Formation and spreading of subtropical mode water in the North Pacific
Authors: Bingham Frederick M.
Published: February 2008

A time‐averaged description of the formation and spreading of North Pacific subtropical mode water (NPSTMW) is presented. The formation of NPSTMW is studied using TRANSPAC expendable bathythermograph data. The data are transformed into a stream coordinate system and averaged. The coordinate system uses the position of the Kuroshio front as its origin. Maps are presented of the temperature structure and mixed layer distribution in winter relative to the Kuroshio front. The formation of NPSTMW occurs in two separate stages, cooling and thickening. As water flows at the surface in the Kuroshio in winter, the water is cooled by the intense ocean‐atmosphere heat flux taking place in the area. When water leaves the Kuroshio to the south, the thermocline underneath becomes deeper, convection is able to penetrate deep into the water column, and the heat flux acts more to thicken the surface mixed layer rather than reduce the temperature. The area south of the Kuroshio, where relatively homogeneous waters are found, is the source region for NPSTMW. At the end of the winter the thick mixed layers are assumed to be covered over by a seasonal thermocline and become NPSTMW. The Levitus (1982) data are used to study the way in which NPSTMW is spread away from its source region by the subtropical circulation. The distribution of potential vorticity (PV) on four NPSTMW isopycnals is studied, with low PV used as a tracer for NPSTMW. The PV reveals a pattern of “differential spreading,” whereby NPSTMW formed in the western part of the source region is advected to the west after its formation. NPSTMW formed farther east is advected more to the south or east after formation. Differential spreading allows NPSTMW to be found in a large part of the northwestern subtropical gyre, well away from its source region. The distribution of NPSTMW described by Masuzawa (1969), with NPSTMW getting colder from west to east, is explained as a combination of the distribution of NPSTMW at its source and the way it is spread after it is formed.
Unidirectional flow over asymmetric and symmetric ripples
Authors: Wiberg Patricia L. , Nelson Jonathan M.
Published: February 2008

Detailed measurements of velocity and turbulence over fixed sets of two‐dimensional asymmetric and symmetric ripples were collected in a flume equipped with a laser‐Doppler velocimeter. The measured velocity profiles show a region of strong wake influence extending 2–3 bedform heights above the bed and an outer, spatially uniform flow that has adjusted to the hydrodynamic roughness of the ripples. The measured velocities over ripples, when compared to measurements of flow over larger‐scale dunes of a similar geometry made by Nelson and Smith (1989), differ in two major respects: the velocity gradients are significantly larger in the outer region of the flow, and the velocity profiles exhibit no sharp inflection at the top of the lowest wake. A model for flow over bedforms that had provided excellent agreement with the dune measurements is modified herein in a physically reasonable manner to represent better the observed flow over ripples. The predictions of the modified model compare well with the velocity measurements made over sets of asymmetric and symmetric ripples in a unidirectional flow when the appropriate drag coefficients for the two bed geometries are used. Drag coefficients deduced from the measurements suggest a possible dependence on relative depth as well as ripple geometry. Hydrodynamic ripple roughnesses determined from the measured and calculated profiles have values of the same order as estimates made using several existing expressions for the roughness of bedforms and regular roughness arrays. However, the measurements and calculations also indicate that bottom roughness depends on more than the ripple height times ripple steepness length scale used in these formulations.
Global sea level change from satellite altimetry
Authors: Wagner Carl A. , Cheney Robert E.
Published: February 2008

We examine the feasibility of using satellite altimeter data to measure the long‐term change of global sea level (estimated from tide gauge data to be a rise of approximately 0.2 cm yr−1). Two and one‐half years of collinear Geosat altimeter data (1986–1989) are used together with a 17‐day set of Seasat altimetry (July–August 1978) having nearly the same ground track. A consistent set of precise orbits was used throughout, and residual orbit error was removed as a sinusoidal fit to approximately 3‐day arcs of sea level collinear differences. The globally averaged Geosat data show sea level falling at 1.2 ± 0.3 cm yr−1over the first 2 years, even after removal of tide errors and instrument biases not accounted for in the Geosat geophysical data records. This unrealistic result is found to be due largely to long‐term error in the ionospheric model for the single‐frequency Geosat altimeter. The Geosat‐Seasat comparison, based on data 10 years apart, shows an apparent sea level rise of 1.0 cm yr−1. Assuming this result is also unrealistic, a possible explanation is a biased scale to the Doppler‐determined Geosat orbit which, unlike Seasat, did not have the benefit of laser tracking. It is also possible that the Geosat altimeter (without external in‐orbit calibration) had a bias of the order of 10 cm. We conclude that for satellite altimetry to make a fundamental contribution to monitoring global mean sea level change, both the altimeter (including its media corrections) and the orbit model which provides a geocentric reference for the ocean surface will need continuing and careful calibration with absolute standards.
Three‐dimensional analytical model for the mixed layer depth
Authors: Yan Xiao‐Hai , Okubo Akira
Published: February 2008

