bluuebunny/crossref_metadata_2025 huggingface dataset:
Eemian and Early Weichselian (140–60 ka) Paleoceanography and paleoclimate in the Nordic Seas with comparisons to Holocene conditions
Authors: Fronval Torben , Jansen Eystein
Published: February 2004

Paleoceanographic and paleoclimatic time series were extracted from Norwegian, Iceland, and Greenland Sea (the Nordic seas) sediment sequences covering the time intervals 60–140 ka and 0–15 ka on the basis of stable isotope stratigraphy, planktonic foraminiferal faunas, and sedimentological methods. The proxy temperature records from the Norwegian‐Iceland Sea display a number of high‐frequency and high‐amplitude variations within the last interglacial (the Eemian) indicating marked changes in heat flux at high northern latitudes. Eemian‐Holocene comparisons document that the climate of the Norwegian‐Iceland Sea in the last interglacial was less stable than that of the present interglacial and that the distribution of surface water masses in the Eemian differed from that of the Holocene. Within the Greenland Sea, uniform cold conditions prevailed throughout both interglacials. Cooling of the Nordic seas, as well as the initial growth of the surrounding ice sheets, began within the Eemian interval, before the main increase in global ice volume. Heat flux to the Nordic seas was significantly higher during the interglacials than at any time during the early Weichselian (60–113 ka), and no marked stadial‐interstadial changes in sea surface temperature and deep water ventilation are documented in the early Weichselian records, contrary to the pattern found in North Atlantic sediment records. However, fluctuations in planktonic foraminiferal content, planktonic δ13C, and ice rafting on suborbital timescales, mostly comparable in number and age to the stadial‐interstadial fluctuations in the Greenland ice cores, are demonstrated in our records. The ice‐rafted detritus records provide a more detailed account of ice sheet variability than previously known for the early Weichselian period and demonstrate that glaciers advanced to sea level somewhere around the Nordic seas several times during this interval.
Thermohaline instability in the North Atlantic during meltwater events: Stable isotope and ice‐rafted detritus records from Core SO75‐26KL, Portuguese Margin
Authors: Zahn Rainer , Schönfeld Joachim , Kudrass Hermann‐Rudolf , Park Myong‐Ho , Erlenkeuser Helmut , Grootes Pieter
Published: February 2004

A benthic isotope record has been measured for core SO75‐26KL from the upper Portuguese margin (1099 m water depth) to monitor the response of thermohaline overturn in the North Atlantic during Heinrich events. Evaluating benthic δ18O in TS diagrams in conjunction with equilibrium δc fractionation implies that advection of Mediterranean outflow water (MOW) to the upper Portuguese margin was significantly reduced during the last glacial (< 15% compared to 30% today). The benthic isotope record along core SO75‐26KL therefore primarily monitors variability of glacial North Atlantic conveyor circulation. The 14C‐accelerator mass spectrometry ages of 13.54±.07 and 20.46±.12 ka for two ice‐rafted detritus (IRD) layers in the upper core section and an interpolated age of 36.1 ka for a third IRD layer deeper in the core are in the range of published 14C ages for Heinrich events H1, H2, and H4. Marked depletion of benthic δ13C by 0.7–1.1‰ during the Heinrich events suggests reduced thermohaline overturn in the North Atlantic during these events. Close similarity between meltwater patterns (inferred from planktonic δ18O) at Site 609 and ventilation patterns (inferred from benthic δ13C) in core SO75‐26KL implies coupling between thermohaline overturn and surface forcing, as is also suggested by ocean circulation models. Benthic δ13C starts to decrease 1.5–2.5 kyr before Heinrich events Hl and H4, fully increased values are reached 1.5–3 kyr after the events, indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor's full strength well after the events. Benthic δ13C changes in the course of the Heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation, a distinct feature of thermohaline instability in numerical models. Inferrred gradual spin‐up of thermohaline circulation after Hl and H4 is in contrast to abrupt wanning in the North Atlantic region that is indicated by sudden increases in Greenland ice core δ18O and in marine faunal records from the northern North Atlantic. From this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the Greenland ice core record.
Spatial and temporal oceanographic variability of the eastern equatorial Pacific during the Late Pleistocene: Evidence from radiolaria microfossils
Authors: Pisias Nicklas G. , Mix Alan C.
Published: February 2004

