bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical distributions of lightning NOx for use in regional and global chemical transport models
Authors: Pickering Kenneth E. , Wang Yansen , Tao Wei‐Kuo , Price Colin , Müller Jean‐Francois
Published: February 2004

We have constructed profiles of lightning NOχ mass distribution for use in specifying the effective lightning NOχ source in global and regional chemical transport models. The profiles have been estimated for midlatitude continental, tropical continental, and tropical marine regimes based on profiles computed for individual storms in each regime. In order to construct these profiles we have developed a parameterization for lightning occurrence, lightning type, flash placement, and NOχ production in a cloud‐scale tracer transport model using variables computed in the two‐dimensional Goddard Cumulus Ensemble (GCE) model. Wind fields from the GCE model are used to redistribute the lightning NOχ throughout the duration of the storm. Our method produces reasonable results in terms of computed flash rates and NOχ mixing ratios compared with observations. The profiles for each storm are computed by integrating the lightning NOχ mass across the cloud model domain for each model layer at the end of the storm. The results for all three regimes show a maximum in the mass profile in the upper troposphere, usually within 2–4 km of the tropopause. Downdrafts appear to be the strongest in the simulated midlatitude continental systems, evidenced by substantial lightning NOχ mass (up to 23%) in the lowest kilometer. Tropical systems, particularly those over marine areas, tended to have a greater fraction of intracloud flashes and weaker downdrafts, causing only minor amounts of NOχ to remain in the boundary layer following a storm. Minima appear in the profiles typically in the 2–5 km layer. Even though a substantial portion of the NOχ is produced by cloud‐to‐ground flashes in the lowest 6 km, at the end of the storm most of the NOχ is in the upper troposphere (55–75% above 8 km) in agreement with observations. With most of the effective lightning NOχ source in the upper troposphere where the NOχ lifetime is several days, substantial photochemical O3 production is expected in this layer downstream of regions of deep convection containing lightning. We demonstrate that the effect on upper tropospheric NOχ and O3 is substantial when the vertical distribution of the lightning NOχ source in a global model is changed from uniform to being specified by our profiles. Uncertainties in a number of aspects of our parameterization are discussed.
Enhanced absorption of UV radiation due to multiple scattering in clouds: Experimental evidence and theoretical explanation
Authors: Mayer B. , Kylling A. , Madronich S. , Seckmeyer G.
Published: February 2004

Measurements of spectral ultraviolet‐B irradiance under optically thick clouds show strongly enhanced attenuation by molecular and particulate absorbers. The atmospheric photon path is enhanced owing to the presence of a highly scattering medium, leading to an amplification of absorption by tropospheric ozone and aerosol. Calculations with discrete ordinate and Monte Carlo models show that photon paths in realistic water clouds may be enhanced by factors of 10 and more compared to cloudless sky. Model calculations further show that UV spectra measured under thick clouds can be well simulated within 10%, indicating that the involved processes are quantitatively described by current models. These findings are of important consequence for all ground‐based remote sensing applications which take advantage of measuring scattered radiation in order to infer atmospheric trace gas abundances. These algorithms, for example, the calculation of total ozone from global irradiance or zenith sky radiance, are subject to large errors, when neglecting the influence of cloud scattering on the derived data. In the present study, errors of more than 300 Dobson Units (DU) have been found, if such methods were applied without care in the presence of thick clouds.
OPAL: Network for the Detection of Stratospheric Change ozone profiler assessment at Lauder, New Zealand 2. Intercomparison of revised results
Authors: McDermid I. S. , Bergwerff J. B. , Bodeker G. , Boyd I. S. , Brinksma E. J. , Connor B. J. , Farmer R. , Gross M. R. , Kimvilakani P. , Matthews W. A. , McGee T. J. , Ormel F. T. , Parrish A. , Singh U. , Swart D. P. J. , Tsou J. J.
Published: February 2004

Following a blind intercomparison of ozone profiling instruments in the Network for the Detection of Stratospheric Change at Lauder, New Zealand, revisions to the analyses were made resulting in a new data set. This paper compares the revised results from two differential absorption lidars (RIVM and GSFC), a microwave radiometer (Millitech/LaRC), and electrochemical concentration cell (ECC) balloon sondes (NIWA). In general, the results are substantially improved compared to the earlier blind intercomparison. The level of agreement was similar both for single profiles and for the campaign average profile and was approximately 5% for the lidars and the sondes over the altitude range from 15 to 42 km (32 km for sondes). The revised microwave data show a bias of 5–10% high in the region from 22 to 42 km. Starting at 42 km, the lidar errors increase significantly, and comparisons of the microwave results were not possible above this altitude.
Simulation of 20th century temperature trends
Authors: Graham Nicholas E. , Tyree Mary
Published: February 2004