We have recently developed a three‐dimensional semi‐analytical mixed layer remote sensing model which includes the horizontal advection and diffusion processes in our previous one dimensional model (Yan et al., 1991a). The three‐dimensional (3‐D) model can be used as a supplement to the one‐dimensional (1‐D) model in the area where advection is important and for a finer resolution grid prediction. The 3‐D thermal inertia model is based on the 3‐D thermal energy conservation equation. Using sine transformation and inverse sine transformation methods, we obtained the solution for mixed layer thermal inertia which, in turn, can be used to calculate mixed layer depth using a similar method to that of Yan et al. (1991a). In the solution, the thermal inertia (which is proportional to the mixed layer depth) is expressed as functions of sea surface temperature changes (δSST), heat flux (Q) and surface layer velocity (u, v, and w). The vertical entrainment and wind effects are accounted for in the model by eddy diffusivity while changes of δSST result from entrainment, e.g., if vertical entrainment is larger, δSST will be smaller and vice versa. The advantages of this model are that it can be easily forced by remotely sensed data and that it is much simpler to compute than numerical 3‐D mixed layer turbulence closure models. The model was tested using the data from the advanced very high resolution radiometer/multichannel sea surface temperature data set and the Cooperative Ocean Atmosphere Data Set for the North Pacific Ocean. The model‐predicted mixed layer depths compared favorably with the mixed layer depths calculated from 14 years of Volunteer Observing Ship/expendable bathythermograph observation data. The model/data comparison also exhibited a number of mesoscale features of seasonal changes and anomalies of the mixed layer depth.
Using the extended Kalman filter with a multilayer quasi‐geostrophic ocean model
Authors: Evensen Geir
Published: February 2008

The formulation of the extended Kalman filter for a multilayer nonlinear quasi‐geostrophic ocean circulation model is discussed. The nonlinearity in the ocean model leads to an approximative equation for error covariance propagation, where the transition matrix is dependent on the state trajectory. This nonlinearity complicates the dynamics of the error covariance propagation, and effects which are nonexistent in linear systems contribute significantly. The transition matrix can be split into two parts, where one part results in pure evolution of error covariances in the model velocity field, and the other part contains a statistical correction term caused by the nonlinearity in the model. This correction term leads to a linear unbounded instability, which is caused by the statistical linearization of the nonlinear error propagation equation. Different ways of handling this instability are discussed. Further, nonlinear small‐scale instabilities also develop, since energy is accumulated at wavelengths 2Δx, owing to the numerical discretization. These small‐scale oscillations are removed with a Shapiro filter, and the effect they have on the error covariance propagation is discussed. Some data assimilation experiments are performed using the full extended Kalman filter, to examine the properties of the filter. An experiment where only the first part of the transition matrix is used to propagate the error covariances is also performed. This simplified experiment actually performs better than the full extended Kalman filter because the unbounded instability associated with the statistical correction term is avoided.
A finite element model for tides and resonance along the north coast of British Columbia
Authors: Foreman M. G. G. , Henry R. F. , Walters R. A. , Ballantyne V. A.
Published: February 2008

A finite element, barotropic, tidal model is developed for the north coast of British Columbia. The model is run with eight tidal constituents and the results are compared with the Flather (1987) finite difference model, and with extensive tide gauge and current meter observations. Although the tidal potential, Earth tide, and loading tide are included in the forcing, their inclusion is shown to change the largest M2 amplitudes by only 2.5% and the largest K1 amplitudes by less than 1%. Root mean square differences between observed and calculated sea level amplitudes and phases are within 1.9 cm and 2.9° for all but one constituent, but the model currents do not in general, compare as favourably. The barotropic currents observed in Hecate Strait are reproduced well, but elsewhere evidence is shown that model inaccuracies are due to baroclinic effects. Tidal residual currents calculated by the model suggest the existence of eddies off the tip of Cape St. James, Cape Chacon, and around Goose Island and Learmonth Banks. The shallow water constituents in Hecate Strait are shown to have significant contributions from the constructive interference of signals propagating into Dixon Entrance and Queen Charlotte Sound. Using the model, the longest resonant period of the system is estimated to be 7.6 hours with an energy dissipation parameter, Q, of 9.5.
Modeling short sea wave energy distributions in the far wakes of ships
Authors: Milgram J. H. , Skop Richard A. , Peltzer Rodney D. , Griffin Owen M.
Published: February 2008

