bluuebunny/crossref_metadata_2025 huggingface dataset:
Sensitivity of two‐dimensional model predictions of ozone response to stratospheric aircraft: An update
Authors: Considine David B. , Douglass Anne R. , Jackman Charles H.
Published: February 2004

The Goddard Space Flight Center two‐dimensional model of stratospheric photochemistry and dynamics has been used to calculate the O3 response to stratospheric aircraft (high‐speed civil transport (HSCT)) emissions. The sensitivity of the model O3 response was examined for systematic variations of five parameters and two reaction rates over a wide range, expanding on calculations by various modeling groups for the NASA High Speed Research Program and the World Meteorological Organization. In all, 448 model runs were required to test the effects of variations in the latitude, altitude, and magnitude of the aircraft emissions perturbation, the background chlorine levels, the background sulfate aerosol surface area densities, and the rates of two key reactions. No deviation from previous conclusions concerning the response of O3 to HSCTs was found in this more exhaustive exploration of parameter space. Maximum O3 depletions occur for high‐altitude, low‐latitude HSCT perturbations. Small increases in global total O3 can occur for low‐altitude, high‐latitude injections. Decreasing aerosol surface area densities and background chlorine levels increases the sensitivity of model O3 to the HSCT perturbations. The location of the aircraft emissions is the most important determinant of the model response. Response to the location of the HSCT emissions is not changed qualitatively by changes in background chlorine and aerosol loading. The response is also not very sensitive to changes in the rates of the reactions NO + HO2 → NO2 + OH and HO2 + O3 → OH + 2O2 over the limits of their respective uncertainties. Finally, levels of lower stratospheric HOχ generally decrease when the HSCT perturbation is included, even though there are large increases in H2O due to the perturbation.
Biogenic sulfur emissions and aerosols over the tropical South Atlantic: 3. Atmospheric dimethylsulfide, aerosols and cloud condensation nuclei
Authors: Andreae Meinrat O. , Elbert Wolfgang , de Mora Stephen J.
Published: February 2004

We measured dimethylsulfide in air (DMSa) and the number concentration, size distribution, and chemical composition of atmospheric aerosols, including the concentration of cloud condensation nuclei (CCN), during February‐March 1991 over the tropical South Atlantic along 19°S (F/S Meteor, cruise 15/3). Aerosol number/size distributions were determined with a laser‐optical particle counter, condensation nuclei (CN) concentrations with a TSI 3020, and cloud condensation nuclei (CCN) with a Hudson‐type supersaturation chamber. Aerosol samples were collected on two‐stage stacked filters and analyzed by ion chromatography for soluble ion concentrations. Black carbon in aerosols was measured by visible light absorption and used to identify and eliminate periods with anthropogenic pollution from the data set. Meteorological analysis shows that most of the air masses sampled had spent extended periods over remote marine areas in the tropical and subtropical region. DMSa was closely correlated with the sea‐to‐ air DMS flux calculated from DMS concentrations in seawater and meteorological data. Sea salt made the largest contribution to aerosol mass and volume but provided only a small fraction of the aerosol number concentration. The submicron aerosol had a mean composition close to ammonium bisulfate, with the addition of some methanesulfonate. Aerosol (CN and CCN) number and non‐sea‐salt sulfate concentrations were significantly correlated with DMS concentration and flux. This suggests that DMS oxidation followed by aerosol nucleation and growth in the marine boundary layer is an important, if not dominating, source of CN and possibly CCN. The degree of correlation between DMS and particle concentrations in the marine boundary layer may be strongly influenced by the different time scales of the processes regulating these concentrations. Our results provide strong support for several aspects of the CLAW hypothesis, which proposes the existence of a feedback loop linking DMS emission from marine plankton to sulfate aerosol and global climate.
Sensitivity analysis of a model of CO2 exchange in tundra ecosystems by the adjoint method
Authors: Waelbroeck C. , Louis J.‐F.
Published: February 2004

A model of net primary production (NPP), decomposition, and nitrogen cycling in tundra ecosystems has been developed. The adjoint technique is used to study the sensitivity of the computed annual net CO2 flux to perturbations in initial conditions, climatic inputs, and model's main parameters describing current seasonal CO2 exchange in wet sedge tundra at Barrow, Alaska. The results show that net CO2 flux is most sensitive to parameters characterizing litter chemical composition and more sensitive to decomposition parameters than to NPP parameters. This underlines the fact that in nutrient‐limited ecosystems, decomposition drives net CO2 exchange by controlling mineralization of main nutrients. The results also indicate that the short‐term (1 year) response of wet sedge tundra to CO2‐induced warming is a significant increase in CO2 emission, creating a positive feedback to atmospheric CO2 accumulation. However, a cloudiness increase during the same year can severely alter this response and lead to either a slight decrease or a strong increase in emitted CO2, depending on its exact timing. These results demonstrate that the adjoint method is well suited to study systems encountering regime changes, as a single run of the adjoint model provides sensitivities of the net CO2 flux to perturbations in all parameters and variables at any time of the year. Moreover, it is shown that large errors due to the presence of thresholds can be avoided by first delimiting the range of applicability of the adjoint results.
Partitioning of ocean and land uptake of CO2 as inferred by δ13C measurements from the NOAA Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network
Authors: Ciais Philippe , Tans Pieter P. , White James W. C. , Trolier Michael , Francey Roger J. , Berry Joe A. , Randall David R. , Sellers Piers J. , Collatz James G. , Schimel David S.
Published: February 2004