Eight 150,000 year long records of sea surface temperatures combined with two additional records spanning 400,000 years constrain the spatial and temporal patterns of oceanographic change in the eastern equatorial Pacific and possible mechanisms of variability in the region. Empirical orthogonal function analysis shows two important modes of variability, one associated with the eastern boundary current and another associated with the North Equatorial Countercurrent. The two long time series located in the equatorial divergence and within the Peru Current have very different patterns of change. The spectrum for the time series from the Peru Current is dominated by orbital periods of 100, 41, and 23 kyr and is similar in variance distribution and phase to records from the Southern Ocean. In contrast, the equatorial divergence site has spectral concentrations at the orbital frequencies and also concentration of variance at the nonorbital 31,000 year period. The phase and amplitude spectra of these two sites support the importance of changes in eastern boundary advection and also document a nonlinear response of the equatorial Pacific to orbital changes. Finally, these data provide a new evaluation of the temperature change in the eastern equatorial Pacific during the last glacial maximum. Cooling in the Peru Current region is predicted to be about 4°C, and cooling in the equatorial divergence is estimated to be 3° to 5°C. The estimated cooling of the region is of the order of 2°C greater than the cooling predicted by Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP).
Reconstruction of past nutrient utilization in the Eastern Angola Basin based on sedimentary 15N/14N ratios
Authors: Holmes M. Elizabeth , Schneider Ralph R. , Müller Peter J. , Segl Monika , Wefer Gerold
Published: February 2004

The δ15N records of two sediment cores from the Angola Basin have been used to investigate past changes in nutrient utilization in the surface waters of this highly productive region. Low 15N/14N ratios in the cores (from >3000 m water depth) generally correspond to low sea surface temperatures and high sedimentary organic content, demonstrating a link between upwelling, productivity, and the degree of nitrate depletion. Glacial δ15N was lower by 0.5–0.7‰, and paleoproductivity was elevated by around 30% relative to interglacial periods, indicating that although productivity was higher, nitrate was less depleted during glacial than interglacial periods. More pronounced than the glacial/interglacial changes in δ15N values of both cores are precession‐related (23 kyr) fluctuations. These cyclic δ15N variations, which are in phase with those of paleoproductivity and sea surface temperature, indicate that changes in trade wind driven upwelling intensity and the advection of cold, nutrient‐rich water from the south drive nutrient availability and productivity off the southwest coast of Africa. No strong evidence was found from sedimentary δ15N values for denitrification in the water column in the Angola Basin during the past 180 kyr.
Biomarker temperature estimates for modern and last glacial surface waters of the California Current System between 33° and 42°N
Authors: Doose Heidi , Prahl Fredrick G. , Lyle Mitch W.
Published: February 2004

Sea surface temperature (SST) estimates were made using data for C37‐39alkenones analyzed in modern and glacial‐age intervals of sediment cores collected along an east‐west (∼42°N from 125° to 132°W) and a north‐south (41°–33°N) transect of the California Current system (CCS). The prymnesiophyte biomarker results suggest that surface waters warmed significantly throughout this region since the last glacial maximum (LGM) but the magnitude of warming varied spatially. Stratigraphic profiles from four sediment cores along the north‐south transect indicate the warming period was confined to the glacial/interglacial transition (15‐10 ka), with SST reaching a maximum value at ∼10 ka and maintaining a uniformly high value throughout the Holocene. Comparison of estimates derived from analysis of modern and LGM sediment intervals indicates the magnitude of the SST change was minimal for locations south of about 36°N (≤1°–2°C) and increased significantly (3°–5°C) north of this latitude. Using a simple heat balance model, we calculate from the latitudinal gradient in SST that southward flow in the California Current system during the LGM was about 60% of that measured today at 36°N. Our findings support the conclusion of others based on pollen data that coastal upwelling in the region of the northeast Pacific off northern California was significantly reduced or even completely shutdown during the LGM.
The timing of major climate terminations
Authors: Raymo M. E.
Published: February 2004

A simple, untuned “constant sedimentation rate” timescale developed using three radiometric age constraints and eleven δ18O records longer than 0.8 Myr provides strong support for the validity of the SPECMAP timescale of the late Quaternary [Imbrie et al., 1984]. In particular, the present study independently confirms the link between major deglaciations (terminations) and increases in northern hemisphere summer radiation at high latitudes and shows that this correlation is not an artifact of orbital tuning. In addition, the excess ice characteristic of late Quaternary “100‐kyr” climate cycles typically accumulates when July insolation at 65°N has been unusually low for more than a full precessional cycle, or >21 kyr, and once established does not last beyond the next increase in summer insolation. Thus, the timing of the growth and decay of large 100‐kyr ice sheets, as depicted in the deep sea δ18O record, is strongly (and semipredictably) influenced by eccentricity through its modulation of the orbital precession component of northern hemisphere summer insolation.
Geochemical and paleomagnetic evidence for the occurrence of “missing” sapropels in eastern Mediterranean sediments
Authors: van Santvoort P. J. M. , de Lange G. J. , Langereis C. G. , Dekkers M. J. , Paterne M.
Published: February 2004