A mathematical model based on the spectral energy balance equation for suppression of shortwave energy in centerline ship wakes is developed. At distances behind a ship of more than a few ship lengths, the dominant influences on short wave energy are the growing effects of wind and nonlinear energy transfer from longer waves, and the attenuating effects of turbulence and viscous attenuation which can be greatly enhanced by surface film distributions which are modified by passage of the ship. These influences are modeled as “source terms” in the energy balance equation. The model is exercised using turbulence data from scale model experiments and surface film distributions measured in an actual ship wake. Agreement between the resulting distribution of Bragg‐scattering wave energy and radar image intensity in the far wake is excellent. The influences of variations in the source term model parameters are explored numerically.
Observations in the Canary Basin and the Azores frontal region using Geosat data
Authors: Tokmakian R. T. , Challenor P. G.
Published: February 2008

This paper discusses observations of the sea level in the Azores frontal region and the Canary Basin as seen by the Geosat altimeter frorn 5° to 45°W and 25° to 45°N. Two years of Geosat exact repeat data were processed by colocating the data on a 0.06° grid then removing the mean by the along track difference method and removing the orbit error by a crossover analysis technique. The data set was then interpolated by the successive correction method onto 15‐day, 0.5° maps to produce 47 residual height fields which were analyzed using complex empirical orthogonal functions (EOF) and spectral analysis. The results show that there is a region of high variability in the northwest corner reflecting the Gulf Stream extension and a region of medium variability at approximately 35°N reflecting the Azores Current. The variability changes only slightly throughout the year, with the winter season being the highest. The data set also shows first mode baroclinic Rossby waves between 30° and 35°N. There is evidence that these Rossby waves have different wavenumbers to the east and west of the Mid‐Atlantic Ridge. It is postulated that these waves have different generation mechanisms, the Mid‐Atlantic Ridge topography in the west and some other generation mechanism in the Canary Basin. Rossby waves are not apparent at 40°N.
Passive microwave signatures of fractures and ridges in sea ice at 33.6 GHz (vertical polarization) as observed in aircraft images
Authors: Farmer L. Dennis , Eppler Duane T. , Lohanick Alan W.
Published: February 2008

An aircraft data set of coincident Kaband (33.6 GHz, vertical polarization) passive microwave images and aerial photographs acquired in the Chukchi‐Beaufort Sea region in March 1983 was analyzed to evaluate radiometric signatures of deformational features that occur in sea ice. A total of 115 fractures and 197 pressure ridges were examined with respect to physical appearance (relative age, snow cover, ice type, width, orientation) as observed in photographs, and radiometric character (brightness temperature, radiometric contrast with respect to adjacent ice, radiometric profile across the feature) as measured from digital passive microwave images. Of the deformational features that were observed in aerial photographs, 82% had radiometric signatures of sufficient contrast to be observed in passive microwave images. Fractures and ridges have equal chance of detection, but fractures cannot be distinguished from pressure ridges on the basis of brightness temperature, radiometric contrast, or characteristics of radiometric profiles measured across these features. However, visual analysis of passive microwave images, which takes into account contextual information, does allow fractures to be distinguished from pressure ridges in many instances. Radiometric signatures of both fractures and ridges are more likely to be radiometrically warmer (as opposed to cooler) than adjacent ice, which suggests that saline ice is a significant constituent of most deformational features. New ridges are more likely to be radiometrically warm than old ridges, probably because brine drains from the ridge as it ages (which reduces emissivity) and snow accumulates in drifts along the ridge trend (which enhances scattering). However, brightness temperatures of snow‐covered ridges extend across a range that is approximately 15 K cooler, and 10 K warmer than the range observed for snow‐free ridges. Old features show higher radiometric contrast with respect to adjacent ice than new features, which increases their probability of detection.
Determination of dielectric constant of young sea ice using microwave spectral radiometry
Authors: St. Germain Karen M. , Swift Calvin T. , Grenfell Thomas C.
Published: February 2008

