bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of mixing on extinction by carbonaceous particles
Authors: Fuller Kirk A. , Malm William C. , Kreidenweis Sonia M.
Published: September 2002

Reported values for the absorption cross section of particulate carbon per unit mass range from under 4 to over 20 m2/g, and the intermediate value of 10 m2/g is used by many as a standard gram‐specific absorption cross section for atmospheric soot. In order to better understand the possible variations in absorption by atmospheric carbon, we reevaluated its optical properties in terms of the material composition and morphology of soot and the electrodynamics of spherules agglomerated into loose (ramiform) aggregates. Primary particles ranging in composition from paracrystalline graphite to low‐density air/graphite volume mixtures are considered. The effects on extinction efficiency of aggregation and of internal mixing of carbon with sulfate are considered in detail. We also compare our results with estimates of specific absorption of internally mixed soot that are based on several homogeneous mixing rules (effective medium approximations), On the basis of our modeling of the optical properties of aggregates of graphitic carbon grains, we conclude that 10 m2/g may be over 50% too high in many cases, and we suggest that the mass absorption coefficient for the light‐absorbing carbon in diesel soot at a wavelength of 0.550 μm may often be less than 7 m2/g, although variations in optical constants and, especially, the specific gravity of the absorbing material make it difficult to assign a specific numerical value. Adhesion of carbon grains to sulfate droplet surfaces is expected to enhance their absorption by no more than about 30%. Soot randomly positioned within droplets, however, can display averaged absorption enhancement factors of about 2.5–4 for hosts with refractive indices ranging from 1.33–1.53, respectively, and radii ≳0.20 μm. Nonetheless, calculations indicate that for realistic dry particle populations, αa < 10 m2/g for graphitic carbon in the atmosphere unless (1) most of it is encapsulated, and (2) the geometric mean radius of the hosts is larger than about 0.06 μm (which corresponds to a mass median diameter of 0.34 μm). These results suggest the importance of the determination of the physical state of the soot particles and their immediate environment when ascribing characteristic values for their absorption and scattering efficiencies.
A climatology of total ozone mapping spectrometer data using rotated principal component analysis
Authors: Eder Brian K. , LeDuc Sharon K. , Sickles Joseph E.
Published: September 2002

The spatial and temporal variability of total column ozone (Ω) obtained from the total ozone mapping spectrometer (TOMS version 7.0) during the period 1980–1992 was examined through the use of a multivariate statistical technique called rotated principal component analysis. Utilization of Kaiser's varimax orthogonal rotation led to the identification of 14, mostly contiguous subregions that together accounted for more than 70% of the total Ω variance. Each subregion displayed statistically unique Ω characteristics that were further examined through time series and spectral density analyses, revealing significant periodicities on semiannual, annual, quasi‐biennial, and longer term time frames. This analysis facilitated identification of the probable mechanisms responsible for the variability of Ω within the 14 homogeneous subregions. The mechanisms were either dynamical in nature (i.e., advection associated with baroclinic waves, the quasi‐biennial oscillation, or El Niño‐Southern Oscillation) or photochemical in nature (i.e., production of odd oxygen (O or O3) associated with the annual progression of the Sun). The analysis has also revealed that the influence of a data retrieval artifact, found in equatorial latitudes of version 6.0 of the TOMS data, has been reduced in version 7.0.
Findings on NOy as an indicator for ozone sensitivity based on different numerical simulations
Authors: Vogel B. , Riemer N. , Vogel H. , Fiedler F.
Published: September 2002

The O3‐NOy relation for a polluted planetary boundary layer is investigated using numerical simulations and observations. Box model runs are carried out to study the sensitivity of the transition value of NOy, which separates the low‐NOx. and the high‐NOx regimes, to changes in emissions and in ambient conditions. It is shown that the transition value of NOy depends on the hydrocarbon and NOx emissions, on water vapor, on radiation, and on temperature. Increasing hydrocarbon emissions, increasing water vapor, increasing radiation, or increasing temperature shifts the transition value to higher levels. One‐dimensional simulations, which include vertical mixing and dry deposition, show that increasing dry deposition reduces the transition value of NOy. Sensitivity studies with a three‐dimensional nonhydrostatic model system are carried out to investigate the effects of transport. A smoothing procedure is applied to reduce the large scatter of the O3‐NOy relation which is found in these cases. After smoothing, a transition value can be identified. For the southwestern part of Germany it is shown that this transition value indicates areas where NOx reduction leads to the most effective ozone reduction. Runs with various scenarios show the dependence of the transition value on emissions and ambient conditions. A normalization procedure is introduced. The normalization procedure is applied to the model results and to observations. Under high‐NOx conditions a large difference between the cases with and without advection is found. It shows that in the three‐dimensional case the ozone concentration is less sensitive to changes in the NOx emissions than in the case of pure chemistry.
Reassessment of the moisture source over the Sahara Desert based on NASA reanalysis
Authors: Shay‐El Y. , Alpert P. , da Silva A.
Published: September 2002