Results are reported from simulations with an atmospheric general circulation model (GCM) covering the 20th century. These results focus primarily on the continental surface air temperature (SAT) record, with some discussion of large spatial scale precipitation averages as well. These experiments were conducted using the Max Planck Institute for Meteorology ECHAM3 GCM configured at triangular‐21 truncation, giving a spatial resolution of approximately 5.5°. The surface boundary conditions for the model were constructed by forming decadal climatologies of sea surface temperature (SST) data from the Global SST Atlas (GOSSTA) data set. No changes in greenhouse gas concentrations were made during the experiments. The GCM was then integrated for 5 years with each climatology. Because the high southern latitudes are poorly sampled and show considerable high‐amplitude variability in the GOSSTA data, two complete experiments were conducted. In one experiment the time‐varying GOSSTA data were prescribed globally (the GLBL simulation), in the other, SSTs in the high southern latitudes were held at the 1950–1969 climatology (the NO_SH simulation). The results show both GCM simulations reproduce important features of the observed 20th century global temperature record, suggesting that the low‐frequency variability in global average SAT is modulated to an important degree by variability in SSTs. In the model results, the changes in SST are transmitted to the atmosphere via latent heat flux, i.e., by changes in the flux of water through the hydrologic cycle. The most notable difference between the GLBL and NO_SH SAT records is that during the 1920–1940 period the NO_SH results agree more closely with observations, while the GLBL results are superior from the 1950s forward. At least in part, these differences appear to be linked to differences in simulated sea‐ice extent in the two simulations. Analyses of large‐scale averages in continental precipitation show good model‐observation agreement in the tropics (±30° latitude) and in the extratropical southern hemisphere; however, the upward trend in extratropical NH continental precipitation seen in the observed record is not reproduced in either simulation.
Heat transport efficiency for stagnant lid convection with dislocation viscosity: Application to Mars and Venus
Authors: Reese C. C. , Solomatov V. S. , Moresi L.‐N.
Published: February 2004

Mantle convection on Mars and Venus is likely to occur in the regime known as stagnant lid convection. We perform thermal boundary layer analyses as well as finite element simulations of stagnant lid convection with non‐Newtonian viscosity (which is believed to be more appropriate for the lithosphere and upper mantle) and discuss one particular application of the results, the efficiency of heat transport on the terrestrial planets. As in the case of Newtonian viscosity, the efficiency of heat transfer in the stagnant lid regime is extremely low compared to plate tectonics: For example, in the absence of plate tectonics, the mantle temperature on Earth, which is already close to the solidus, would be about 700–1500 K higher for the present‐day value of the surface heat flux. For Venus, the critical heat flux which can be removed without widespread melting is only 10–20 m W/m2. For Mars, it is 15–30 m W/m2. Therefore, there are no doubts that in the absence of mobile plates, the mantle temperature would significantly exceed solidus during planetary evolution. It is hypothesized that this could cause one, or a combination, of two possible processes: (1) differentiation of radiogenic isotopes into the crust during early planetary magmatism and (2) initiation of some kind of plate tectonics as a result of plate weakening due to melting.
Style and sequence of extensional structures in tessera terrain, Venus
Authors: Gilmore Martha S. , Collins Geoffrey C. , Ivanov Mikhail A. , Marinangeli Lucia , Head James W.
Published: February 2004

Recent studies have focused on the question of the stratigraphic sequence and thus the stages of tessera formation, specifically, if tessera are formed by contractional deformation followed by extensional deformation or vice versa. A major question centers on the interpretation of specific lineaments within tesserae as graben (bounded by faults ∼60°) or, alternatively, open tension fractures (dipping ∼90°). We document and assess the origin of extensional structures in tesserae at several locations on Venus, noting the morphology, continuity along strike, parallelism of walls, stratigraphic position and interaction with other structures, and variability due to radar viewing geometry. In each study area, our analyses demonstrate that (1) the extensional structures have variable widths, interior subparallel lineaments, and ramp terminations; (2) ridges and lineaments are continuous across the troughs, where the floors of many of these structures contain the lowered sections of preexisting structures; and (3) intratessera plains are seen to embay ridges and an impact crater is superposed on a ridge and in both cases these features are subsequently deformed by the extensional structures. We conclude that the morphology of these extensional structures is consistent with an origin as graben, not open tension fractures, and that these graben postdate the ridges in each study area. Both the graben and the ridges of the sizes found in our survey can be formed when the brittle crust is of the order of 1 to 10 km thick. To further test the tension fracture model, we examine the conditions of a Venus that could produce tension fractures of the dimension (∼1 km width) of extensional structures found in tessera terrain and find that thermal gradients of a minimum of 400 to 1500 K km−1 (heat flows of 800 to 3000 mW m−2) are required for a range of diabase rheologies and strain rates thought typical of Venus during tessera formation. Such a thermal structure would favor partial melting at depths <1 km. Dike propagation from this region of shallow melting within the tensile stress field would produce vast quantities of volcanism, mitigating against the preservation of the closely associated tension fractures; this volcanism is not observed. Both the amplitude and sign of changes in surface temperature induced by atmospheric warming due to massive outpourings of lava are not consistent with the hotspot model. On the basis of our analysis of tesserae, we conclude that the ridges formed first in response to large‐scale contraction of the crust and that the graben formed contemporaneously and largely following this phase as the thickened crust relaxed in a manner to what is predicted and observed for plateau regions on Earth such as Tibet and the Altiplano.
Erosional valleys in the Thaumasia region of Mars: Hydrothermal and seismic origins
Authors: Tanaka Kenneth L. , Dohm James M. , Lias Juan H. , Hare Trent M.
Published: February 2004