Using δ13C measurements in atmospheric CO2 from a cooperative global air sampling network, we determined the partitioning of the net uptake of CO2 between ocean and land as a function of latitude and time. The majority of δ13C measurements were made at the Institute of Arctic and Alpine Research (INSTAAR) of the University of Colorado. The network included 40 sites in 1992 and constitutes the most extensive data set available. We perform an inverse deconvolution of both CO2 and δ13C observations, using a two‐dimensional model of atmospheric transport. New features of the method include a detailed calculation of the isotopic disequilibrium of the terrestrial biosphere from global runs of the CENTURY soil model. Also, the discrimination against 13C by plant photosynthesis, as a function of latitude and time, is calculated from global runs of the SiB biosphere model. Uncertainty due to the longitudinal structure of the data, which is not represented by the model, is studied through a bootstrap analysis by adding and omitting measurement sites. The resulting error estimates for our inferred sources and sinks are of the order of 1 GTC (1 GTC = 1015 gC). Such error bars do not reflect potential systematic errors arising from our estimates of the isotopic disequilibria between the atmosphere and the oceans and biosphere, which are estimated in a separate sensitivity analysis. With respect to global totals for 1992 we found that 3.1 GTC of carbon dissolved into the ocean and that 1.5 GTC were sequestered by land ecosystems. Northern hemisphere ocean gyres north of 15°N absorbed 2.7 GTC. The equatorial oceans between 10°S and 10°N were a net source to the atmosphere of 0.9 GTC. We obtained a sink of 1.6 GTC in southern ocean gyres south of 20°S, although the deconvolution is poorly constrained by sparse data coverage at high southern latitudes. The seasonal uptake of CO2 in northern gyres appears to be correlated with a bloom of phytoplankton in surface waters. On land, northern temperate and boreal ecosystems between 35°N and 65°N were found to be a major sink of CO2 in 1992, as large as 3.5 GTC. Northern tropical ecosystems (equator‐30°N) appear to be a net source to the atmosphere of 2 GTC which could reflect biomass burning. A small sink, 0.3 GTC, was inferred for southern tropical ecosystems (30°S‐equator).
Ground‐based microwave monitoring of middle atmosphere ozone: Comparison to lidar and Stratospheric and Gas Experiment II satellite observations
Authors: Tsou J. J. , Connor Brian J. , Parrish Alan , McDermid I. Stuart , Chu William P.
Published: February 2004

A dedicated ground‐based microwave radiometer was in operation to monitor the middle atmospheric ozone concentration at Table Mountain Facility (TMF) of the Jet Propulsion Laboratory (JPL), California (34.4°N, 117.7°W) from July 1989 to June 1992, as a part of the Network for Detection of Stratospheric Change. Ozone profiles from 56 to 0.04 mbar (∼20–70 km) were retrieved from the microwave data. The focus in this paper is to validate the microwave ozone observations from 56 to 1 mbar by comparing the results from a JPL ground‐based lidar located at the same site and from the Stratospheric Aerosol and Gas Experiment II (SAGE II) satellite overpasses within 1000 km of TMF and to examine the ability of these instruments to detect short‐term, seasonal, and annual variations in ozone. The profile comparison results show that the mean differences of microwave ozone from lidar and SAGE II are about 5% or less and the root‐mean‐square scatter about the mean is mainly from the precision of the instruments. A correlation analysis of ozone time series suggests highly significant correlations up to 2.4 mbar between lidar and microwave measurements and up to 1 mbar between SAGE II and microwave. The short‐term and seasonal variation of the ozone profile seen in the microwave measurements is shown to be consistent with the observations of lidar and SAGE II, and the interannual variation of ozone appears to be detectable within an accuracy of a few percent with the microwave instrument.
Observation of thunderstorms by multilevel electric field measurement system and radar
Authors: Soula S. , Sauvageot H. , Saïssac M. P. , Chauzy S.
Published: February 2004