The cyclic occurrence of sapropels appears to be associated with maxima in the 65°N summer insolation target curve. Studies of the most recent sapropel have revealed extensive oxidation of organic matter or even the complete removal of this unit. Geochemical and magnetic signals allow for the detection of such a “missing” sapropel. In older sediments, these signals are altered by ongoing diagenetic processes. An important parameter to reveal missing sapropels is Ba. Elevated concentrations of Ba are always found in visible sapropels, and even after oxidation of part of the sapropel, the initial Ba profile remains visible. We use this tool to trace down missing sapropels in KC01B, a 37‐m Kullenberg core from the Calabrian Ridge, eastern Mediterranean. At least 8 of 11 sampled intervals around insolation maxima contain evidence of a completely oxidized sapropel: Ba, trace metal distributions, and magnetic parameters are similar to those observed around visible sapropels.
Tropical Pacific sea surface temperatures and upper water column thermal structure during the Last Glacial Maximum
Authors: Patrick Andrew , Thunell Robert C.
Published: February 2004

Using two cores from the eastern and western Pacific, we have attempted to better quantify tropical ocean temperatures during the last glacial in order to determine how this climatically‐important region responds to large scale changes in climate forcing. By analyzing the oxygen isotopes of surface dwelling (G. sacculifer, G. ruber), thermocline dwelling (N. dutertrei, G. menardii, P. obliquiloculata) and sub‐thermocline dwelling (G. inflata) planktonic foraminifera, both relative and absolute estimates of the changes in the temperature gradient over this depth interval have been made. Owing to poor carbonate preservation in the Holocene section of both cores, relative temperature estimates suggest only a slight glacial cooling (∼2°C) at these locations, similar to that reported by CLIMAP [1976, 1981]. However, absolute temperature estimates determined from calcite‐seawater paleothermometry indicate the eastern equatorial Pacific (EEP) was ∼3°C cooler during the last glacial maximum (LGM), while the western equatorial Pacific (WEP) was ∼4°C cooler. The upper water column appears to have been less stratified in the EEP, with a steeper thermocline, interpreted as indicating an increase in upwelling during the LGM. The WEP maintained a well developed mixed layer and deep thermocline, similar to today. These results are consistent with a variety of recent tropical temperature estimates for the LGM.
An ice shelf mechanism for Heinrich layer production
Authors: Hulbe Christina L.
Published: February 2004

The effect of an ice shelf in the Labrador Sea on ice‐rafted sediment delivery to the glacial North Atlantic is investigated using a finite element numerical model of ice shelf flow. Discharge into the shelf from Hudson Strait creates a thick central core, extending downstream into the shelf, that is flanked by relatively thin ice. Melting at the base of the deep keel would produce cool, fresh water which would rise to refreeze along the keel's flanks. Debris deposited by melting deep central ice could create the Heinrich layers observed in the Labrador Sea while the debris‐rich ice protected by basal freezing on the flanks of the thick ice plume could transport sediment over large distances. Heinrich layers would be produced solely by external climate forcing, without changes in ice sheet flow. The mechanism is plausible if the lifetime of the ice shelf is about 1000 years.
Origin of the Middle Pleistocene Transition by ice sheet erosion of regolith
Authors: Clark Peter U. , Pollard David
Published: February 2004

The transition in the middle Pleistocene (∼0.9 Ma) seen in δ18O deep‐sea‐core records from relatively low‐amplitude, high‐frequency (41 kyr) to high‐amplitude, low‐frequency (100 kyr) ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft‐bedded to a mixed hard‐soft bedded Laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock. A one‐dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant (23 and 41 kyr) orbital forcing. Progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change (100 kyr) reflects nonlinear deglaciation processes. In model simulations over the last 3 Ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records.
Central Arctic surface ocean environment during the past 80,000 years
Authors: Nørgaard‐Pedersen Niels , Spielhagen Robert F. , Thiede Jörn , Kassens Heidemarie
Published: February 2004

Stable oxygen and carbon isotope and sedimentological‐paleontological investigations supported by accelerator mass spectrometry 14C datings were carried out on cores from north of 85°N in the eastern central Arctic Ocean. Significant changes in accumulation rates, provenance of ice‐rafted debris (IRD), and planktic productivity over the past 80,000 years are documented. During peak glacials, i.e., oxygen isotope stages 4 and 2, the Arctic Ocean was covered by sea ice with decreased seasonal variation, limiting planktic productivity and bulk sedimentation rates. In early stage 3 and during Termination I, major deglaciations of the circum‐Arctic regions caused lowered salinities and poor oxygenation of central Arctic surface waters. A meltwater spike and an associated IRD peak dated to ∼14–12 14C ka can be traced over the southern Eurasian Basin of the Arctic Ocean. This event was associated with the early and rapid deglaciation of the marine‐based Barents Sea Ice Sheet. A separate Termination Ib meltwater event is most conspicuous in the central Arctic and is associated with characteristic dolomitic carbonate IRD. This lithology suggests an origin of glacial ice from northern Canada and northern Greenland where lower Paleozoic platform carbonates crop extensively out.
Changing depth distribution of hiatuses during the Cenozoic
Authors: Spencer‐Cervato Cinzia
Published: February 2004