Analysis of erosional valleys, geologic materials and features, and topography through time in the Thaumasia region of Mars using co‐registered digital spatial data sets reveals significant associations that relate to valley origin. Valleys tend to originate (1) on Noachian to Early Hesperian (stages 1 and 2) large volcanoes, (2) within 50–100 km of stages 1 and 2 rift systems, and (3) within 100 km of Noachian (stage 1) impact craters >50 km in diameter. These geologic preferences explain observations of higher valley‐source densities (VSDs) in areas of higher elevations and regional slopes (>1°) because the volcanoes, rifts, and craters form high, steep topography or occur in terrain of high relief. Other stage 1 and stage 2 high, steep terrains, however, do not show high VSDs. The tendency for valleys to concentrate near geologic features and the overall low drainage densities in Thaumasia compared to terrestrial surfaces rule out widespread precipitation as a major factor in valley formation (as is proposed in wann, wet climate scenarios) except perhaps during the Early Noachian, for which much of the geologic record has been obliterated. Instead, volcanoes and rifts may indicate the presence of shallow crustal intrusions that could lead to local hydrothermal circulation, melting of ground ice and snow, and groundwater sapping. However, impact‐crater melt would provide a heat source at the surface that might drive away water, fonning valleys in the process. Post‐stage 1 craters mostly have low nearby VSDs, which, for valleys incised in older rocks, suggests burial by ejecta and, for younger valleys, may indicate desiccation of near‐surface water and deepening of the cryosphere. Later Hesperian and Amazonian (stages 3 and 4) valleys originate within 100–200 km of three young, large impact craters and near rifts systems at Warrego Valles and the southern part of Coprates rise. These valleys likely developed when the cryosphere was a couple kilometers or more thick, inhibiting valley development by hydrothermal circulation. However, eruption of groundwater may have occurred from impact‐induced fracturing and lateral and perhaps minor upward transport of water due to seismic pumping. The two smaller craters formed along the plateau margin where the highest potential hydraulic head would occur in aquifers beneath the plateau. In the case of the larger crater (Lowell, 200 km in diameter), potential aquifers would likely be at depths of kilometers below the cryosphere. Seismic energy generated by the Lowell impactor would have been much greater, pumping both groundwater and perhaps fluidized slurry to the surface from beneath the cryosphere to form the young valleys and flow deposit. Along the margin of Thaumasia, tectonic pressurization of groundwater also may have contributed to valley formation. Dissection of rim materials of the Argyre impact may relate to tectonic activity and the unconsolidated state of basin ejecta.
Non‐local thermodynamic equilibrium in general circulation models of the Martian atmosphere 1. Effects of the local thermodynamic equilibrium approximation on thermal cooling and solar heating
Authors: López‐Valverde Miguel A. , Edwards David P. , López‐Puertas Manuel , Roldán Cristina
Published: February 2004

Calculations of CO2 thermal cooling and near‐IR solar heating rates under non‐local thermodynamic equilibrium (non‐LTE) situations have been performed to understand and evaluate the effects of non‐LTE on the energy balance of the upper atmosphere of Mars. We find that the 15‐μm cooling rates can be in error if LTE is assumed above 80 km. In general, the correct non‐LTE values are significantly smaller than the LTE values above about 85 km, but the magnitude and sign of the error depend on the temperature structure and the top altitude of the model and, to a lesser extent, on the collisions with atomic oxygen. A detailed analysis of the relevance of the upper boundary layer and a suggested buffer region are presented for both LTE and non‐LTE. Based on general considerations of the thermal profile in the mesosphere and lower thermosphere, recommendations for general circulation models (GCM) are presented as a first guide for minimizing the LTE cooling rates inaccuracies. The error of assuming LTE on the CO2 near‐IR solar heating rates is found to be about 20% at 85 km and increases strongly above this altitude. The dependences of this LTE‐non‐LTE difference on rate coefficients, thermal structure, surface pressure, and solar zenith angle (SZA) are studied. In contrast to the large effect of the SZA on the solar heating rate, we find it is not important for the LTE‐non‐LTE relative difference, which permits a simple tabulation of the non‐LTE effect as a function of pressure only. A table of LTE correction factors for the solar heating rate is included for its potential use as a fast yet accurate operative scheme within GCMs.
The search for life on Mars: What we learned from Viking
Authors: Klein Harold P.
Published: February 2004

The Viking mission has profoundly influenced scientific thinking about the question of life on Mars. Based on the results obtained from a number of Viking investigations, it is clear that the surface of the planet is too hostile to support a carbon‐based biota. While the existence of biological “oases” elsewhere on the planet have been proposed, none have yet been found. Other information based on Viking data has focused on the possibility that life may have been present on Mars in the distant past, and Viking images have revealed sites on Mars where further exploratory studies for extinct biology are warranted.
Refugia from asteroid impacts on early Mars and the early Earth
Authors: Sleep Norman H. , Zahnle Kevin
Published: February 2004