The components of the moisture balance equation are calculated for the Middle East/North Africa regions based on NASA/GEOS‐1 multiyear reanalysis data set. These include the Evaporation (E), Precipitation (P), moisture flux divergence (∇ · Q), and errors associated with the incremental analysis updates of the specific humidity, or IAU(q). The Annual mean ∇ · Q corresponds well to the results of Vitart et al. [1996], based on NCEP data. IAU (q) reveals a strong moisture source over the eastern Mediterranean and also confirms the paradoxical net moisture sink over the Arabian‐Iraqi desert found by Alpert and Shay‐El [1993]. Over the North African Sahara Desert the moisture flux was shown to converge through the northern and southern boundaries mainly at low levels (∼900 hPa) and to diverge through the eastern and western boundaries at higher levels (∼700 hPa). Starr and Peixoto [1958] have classified North Africa as a net moisture source. Area averaging of ∇ · Q over a box with varying dimensions reveals that it can be classified as a net sink if the box is small enough and located over the center of the desert. If the box is big enough to include the boundaries of the continent only then can it be classified as net source or divergence zone. Inspection of the intermonthly and diurnal variability, as well as the model biases, weakens also the net source argument. It is suggested that the earlier finding of a net source might be due to the smoothing of the water/land boundary, or due to various atmospheric diffusion processes such as the sea breeze cycle and cloud intrusion and evaporation.
Processes of buildup and retreat of the Greenland Ice Sheet
Authors: van de Wal R. S. W.
Published: September 2002

Sensitivity experiments were conducted to study the influence of several physical processes on the evolution of the Greenland ice sheet over the last 250,000 years. The experiments were carried out by means of a three‐dimensional thermodynamical model. The mass balance is modeled by a surface energy balance model. This means that variations in orbital parameters following from the astronomical (Milankovitch) theory can be incorporated in the ablation calculations. It is shown that with regard to the Greenland ice sheet, variations in shortwave radiation are only of minor importance for the changes in volume over time. Calculations of the various components of the energy balance under glacial and interglacial conditions show that the changes in absorbed shortwave radiation resulting from changes in orbital parameters are considerably smaller than the variations in the sensible heat flux caused by temperature variations. Thermodynamics are only of importance for the volume changes of the ice sheet if the climate, is on average, stable over a long period. In between 110 kyr B.P. and 20 kyr B.P. the ice volume increased slowly due to the cooling of the ice, which resulted in a higher effective viscosity and thus a thicker ice sheet. The long response time of the thermodynamics means that the thermodynamics cannot play a crucial role during transition from a glacial to a deglacial period which occur typically within several thousands of years. The mass balance height feedback has an important function in the buildup of the ice sheet after a warm period like the Eemian. Without isostatic bedrock compensation the ablation will remain very high in some marginal parts, yielding a considerably smaller ice sheet volume for the last glacial period.
Downwelling spectral radiance observations at the SHEBA ice station: Water vapor continuum measurements from 17 to 26μm
Authors: Tobin D. C. , Best F. A. , Brown P. D. , Clough S. A. , Dedecker R. G. , Ellingson R. G. , Garcia R. K. , Howell H. B. , Knuteson R. O. , Mlawer E. J. , Revercomb H. E. , Short J. F. , van Delst P. F. W. , Walden V. P.
Published: September 2002

Earth loses energy to space in the form of longwave (or infrared) radiation. Much of this energy is radiated through the transparent portion of the water vapor rotational band from 17 to 33 μm (300 to 600 cm−1). Very few measurements have been made in this spectral region to characterize how water vapor absorbs and emits longwave radiation. An Atmospheric Emitted Radiance Interferometer (AERI) with extended longwave spectral coverage has been deployed at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice station 300 miles north of the Alaskan coast to measure downwelling radiances at wavelengths of 3 to 26 μm (380 to 3000 cm−1). The spectral and radiometric performance of the instrument, installation at the ice station, and initial observations are shown. Comparisons to line‐by‐line radiative transfer calculations for selected clear‐sky cases are presented, and air‐broadened water vapor continuum absorption coefficients are determined in the wing of the pure rotational band from 17 to 26 μm (380 to 600 cm−1). Comparisons of the coefficients with the widely used Clough Kneizys Davies (CKD) water vapor continuum model suggest empirical modifications to this model are necessary. Comparisons to laboratory measurements of Burch et al. [1974] made at room temperature suggests little or no temperature dependence of the continuum from 400 to 550 cm−1. Implications of these modifications on top‐of‐atmosphere and surface fluxes, as well as atmospheric cooling rates, are discussed.
Comparative responses of EPIC and CERES crop models to high and low spatial resolution climate change scenarios
Authors: Mearns L. O. , Mavromatis T. , Tsvetsinskaya E. , Hays C. , Easterling W.
Published: September 2002