The differential effects of climate change, sea level, and water mass circulation on deposition/erosion of marine sediments can be constrained from the distribution of unconformities in the world's oceans. I identified temporal and depth patterns of hiatuses (“hiatus events”) from a large and chronologically well constrained stratigraphic database of deep‐sea sediments. The Paleogene is characterized by few, several million year long hiatuses. The most significant Cenozoic hiatus event spans most of the Paleocene. The Neogene is characterized by short, frequent hiatus events nearly synchronous in shallow and deep water sediments. Epoch boundaries are characterized by peaks in deep water hiatuses possibly caused by an increased circulation of corrosive bottom water and sediment dissolution. The Plio‐Pleistocene is characterized by a gradual decrease in the frequency of hiatuses. Future studies will focus on the regional significance of the hiatus events and their possible causes.
Modeling the composition of liquid stratospheric aerosols
Authors: Carslaw Kenneth S. , Peter Thomas , Clegg Simon L.
Published: February 2004

There is extensive evidence to suggest that stratospheric aerosols can remain liquid to very low stratospheric temperatures, despite being highly supercooled. Even polar stratospheric clouds, which are a key factor in the depletion of ozone in polar regions, can often consist of liquid rather than solid particles. It has been known since the 1960s that stratospheric aerosols are mostly concentrated sulfuric acid‐water droplets, but the combination of recent laboratory measurements, field observations, and thermodynamic model calculations has led to a recognition that many species other than water vapor can partition into the aerosols, particularly at low temperatures. This has been shown to increase the aerosol size, to control their freezing properties, and to affect the rates of important liquid phase reactions. This in turn influences the formation of polar stratospheric clouds and the subsequent extent and duration of seasonal ozone depletion in the polar regions. We review thermodynamic models of the liquid phase that enable the partitioning of gases such as HCl, HBr, HOCl, and HNO3 into sulfuric acid aerosols to be calculated over the full range of stratospheric conditions. Such models have been used to show that the uptake of nitric acid vapor can lead to a rapid transition from mainly sulfuric‐acid‐ to mainly nitric‐acid‐based liquid aerosols at low temperatures, a process that has changed our view of how polar stratospheric clouds form. Liquid aerosol composition at these low temperatures is still known largely from predictions made by thermodynamic models, rather than from observations, and even laboratory data under these conditions are limited. This and other uncertainties in calculated aerosol composition are estimated, and their effect on the interpretation of particle observations and predictions made by chemical stratospheric models is described.
Representation of heterogeneity effects in Earth system modeling: Experience from land surface modeling
Authors: Giorgi Filippo , Avissar Roni
Published: February 2004

The land surface is characterized by pronounced spatial heterogeneity that spans a wide range of scales. This heterogeneity affects the surface energy and water budgets, as well as the land‐atmosphere exchanges of momentum, heat, water and other constituents, through a number of highly nonlinear processes. The resolution of present‐day Earth (or climate) system models is still too coarse to explicitly capture the effects of surface heterogeneity, which therefore needs to be parameterized within the framework of complex and nonlinear land surface process schemes. The effects of surface heterogeneity are here grouped in two categories, which we define as “aggregation” and “dynamical” effects. Models of aggregation effects attempt to calculate the contribution of different subgrid scale surface types to the grid box average energy and water budgets and surface‐atmosphere exchanges. Such models have been based on discrete approaches, whereby heterogeneity is described in terms of a finite number of subgrid “tiles” or “patches,” and on continuous approaches, in which heterogeneity is described in terms of probability density functions. Subgrid scale aggregation has been shown to especially affect the surface latent and sensible heat fluxes, the simulation of snow, and the dynamics of soil moisture and runoff. Dynamical heterogeneity effects are associated with microscale and mesoscale circulations induced by heterogeneous surfaces. These circulations can influence boundary layer structure, cloud formation, precipitation, and vertical transfer of momentum, energy, and water up to the midtroposphere. In the last decade or so, the importance of land surface heterogeneity representation has been increasingly recognized in a large number of new studies. This paper reviews and critically discusses different approaches that have been proposed to represent aggregation and dynamical effects of surface heterogeneity and their incorporation in land surface process schemes. Some of the methodologies discussed in this paper are of general nature and therefore can be of interest for problems of subgrid scale process description in other geophysical disciplines.
Permeability within basaltic oceanic crust
Authors: Fisher Andrew T.
Published: February 2004