Impacts of asteroids and comets posed a major hazard to the continuous existence of early life on Mars, as on Earth. The chief danger was presented by globally distributed ejecta, including transient thick rock vapor atmospheres. On Earth, much of the thermal radiation is absorbed by boiling the oceans. Any surviving life is either in deep water or well below the surface. Global thermal excursions are buffered by the heat capacity of the oceans. But when impacts are large enough to vaporize the oceans (>l028J), thermal buffering serves only to prolong the disaster for thousands of years, while the oceans rain out. Without oceans, thermal buffering does not occur on Mars. Relatively small impacts (1026J) frequently heat the surface everywhere to the melting point. However, owing to the low Martian escape velocity, the most energetic ejecta (including the rock vapor) more easily escape to space, while massive quantities of less energetic ejecta are globally distributed. Survival in deep subsurface environments is more likely on Mars because (1) Mars' lower background heat flow and lower gravity allow deeper colonies, and (2) the thermal heat pulse from a major impact is briefer. Only thermophile organisms could have survived impacts of asteroids large enough to leave heat or boil the entire terrestrial ocean. Studies of terrestrial microorganisms indicate that the last common ancestor may have been thermophile and the survivor of such a catastrophe. This organism should be distinguished from the first common ancestor. An additional refugium is ejection of rock to space by impacts and thier return to a habitable planet.
On the thickness of the oxidized layer of the Martian regolith
Authors: Zent Aaron P.
Published: February 2004

A revised model of the diffusion of H2O2 through the Martian regolith is presented, which argues that oxidant diffusion may be more efficient than previously thought. Recent models of the adsorption of H2O at Mars‐like conditions indicate that it adsorbs more poorly than previously believed. H2O adsorption is a necessary proxy for peroxide adsorption; hence the adsorptive slowing of peroxide diffusion is modeled as less efficient. Because the peroxide has a finite lifetime, it has a finite extinction depth as well. The effects of regolith gardening by impacts are quantitatively estimated and combined with the effects of oxidation by atmospheric gases to produce estimates of the degree of oxidation of the Martian surface with depth. We explore the effects of different crater production populations along with variations in H2O2 extinction depths, and hydrothermal oxidation of ejecta. In very select circumstances involving very early onset of oxidizing conditions during heavy bombardment, 150–200 m of regolith could be fully oxidized. More likely scenarios for the crater production population, onset of oxidizing conditions, and oxidant extinction depth yield estimates of no more than a few meters to putative reducing material. In addition, uncertainties remain regarding the degree to which hydrothermal or other high‐temperature chemistry might oxidize materials in ejecta blankets. The trade‐off between accessing unlithified sediments or rock interiors must be considered.
Effect of surface roughness on bidirectional reflectance of Antarctic snow
Authors: Warren Stephen G. , Brandt Richard E. , O'Rawe Hinton Patricia
Published: February 2004

The angular pattern of sunlight reflected by snow is altered by surface roughness, which in the interior of Antarctica is usually in the form of meter‐scale longitudinal erosional features (sastrugi), whose axes align with the direction of strong winds. The bidirectional reflectance distribution function (BRDF) changes over the course of a day as the solar azimuth changes relative to the sastrugi axis. The normalized BRDF, or “anisotropic reflectance factor” R, was measured at South Pole Station from a 22‐m tower at 600, 660, and 900 nm wavelengths. The R pattern was similar at the three wavelengths; it probably varies little from 300 to 900 nm. Measurements were made at solar zenith angles θ0 from 67° to 90°, over the full range of viewing zenith angle (θr), azimuth angle between Sun and view (ϕ), and azimuth angle between Sun and sastrugi (ϕsas). Variation of R with ϕsas was notable; sastrugi oriented perpendicular to the solar beam cause a reduction of the forward peak, and sastrugi at an oblique angle cause R to lose its symmetry about the solar azimuth. However, the effects of sastrugi are mostly restricted to large viewing zenith angles, so remote sensing of albedo and atmospheric properties can be carried out accurately without knowledge of sastrugi height and orientation if only near‐nadir views are used. This recommendation is opposite that for observations of broken clouds over dark surfaces, for which large θr is preferred. A parameterization of R is developed, valid for viewing angles θr ≤ 50°. Sastrugi can cause a reduction of the snow albedo by altering the angle of incidence and by trapping of photons. For the small sastrugi of the Antarctic Plateau, the albedo is unaffected at visible wavelengths but can be reduced by a few percent at near‐infrared wavelengths when the Sun is low.
The Shergottite age paradox and the relative probabilities for Martian meteorites of differing ages
Authors: Nyquist L. E. , Borg L. E. , Shih C.‐Y.
Published: February 2004