We compared the responses of the CERES and EPIC crop models, for wheat and corn, to two different climate change scenarios of different spatial scales applied to the central Great Plains. The scenarios were formed from a high‐resolution regional climate model (RegCM) and a coarse resolution general circulation model, which provided the initial and boundary conditions for the regional model. Important differences in yield were predicted by the two models for the two different scenarios. For corn, CERES simulated moderate yield decreases for both scenarios, while EPIC simulated a decrease for the coarse scenario but no change for the fine scale scenario. Differences in the simulation of wheat yields were also found. These differences were traced to the contrasting ways in which the models form final yield, even though their strategies for simulating potential total biomass are similar. We identify the crop model type as an important uncertainty in impacts assessment in addition to the spatial resolution of climate change scenarios.
In situ measurements of effective diameter and effective droplet number concentration
Authors: Korolev Alexei V. , Isaac George A. , Strapp J. Walter , Nevzorov Anatoly N.
Published: September 2002

The effective diameter of cloud droplets is usually derived from measurements of droplet size distribution measured by Particle Measuring Systems (PMS) probes. The disadvantage of this method is that PMS probes have a truncated size range. During the RACE project, an alternative method to measure the effective diameter used a cloud extinction meter and King and Nevzorov hot wire liquid water content and total water content (LWC/TWC) probes installed on the National Research Council (NRC) Twin Otter. The effective diameter was derived from direct in situ measurements of the extinction coefficient (ε) and liquid water content (W) as Deff=k1W/ε. This method of calculation of Deff is free of problems related to deriving Deff from the truncated particle size distribution. Since measurements of ε and W cover the whole size range of cloud particles, this method gives an accurate value of Deff. This method can also be successfully applied for mixed and ice phase clouds, since the Nevzorov TWC probe provides measurements of total (ice plus liquid) water content. Effective number concentration was calculated as Neff = k2ϵ3/W2. Comparisons of Deff and Neff, calculated by this method, and directly from PMS Forward Scattering Spectrometer Probe (FSSP) spectra, are favorable in the subset of conditions when the FSSP is considered to measure the spectra fully and accurately.
Climatology and small‐scale structure of lower stratospheric N2O based on in situ observations
Authors: Strahan S. E. , Loewenstein M. , Podolske J. R.
Published: September 2002

We derive seasonal mean distributions of N2O as a function of latitude and height from in situ measurements made by the airborne tunable laser absorption spectrometer (ATLAS) instrument on board the NASA ER‐2 aircraft. Seasonal means are also derived using an equivalent latitude coordinate which maximizes the information content and produces nearly pole‐to‐pole coverage. The mean N2O fields are consistent with the Brewer‐Dobson circulation, showing tropospheric values of N2O entering the tropical lower stratosphere and low values at the winter hemisphere poles. The analysis presented here is intended for use as a climatology for two‐ and three‐dimensional modelers wishing to evaluate the balance of transport processes affecting their models' lower stratosphere. We also present some interesting features that have been revealed by the high spatial resolution of the ATLAS measurements. These include large sgradients of N2O observed at the equatorward and poleward edges of the surf zone and filaments less than 1 km thick identified during aircraft profiling. The results presented here demonstrate the unique nature and high value of aircraft in situ data sets.
Atmospheric sulfur and deep convective clouds in tropical Pacific: A model study
Authors: Andronache C. , Donner L. J. , Seman C. J. , Ramaswamy V. , Hemler R. S.
Published: September 2002

A high‐resolution limited area nonhydrostatic model was used to simulate sulfate‐cloud interactions during the convective activity in a case study from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment, December 20–25, 1992. The model includes a new detailed sulfate‐cloud microphysics scheme designed to estimate the effects of sulfate on cloud microphysics and radiative properties and the effects of deep convection on the transport and redistribution of aerosol. The data for SO2 and SO4(2−) species were taken from the Pacific Exploratory Mission West B observations during February‐March 1994. Results show that a change in sulfate loading from the minimum to the maximum observed value scenarios (i.e., from about 0.01 to 1 μg m−3) causes a significant decrease of the effective radius of cloud droplets (changes up to 2 μm on average) and an increase of the diagnostic number concentration of cloud droplets (typical changes about 5–20 cm−3). The change in the average net shortwave (SW) radiation flux above the clouds was estimated to be on average −1.5 W m−2, with significant spatial and temporal variations. The horizontal average of the changes in the net SW radiation fluxes above clouds has a diurnal cycle, reaching typical values approximately −3 W m−2. The changes in the average net longwave radiation flux above the clouds were negligible, but they showed significant variations, typically between −10 W m−2 and 10 W m−2 near the surface. These variations were associated mainly with the changes in the distribution of cloud water, which showed typical relative changes of cloud water path of about 10–20%. Other notable changes induced by the increase of aerosol were the variations in air temperature of the order of 1°C. The case study presented here suggests that characteristics of convective clouds in tropical areas are sensitive to atmospheric sulfate loading, particularly during enhanced sulfate episodes.
Seasonal cycle and interannual variability in the Amazon hydrologic cycle
Authors: Zeng Ning
Published: September 2002