Water‐rock interactions within the seafloor are responsible for significant energy and solute fluxes between basaltic oceanic crust and the overlying ocean. Permeability is the primary hydrologic property controlling the form, intensity, and duration of seafloor fluid circulation, but after several decades of characterizing shallow oceanic basement, we are still learning how permeability is created and distributed and how it changes as the crust ages. Core‐scale measurements of basaltic oceanic crust yield permeabilities that are quite low (generally 10−22 to 10−17 m²), while in situ measurements in boreholes suggest an overlapping range of values extending several orders of magnitude higher (10−18 to 10−13 m²). Additional indirect estimates include calculations made from borehole temperature and flow meter logs (10−16 to 10−11 m²), numerical models of coupled heat and fluid flow at the ridge crest and within ridge flanks (10−16 to 10−9 m²), and several other methods. Qualitative indications of permeability within the basaltic oceanic crust come from an improved understanding of crustal stratigraphy and patterns of alteration and tectonic modification seen in ophiolites, seafloor samples and boreholes. Difficulties in reconciling the wide range of estimated permeabilities arise from differences in experimental scale and critical assumptions regarding the nature and distribution of fluid flow. Many observations and experimental and modeling results are consistent with permeability varying with depth into basement and with primary basement lithology. Permeability also seems to be highly heterogeneous and anisotropic throughout much of the basaltic crust, as within crystalline rocks in general. A series of focused experiments is required to resolve permeability in shallow oceanic basement and to directly couple upper crustal hydrogeology to magmatic, tectonic, and geochemical crustal evolution.
Radar interferometry and its application to changes in the Earth's surface
Authors: Massonnet Didier , Feigl Kurt L.
Published: February 2004

Geophysical applications of radar interferometry to measure changes in the Earth's surface have exploded in the early 1990s. This new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times. The resulting interferogram is a contour map of the change in distance between the ground and the radar instrument. These maps provide an unsurpassed spatial sampling density (∼100 pixels km−2), a competitive precision (∼1 cm), and a useful observation cadence (1 pass month−1). They record movements in the crust, perturbations in the atmosphere, dielectric modifications in the soil, and relief in the topography. They are also sensitive to technical effects, such as relative variations in the radar's trajectory or variations in its frequency standard. We describe how all these phenomena contribute to an interferogram. Then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments, including the four satellites currently in orbit: ERS‐1, ERS‐2, JERS‐1, and RADARSAT. The next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement: respecting the limits of the technique, assessing its uncertainty, recognizing artifacts, and discriminating different types of signal. We then review the geophysical applications published to date, most of which study deformation related to earthquakes, volcanoes, and glaciers using ERS‐1 data. We also show examples of monitoring natural hazards and environmental alterations related to landslides, subsidence, and agriculture. In addition, we consider subtler geophysical signals such as postseismic relaxation, tidal loading of coastal areas, and interseismic strain accumulation. We conclude with our perspectives on the future of radar interferometry. The objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it.
Midlatitude cyclonic cloud systems and their features affecting large scales and climate
Authors: Stewart Ronald E. , Szeto Kit K. , Reinking Roger F. , Clough Sid A. , Ballard Susan P.
Published: February 2004

Midlatitude cyclonic cloud systems are common occurrences that significantly impact our climate. In this review, attention is paid to those physical characteristics of these cloud systems with large‐scale impacts that must be accounted for in climate simulations. Such attributes include atmospheric forcing, internal structure, surface influences, cloud layering, microphysics, precipitation, water cycling, and radiation. Because of their present limitations associated with, for example, grid sizes and simplified parameterizations, climate models cannot account for all the crucial impacts of these cloud systems. Future advances in the representation of these systems within climate models will need to rely in part on rigorous assessments of model capabilities in a variety of conditions.
HF propagation and scattering in a turbulent ionosphere with an anisotropic and inhomogeneous background
Authors: Liang Jun , Wan Weixing
Published: February 2004

The problem of HF propagation and scattering in a turbulent ionosphere With an anisotropic and inhomogeneous background is studied in this paper. By constructing the asymptotic form of the dyadic Green's function of the wave equation in an anisotropic and stratified ionosphere, the integral representation expressing the fluctuation of the HF field caused by ionospheric irregularities is derived. In the case of vertical sounding, the spectra of phase and amplitude fluctuation of reflected waves are obtained. Numerical calculations are also performed to reveal some effects of background anisotropy on HF scattering.
Electromagnetic scattering from a group of thin conducting cylinders
Authors: Rodger Craig J. , Wait James R. , Dowden Richard L.
Published: February 2004

Solutions are given for the problem of scattering of plane waves from an arbitrary number of vertical columns that are thin in terms of the incident wavelength. The electric properties of the cylinders are taken as homogenous and isotropic but otherwise arbitrary, and the incident electric field vector is taken to be parallel to the axis of the cylinders. The scattering solution for individual columns is described, which is used to introduce that for multiple columns under the Born approximation. A more general approach is presented that leads to a solution which includes multiple scattering (coupling) between the cylinders. Comparison is made between this solution for a ring of cylindrical elements and one that treats the scattering from the ring as a whole. The theory described here is now being extended and applied to the scattering from vertical ionized columns known as “sprites”. These are associated with lightning discharges between the ionosphere and cloud tops.
First observations of Kelvin‐Helmholtz billows in an upper level jet stream using VHF frequency domain interferometry
Authors: Chilson Phillip B. , Muschinski Andreas , Schmidt Gerhard
Published: February 2004