The apparent paradox that the majority of impacts yielding Martian meteorites appear to have taken place on only a few percent of the Martian surface can be resolved if all the shergottites were ejected in a single event rather than in multiple events. The apparent degree by which shergottites are overrepresented among Martian meteorites is thereby reduced to about fourfold relative to other types of Martian meteorites for nominal cratering rate models, and further reduced for higher cratering rate models. If the shergottite‐ejection event is assigned to one of three craters in the vicinity of Olympus Mons that were previously identified as candidate source craters for the SNC (shergottites, nakhlites, Chassigny) meteorites, and the nakhlite/chassignite event is assigned to a candidate crater in the vicinity of Ceraunius Tholus, the implied ages of the surrounding terrains agree well with crater density ages. Because of their young crystallization ages, the likely origin of the shergottites is in the Tharsis region even for cratering rates up to twice the nominal value (minimum terrain ages). For these higher cratering rates, the nakhlite source need not be in the Tharsis region. Since the source of the ancient orthopyroxenite, ALH84001, was probably in the ancient southern uplands, meteorite‐yielding impacts may have been widely dispersed on the Martian surface.
Global dominance of Gephyrocapsa coccoliths in the Late Pleistocene: Selective dissolution, evolution, or global environmental change?
Authors: Bollmann Jörg , Baumann Karl‐Heinz , Thierstein Hans R.
Published: February 2004

Individual species of marine calcareous phytoplankton are known to occur in blooms in today's ocean and to have dominated fossil assemblages. Interest in the study of marine phytoplankton‐ocean‐climate interactions has increased because of the potential influence of phytoplankton species like Emiliania huxleyi on global climate and on the global carbon cycle. There is evidence that Gephyrocapsa caribbeanica, which is closely related to E. huxleyi, was globally dominant in the late Pleistocene (480–262 ka). Morphological analyses of Gephyrocapsa coccoliths reveal that only two of six Holocene morphological associations occurred during this time interval. We examine three potential causes for the dominance: preservation, environment, and evolutionary adaptation. We conclude that evolutionary adaptation was the likely process responsible for Gephyrocapsa dominance, although there are indications in the mid‐Brunhes for warmer climates than today.
Quaternary surface water temperature estimations: Calibration of a diatom transfer function for the Southern Ocean
Authors: Zielinski U. , Gersonde R. , Sieger R. , Fütterer D.
Published: February 2004

The quantitative diatom analysis of 218 surface sediment samples recovered in the Atlantic and western Indian sector of the Southern Ocean is used to define a base of reference data for paleotemperature estimations from diatom assemblages using the Imbrie and Kipp transfer function method. The criteria which justify the exclusion of samples and species out of the raw data set in order to define a reference database are outlined and discussed. Sensitivity tests with eight data sets were achieved evaluating the effects of overall dominance of single species, different methods of species abundance ranking, and no‐analog conditions (e.g., Eucampia Antarctica) on the estimated paleotemperatures. The defined transfer functions were applied on a sediment core from the northern Antarctic zone. Overall dominance of Fragilariopsis kerguelensis in the diatom assemblages resulted in a close affinity between paleotemperature curve and relative abundance pattern of this species downcore. Logarithmic conversion of counting data applied with other ranking methods in order to compensate the dominance of F. kerguelensis revealed the best statistical results. A reliable diatom transfer function for future paleotemperature estimations is presented.
Temporal variability in the northern Arabian Sea oxygen minimum zone (OMZ) during the last 225,000 years
Authors: Reichart G. J. , Lourens L. J. , Zachariasse W. J.
Published: February 2004

The northern Arabian Sea is one of the few regions in the open ocean where thermocline water is severely depleted in oxygen. The intensity of this oxygen minimum zone (OMZ) has been reconstructed over the past 225,000 years using proxies for surface water productivity, water column denitrification, winter mixing, and the aragonite compensation depth (ACD). Changes in OMZ intensity occurred on orbital and suborbital timescales. Lowest O2 levels correlate with productivity maxima and shallow winter mixing. Precession‐related productivity maxima lag early summer insolation maxima by ∼6 kyr, which we attribute to a prolonged summer monsoon season related to higher insolation at the end of the summer. Periods with a weakened or even non‐existent OMZ are characterized by low productivity conditions and deep winter mixing attributed to strong and cold winter monsoonal winds. The timing of deep winter mixing events corresponds with that of periods of climatic cooling in the North Atlantic region.
Interhemispheric appraisal of the value of alkenone indices as temperature and salinity proxies in high‐latitude locations
Authors: Rosell‐Melé Antoni
Published: February 2004

Alkenone sediment data from the Nordic seas and North Atlantic are compared to those from Sikes et al.[1997] for the Southern Ocean to evaluate further UK37 and UK37′ as proxies to estimate cold temperatures (<10°C) and the effect of salinity and temperature in the relative abundance of 37∶4 to the total abundance of C37 alkenones (37∶4%). UK37 and UK37′ are found to be equally viable as proxies, but there are significant regional differences in their cold temperature dependence. The measurement of 37∶4% in cores from the North Atlantic region can be used to identify situations when UK37′ is not a reliable paleothermometer. Variations in salinity are probably responsible for changes in the sedimentary record of 37∶4%, and a preliminary calibration has been obtained for 37:4%=f(salinity). This new relationship should be further confirmed through field or laboratory experiments, but it paves the way to derive a molecular proxy to reconstruct paleosalinity in surface waters.
Atmospheric and oceanic evidences of El Niño‐Southern Oscillation events in the south central Pacific Ocean from coral stable isotopic records over the last 137 years
Authors: Boiseau Muriel , Juillet‐Leclerc Anne , Yiou Pascal , Salvat Bernard , Isdale Peter , Guillaume Mireille
Published: February 2004