Microwave brightness temperature measurements across C band during the growth of a saline ice sheet are presented. The data were collected with the Stepped Frequency Microwave Radiometer (SFMR) as part of a cooperative experiment at the U.S. Army Cold Regions Research and Engineering Laboratory, CRRELEX '88. Data at the six frequencies of the SFMR exhibited oscillatory behavior as the ice sheet developed, which indicates a coherent effect in the emission. Ice thickness and angle of incidence were monitored continuously, making it possible to solve for the dielectric constant of the sheet at ice thicknesses corresponding to local brightness temperature maxima and minima. The resulting values for dielectric constant are large for extremely thin ice but then decrease to approach the accepted first‐year ice value as the ice becomes optically thick.
Seasonal distribution of nutrients and primary productivity on the eastern continental shelf of Venezuela as influenced by the Orinoco River
Authors: Bonilla Jaime , Senior William , Bugden John , Zafiriou Oliver , Jones Ronald
Published: February 2008

Nitrogenous nutrients, dissolved silicate, and salinity were measured in surface waters and shallow hydrocasts along similar cruise tracks during the spring (dry season) and fall (wet season) of 1988. Both cruises transected the eastern Caribbean, transited the Gulf of Paria, ran parallel to the Orinoco Delta and into the main channel of the Orinoco River. Trends in primary productivity were also measured by daily carbon 14 incubations. In both seasons, samples covered the range from highly oligotrophic and transparent to highly productive and rich in biogenic and abiogenic particulate matter. Most of the Orinoco outflow appears to turn N to NW and remains in shallow waters off Venezuela and surrounding Trinidad, permitting benthic regeneration of river‐borne nutrients. However, the role of the Orinoco and associated low‐salinity coastal waters in fertilizing large areas of the eastern Caribbean basin, as suggested by satellite imagery, can be approximated crudely from the nutrient composition at Boca de Dragon, which is representative of the nutrient status of these waters as they flow into deeper Caribbean waters. Additional nutrients may be supplied to the area primarily from Amazon‐derived water entering the Caribbean Basin further north, with some coastal upwelling along the continental shelf in the dry season.
Sea surface height variability during 1986–1988 in the tropical Pacific Ocean
Authors: Chao Yi , Halpern David , Perigaud Claire
Published: February 2008

Sea surface height variabilities during 1986–1988 in the tropical Pacific Ocean are described, based upon the Geosat altimeter data and a tropical Pacific Ocean general circulation model (OGCM) simulation forced with observed winds from Florida State University. Both data sets are validated against in situ sea level measurements at 12 selected sea level stations in the equatorial Pacific and were found to be in good agreement with these measurements with correlation coefficients (averaged over the 12 sea level stations in the equatorial Pacific) of 0.69 for Geosat and 0.71 for OGCM. Both the Geosat data and OGCM simulation capture significant amounts of variance of in situ measurements with standard deviations (averaged over the 12 sea level stations) of 7.7 cm for Geosat and 7.1 cm for OGCM as compared with that of 9.8 cm for in situ measurements. The Geosat altimeter data, given their uncertainties, are capable of describing sea surface height variabilities during 1986–1988 in the tropical Pacific Ocean, in particular the seasonal cycle, the 1987 El Niño and 1988 La Niña episodes, and the December 1986 equatorial Kelvin wave event. The OGCM‐simulated surface dynamic height is found sensitive to different surface wind products: The standard deviation of the surface dynamic height simulated by the European Center for Medium‐Range Weather Forecasting (ECMWF) 10‐m wind product is only 5.5 cm.
A three‐dimensional numerical model of cloud dynamics, microphysics, and chemistry: 1. Concepts and formulation
Authors: Wang Chien , Chang Julius S.
Published: February 2008

We have developed a three‐dimensional, comprehensive cloud modeling system with detailed descriptions of dynamics, microphysics, and chemistry. This system can be used to study not only the evolution of dynamical and microphysical processes, but also those of chemical processes and the interactions or relationships among them. The primary objective of this model is the simulation of deep convective events and severe storms, although in principle the stratiform clouds and clouds in frontal systems could also be included with some improvements of the model. The physical component of this model starts with a pseudoelastic form of the continuity equation. The overall model is a self‐consistent, time‐dependent system which is easier to solve. A so‐called ice‐liquid potential temperature is used as a thermodynamic conservative variable. Seven different water categories in three phases are considered in this model. Both the mass mixing ratio and concentration of each liquid or solid phase category are predicted so that a semi‐spectra microphysical parameterization scheme can be formulated. More than 40 microphysical processes have been parameterized based on the spectrum‐integration and the behavior of the individual particles in the microphysical interactions. Concentrations of seven chemical groups in air and in six liquid‐ or solid‐phase water carriers are predicted by our chemical model. Their evolution in this model came from not only the dynamical and subgrid‐scale transports, but also the chemical reactions and microphysics‐related conversions. Modeling results include the dynamical features and appropriate detailed descriptions of microphysical and chemical evolution of the modeled clouds. This allows more direct comparison between model results and observational data. A potential use of this model would be in the analysis and interpretation of field observations.
Paleoclimatic tracers: An investigation using an atmospheric general circulation model under ice age conditions: 2. Water isotopes
Authors: Joussaume Sylvie , Jouzel Jean
Published: February 2008