In this paper we report the first high‐resolution observations of upper tropospheric Kelvin‐Helmholtz billows using VHF frequency domain interferometry (FDI). The measurements were made using the sounding system VHF radar located in the German Harz Mountains operating at the frequencies of 53.25 and 53.75 MHz. Through an application of the FDI technique it has been possible to track the altitude of thin scattering layers with an accuracy of a few tens of meters and a temporal resolution of 13 s. Taking advantage of the FDI technique, we found several examples of Kelvin‐Helmholtz instability (KHI) in regions coinciding with large values of wind shear. One particularly good example is presented in detail. The KHI occurred just below the axis of the jet at an altitude of 9.1 km. It produced oscillations in the vertical velocity field in the altitude range of 8–10 km. The oscillations had a local period of 90 s and persisted over a time interval of 10 min. On the basis of the magnitude of the horizontal wind at the time and height of the KHI, the billow train had a horizontal extent of about 27 km, while the horizontal length of the individual billow amounted to about 4.0 km. The 90‐s oscillations were also observed in the radar signal power, the FDI‐derived layer height, and the coherence between the received radar signals for the two frequencies. Using the FDI data, we were able to identify the individual billows located within the train and observe their maximum crest‐to‐trough amplitudes to be between 220 and 230 m. This was less than the conventional radar resolution of 300 m used during the experiment.
Aspect sensitivity of the received radar backscatter at VHF: Preliminary observations using the Indian MST radar
Authors: Jain A. R. , Jaya Rao Y. , Rao P. B.
Published: February 2004

The main mechanisms that give rise to atmospheric radar echoes at VHF are isotropic, anisotropic backscattering and Fresnel reflection/scattering. Anisotropic turbulence and Fresnel reflection/scattering make the radar echo aspect sensitive, which in turn influences the beam pointing angle and beam width and thereby the determination of horizontal wind components and turbulence parameters. Therefore a series of experiments are conducted on 4 days, using the newly established Indian MST radar, located at Gadanki and operating at 53 MHz, to study the aspect sensitivity in the troposphere‐stratosphere region. Results of these experiments have shown that aspect sensitivity is more prominent between the altitude range of 15 and 21 km, and the same is noticed to be reduced above 21 km. Results also show the presence of scatterers with different horizontal correlation lengths. Aspect sensitivity measurements for different beam combinations show significant differences for two orthognal planes, i.e., E‐W and N‐S, indicating the azimuthal anisotropy of the scatterers. The effect of aspect sensitivity on beam pointing angle and wind measurements is estimated. For beams with small zenith angle (i.e., ∼4°), the underestimation of winds can be as much as 30%, whereas for larger zenith angles (≥8°), the same would be ≤5%. Results presented thus give some insight into the scattering processes and also yield valuable information on the structure of the scatterers.
Imaging of electron density troughs by tomographic techniques
Authors: Kersley L. , Pryse S. E. , Walker I. K. , Heaton J. A. T. , Mitchell C. N. , Williams M. J. , Willson C. A.
Published: February 2004

Troughs in the latitudinal distribution of electron density are a well‐known feature of the ionosphere from subauroral to polar latitudes. The location and depth of the trough minimum, the width of the feature, and the horizontal gradients in electron density associated with the trough walls are all quantities of interest or concern to practical applications of radio systems involving the ionosphere. In practice, the precise characteristics of trough‐like structures have been difficult to monitor using ground‐based methods. Ionospheric tomography represents a new development that is maturing into a technique ideally suited to the study of electron density troughs. Results are presented from a variety of observations made during tomographic campaigns in northern Europe. A long‐term investigation has been made of the main trough from a network of stations in the United Kingdom. The position of the trough minimum and the wall gradients have been studied on a diurnal basis using tomographic images reconstructed from measurements for a succession of passes of Navy Navigation Satellite System satellites. With stations deployed for more than 6 months, the average behavior has also been studied. Examples are shown of extreme behavior of the trough under very disturbed geomagnetic conditions, during which tomography continues to yield images while the limitations of ionosondes are exposed. Studies of narrow troughs with very steep gradients seen at auroral latitudes have been used to investigate some of the successes and limitations of the tomographic method. Measurements made in the polar cap show the depleted densities of the polar hole in the center of the dawn convection cell and illustrate the power of the tomographic method at high latitudes. Finally, the dayside trough at the high‐latitude boundary between corotating and counterstreaming flux tubes in the afternoon sector has been revealed in a tomographic image extending over some 30° latitude, made using a chain of six stations in Scandinavia.
Neural network modeling of the ionospheric electron content at global scale using GPS data
Authors: Hernández‐Pajares M. , Juan J. M. , Sanz J.
Published: February 2004