We measured δ18O and δ13C in Porites lutea collected in the Moorea lagoon where the instrumental records show that an El Niño‐Southern Oscillation (ENSO) event implies both a cloud cover decrease and a weak sea surface temperature (SST) increase. Two proxies allow an ENSO record to be reconstructed at Moorea: the annual δ13C anomaly, associated with the South Pacific Convergence Zone motion, and an annual δ18O anomaly showing increased SST. The frequency bands exhibited by the singular spectral analysis (SSA) and the multitaper method of the annual δ18O and δ13C records are centered on 2.5 and 5.2 years and 2.4 and 3.2 years, respectively, which are the main ENSO modes. We used SSA to reconstruct ENSO events at Moorea over the past 137 years. Results indicate that climate variability at this site is strongly affected by ENSO events.
Scattering function of the fluctuating ionosphere in the HF band
Authors: Gherm Vadim E. , Zernov Nikolay N.
Published: February 2004

This paper is devoted to the investigation of the two‐frequency, two‐position, time coherence function and the ionospheric scattering function describing the HF ionospheric fluctuating radio channel. The complex phase method is applied to obtain the analytical expressions for the coherence and correlation functions, which are then calculated numerically for the realistic models of the fluctuating ionosphere. The numerical Fourier transformation of the correlation function gives the ionospheric scattering function. The numerical results obtained lead to the conclusion that in the general case the large variability of shapes of the scattering function of the fluctuating ionosphere exists depending on the concrete conditions of propagation. In particular, the well‐known delay‐Doppler coupling can be more or less pronounced in different propagation conditions. We have shown that the presence of the coupling is exclusively due to the nonzero imaginary part of the correlation function of the scattered field, which means that this effect has a purely diffractional nature and cannot be obtained in the geometrical optics approximation.
Improved relocatable over‐the‐horizon radar detection and tracking using the maximum likelihood adaptive neural system algorithm
Authors: Perlovsky Leonid I. , Webb Virgil H. , Bradley Scott R. , Hansen Christopher A.
Published: February 2004

An advanced detection and tracking system is being developed for the U.S. Navy's Relocatable Over‐the‐Horizon Radar (ROTHR) to provide improved tracking performance against small aircraft typically used in drug‐smuggling activities. The development is based on the Maximum Likelihood Adaptive Neural System (MLANS), a model‐based neural network that combines advantages of neural network and model‐based algorithmic approaches. The objective of the MLANS tracker development effort is to address user requirements for increased detection and tracking capability in clutter and improved track position, heading, and speed accuracy. The MLANS tracker is expected to outperform other approaches to detection and tracking for the following reasons. It incorporates adaptive internal models of target return signals, target tracks and maneuvers, and clutter signals, which leads to concurrent clutter suppression, detection, and tracking (track‐before‐detect). It is not combinatorial and thus does not require any thresholding or peak picking and can track in low signal‐to‐noise conditions. It incorporates superresolution spectrum estimation techniques exceeding the performance of conventional maximum likelihood and maximum entropy methods. The unique spectrum estimation method is based on the Einsteinian interpretation of the ROTHR received energy spectrum as a probability density of signal frequency. The MLANS neural architecture and learning mechanism are founded on spectrum models and maximization of the “Einsteinian” likelihood, allowing knowledge of the physical behavior of both targets and clutter to be injected into the tracker algorithms. The paper describes the addressed requirements and expected improvements, theoretical foundations, engineering methodology, and results of the development effort to date.
A new solution to the problem of ionospheric tomography using quadratic programing
Authors: Spencer P. , Kersley L. , Pryse S. E.
Published: February 2004

A new independent method for reconstructing tomographic images of the ionosphere is described. The mathematical approach makes use of an interior‐point algorithm for solving quadratic programing problems. The procedure has been designed to be fully automated and to work in either two or three dimensions. Results are presented using experimental data obtained during a campaign carried out in Scandinavia in November 1995 for which independent verification from the European incoherent scatter (EISCAT) radar and the Kiruna ionosonde are available. A comparison is made between the mean diurnal variation of maximum electron density obtained from the tomographic reconstructions and the parameterized ionospheric model.
Separation of multiple echoes using a high‐resolution spectral analysis for SuperDARN HF radars
Authors: Barthes L. , André R. , Cerisier J.‐C. , Villain J.‐P.
Published: February 2004