The linear relationship observed between the water isotopic contents of precipitation and surface air temperatures leads to the use of the water isotopes, H218O and HDO, in paleoclimatology. Applied to the measurements of the isotopic content of paleowaters, like groundwaters and deep ice cores, this relationship is used to infer paleotemperatures. However, this interpretation of paleo‐isotopic contents is only valid if the isotope‐temperature relationship is not affected by climate change. To address this problem, we have developed a water isotope modeling inside an atmospheric general circulation model (AGCM) and performed simulations of both the present‐day and Last Glacial Maximum (LGM) climatic conditions. AGCMs are indeed the only appropriate tools able to account the whole complexity of the atmospheric circulation. For the present‐day climate, preliminary results for January were presented by Joussaume et al. (1984) and are complemented by new simulations performed for both February and August climatic conditions with a higher‐resolution version of the model. Model results are well corroborated by observations. They also exhibit some effects of the atmospheric circulation on the isotopic fields. For the simulated LGM climate, the model results compare well with paleoclimatic data of water isotopic contents, except for a higher than observed spatial variability. The overall patterns of the simulated δ18O‐temperature relationship for the LGM climate are practically unchanged, which tends to comfort the use of water isotopes in paleoclimatology. However, concerning the deuterium excess, i.e., the relationship between oxygen 18 and deuterium, the model results are not sufficiently valid to allow a discussion of the use of deuterium excess in paleoclimatology.
Solar radiation absorption by CO2, overlap with H2O, and a parameterization for general circulation models
Authors: Freidenreich S. M. , Ramaswamy V.
Published: February 2008

Line‐by‐line (LBL) solar radiative transfer solutions are obtained for CO2‐only, H2O‐only, and CO2 + H2O atmospheres, and the contributions by the major CO2 and H2O absorption bands to the heating rates in the stratosphere and troposphere are analyzed. The LBL results are also used to investigate the inaccuracies in the absorption by a CO2 + H2O atmosphere, arising due to a multiplication of the individual gas transmissions averaged over specific spectral widths (Δν). Errors in absorption generally increase with the value of Δν chosen. However, even when the interval chosen for averaging the individual gas transmissions is the entire solar spectrum, there is no serious degradation in the accuracy of the atmospheric absorbed flux (error < 3%) and the heating rates (errors < 10%). A broadband parameterization for CO2 absorption, employed in several weather prediction and climate models, is found to substantially underestimate the LBL heating rates throughout the atmosphere, most notably in the stratosphere (errors > 40%). This parameterization is modified such that the resulting errors are less than 20%. When this modified CO2 parameterization is combined with a recently modified formulation for H2O vapor absorption, the resulting errors in the heating rates are also less than 20%. The application of the modified solar absorption parameterizations in a general circulation model (GCM) causes an increase in the simulated clear sky diabatic heating rates, ranging from nonnegligible (middle stratosphere and lower troposphere) to significant (lower stratosphere and upper troposphere) additions. The results here should enable a continued use of the older broadband parameterizations in GCMs, albeit in modified forms.
A three‐dimensional modeling study of trace species in the Arctic lower stratosphere during winter 1989–1990
Authors: Chipperfield M. P. , Cariolle D. , Simon P. , Ramaroson R. , Lary D. J.
Published: February 2008