The adaptative classification of the rays received from a constellation of geodetic satellites (the Global Positioning System (GPS)) by a set of ground receivers is performed using neural networks. This strategy allows us to improve the reliability of reconstructing the ionospheric electron distribution from GPS data. As an example, we present the evolution at global scale of the radially integrated electron density (total electron content (TEC)) for October 18, 1995, coinciding with an important geomagnetic storm.
Prediction of fade durations due to rain in satellite communication systems
Authors: Matricciani Emilio
Published: February 2004

The paper discusses the prediction of long‐term fade duration statistics in slant paths, obtained from rain attenuation time series simulated with the synthetic storm technique (with input from 1‐min rain rate time series). The results of the prediction were tested against the experimental fade duration statistics collected at three sites of the Sirio experiment in Italy at 11.6 GHz. Predicted and measured statistics are very similar for fade durations longer than about 64–128 s, which represent a fundamental lower limit to fade duration prediction using 1‐min rain rate time series. The short durations have not been reproduced because they are mainly caused by tropospheric turbulence, not by the space‐time structure of rain, well described statistically by the synthetic storm technique. The prediction is not sensitive to rainstorm speed.
Validation of a UHF spaced antenna wind profiler for high‐resolution boundary layer observations
Authors: Cohn Stephen A. , Holloway Christopher L. , Oncley Steven P. , Doviak Richard J. , Lataitis Richard J.
Published: February 2004

In this paper we apply a spaced antenna technique derived from the recent work of Doviak et al. [1996a] and Holloway et al. [this issue] to wind measurement with a small UHF boundary layer profiler. We discuss the implementation of the technique, averaging and quality control strategies, and some advantages and limitations of spaced antenna methods over conventional Doppler beam swinging wind profilers in the boundary layer. Such advantages include a relaxation of the assumption of a horizontally uniform wind field and the possibility of high temporal resolution wind profiles. In this regard we present velocity measurements derived from this UHF system with time resolution of about 30 s and compare these measurements with in situ sonic anemometer data taken on a 300‐m tower. Finally, we present an example of a high‐resolution time‐height cross section of atmospheric winds. This example, collected in stratiform precipitation, shows the intriguing situation of a wind speed maximum (jet) which closely follows the height of the melting layer over several hours even as this height changes by several hundred meters.
A comparative study of ionospheric total electron content measurements using global ionospheric maps of GPS, TOPEX radar, and the Bent model
Authors: Ho C. M. , Wilson B. D. , Mannucci A. J. , Lindqwister U. J. , Yuan D. N.
Published: February 2004

Global ionospheric mapping (GIM) is a new and emerging technique for determining global ionospheric TEC (total electron content) based on measurements from a worldwide network of Global Positioning System (GPS) receivers. In this study, GIM accuracy in specifying TEC is investigated by comparison with direct ionospheric measurements from the TOPEX altimeter. A climatological model (Bent model) is also used to compare with the TOPEX altimeter data. We find that the GIM technique has much better agreement with TOPEX in TEC measurements, compared with the predictions of the climatological model. The difference between GIM and TOPEX in TEC measurements is very small (less than 1.5 TEC units (TECU)) within a 1500‐km range from a reference GPS station. The RMS gradually increases with increasing distance from the station, while the Bent model shows a constant large RMS, unrelated to any station location. Within a 1000‐km distance of a GPS site (elevation angle > 25°), GIM has a good correlation (R > 0.93) to TOPEX with respect to TEC measurements. The slope of the linear fitting line to the data set from two TOPEX cycles is 44.5° (near the ideal 45°). In the northern hemispheric regions, ionospheric specification by GIM appears to be accurate to within 3‐10 TECU up to 2000+ km away from nearest GPS station (corresponding to ∼1° elevation angle cutoff). Beyond 2000 km, GIM accuracy, on average, is reduced to the Bent model levels. In the equatorial region, the Bent model predictions are systematically lower (∼5.0 TECU) than TOPEX values and often show a saturation at large TEC values. During ionospheric disturbed periods, GIM sometimes shows differences from TOPEX values due to transient variations of the ionosphere. Such problems may be improved by the continuous addition of new GPS stations in data‐sparse regions. Thus, over a GPS station's measurement realm (up to 2000 km in radius), GIM can produce generally accurate TEC values. Through a spatial and temporal extrapolation of GPS‐derived TEC measurements, the GIM technique provides a powerful tool for monitoring global ionospheric features in near real time.
Studies of polar mesosphere summer echoes over EISCAT using calibrated signal strengths and statistical parameters
Authors: Hocking W. K. , Röttger J.
Published: February 2004