An analysis of the Amazon basin hydrologic cycle has been carried out using the NASA/Goddard Earth Observing System (GEOS‐1) atmospheric reanalysis, observed rainfall of Xie and Arkin [1996], and historical Amazon River discharge. Over a seasonal cycle the precipitation is found to vary by 5 mm d−1, and the runoff is found to vary by 2 mm d−1, while the evaporation largely remains constant. On interannual timescales the hydrologic variability both in the atmosphere and at the land surface is found to be closely related to El Niño‐Southern Oscillation (ENSO). The correlation between the Southern Oscillation Index and Xie and Arkin precipitation is 0.8 for the period 1985–1993 and 0.56 for the period 1979–1996. The precipitation lags behind the Southern Oscillation Index by 3–4 months while the Amazon River discharge lags behind the precipitation by another 3 months. The lagged relationship suggests interesting dynamic mechanisms. The reanalysis moisture convergence and observed discharge are used to diagnose basin average soil water storage. The year to year variation in the annual mean soil water storage is ∼200 mm, comparable to the change within a climatological seasonal cycle. In one case, the basin soil water storage increases by 462 mm from September 1987 to March 1989, suggesting the remarkable ability of the tropical rain forest environment to store and take up water.
Gravity wave interactions with the diurnal propagating tide
Authors: Meyer Christian K.
Published: September 2002

Observations of the diurnal propagating solar tide in the mesosphere and lower thermosphere (∼ 50–150 km) indicate substantial annual variations in tidal amplitudes. Observations and modeling efforts suggest that interactions with internal gravity waves can result in significant modifications of the diurnal tide. Herein a two‐dimensional (2‐D) linear steady‐state tidal model is used in conjunction with hybrid Lindzen/Matsuno gravity wave parameterization to examine the role of gravity wave interactions in the seasonal variability of the diurnal tide in the MLT. The analysis indicates that annual variations of the background atmosphere result in significant annual variations in the eddy diffusion and the diurnal harmonic of the momentum flux divergence (wave stress) produced by breaking gravity waves. The seasonal variability in these two mechanisms can explain the observed semi‐annual variability in the diurnal tide: amplitude minima at solstice and maxima at equinox. The relative roles of wave stress and diffusion are examined and it is shown that both play important roles in tidal modification by damping its amplitude. The effects of varying inputs to the gravity wave parameterization are explored.
Precipitation charge and size measurements inside a New Mexico mountain thunderstorm
Authors: Bateman Monte G. , Marshall Thomas C. , Stolzenburg Maribeth , Rust W. David
Published: September 2002

We measured the charge and size of precipitation particles with an instrumented free balloon in a convective mountain thunderstorm over Langmuir Laboratory in central New Mexico. Using an instrument that measured precipitation charge from 2 to 220 pC and equivalent particle diameters from 0.6 to 3.8 mm, we deduced that (1) the charge of the main positive charge region is carried by cloud particles, the charge of the main negative charge region is carried by a mixture of negative charge bearing cloud particles and precipitation particles, and the charge of the lower positive charge region is carried almost entirely by precipitation; (2) there was a mixture of both polarities of precipitation charge at nearly all altitudes; (3) there was no relationship between precipitation charge and size; (4) our charge data appear to support the noninductive ice‐ice collisional charging mechanism; and (5) the sign of charge carried by the precipitation reverses with ambient temperature. In comparing these data to previous measurements that were collected in New Mexico mountain thunderstorms with different instrumentation, we found that in most ways our precipitation charge data are similar to the previous measurements. In comparing these data with data that we previously collected in the trailing stratiform regions of mesoscale convective systems, we found that there are substantial differences in the precipitation charge data between these different cloud types.
Seasonal variations of upper tropospheric water vapor and high clouds observed from satellites
Authors: Chen Minghang , Rood Richard B. , Read William G.
Published: September 2002

Multiyear satellite measurements of specific humidity at 215 mbar from the Microwave Limb Sounder aboard the Upper Atmosphere Research Satellite and cloud amount from the International Satellite Cloud Climatology Project have been used to investigate seasonal variations of upper tropospheric water vapor (UTWV), high clouds, and deep convection. The tropical and extratropical UTWV for each hemisphere have maximum values in summer and minimum values in winter because of the moistening effect of the tropical deep convection. The seasonal change of high cloud amount is similar to UTWV in the tropics but very different in the extratropics. Implications of the present results for the water vapor feedback in the climate system are discussed.
A chemical model for evaporites on early Mars: Possible sedimentary tracers of the early climate and implications for exploration
Authors: Catling David C.
Published: September 2002

Martian geomorphology seems to indicate extensive hydrological activity during the Noachian era. Liquid water at the surface would require a large greenhouse effect that is widely hypothesized to have been caused by a high partial pressure of atmospheric carbon dioxide,PCO2. A sedimentation model driven by sequential evaporation is used to calculate the evaporite mineral sequence in a closed basin lake subject to highPCO2. The initial fluid is derived from weathered igneous rock similar to Martian meteorite basalts. Siderite (FeCO3) is always the first major carbonate to precipitate. Thus siderite is predicted to be an important fades component in ancient Martian sediments along with silica, which is also an early precipitate. These would form varves in lakes that undergo cycles of evaporation and water recharge. After silica and siderite, the sequence is magnesian calcite, nearly pure hydromagnesite, and gypsum, followed by highly soluble salts like NaCl. The presence of siderite sediments generally requires an atmosphericPCO2level in excess of ∼0.1 bar, otherwise iron silicates (such as greenalite) would form. This may be used in exploration as an observational test on the past atmospheric composition of Mars, subject to consideration of the depositional environment. AtPCO2of approximately several bar, gypsum precipitation could occur before calcite upon evaporation if the initial SO42−:Ca2+ratio is high and there is no water recharge. The predicted carbonate sequence upon evaporation is generally consistent with recent hypotheses suggesting traces of evaporite carbonates in some Martian meteorites. Several mechanisms destroy or obscure carbonates at the Martian surface. Consequently, in situ analysis of the interior of ejecta from recent impact craters lying within sedimentary basins may offer the most practical approach to future exploration of ancient carbonate sediments.
Properties of dust in the Martian atmosphere from the Imager on Mars Pathfinder
Authors: Tomasko M. G. , Doose L. R. , Lemmon M. , Smith P. H. , Wegryn E.
Published: September 2002