A three‐dimensional (3D) radiative‐dynamical‐chemical model has been developed and used to study the evolution of trace gases in the Arctic lower stratosphere during winter 1989–1990. A series of 10‐day model integrations were performed throughout this period. The model includes a comprehensive scheme of gas phase chemical reactions as well as a parameterization of heterogeneous reactions occurring on polar stratospheric cloud (PSC) surfaces. An important element of a 3D chemical model is the transport scheme. In this study the transport of chemical species is achieved by a non diffusive method well suited to the preservation of sharp gradients. During the winter studied temperatures were cold enough for the formation of both type I and type II polar stratospheric clouds from early December to early February. Model simulations in late December show that inside the polar vortex air is rapidly processed by polar stratospheric clouds converting HCl and ClONO2 to active chlorine. The possibility of ozone destruction depends strongly on the amount of sunlight. In early February an average ozone loss of 15 ppbv (parts per billion by volume) /day is predicted in PSC‐processed air at 50 hPa, giving a column loss of just under 1 DU/day. This loss increases to 25 ppbv/day if PSCs persist until March with a column loss of around 1.5 DU/day. The relatively small magnitude of the ozone loss predicted in the model, compared to the variability of ozone induced by dynamics, highlights the problems in identifying the signature of chemical ozone loss in the Arctic. In future years significant ozone depletion could occur if PSCs persist until late March. The efficiency of the catalytic cycles responsible for the ozone loss has been analyzed as a function of latitude, altitude and time. In general, the cycle involving ClO + ClO is the dominant loss mechanism in the polar lower stratosphere. Cycles involving BrO can make a relatively large contribution early in the season and when the levels of ClO are low. The cycle initiated by ClO + O destroys ozone at altitudes above 30 hPa but the loss is compensated, to some extent, by in situ ozone production. The results for trace species are validated, where possible, by comparison with the available measurements, although the sparse nature of the observations does not effectively constrain the model.
The magma ocean as an impediment to lunar plate tectonics
Authors: Warren Paul H.
Published: February 2008

Potential for plate tectonics on a planet is determined by several factors besides the obvious one of temperature. One of the most important of these is the ratio zc/zl, where zc and zl are the thicknesses of the crust and tectonic lithosphere, respectively. A high zc/zl ratio imparts a low bulk density, and thus a high buoyancy, to the lithosphere. Cooling of the lithosphere can offset a limited degree of compositional buoyancy, but for realistic assumptions regarding crustal and mantle densities, lithosphere geotherms, and the spatial variability of the zc/zl ratio, systematic lithosphere subduction (i.e., plate tectonics) is unlikely on any planet with average zc/zl ratio greater than about 0.25. During a “magma ocean” phase, zc/zl must be ∼ 1, so systematic lithosphere subduction is impossible. In general, zl is closely linked to the average planet temperature, whereas zc is only weakly related to T but is sensitive to the planet's pressure‐depth gradient dP/dz (i.e., its size), which governs the fate of Al during deep melting/crystallization processes. The Moon has probably always had zc/zl too high, or in later times lithosphere thickness too high, to sustain plate tectonics. Conceivably, plate tectonics operated over the first few million years of lunar evolution, particularly if the early interior was cooler than generally assumed and a true magma “ocean” never developed, or only developed after gradual warming of the deep interior. However, weighing against this scenario, in addition to the wide array of evidence in favor of the magma ocean hypothesis, are the consistently moderate to young (by lunar standards) ages of geochemically evolved lunar rocks.
Thermally altered palagonitic tephra: A spectral and process analog to the soil and dust of Mars
Authors: Bell James F. , Morris Richard V. , Adams John B.
Published: February 2008

We studied six palagonitic soil samples (PH‐1 through PH‐6) which were collected at 30‐cm intervals from a lava slab on Mauna Kea, Hawaii. The samples represent an alteration sequence caused by heating during emplacement of molten lava over a preexisting tephra cone. They are both spectral and weathering/alteration process analogs to the Martian surface. Techniques employed included visible and near‐IR spectroscopy, Mössbauer spectroscopy, and magnetic analysis. The two samples collected from beyond 90 cm from the slab (PH‐5 and PH‐6) did not see the transient heating event and are composed of coarse‐grained glassy basaltic (hawaiitic) particles, some of which have been moderately palagonitized since formation of the cone in the late Pleistocene. The four samples closest to the slab (PH‐1 through PH‐4) have been strongly altered in response to heating during its emplacement; their iron oxide mineralogy is dominated by nanophase ferric oxide. The sample adjacent to the slab (PH‐1), which would have seen the highest temperatures, has a factor of 3 less H2O and contains crystalline hematite and magnetite in addition to nanophase ferric oxide. Lesser amounts of magnetite, but not hematite, are present in samples 30 cm (PH‐2) and 60 cm (PH‐3) from the slab. The reflectivity spectra of samples PH‐2 through PH‐4 are similar to spectra of palagonites reported by other workers to be good Mars visible to near‐IR spectral analogs. The reflectivity spectrum of PH‐1 is an even better spectral analog to Mars in that it exhibits absorption features indicative of both nanophase and crystalline ferric oxides (similar to several new Martian telescopic data sets). Our sampling site may also be a process analog for Mars, in that heating episodes by volcanism and/or impact cratering could produce crystalline ferric oxides from poorly crystalline palagonitic material. Thus, localized thermal alteration events may provide a volumetrically important mechanism for the palagonitization of basaltic glass and the production of crystalline ferric oxides on Mars.
A minimal cost function method for optimizing the age‐Depth relation of deep‐sea sediment cores
Authors: Brüggemann Wolfgang
Published: February 2008