During the summer of 1992, an experiment was conducted in southwestern France, close to the Pyrénées, at the Centre de Recherches Atmosphériques (CRA) in order to study the evolution of the electric field measured at several levels below thunderclouds. We used a field mill flush to the ground and four field sensors, suspended from an insulated cable and distributed between 0 and 48 m. These altitude sensors separately measure the ambient electric field and the field created by the sensor itself. The Rabelais millimetric radar provides reflectivities and Doppler velocities of cloud and rain systems. Meteorological data like wind velocity, humidity, temperature, and rainfall rate are recorded at the site. Two storm intervals are studied, one on July 30 and one on August 6. Both examples give an idea on how the electric field signature during the development or advection of a convective cloud can be different at the ground and at altitudes of a few tens of meters. The maxima in the electrostatic activity are visible only on the altitude electric field evolution. The maximum value of the electric field between lightning flashes, close to 30 kV m−1, is detected at 48 m above ground. A vertical gradient of the electric field is observed in this 48‐m‐thick layer, especially when the field is high. The upward progressive development of this gradient is interpreted in terms of a local charge generated by corona at the ground and rising by conduction. The average charge density in the whole layer reaches around 5 nC m−3 between lightning flashes when the field keeps large values of the same polarity. This density is weaker when the electric field has taken large values of both polarities. Long periods of different electrical activity are observed during both storms (those with large electrostatic fields without any lightning flash and those with lightning activity). The former are associated with a thick stratiform cloud structure, possibly with strong radar echoes above the melting level but without rain reaching the ground; the latter are associated with rain reaching the ground close to the measurement area. These observations suggest that the water liquid phase could play an important role in lightning initiation.
Atmospheric branch of the global hydrologic cycle in the Goddard Laboratory for Atmospheres interactive forecast retrieval analysis system
Authors: Wu Man Li C. , Lau K.‐M.
Published: February 2004

In this paper, we present a description of the atmospheric branch of the hydrologic cycle (AHC) in the Goddard Laboratory for Atmospheres interactive forecast retrieval analysis (IFRA) system. Global hydrologic parameters are estimated using a mix of satellite‐retrieved geophysical parameters and general circulation model gridded wind fields. The global distribution of components of the AHC and their related fields such as the total precipitable water, rainfall estimate, latent heat flux/evaporation over the ocean, moisture transport, and associated seasonal and low‐frequency variability (timescale ≥10 days) are studied. Results show that the seasonal variations of these variables are consistent with climatological observations. Positive surface hydrologic forcing (E ‐ P > 0) is found over extended regions in the subtropics and in the oceanic upwelling regions, while negative surface hydrologic forcing (E ‐ P < 0) is concentrated in small regions of heavy precipitation over warm water. Moisture is transported from the regions of positive E ‐ P (evaporation‐precipitation) toward regions of negative E ‐ P. The moisture recycling timescale is much larger in the former compared with the latter. On the large scale, rainfall and moisture convergence are highly correlated. Also presented is an analysis of the IFRA tropical rainfall variability. Principal modes of tropical rainfall variation can be identified with the well‐known intraseasonal or 30‐ to 60‐day oscillations. Regional characteristics of these oscillations and relationship with other components of the atmospheric hydrologic cycle are investigated. The analysis reveals clearly a premoistening of the tropical atmosphere approximately 10 days prior to enhanced convection over the western Pacific followed by drying out of equatorial central and eastern Pacific. Results show that much of the variability of rainfall in the maritime continent associated with the 30‐ to 60‐day oscillation can be connected with the zonal transport of moisture from the Indian Ocean and the eastern Pacific associated with Walker‐type circulation anomalies. The water budget of the tropical western Pacific is strongly influenced by rainfall variability associated with these intraseasonal oscillations.
On the HO2 + HO2 reaction: Its misapplication in atmospheric chemistry models
Authors: Stockwell William R.
Published: February 2004

The self‐reaction of the HO2 radical is an important termination reaction for HOχ and it is the major gas phase source of hydrogen peroxide in the atmosphere. It is well established that the rate of this reaction is sensitive to both pressure and water vapor concentrations. The NASA recommended rate parameter (DeMore et al., 1992) has been utilized in several detailed gas phase chemical mechanisms for modeling the troposphere, but several other recent studies have omitted one or both of the terms for the rate constant. If both the pressure and the water dependent terms are omitted the relative error can be as high as 75% at the surface to near 30% at 10 km for water vapor saturated conditions. Models which omit these terms will predict incorrect HOχ budgets, underestimate gas phase hydrogen peroxide formation rates, and overestimate ozone and organic peroxide formation rates especially in clouds or under marine conditions.
Global perspective of nitrate flux in ice cores
Authors: Yang Qinzhao , Mayewski Paul A. , Whitlow Sallie , Twickler Mark , Morrison Michael , Talbot Robert , Dibb Jack , Linder Ernst
Published: February 2004

The relationships between the concentration and the flux of chemical species (Cl−, NO3−, SO42−, Na+, K+, NH4+, Mg2+, Ca2+) versus snow accumulation rate were examined at GISP2 and 20D in Greenland, Mount Logan from the St. Elias Range, Yukon Territory, Canada, and Sentik Glacier from the northwest end of the Zanskar Range in the Indian Himalayas. At all sites, only nitrate flux is significantly (α = 0.05) related to snow accumulation rate. Of all the chemical series, only nitrate concentration data are normally distributed. Therefore we suggest that nitrate concentration in snow is affected by postdepositional exchange with the atmosphere over a broad range of environmental conditions. The persistent summer maxima in nitrate observed in Greenland snow over the entire range of record studied (the last 800 years) may be mainly due to NOxreleased from peroxyacetyl nitrate by thermal decomposition in the presence of higher OH concentrations in summer. The late winter/early spring nitrate peak observed in modern Greenland snow may be related to the buildup of anthropogenically derived NOyin the Arctic troposphere during the long polar winter.
Calculation of divergence and vertical motion from volume imaging lidar data
Authors: Young P. W. , Eloranta E. W.
Published: February 2004