Data obtained with coherent HF backscatter radars of the Super Dual Auroral Radar Network (SuperDARN) are routinely analyzed with a standard algorithm based upon the simplifying assumption that the backscattered signal consists of a single source, characterized by its Doppler velocity and spectral width. More complex situations are often encountered where the signal includes several sources, either due to the additional existence of ground scatter or due to multiple ionospheric lines, related to a strongly inhomogeneous velocity field. We analyze the response of the standard algorithm to such signals and we propose to use high‐resolution spectral analysis methods, namely, the multiple signal classification (MUSIC) method, to separate multiple echoes with different velocity and spectral width. We analyze theoretically the autocorrelation function of the received signal, and we show that its structure satisfies the criteria for processing by the MUSIC algorithm. A statistical numerical simulation of SuperDARN data processing by the MUSIC method allows us to evaluate the performances and the limits of applicability of the method. We show and illustrate with examples taken from experimental data that the main improvements are (1) the correct separation of mixed echoes from ground and ionosphere, which enhances the quality of ionospheric convection measurements, and (2) the capability to resolve multiple ionospheric sources which appear in regions of inhomogeneous convection. These multiple sources can be used to resolve small‐scale structures in the velocity field.
Interpretation of angle‐of‐arrival measurements in the lower atmosphere using spaced antenna radar systems
Authors: Chau Jorge L. , Balsley Ben B.
Published: February 2004

On the basis of scattering theory, we present four different contributions to angle‐of‐arrival (AOA) measurements using spaced antenna (SA) radar systems. We show that the measurement of the scattering parameters is needed to estimate most AOA contributions (e.g., tilt angle of layers). We analyze a general SA system (i.e., receiving antennas not necessarily symmetrically placed about the transmitter) and show, theoretically, that such systems can be used for vertical velocity corrections without explicitly knowing the scattering parameters (e.g., correlation lengths). The theoretical AOA expressions are compared with lower atmospheric data (3–21 km) obtained with the Jicamarca VHF radar system in Perú. We compare the AOAs obtained by a time and a frequency domain method, giving both methods essentially the same information. While the results are comparable, frequency domain AOAs tend to exhibit more “outliers.” From examination of time series of these AOA measurements, we find some short, but predominantly long‐period oscillations (>12 hours) with amplitudes of ∼1°.
HF cross section of field‐aligned irregularities: Finite difference time domain tests of the Born theory formula
Authors: Nickisch L. J. , Franke P. M.
Published: February 2004

A model for aspect‐sensitive clutter from field‐aligned ionospheric irregularity structure has been incorporated into the over‐the‐horizon radar performance prediction code clutter effects model [Lauer et al., this issue]. These coherent scatter contributions are modeled using scattering cross sections computed in the Born approximation. It has not been known, however, whether the Born formula for the scattering cross section adequately predicts the correct level of backscatter return at HF. The model required validation, and this has now been accomplished by computing scattering amplitudes using the finite difference time domain (FDTD) method. No approximations are made in the FDTD computations other than that implicit in the use of discrete temporal and spatial grids, so these computations serve to test the validity of the approximations used to develop the Born result. We find that under certain conditions it is necessary to use a generalization of the Born formula which incorporates the profile function of the irregularity region.
Influence of cloud temperature on brightness temperature and consequences for water retrieval
Authors: Bosisio Ada Vittoria , Mallet Cécile
Published: February 2004

In this paper we show that to improve existing radiometric methods to estimate path‐integrated cloud liquid water, the physical approximations commonly used concerning the direct model have to be reconsidered. In fact, even if the problem that consists in deducing the brightness temperatures from atmospheric quantities is rigorously solved (for a nonscattering atmosphere) by means of the radiative transfer equation, the relation between radiometric temperatures and one physical global quantity integrated content is not always determined from theoretical considerations. In all the dual‐frequency retrieval models commonly used, the method always consists in the separation of the two water phases based on two radiometric measurements: one at a vapor‐sensitive frequency and one at a cloud‐sensitive frequency. The variations in the vertical absorption profile are neglected, in particular the temperature one, thus leading to linear expressions that allow an easier resolution of the inverse problem. Through an important set of simulated data, the authors have proved that a new global variable, which allows us to take the temperature profile into account, can greatly improve the modelization of the direct problem. The introduction of this quantity helped the authors to show that the relationship between brightness temperature and liquid water content is a nonlinear one. Even if the analytical inversion of the direct model allows only a relatively small improvement in water vapor and liquid water prediction, the consequences for the inverse problem are also discussed. To improve the existing dual‐frequency algorithm, additional information sensitive to temperature profile is needed. For example, a third radiometer frequency can improve the inverse algorithm, but it should be used in a nonlinear expression.
Mesospheric wind observations derived from Super Dual Auroral Radar Network (SuperDARN) HF radar meteor echoes at Halley, Antarctica: Preliminary results
Authors: Jenkins B. , Jarvis M. J. , Forbes D. M.
Published: February 2004