The question of an optimal age‐depth relation for deep‐sea sediment cores has been raised frequently. The data from such cores (e.g., δ18O values) are used to test the astronomical theory of ice ages as established by Milankovitch in 1938. In this work, we use a minimal cost function approach to find simultaneously an optimal age‐depth relation and a linear model that optimally links solar insolation or other model input with global ice volume. Thus a general tool for the calibration of deep‐sea cores to arbitrary tuning targets is presented. In this inverse modeling type approach, an objective function is minimized that penalizes: (1) the deviation of the data from the theoretical linear model (whose transfer function can be computed analytically for a given age‐depth relation) and (2) the violation of a set of plausible assumptions about the model, the data and the obtained correction of a first guess age‐depth function. These assumptions have been suggested before but are now quantified and incorporated explicitly into the objective function as penalty terms. We formulate an optimization problem that is solved numerically by conjugate gradient type methods. Using this direct approach, we obtain high coherences in the Milankovitch frequency bands (over 90%). Not only the data time series but also the the derived correction to a first guess linear age‐depth function (and therefore the sedimentation rate) itself contains significant energy in a broad frequency band around 100 kyr. The use of a sedimentation rate which varies continuously on ice age time scales results in a shift of energy from 100 kyr in the original data spectrum to 41, 23, and 19 kyr in the spectrum of the corrected data. However, a large proportion of the data variance remains unexplained, particularly in the 100 kyr frequency band, where there is no significant input by orbital forcing. The presented method is applied to a real sediment core and to the SPECMAP stack, and results are compared with those obtained in earlier investigations.
Calibration of Upper Pliocene‐Lower Pleistocene Nannofossil Events with Oxygen Isotope Stratigraphy
Authors: Wei Wuchang
Published: February 2008

Twelve upper Pliocene‐lower Pleistocene calcareous nannofossil events are calibrated with oxygen isotope stratigraphy at eight DSDP/ODP sites ranging from 53°N to 41°S latitudes in different oceans in order to determine their ages and to reveal their synchrony or diachrony within a significantly higher‐resolution time scale than the geomagnetic polarity time scale allows. Six nannofossil events appear to be synchronous from low to middle latitudes (the last occurrences of Reticulofenestra asanoi (oxygen isotope stage 22), Gephyrocapsa spp. A‐B (stage 37), Discoaster brouweri (stage 71), Discoaster surculus (stage 99), Discoaster tamalis (stage 111), and the first occurrence of Gephyrocapsa spp. A‐B (stage 59)), five appear to be diachronous (the first occurrences of Gephyrocapsa spp. C‐D, Reticulofenestra asanoi, and Gephyrocapsa spp. A‐B larger than 5.5 µm, and the last occurrences of Helicosphaera sellii and Discoaster pentaradiatus), and one is synchronous at low latitudes (the last occurrence of Calcidiscus macintyrei).
Electromagnetic scattering by arbitrarily shaped surfaces with impedance boundary conditions
Authors: Glisson Allen W.
Published: February 2008

The impedance boundary condition has been implemented in the electric field integral equation triangular patch model for scattering from arbitrarily shaped, imperfectly conducting surfaces. The impedance boundary condition function has been assumed linear within each triangular face, and discontinuous changes in the impedance value are allowed from face to face. The procedure has been validated by comparison of numerical results obtained with the model and exact solutions for scattering by spheres having uniform impedance boundary conditions. Validation for scattering by objects with discontinuous or nonuniform impedance boundary conditions has been obtained by comparison of triangular patch models with body of revolution models.
The dynamic ionospheric polar hole
Authors: Crowley G. , Carlson H. C. , Basu S. , Denig W. F. , Buchau J. , Reinisch B. W.
Published: February 2008