Boundary layer divergences are calculated from a sequence of images of the inhomogeneities in aerosol backscattering observed with volume imaging lidar. Two‐dimensional spatial cross correlations are calculated between successive horizontal plane images, 10 km×5 km, which are interpolated from the lidar returns. The image at the first time represents the actual aerosol pattern. An assumed divergence is included in the interpolation of the second image. The divergence is determined by calculating correlations for a matrix of assumed divergences and selecting that value which optimizes the correlation. The results from this process are consistent with predictions for divergence over the spatial scale measured, and the apparent resolution of the results agrees with the image resolution. A higher pulse‐rate volume imaging lidar system would allow for increased image resolution and increased image area, both of which should improve the resolution in the divergence measurements.
Lightning‐channel morphology revealed by return‐stroke radiation field waveforms
Authors: Willett J. C. , Le Vine D. M. , Idone V. P.
Published: February 2004

Simultaneous video and wideband electric field recordings of 32 cloud‐to‐ground lightning flashes in Florida were analyzed to show that the formation of new channels to ground can be detected by examination of the return‐stroke radiation fields alone. The return‐stroke E and dE/dt waveforms were subjectively classified according to their fine structure. Then the video images were examined field by field to identify each waveform with a visible channel to ground. Fifty‐five correlated waveforms and channel images were obtained. Of these, all 34 first‐stroke waveforms (multiple jagged E peaks, noisy dE/dt), 8 of which were not radiated by the chronologically first stroke in the flash, came from new channels to ground (not previously seen on video). All 18 subsequent‐stroke waveforms (smoothly rounded E and quiet dE/dt after the initial peak) were radiated by old channels (illuminated by a previous stroke). Two double‐ground waveforms (two distinct first‐return‐stroke pulses separated by tens of microseconds or less) coincided with video fields showing two new channels. One “anomalous‐stroke” waveform (beginning like a first stroke and ending like a subsequent) was produced by a new channel segment to ground branching off an old channel. This waveform classification depends on the presence or absence of high‐frequency fine structure. Fourier analysis shows that first‐stroke waveforms contain about 18 dB more spectral power in the frequency interval from 500 kHz to at least 7 MHz than subsequent‐stroke waveforms for at least 13 μs after the main peak.
Stratospheric Aerosol and Gas Experiments I and II comparisons with ozonesondes
Authors: Veiga Robert E. , Cunnold Derek M. , Chu William P. , McCormick M. Patrick
Published: February 2004

Ozone profiles measured by the Stratospheric Aerosol and Gas Experiments (SAGE) I and II are compared with ozonesonde profiles at 24 stations over the period extending from 1979 through 1991. Ozonesonde/satellite differences at 21 stations with SAGE II overpasses were computed down to 11.5 km in the midlatitudes, to 15.5 km in the lower latitudes, and for nine stations with SAGE I overpasses down to 15.5 km. The set of individual satellite and ozonesonde profile comparisons most closely colocated in time and space shows mean absolute differences relative to the satellite measurement of 6±2% for SAGE II and 8±3% for SAGE I. The ensemble of ozonesonde/satellite differences, when averaged over all altitudes, shows that for SAGE II, 70% were less than 5%, whereas for SAGE I, 50% were less than 5%. The best agreement occurred in the altitude region near the ozone density maximum where almost all the relative differences were less than 5%. Most of the statistically significant differences occurred below the ozone maximum down to the tropopause in the region of steepest ozone gradients and typically ranged between 0 and −20%. Correlations between ozone and aerosol extinction in the northern midlatitudes indicate that aerosols had no discernible impact on the ozonesonde/satellite differences and on the SAGE II ozone retrieval for the levels of extinction encountered in the lower stratosphere during 1984 to mid‐1991.
Lithospheric‐scale buckling and thrust structures on Mars: The Coprates rise and south Tharsis ridge belt
Authors: Schultz Richard A. , Tanaka Kenneth L.
Published: February 2004

Detailed photogeologic mapping documents for the first time the deformation sequence of the Coprates rise. Materials of Middle Noachian through perhaps Early Hesperian age were involved in the horizontal shortening of crustal and lithospheric rocks that produced uplift and asymmetric, east‐vergent folding of these layers during the Late Noachian and perhaps Early Hesperian. Previous suggestions of normal faulting as the deforming mechanism are not supported by the new mapping or by dislocation models of rise topography. The most likely mechanism for producing the Coprates rise is folding due to buckling instability and perhaps thrust faulting. An extensive ridge belt concentric to south Tharsis is defined by a newly recognized system of approximately coeval long‐wavelength topographic ridges and swales that include the Coprates rise. Horizontal shortening of crust and lithosphere is manifested by periodically spaced structures at two different scales: wrinkle ridges spaced tens of kilometers apart and the south Tharsis system of ridges spaced hundreds of kilometers apart. Both types of structures may be associated with early volcanotectonic activity and crustal thickening in south Tharsis. These and other large ridges indicate that lithospheric buckling and thrust faulting may have been common on early Mars.
A non‐local thermodynamic equilibrium radiative transfer model for infrared emissions in the atmosphere of Mars: 1. Theoretical basis and nighttime populations of vibrational levels
Authors: López‐Valverde M. A. , López‐Puertas M.
Published: February 2004