The Imager for Mars Pathfinder (IMP) returned sequences of images of the Martian sky characterizing the size distribution, optical constants, and nature of the aerosols suspended in the atmosphere of Mars. These sequences were executed when the solar elevation angle was approximately 15° and consisted of images near the elevation of the Sun, spanning a range in azimuth from about 4° to 180° from the Sun. Images were obtained at four wavelengths from 444 to 965 nm. From one sequence of observations, results are shown from a comparison of absolute photometry of the Martian sky with multiple scattering models. Results include the following. (1) The geometric cross‐section‐weighted mean particle radius is 1.6 ± 0.15 μm almost independent of the assumed width (variance) of the size distribution. (2) The imaginary refractive index shows a steep increase with wavelength from 670 nm to shorter wavelengths, and a shallow increase toward longer wavelengths, consistent with the reflection spectrum observed by IMP for Martian soil. (3) For each assumed variance, two parameters governing the slope and curvature of the portion of the phase function due to internally transmitted light are found uniquely as functions of wavelength. (4) The variance of the gamma size distribution is difficult to constrain from these observations alone. The shape of the single scattering phase functions derived from the IMP observations is compared to laboratory measurements of powder samples. One sample of irregular particles has a single scattering phase function quite similar to that derived for Mars. Overall, the results for the mean cross‐section‐weighted size and imaginary refractive index as a function of wavelength are in remarkably good agreement with the revised analysis by Pollack et al. [1995] of the observations made by the Viking lander 20 years earlier.
Opacity of the Martian atmosphere measured by the Imager for Mars Pathfinder
Authors: Smith Peter H. , Lemmon Mark
Published: September 2002

The Imager for Mars Pathfinder (IMP) has obtained 1733 images of the Sun during its 83‐sol mission starting on July 4, 1997, which have been used to determine the atmospheric opacity at wavelengths of 450, 670, 883, and 989 nm. Solar images were obtained hourly between 0700 and 1700 hours, allowing detection of changes within a sol as well as from one sol to another. Atmospheric opacities were similar to those determined by the Viking landers: τ = 0.4–0.5 in all filters early in the mission (near Ls 145) increasing to τ = 0.6 late in the mission (near Ls 185). Opacity was variable in the mornings, especially in the blue filter; less variability was seen in the afternoons and in the longer wavelength filters. We conclude that submicron water ice particles with opacities as large as 0.14 in blue light are the cause of the diurnal variations. Orbital observations of the limb have been modeled by water ice detached hazes above a dust layer. Using this model, detached hazes of small particles provide the extra limb brightening that is observed by images in the UV taken by the Hubble Space Telescope. Thus the presence of about one‐half optical depth of dust during the interval between Viking and Pathfinder is consistent with UV observations, suggesting that Viking‐ and Pathfinder‐like dust conditions are typical for northern subtropical latitudes.
Cloud formation under Mars Pathfinder conditions
Authors: Colaprete Anthony , Toon Owen B. , Magalhães Julio A.
Published: September 2002

Temperature data taken during the Mars Pathfinder entry are applied to a time dependent microphysical model of cloud formation. Our goal is to reproduce the characteristics of clouds observed at the Pathfinder site. We assume that the total water column abundance is 11 percipitable microns. We also assume that the atmospheric dust initially has a lognormal size distribution with a modal radius of 0.6 μm, reff =1.5 μm, and an optical depth of τ = 0.5. Three temperature profiles are used in the model calculations. The first is the Pathfinder entry observation. The second is this same profile but with the inversion at 10 km removed. The third simulation uses the temperature profiles calculated by the Ames Mars general circulation model for Mars Pathfinder conditions. In all cases clouds form at altitudes of 35–45 km. In addition lower thicker clouds form within the 10 km inversion of the unaltered profile, while peak cloud formation occurred near local noon at 20 km for the GCM calculated profiles. Given a combination of all three temperature scenarios, cloud optical depths, particle sizes, and colors are consistent with IMP observations.
Analyzing Pathfinder data using virtual reality and superresolved imaging
Authors: Stoker Carol R. , Zbinden Eric , Blackmon Theodore T. , Kanefsky Bob , Hagen Joel , Neveu Charles , Rasmussen Daryl , Schwehr Kurt , Sims Michael
Published: September 2002