The implementation of a technique to derive mesospheric winds from HF radar observations of meteor echoes at Halley (76°S, 27°W), Antarctica, is described. The meteor echoes are observed at near ranges (less than 400 km) and have characteristics distinctly different from echoes backscattered from plasma irregularities in the E and F regions of the ionosphere. A Lorentzian model is used to fit the echo spectrum. The echo occurrence rate has the diurnal variation expected of meteors with a minimum in the afternoon. There also appears to be an annual variation which may be related to seasonal changes in the atmosphere. At present, results are confined to a single beam; that directed toward the south geographic pole is presented here, as this will give the meridional component of the wind and can be compared with other studies. The meridional wind is found to be dominated by the semidiurnal tide most of the year, with maxima in spring and autumn. Data for May 1996 show that the semidiurnal tide is dominant, but there are significant contributions from the 24‐hour and 8‐hour tides. A moving window spectral analysis technique allows the temporal behavior of the waves over a 10‐day period to be studied. A quasi 2‐day wave is observed during this interval, and slight changes of period with time can be observed. Planetary waves with periods of 5 and 10 days are observed during the winter of 1996. The radar has been operating since 1988, and so these data form a substantial and valuable database of mesospheric observations in the high‐latitude southern hemisphere.
A ray tracing formulation and its application to some problems in over‐the‐horizon radar
Authors: Coleman C. J.
Published: February 2004

A simple, two‐dimensional ray tracing scheme is developed and is applied to several propagation problems of importance to over‐the‐horizon radar. In particular, it is used as the basis of schemes for the simulation of oblique and backscatter ionograms. The efficiency of the scheme makes it ideal for applications where fast calculations are required, and an algorithm for inverting backscatter ionogram leading edges is developed as a particular example.
A comparison of reconstruction techniques used in ionospheric tomography
Authors: Pryse S. E. , Kersley L. , Mitchell C. N. , Spencer P. S. J. , Williams M. J.
Published: February 2004

This paper compares the performance of three different reconstruction algorithms used for tomographic imaging of the ionosphere. The test cases chosen make use of experimental electron content observations obtained from a meridional chain of receivers in Scandinavia at times when the European incoherent scatter radar facility provided independent measurements of electron density for verification of the reconstructions. The examples include ionospheres with layers peaking at both anomalously high and very low altitudes, in addition to a smooth horizontally stratified layer. The results expose the limitations of an algorithm based on use of a standard ionospheric model but demonstrate that with appropriate choice of background a priori profiles, reliable images are obtained from both discrete inverse theory and quadratic programing. For these two latter algorithms the resultant reconstructions are essentially independent of the algorithm used. The study highlights the critical role of the range of the background input to the algorithms. A density value at a single grid cell provides sufficient additional a priori information to constrain the imaging of a horizontally stratified ionosphere.
Three‐dimensional nonlinear simulations of the gradient drift instability in the high‐latitude ionosphere
Authors: Guzdar P. N. , Gondarenko N. A. , Chaturvedi P. K. , Basu S.
Published: February 2004

Nonlinear three‐dimensional (3‐D) simulations of the gradient drift instability (GDI) are presented for a two‐dimensional equilibrium density gradient representing a polar cap plasma patch. The overall evolution of structuring of the plasma patch is influenced markedly by the effects caused by dynamics parallel to the geomagnetic field. The long wavelengths (kLn ≤ 1) are strongly stabilized in the 3‐D case, and the nonlinear state is dominated by smaller mesoscales (kLn » 1) (where k and Ln are the mode number and density gradient scale length, respectively). The results provide an interpretation to the observations at the high‐latitude ionosphere indicating a generation of mesoscale irregularities associated with the polar cap patches when the interplanetary magnetic field (IMF) BZ is southward and antisunward convection prevails over the polar cap. The polar cap patches are observed to convect to large distances (∼3000 km) undergoing structuring while maintaining their distinct identity. Thus we suggest here that the inclusion of three‐dimensional effects is key to a successful interpretation of high‐latitude irregularities, as well as a prerequisite for a credible simulation of these processes.
Utilization of a nonrecurrent waveform to mitigate range‐folded spread Doppler clutter: Application to over‐the‐horizon radar
Authors: Hartnett Michael P. , Clancy John T. , Denton Robert J.
Published: February 2004

This paper describes the application of a new nonrecurrent waveform (NRWF) to a relocatable over‐the‐horizon radar (ROTHR) located in southern Virginia. The waveform is designed to improve the detection of aircraft targets through the mitigation of long‐range spread Doppler clutter (SDC) encountered during nighttime operations. A discussion of the NRWF and its principle of operation are included, along with a brief description of ROTHR modifications. The focus of the paper is the presentation of test results from engineering, clutter mitigation, and target detection tests conducted during January through April of 1997.
Dynamical effects of ionospheric conductivity on the formation of polar cap arcs
Authors: Zhu L. , Sojka J. J. , Schunk R. W. , Crain D. J.
Published: February 2004

By using a magnetosphere‐ionosphere (M‐I) coupling model of polar cap arcs [Zhu et al., 1993], a systematic model study of the effects of ionospheric background conductivity on the formation of polar cap arcs has been conducted. The variations of the ionospheric background conductivity in the model study cover typical ionospheric conditions, including solar minimum, solar maximum, winter, and summer. The simulation results clearly indicate that the ionospheric background conductivity can dynamically affect the mesoscale features of polar cap arcs through a nonlinear M‐I coupling process associated with the arcs.