A radiative transfer model to study the infrared (1–20 μm) emissions of the CO and CO2 molecules in the atmosphere of Mars has been developed. The model runs from the planet's surface up to 180 km and has been especially elaborated to study non‐local thermodynamic equilibrium (non‐LTE) situations. It includes the most important energy levels and vibration‐rotation bands able to give a significant atmospheric emission or produce a significant cooling/heating rate. Exchanges of energy in thermal and nonthermal (vibrational‐vibrational) collisions as well as by radiative processes have been included. An exhaustive review of the rate constants for vibrational‐thermal and vibrational‐vibrational collisional exchanges has been carried out. Radiative transfer processes have been treated by using a modified Curtis matrix formulation. The populations of the excited vibrational levels for nighttime conditions are presented along with a sensitivity study of their variations to the kinetic temperature profile and to collisional rate constants. The populations of the CO2(0,ν2,0) levels follow LTE up to about 85 km with the radiative transfer processes playing a very important role in maintaining this situation above the tropopause. This result is practically insensitive to plausible variations in the kinetic temperature of the troposphere. The uncertainties in the rate constants play an important role in determining the populations of the levels at thermospheric altitudes, but they are of little significance for the heights where they start departing from LTE. The CO2(0,0°,1) level breaks down from LTE at about 60 km, the laser bands at 10 μm giving a significant contribution to its population in the Martian mesosphere. The CO(1) level starts departing around 50 km and is noticeably enhanced in the upper thermosphere by absorption of upwelling flux from the planet's surface.
Sinuosity of Martian rampart ejecta deposits
Authors: Barlow Nadine G.
Published: February 2004

The sinuosities of 2213 Martian rampart ejecta craters are quantified through measurement of the ejecta flow front perimeter and ejecta area. This quantity, called lobateness, was computed for each complete lobe of the 1582 single lobe (SL), 251 double lobe (DL), and 380 multiple lobe (ML) craters included in this study. A lobateness value of 1 indicates a circular ejecta blanket, whereas more sinuous ejecta perimeters have lobateness values >1. Although resolution does have an effect on the absolute values of lobateness, the general relationships between lobateness and morphology exist regardless of resolution. Evaluation of the lobateness values reveals that the outer lobes of DL and ML craters have higher median lobateness values (i.e., are more sinuous) than the inner lobes. The outermost lobe of ML craters displays higher lobateness values than the outer lobe of DL craters or the single lobe of SL craters. Previous reports of lobateness‐diameter, lobateness‐latitude, and lobateness‐terrain relationships for rampart craters are not supported by this study. Many of the differences between the results of this study and the previous lobateness analyses can be attributed to the inclusion of resolution effects and the distinction between different ejecta morphologies in this study. The results of this study taken together with a previous analysis of the distribution and diameter dependence of different ejecta morphologies are most consistent with the theory that Martian lobate ejecta morphologies form from impact into subsurface volatiles.
Assimilation by lunar mare basalts: Melting of crustal material and dissolution of anorthite
Authors: Finnila A. B. , Hess P. C. , Rutherford M. J.
Published: February 2004

We discuss techniques for calculating the amount of crustal assimilation possible in lunar magma chambers and dikes based on thermal energy balances, kinetic rates, and simple fluid mechanical constraints. Assuming parent magmas of picritic compositions, we demonstrate the limits on the capacity of such magmas to melt and dissolve wall rock of anorthitic, troctolitic, noritic, and KREEP (quartz monzodiorite) compositions. Significant melting of the plagioclase‐rich crustal lithologies requires turbulent convection in the assimilating magma and an efficient method of mixing in the relatively buoyant and viscous new melt. Even when this occurs, the major element chemistry of the picritic magmas will change by less than 1–2 wt %. Diffusion coefficients measured for Al2O3 from an iron‐free basalt and an orange glass composition are 10−12 m2 s−1 at 1340°C and 10−11 m2 s−1 at 1390°C. These rates are too slow to allow dissolution of plagioclase to significantly affect magma compositions. Picritic magmas can melt significant quantities of KREEP, which suggests that their trace element chemistry may still be affected by assimilation processes; however, mixing viscous melts of KREEP composition with the fluid picritic magmas could be prohibitively difficult. We conclude that only a small part of the total major element chemical variation in the mare basalt and volcanic glass collection is due to assimilation/fractional crystallization processes near the lunar surface. Instead, most of the chemical variation in the lunar basalts and volcanic glasses must result from assimilation at deeper levels or from having distinct source regions in a heterogeneous lunar mantle.
Petrogenesis of lunar troctolites
Authors: Hess Paul C.
Published: February 2004