The Mars Pathfinder mission used a unique capability to rapidly generate and interactively display three‐dimensional (3‐D) photorealistic virtual reality (VR) models of the Martian surface. An interactive terrain visualization system creates and renders digital terrain models produced from stereo images taken by the Imager for Mars Pathfinder (IMP) camera. The stereo pipeline, an automated machine vision algorithm, correlates features between the left and right images to determine their disparity and computes the corresponding positions using the known camera geometry. These positions are connected to form a polygonal mesh upon which IMP images are overlaid as textures. During the Pathfinder mission, VR models were produced and displayed almost as fast as images were received. The VR models were viewed using MarsMap, an interface that allows the model to be viewed from any perspective driven by a standard three‐button computer mouse. MarsMap incorporates graphical representations of the lander and rover and the sequence and spatial locations at which rover data were taken. Graphical models of the rover were placed in the model to indicate the rover position at the end of each day of the mission. Images taken by Sojourner cameras are projected into the model as 2‐D “billboards” to show their proper perspective. Distance and angle measurements can be made on features viewed in the model using a mouse‐driven 3‐D cursor and a point‐and‐click interface. MarsMap was used to assist with archiving and planning Sojourner activities and to make detailed measurements of surface features such as wind streaks and rock size and orientation that are difficult to perform using 2‐D images. Superresolution image processing is a computational method for improving image resolution by a factor of n1/2 by combining n independent images. This technique was used on Pathfinder to obtain better resolved images of Martian surface features. We show results from superresolving IMP camera images of six targets including near‐ and far‐field objects and discuss how the resolution improvement aids interpretation. Similar flood deposits can be seen on both of the Twin Peaks that cannot be resolved in raw images. Millimeter‐sized pits are resolved on the rocks Wedge and Halfdome. Other rocks at the Pathfinder site exhibit fine‐scale layering that is otherwise invisible. Use of the method resulted in the probable discovery of an artifact of intelligent life on Mars: a part of the Pathfinder spacecraft.
Stable isotopic measurements of the low‐temperature nitrogen components in ALH 84001
Authors: Wright I. P. , Grady Monica M. , Pillinger C. T.
Published: September 2002

The distribution and stable isotopic composition of nitrogen released at low temperatures from the ALH 84001 Martian meteorite have been appraised. The results demonstrate an apparently complex geochemistry of the element, with at least three, and possibly four, separate components present. While the isotopic composition of most of the nitrogen is within the range of typical terrestrial materials (and thus is probably contamination), nitrogen in the balance of the material has a very different δ15N value. Isotopically light nitrogen (component N2; 2.2 ppm with δ15N ∼ −30‰ and C/N ∼ 6) is released when a carbonaceous phase combusts below 300°C. Since amino acids and nucleic acid bases have C/N ∼ 2–3, the host phase of N2 could be from a mixture of such compounds. The associated nitrogen isotopic composition is very different from terrestrial contaminants; while this is suggestive of a Martian origin for the components, it is difficult to reconcile without advocating formation under special conditions.
On the equipotential surface hypothesis of lunar maria floors
Authors: Arkani‐Hamed Jafar , Konopliv A. S. , Sjogren W. L.
Published: September 2002

The equipotential surface hypothesis suggests that lunar maria floors lie on a surface parallel to the selenoid. This is examined using the spherical harmonic representations of the Clementine topography and Lunar Prospector gravity data. It is demonstrated that the floors of both circular and noncircular maria significantly deviate from an equipotential surface. Deeper circular maria and the deeper part of the noncircular Maxe Tranquillitatis have been subsided under larger mass loads in the crust. We calculate the mass beneath the maria to be in excess to the mass required for isostatic compensation of the topography at 60 km depth. A global map of this excess mass shows that the noncircular maria are isostatically compensated, unlike the circular maria. The map also reveals seven new sizable mascons: the three largest are associated with Mendel‐Rydberg, Mare Humboldtianum, and Mare Moscoviense.
Barium Cycling in the North Pacific: Implications for the Utility of Ba as a Paleoproductivity and Paleoalkalinity Proxy
Authors: McManus James , Berelson William M. , Hammond Douglas E. , Klinkhammer Gary P.
Published: September 2002

Benthic incubation chambers have been deployed in a variety of geochemical environments that provide a comprehensive geochemical framework from which to address issues related to Ba geochemistry and the use of Ba as a paleoproxy. First order budgets for barium show that in the equatorial Pacific, present rates of Ba rain and benthic remobilization are nearly in balance, indicating that the rate of net accumulation is negligible and is clearly much less than the average for the Holocene; thus any paleoproxy algorithms built on the assumption of steady state are questionable. In contrast, budgets for sediments in the southern California Borderland indicate much higher burial efficiencies, in the range of 50–80%. The Ba:alkalinity (Alk) flux ratio is found to be remarkably constant throughout the environments studied and is indistinguishable from the deep water ratio used for paleoceanographic reconstructions. However, the Ba:organic carbon remobilization ratio is not constant. Combined, these results do not indicate a simple, first‐order direct link between Ba and alkalinity remobilization via organic carbon oxidation; however, the similarities in the Ba and alkalinity source functions conspire to maintain the Ba:Alk ratio near the global water column average. This latter observation provides promise for the use of the Ba:Ca ratio in benthic foraminifera as a paleocirculation tracer.
Land‐ocean linkages over orbital and millennial timescales recorded in Late Quaternary sediments of the Japan Sea
Authors: Tada Ryuji , Irino Tomohisa , Koizumi Itaru
Published: September 2002