At a time when most polar cap ionospheric studies are focused on electron density enhancements caused by patches or arcs, this paper discusses an F region depletion known as the ionospheric polar hole. Statistically, polar holes tend to develop on the nightside from 2100–0600 MLT between 70°–80°Λ. Electron concentrations as small as 2×102 cm−3 at altitudes near 300 km have previously been observed in polar holes during solar minimum winter conditions. Under magnetically quiet conditions, polar holes are thought to form due to slow convection of plasma across the polar cap. The electron concentration is depleted by normal recombination processes during many hours spent in the dark polar cap. In contrast with previous studies, which have tended to be statistical in nature, this paper describes the temporal and spatial development of a single polar hole. During the Geospace Environment Modelling Pilot Program of January 16,1990, the development of the polar hole was monitored by the DMSP F8 and F9 satellites, a digital ionosonde and a 250‐MHz scintillation receiver. The data reveal that the polar hole is a very dynamic phenomenon, and its location can change dramatically within several hours. This is the first study of the polar hole using ground based instruments. The observations presented here are of interest not only because they contribute to a morphological understanding of the polar hole, but also because they illustrate the influence of the polar hole on radio signals propagating in the ionospheric medium. The results are of direct relevance to applications and systems involving ionospheric and transionospheric propagation. The data set is sufficiently complete that it will provide rigorous constraints for future modeling studies and will thus contribute to a better physical understanding of the polar hole.
Effect of overburden thickness on thrust belt geometry and development
Authors: Marshak Stephen , Wilkerson M. Scott
Published: February 2008

Laboratory sand models simulating development of thrust belts indicate that (1) distance between the backstop and the first‐formed thrust is a linear function of initial sand layer thickness; (2) spacing between the first two thrusts is most likely a nonlinear function of initial sand layer thickness; (3) initial dip of successive ramps decreases progressively toward the foreland because of variations in principal stress directions with depth and/or changes of mechanical properties of sand with increasing overburden; (4) faults to the hinterland steepen and become less active as hinterland thrust sheets undergo penetrative shear during sand wedge growth; and (5) new ramp initiation occurs to the hinterland of the basal detachment tip line. These observations can be interpreted both in context of area‐balance considerations and soil mechanics theory. If initial sand layer thicknesses in our models represent syntectonic overburden in natural thrust belts (i.e., distance between the ground surface and the detachment), then our results suggest that locations of some thrust belt salients are controlled by along‐strike variations in stratigraphic thicknesses and that initial thrust spacing decreases progressively to the foreland if the thrust belt involves a foreland‐tapering basin. Furthermore, our sand model observations suggest that the concept of a fixed, rigid backstop in the hinterland of thrust belts may be misleading. As a consequence of thickening, the hinterland portion of the thrust belt becomes stabilized and effectively becomes the backstop itself.
The tectonostratigraphy and structure of the eastern Nepal Himalaya
Authors: Schelling Daniel
Published: February 2008

Six years of geological research in eastern Nepal has resulted in a new geological map of the eastern Nepal Himalaya which includes the region stretching from the Sikkhim border in the east to the Kathmandu Valley in the west, and from the summits of the Higher Himalaya in the north to the Ganges Plain in the south. This research has permitted the determination of the tectonostratigraphy and structure of one section of the central Himalayan arc. South of the Tibetan Plateau the eastern Nepal Himalaya can be divided into three distinct, thrust‐bound tectonic packages: (1) the Higher Himalayan thrust sheet composed of the Higher Himalayan Crystallines, (2) the Lesser Himalayan thrust sheet composed of the Lesser Himalayan Series, and (3) the Sub‐Himalayan imbricate zone composed of sedimentary rocks belonging to the Siwalik Group. The Higher Himalayan thrust sheet of eastern Nepal has been thrust over the Lesser Himalayan Metasediments a minimum of 140 km, and possibly as much as 175–210 km, along the Main Central Thrust (MCT). The Lesser Himalayan thrust sheet is overlain by the MCT and is underlain by the Main Detachment Fault (MDF) and the Main Boundary Thrust (MBT). Out‐of‐sequence thrust faults in the hanging wall of the MBT have breached and offset the presently inactive MCT. The Sub‐Himalayan imbricate zone is an emergent imbricate fan bounded by the MBT to the north and the Main Frontal Thrust (MFT) to the south and is underlain by the MDF which lies at a depth of between 5 km and 7 km. A balanced cross section constructed across the Higher, Lesser, and Sub‐Himalaya of eastern Nepal shows that the eastern Nepal Himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal, north‐south tectonic shortening since the initiation of the MCT. The Lesser and Sub‐Himalaya have absorbed 70 km of north‐south shortening by thrusting along the basal MDF, of which the Sub‐Himalayan imbricate zone has accommodated 25 km, the Sun Kosi Thrust has accommodated about 10 km, and the MBT has accommodated the remaining 35 km of shortening. Since the initiation of the MCT between 15 Ma and 25 Ma shortening across the eastern Nepal Himalaya has occurred at an average rate of 8.4–18.6 mm per year. The structural geometry of the eastern Nepal Himalaya suggests an overall “piggyback” sequence of thrusting, with motion transferred from the MCT to the underlying MDF and its emergent splay thrust, the MBT, and with the MBT rotated to its present steep orientation by imbricate thrusting within the Sub‐Himalaya.