Models of petrogenesis for lunar troctolites are severely constrained by the high Mg* (>88) values of the most primitive members of the Mg‐rich suite. Melts derived by partial melting of olivine cumulates to the lunar magma ocean have high Mg* but are strongly undersaturated with respect to plagioclase. Assimilation of lunar crust cannot bring these melts to the plagioclase liquidus without unduly reducing the Mg* values of olivine. Partial melts derived by melting a Iherzolite primitive interior have higher normative plagioclase contents but require sources with Mg* > 90. More terrestrial‐like Mg* (∼88) values are permitted if rising thermal plumes are capable of entraining the most Mg‐rich cumulates of the magma ocean. The Mg* values of lunar troctolites may, therefore, reflect a hybrid source composed of primitive mantle and Mg‐rich cumulates of the magma ocean. Higher Mg* values may also arise from the reduction of small amounts of FeO to Fe. Alternatively, the parent magmas to lunar troctolites may be impact melts of the anorthositic crust and the Mg‐rich dunite cumulates of the magma ocean; the latter were brought to the upper mantle during the overturn of the unstable cumulate pile of the magma ocean.
Viking Lander image analysis of Martian atmospheric dust
Authors: Pollack James B. , Ockert‐Bell Maureen E. , Shepard Michael K.
Published: February 2004

We have reanalyzed three sets of Viking Lander 1 and 2 (VL1 and VL2) images of the Martian atmosphere to better evaluate the radiative properties of the atmospheric dust particles. The properties of interest are the first two moments of the size distribution, the single‐scattering albedo, the dust single‐scattering phase function, and the imaginary index of refraction. These properties provide a good definition of the influence that the atmospheric dust has on heating of the atmosphere. Our analysis represents a significant improvement over past analyses (Pollack et al. 1977, 1979) by deriving more accurate brightnesses closer to the sun, by carrying out more precise analyses of the data to acquire the quantities of interest, and by using a better representation of scattering by nonspherical particles. The improvements allow us to better define the diffraction peak and hence the size distribution of the particles. For a lognormal particle size distribution, the first two moments of the size distribution, weighted by the geometric cross section, are found. The geometric cross‐section weighted mean radius (reff) is found to be 1.85±0.3 μm at VL2 during northern summer when dust loading was low and 1.52±0.3 μm at VL1 during the first dust storm. In both cases the best cross‐section weighted mean variance (νeff) of the size distribution is equal to 0.5±0.2 μm. The changes in size distribution, and thus radiative properties, do not represent a substantial change in solar energy deposition in the atmosphere over the Pollack et al. (1977, 1979) estimates.
Temperature measurements of a Martian local dust storm
Authors: Kahn Ralph
Published: February 2004

A technique for estimating the ground and near‐ground atmospheric temperatures within a Martian local dust storm is presented. It is applied to soundings taken by the Viking orbiter infrared thermal mapper (IRTM) instrument at four times‐of‐day for one storm. Essentially, a comparison is made between infrared radiances emerging from the storm interior and those from the region surrounding the storm. Particle extinction properties are assumed to be independent of position in the storm region, and scattering properties must be selected arbitrarily. For the storm studied here, the ground temperature in the interior is at least 6 K cooler, whereas the near‐ground atmospheric temperature may be less than or comparable to, those of the surroundings. The thermal structure of the storm interior did not change measurably between 11.5 and 16.6 hours local time. These observations favor theories of dust storm development in which regional winds rather than local, dust‐driven convection inititate the mobilization of dust from the surface. It is also concluded that the optical properties of dust particles in this local storm differ from those observed by Mariner 9 during the 1971–1972 global dust storm.
Probing the interior structure of Mercury from an orbiter plus single lander
Authors: Wu Xiaoping , Bender Peter L. , Rosborough George W.
Published: February 2004

The possibility of measuring the inclination of Mercury's spin axis and the amplitude of its 88‐day forced libration in longitude by a lander‐orbiter measurement system is investigated. The experiment would involve ranging to a small lander placed at a high latitude on Mercury from a satellite in a near‐circular polar orbit, which is accurately tracked from the Earth. These measurements can be used to determine the polar moment of inertia of the anticipated fluid outer core plus solid inner core, and thus to obtain information on the outer radius of the fluid core. Covariance analyses are carried out to assess the performance of a simulated data set consisting of 40 separate arcs with orbiter to lander ranging of about 1‐m accuracy and high‐accuracy Earth to orbiter ranging and Doppler tracking data. The effects of the uncertainties in a number of unestimated parameters are also considered. The force model includes the gravity field up to degree and order 14, solar tidal distortion of Mercury, and a fairly complex formulation for solar and Mercury radiation pressure on the spacecraft body and on the antenna. The total uncertainties for the estimated libration amplitude and the spin axis inclination are shown to be 0.26 and 0.03 arc seconds, respectively, for the model employed. The resulting uncertainty for the fractional moment of inertia of the core is 0.0045. Moreover, the Love number k2 can be determined with an accuracy of 0.01. Thus information on the elastic properties of Mercury's mantle also can be obtained.
Diagnostic calculations of the circulation in the Martian atmosphere
Authors: Santee Michelle L. , Crisp David
Published: February 2004