Late Quaternary sediments of the Japan Sea are characterized by centimeter‐ to meter‐scale alternations of dark and light layers which are synchronous basinwide. High‐resolution analyses of the sediments from Ocean Drilling Program site 797 reveal that deposition of the meter‐scale alternations reflect variations in paleoceanographic conditions which were closely associated with glacio‐eustatic sea level changes through the modulation of the volume and character of the influx to the sea through the Tsushima Strait. The centimeter‐ to decimeter‐scale alternations reflect millennial‐scale variations which are possibly associated with Dansgaard‐Oeschger (D‐O) cycles, with each dark layer appearing to correspond to an interstadial. This variability is attributed to the development of a humid climate in central to eastern Asia and the consequent increase in discharge from the Huanghe and Changjiang Rivers during interstadials. This caused expansion of the East China Sea coastal water (ECSCW), which penetrated more strongly into the Japan Sea. The increased influence of the lower‐salinity, nutrient‐enriched ECSCW reduced deep water ventilation and enhanced the surface productivity, leading to the development of anoxic bottom waters and deposition of the dark layers. Thus the centimeter‐ to decimeter‐scale alternations of the dark and light layers record wet and dry cycles in central to eastern Asia possibly associated with D‐O cycles.
Rapid analysis of high‐precision Sr/Ca ratios in corals and other marine carbonates
Authors: Schrag Daniel P.
Published: September 2002

A method for rapid determination of high‐precision Sr/Ca ratios in scleractinian corals is presented. Using an inductively coupled plasma atomic emission spectrophotometer, samples are corrected for instrument drift using a reference solution, similar to the approach used for analysis of stable isotopes using gas‐source mass spectrometry. Further correction for variation of the Sr/Ca ratio with Ca concentration is accomplished using internal standards. The precision, once all corrections have been made, is better than 0.1% (relative standard deviation, 1σ) for samples of similar Ca concentration and better than 0.2% for samples with variable Ca concentrations. This method increases the sample throughput by approximately a factor of 20 relative to thermal ionization mass spectrometry and significantly reduces instrument and per sample costs. Comparison of Sr/Ca data for a coral from the Galapagos Islands with an instrumental temperature record shows excellent agreement and demonstrates the potential for application of this technique to samples of modern and fossil scleractinian corals and other marine carbonates, including foraminifera.
Organic atmospheric aerosols: Review and state of the science
Authors: Jacobson M. C. , Hansson H. ‐C. , Noone K. J. , Charlson R. J.
Published: September 2002

Atmospheric aerosol particles are known to contain organic carbon material in variable amounts, depending on their location. In some parts of the world, organic compounds make up the majority of the total suspended particle mass. This class of particulate matter is important in a wide range of geophysical and environmental problems, ranging from local issues (e.g., pollution toxicity) to the global scale (e.g., climate change). Unfortunately, the richness of organic chemistry and the highly variable physical properties associated with both natural and anthropogenic organic particles lead to great difficulties in sampling and obtaining complete chemical information on these materials. These obstacles result in an incomplete picture of a potentially significant part of atmospheric chemistry and a correspondingly poor understanding of the geophysical and environmental effects of this aerosol. Given the paucity of quantitative molecular data, the purpose of this paper is not to quantitatively describe the importance of organic aerosols in environmental issues, but rather to present a basis for defining what data are needed. With this goal in mind, we begin with an overview of the major environmental issues known to be affected by organic aerosols, followed by a description of the distribution, sources, and chemical and physical properties of organic aerosols as they are currently understood. Methods used to collect and study organic aerosols are provided, followed by a list of outstanding scientific questions and suggestions for future research priorities.
Microwave imaging of buried objects for ground radar tomography
Authors: Dourthe C. , Pichot C. , Dauvignac J. Y. , Cashman J.
Published: September 2002

The detection and identification of buried inhomogeneities using electromagnetic waves are of great importance for nondestructive testing in geophysical, civil engineering, military, or humanitarian applications. The approach described here uses a linear inversion algorithm based on diffraction tomography to process the backscattered field signal generated by the buried inhomogeneities, measured over a probing area, for computation of a tomographic image of the ground. The technique has been extended to cater for time pulse (synthetic pulse or real pulse) illumination and to incorporate the near‐field radiation pattern transmitted by a broadband antenna. The backscattered field is measured at various locations with a receiving antenna for different positions of the transmitting antenna. The main advantages of such a method are the use of explicit formulas for solving the imaging problem and the possibility of rapid image display thanks to the use of Fast Fourier transform to implement the reconstruction algorithms on PCs. First, a number of tomographic reconstructions of buried objects are given for situations of practical interest. Results of numerical simulations with a Rayleigh incident pulse in the frequency band [0.3,3.0] GHz and the use of a bow tie antenna in the frequency band [0.3,1.3] GHz are presented. Second, tomographic reconstructions of buried objects are presented and discussed, to demonstrate the influence of object dielectric properties versus soil electromagnetic parameters.
Riometer and HF radar signatures of polar patches
Authors: Rodger A. S. , Rosenberg T. J.
Published: September 2002