The circulation of the Martian atmosphere during late southern summer is derived from atmospheric temperature and dust distributions retrieved from a subset of the Mariner 9 infrared interferometer spectrometer (IRIS) thermal emission spectra (Ls = 343°–348°) (Santee and Crisp, 1993). Zonal‐mean zonal winds are calculated by assuming gradient wind balance and zero surface zonal wind. Both hemispheres have intense midlatitude westerly jets with velocities of 80–90 m/s near 50 km; in the southern tropics the winds are easterly with velocities of 40 m/s near 50. km. The net effect of the zonal‐mean meridional circulation and large‐scale waves can be approximated by the diabatic circulation, which is defined from the atmospheric thermal structure and net radiative heating rates. The radiative transfer model described by Crisp (1990) and Santee (1993) is used to compute solar heating and thermal cooling rates from diurnal averages of the retrieved IRIS temperature and dust distributions. At pressures below 4 mbar, there are large net radiative heating rates (up to 5 K/d) in the equatorial region and large net radiative cooling rates (up to 12 K/d) in the polar regions. These net radiative heating rates are used in a diagnostic stream function model which solves for the meridional and vertical components of the diabatic circulation simultaneously. We find a two‐cell circulation, with rising motion over the equator, poleward flow in both hemispheres, sinking motion over both polar regions, and return flow in the lowest atmospheric levels. The maximum poleward velocity is 3 m/s in the tropics at ∼55 km altitude, and the maximum vertical velocity is 2.5 cm/s downward over the north pole at ∼60 km altitude. If these large transport rates are sustained for an entire season, the Martian atmosphere above the 1‐mbar level is overturned in about 38 days. This diabatic circulation is qualitatively similar to the terrestrial diabatic circulation at the comparable season, but is more vigorous.
Glacial conditions in the Red Sea
Authors: Rohling Eelco J.
Published: February 2004

In this paper, results from previous studies on planktonic foraminifera, δ18O, and global sea level are combined to discuss climatic conditions in the Red Sea during the last glacial maximum (18,000 B.P.). First, the influence of 120‐m sea level lowering on the exchange transport through the strait of Bab‐el‐Mandab is considered. This strait is the only natural connection of the Red Sea to the open ocean. Next, glacial Red Sea outflow salinity is estimated (about 48 parts per thousand) from the foraminiferal record. Combined, these results yield an estimate of the glacial net water deficit, which appears to have been quite similar to the present (about 2 m yr−1). Finally, budget calculation of δ18O fluxes suggests that the glacial δ18O value of evaporation was about 50% of the present value. This is considered to have resulted from substantially increased mean wind speeds over the glacial Red Sea, which would have caused a rapid drop in the kinematic fractionation factor for 18O. The sensitivity of the calculated values for water deficit and isotopic fractionation to the various assumptions and estimates is evaluated in the discussion. Improvents are to be expected especially through research on the glacial salinity contrast between the Red Sea and Gulf of Aden. It is argued, however, that such future improvement will likely result in a worsening of the isotopic discrepancy, thus increasing the need for an additional mechanism that influenced fractionation (such as mean wind speed). This study demonstrates the need for caution when calculating paleosalinities from δ18O records under the assumption that the modern S∶δ18O relation has remained constant through time. Previously overlooked factors, such as mean wind speed, may have significantly altered that relation in the past.
Climatic importance of the modulation of the 100 kyr cycle inferred from 16 m.y. long Miocene records
Authors: Beaufort Luc
Published: February 2004

The variations of the amplitude of the 100 kyr cycle are not yet clearly understood. Although the timing of the appearance of these cycles in the Pleistocene is well established, very little is known about the occurrence of such cycles and their evolution in most of the Neogene. Two detailed and continuous 16 m.y. long Miocene records of paleoceanographic significance are analyzed in order to study the dynamics of the 100 kyr cycle. One record results from counts of the relative abundance of the calcareous nannoplankton Coccolithus pelagicus in about 1000 samples taken in the Neogene section recovered from Ocean Drilling Program site 747 (southern Indian Ocean). This record, in which the chronologic resolution is ∼17 kyr, is interpreted as reflecting migrations of the Antarctic polar front through time. The record of the wet‐bulk density in the same section, which is interpreted as reflecting variations of paleoproductivity constitutes the second 16 m.y. record. It has a chronologic resolution of 3.5 kyr. The two series reveal the presence of 100 kyr cycle during the Miocene. This cycle is not stable through time and vary similarly in the two series. The amplitude of the 100 kyr cycle fluctuates quasi‐periodically with primary periods close to 2.38, 1.14, and 0.8 m.y., equivalent to those of the modulation of the orbital parameters (obliquity and eccentricity). In addition, the amplitude of the 100 kyr cycle is stronger during cooler times. The interactions between climate and modulation of the eccentricity or the obliquity cycles are good candidates to explain that change in amplitude. It is concluded that the long term modulation of the orbital parameter has a significant influence on global climate.
Interspecies variation in stable isotopic signals of Maastrichtian planktonic foraminifera
Authors: D'Hondt Steven , Arthur Michael A.
Published: February 2004

We measured stable isotopic signals of the largest specimens of all abundant species of planktonic foraminifera in late Maastrichtian samples from three Atlantic mid‐latitude and high‐latitude sites. Each sample is marked by a relatively small interspecies range of δ18O values (1.2 to 0.8‰). Each also exhibits an interspecies range of δ13C values that resembles those of modern assemblages at similar latitudes (2.8 to 0.9‰, with greater ranges at lower latitudes). These results indicate that Late Cretaceous planktonic foraminifera inhabited water masses characterized by a relatively narrow range of temperatures but may have occupied an array of niches as ecologically diverse as that of modern taxa. Maastrichtian taxa exhibit more complex relationships between skeletal morphology and vertical and seasonal paleohabitat affinities than previous paleoecologic models have assumed. For example, keeled taxa were not limited to cold or deep habitats and serial taxa appear to have occupied both warm near‐surface niches and the coolest or most saline habitats. The isotopic differences between species indicate that apparent variation in Maastrichtian stable isotopic signals strongly depends on the taxa analyzed. Narrowly constrained size fractions of Rugoglobigerina species may provide a good estimate of relative variation in isotopic equilibria of near‐surface summer paleoconditions at low and middle latitudes. Gublerina species, Laeviheterohelix species, and Planoglobulina multicamerata appear the likeliest candidates for consistently tracking relative variation in isotopic equilibria of the coldest or most saline paleohabitats of Maastrichtian planktonic foraminifera. These isotopic results suggest that upper water column thermal gradients may have been only 3 to 5°C in mid‐latitude and high‐latitude regions. They also suggest that the western and central Atlantic mid‐latitude summer seasurface may have been much cooler or saltier in the Maastrichtian than it is today. Finally, they corroborate previous estimates of ∼ 10°C surface water at southern high latitudes.
Surface water temperature, salinity, and density changes in the northeast Atlantic during the last 45,000 years: Heinrich events, deep water formation, and climatic rebounds
Authors: Maslin M. A. , Shackleton N. J. , Pflaumann U.
Published: February 2004

We developed a new method to calculate sea surface salinities (SSS) and densities (SSD) from planktonic foraminiferal δ18O and sea surface temperatures (SST) as determined from planktonic foraminiferal species abundances. SST, SSS, and SSD records were calculated for the last 45,000 years for Biogeochemical Oceanic Flux Study (BOFS) cores 5K and 8K recovered from the northeast Atlantic. The strongest feature is the dramatic drop in all three parameters during the Heinrich “ice‐rafting” events. We modelled the possibility of deepwater formation in die northeast Atlantic from the SSD records, by assuming that the surface waters at our sites cooled as they flowed further north. Comparison with modelled North Atlantic deepwater densities indicates that there could have been periods of deepwater formation between 45,000 and 30,000 14C years B.P. (interrupted by iceberg meltwater input of Heinrich event 3 and 4, at 27,000 and 38,000 14C years B.P.) and during the Holocene. No amount of cooling in the northeast Atlantic between 30,000 and 13,000 years could cause deep water to form, because of the low salinities resulting from the high meltwater inputs from icebergs. Our records indicate that after each Heinrich event there were periods of climatic rebound, with milder conditions persisting for up to 2000 years, as indicated by the presence of warmer and more saline water masses. After these warm periods conditions returned to average glacial levels. These short term cold and warm episodes in the northeast Atlantic are superimposed on the general trend towards colder conditions of the Last Glacial Maximum (LGM). Heinrich event 1 appears to be unique as it occurs as insolation rose and was coeval with the initial melting of the Fennoscandian ice sheet. We propose that meltwater input of Heinrich event 1 significantly reduced North Atlantic Deep Water formation, reducing the heat exchange between the low and high latitudes, thus delaying deglaciation by about 1500 radiocarbon years (2000 calendar years).
Dynamic modeling of orographically induced precipitation
Authors: Barros Ana Paula , Lettenmaier Dennis P.
Published: February 2004

Local orography governs the triggering of cloud formation and the enhancement of processes such as condensation and hydrometeor nucleation and growth in mountainous regions. Intense, lengthy precipitation events are typical upwind of the topographic divide, with sharply decreasing magnitude and duration on the lee side. Differences in mean annual precipitation of several hundred percent between windward slopes of orographic barriers and adjacent valleys or lee side slopes are not unusual. Because much of the streamflow in areas such as the western United States is derived from mountainous areas that are remote and often poorly instrumented, modeling of orographic precipitation has important implications for water resources management. Models of orographically induced precipitation differ by their treatment of atmospheric dynamics and by the extent to which they rely on bulk parameterization of cloud and precipitation physics. Adiabatic ascent and a direct proportionality between precipitation efficiency and orographically magnified updrafts are the most frequent assumptions in orographic precipitation modeling. Space‐time discretization (i.e., resolution) is a major issue because of the high spatial variability of orographic precipitation. For a specific storm, relative errors as large as 50 to 100% are common in the forecast/hindcast of precipitation intensity and can be even larger in the case of catastrophic storms. When monthly or seasonal timescales are used to evaluate model performance, the magnitude of such errors decreases dramatically, reaching values as low as 10 to 15%. Current research is focusing on the development of data assimilation techniques to incorporate radar and satellite observations, and on the development of aggregation and disaggregation methodologies to address the implications of modeling a multiscale problem at restricted spatial and temporal resolutions.