Polar patch was the name given originally to a spatially limited enhancement of the 630‐nm emission observed in the dark polar cap. More recently, the term has been applied to related phenomena observed by other experimental techniques. The interrelationship between patches observed by differing methods has yet to be determined fully. In this paper the signatures of a series of polar patches observed by Halley HF radar and the South Pole broad‐beam and imaging riometers are presented. Most frequently, the HF radar patch signature immediately precedes that of the riometer polar patch signature. The interpretation is that the leading edge of the electron concentration structure that forms the patch is steeper than its trailing edge. On a few occasions, HF radar polar patch signatures coincide with the trailing edge of riometer patches and sometimes are seen throughout the riometer patch. Halley digital ionosonde data are used to show that riometer patches are more common over South Pole when the maximum F region plasma concentration is considerably in excess of 5×1011 m−3, which often occurs in the afternoon plasma convection cell.
Radio wave back‐propagating in polar coordinates: A linear filter in the time‐frequency angle‐frequency domain
Authors: Herique A.
Published: September 2002

The Comet Nucleus Sounding Experiment by Radio Wave Transmission (CONSERT) is a tomography of a comet nucleus for the ROSETTA mission, which consists of the transmission of electromagnetic waves between a lander on the comet nucleus surface and an orbiter through the nucleus to study its internal structure. The data inversion requires back‐propagating the electric field from the orbit to the nucleus surface. In order to take into account the circular geometry of the boundary conditions, we propose a back‐propagation in the angle‐frequency time‐frequency domain. This new method is described in this paper. We analyze similarities and differences with classical methods in Cartesian coordinates in terms of dispersion equations and evanescent waves; and we used this study to stabilize our back‐propagating filter. This method is then tested on CONSERT synthetic data.
Atmospheric effect on microwave polarimetric passive remote sensing of ocean surfaces
Authors: Yeang Chen‐Pang , Yueh Simon H. , Ding Kung‐Hau , Kong Jin Au
Published: September 2002

A theoretical emission model of combined ocean surface and atmosphere is presented to predict the microwave emissivity of the ocean. The modeled ocean surface is one‐dimensional with a random rough profile. The electromagnetic scattering from the surface is calculated based on the extended boundary condition method. Realizations of rough surfaces are created using Monte Carlo simulations. The bistatic scattering coefficients are computed from the ensemble average. The millimeter‐wave propagation model is used to evaluate the absorption of microwave radiation at all height levels in the atmosphere. An expression for the observed brightness temperatures is derived by solving the radiative transfer equations. The radiative transfer model results show a good agreement with the measured data from the 1995 NASA Jet Propulsion Laboratory WIND radiometer (WINDRAD) campaign. An approximate model is provided to estimate the atmospheric effect on the ocean brightness temperatures based on the overall atmospheric attenuation. The approximate model also compares well with the WINDRAD data. Further comparisons are made between the approximate formula and the radiative transfer results on the ratio of the third Stokes parameter in the atmosphere to the one in free space by varying the atmospheric conditions, surface roughness, and radiation frequencies. The approximate formula shows its usefulness for the prediction of the ocean brightness temperatures.
Continental collision and lateral escape deformation in the lower and upper crust: An example from Caledonide Svalbard
Authors: Lyberis Nikos , Manby Geoff
Published: September 2002

Ductile Caledonian structures of eastern Svalbard and the pattern of brittle deformation in eastern Anatolia are examined in an attempt to understand the different mechanical responses of the upper and the lower crust in collision and lateral escape systems. In Svalbard, Early to Middle Proterozoic basement structures were reworked under amphibolite facies conditions, in Late Silurian time, during the Caledonian orogeny when synmetamorphic ductile to postmetamorphic brittle, left‐lateral shearing was concentrated in narrow zones. The widespread distribution of contractional and elongation structures over the northern and northeastern Svalbard indicates that this deformation was the result of E–W convergence and N–S lateral escape of the whole continental mass along an array of deep crustal shear zones. The combination of vertical shear zones with parallel low‐angle compressional shear zones and associated folds in Ny Friesland represents the ductile or plastic flow response of the lower crust to the orthogonal‐lateral escape setting. The westward escape of the Anatolian block, accompanying the active Arabia‐Eurasia collision, is associated with a 150 km wide zone of thrust faulting and NE–SW sinistral strike‐slip faults along the East Anatolian Fault Zone. Arrays of dextral and sinistral strike‐slip faults with thrust and normal faults between the Eastern Anatolian and the North Anatolian Fault Zones represent an elastic response of the upper crust that is considered to be the superficial equivalent of the ductile flow in the lower crust of Caledonide Svalbard in a convergence‐lateral escape setting.