The EISCAT (European incoherent scatter) monostatic VHF radar operating on 224 MHz has been used to investigate characteristics of polar mesosphere summer echoes (PMSE) at Bragg scales of around 0.6 m. In particular, we look at four main parameters, two of which have not been studied in any real detail for these echoes. These parameters are absolute backscatter strengths, fading times, amplitude distributions, and turbulent energy dissipation rates. The calibrated signal strengths, combined with theoretical modeling, are used to demonstrate that specular reflection from even the sharpest background density gradients which have been measured thus far by rocket probes cannot be the cause of these echoes at VHF frequencies. We also demonstrate that specular reflections from the edges of the large electron density “bite‐outs” (which are often observed in association with these echoes) cannot be responsible for the measured backscatter strengths. However, these same gradients can, in association with turbulence, cause measurable backscatter, but only provided that scatter is from within the inertial subrange of turbulent electron density fluctuation spectra. This requires large Schmidt numbers. We also use statistical studies of the amplitude distributions to show that our echoes were due to an ensemble of scatterers in the radar volume, rather than single entities. A new calculation concerning the expected lifetimes of the scattering entities in the presence of turbulence is also demonstrated, and then our measurements are used to show that the scatterers often have lifetimes in excess of those which are predicted for classical neutral turbulence. We use this latter point to supply additional evidence that the Schmidt number substantially exceeds 1.0.
Ionospheric radio tomography using maximum entropy: 2. Results of the Russian‐American Tomography Experiment, including the large magnetic storm of November 3–4, 1993
Authors: Fougere Paul F.
Published: February 2004

Near the end of the Russian‐American Tomography Experiment, November 3 and 4, 1993, the index of magnetic activity (Kp) climbed rapidly from 0 to 7—in a little under 1 day. The quasi‐logarithmic Kp index ranges from a low of 0 for very calm conditions to 9 for the most intense magnetic storms. During the course of this magnetic storm the ionospheric response was studied using data collected and analyzed in 28 passes. The results of this intensive study, using contour charts of electron density as well as the three important parameters of the average Chapman profile, the maximum density, the altitude of maximum, and the scale height, are presented. We had dual‐frequency receivers set up at four locations on the eastern edge of North America: Block Island, Rhode Island; Hanscom Air Force Base, Massachusetts; Jay, Vermont; and Roberval, Quebec, Canada. Using the dual‐frequency (150 and 400 MHz) beacon on the Navy Navigation Satellite System, with a nominal altitude of 1100 km, a tomography pass would typically last about 20 min from horizon to horizon. From a total of 88 passes, 86 passes possessed sufficiently accurate data and have been analyzed using the maximum entropy method, which determines the average vertical profile in the form of an analytical Chapman profile, as well as a set of electron density contours. The Russian tomography experiment, which utilized the same four locations, as well as the incoherent scatter radar results obtained by the Haystack Observatory group, will be discussed at a later date.
Required signal‐to‐interference ratios for shortwave broadcasting
Authors: Lane George
Published: February 2004

The required signal‐to‐Interference (RSI) ratio for a specified grade of HF radio service is the hourly median wanted signal power at the input of the receiver needed relative to the sum of the hourly median unwanted signal power and the hourly median radio noise power in the RF bandwidth of the receiver, adjusted so that the hourly median ratio will not fall below the RSI ratio more than a certain percentage of the time due to minute‐to‐minute fading within the hour. Shortwave listeners are well aware of the deleterious effects of cochannel and adjacent channel interference. This type of interference is especially prevalent in the overcrowded international broadcast bands where it is manifested by cross talk and a beat note produced in the receiver by the carrier of the unwanted signal. Yet little agreement exists as to the magnitude of the amplitude‐modulated, double sideband (AM‐DSB) interfering signal that can be tolerated by the listener. Numerous protection ratios have been proposed in the literature, as well as by elements of the International Telecommunication Union. These values tend to range from 17 dB [International Frequency Registration Board, 1989] to as high as 50 dB for “good commercial quality,” offset in carrier frequency of 500 Hz and 10 dB short‐term fade protection [CCIR, 1970]. In this paper, several significant experiments are reviewed for the purpose of normalizing their findings to a common set of parameters. The parameters relate to articulation scoring, type of noise (if used), fading of wanted and unwanted signals, type of interference, listener skill, bandwidth of the receiver, carrier frequency offset, etc. From this compilation of normalized data, RSI values are recommended as they relate to the desired broadcast quality and the signal‐to‐noise ratio of the wanted signal. The RSI ratios are compatible for use in HF sky wave prediction programs that contain appropriate RF noise and interference combining subroutines. The recommended minimum acceptable for cochannel interference is 36 dB, in order to maintain a fair grade of service where voice reception is understandable but with difficulty for at least 90% of the hour. In order to provide good service ( i.e., the interference is perceived not to be annoying) to 90% of the listeners, the RSI should be 48 dB. The RSI needed for “no perceptible” interference is 53 dB.
Improving IRI‐90 low‐latitude electron density specification
Authors: Decker Dwight T. , Anderson David N. , Preble Amanda J.
Published: February 2004

At low latitudes under moderate to high solar conditions, a number of comparisons between the international reference ionosphere (IRI‐90) model of F region electron density profiles and observed profiles measured by the Jicamarca incoherent scatter radar indicate that during the daytime the observed profile shape can be much broader in altitude than that specified by IRI‐90, while at night, just after sunset, observed F2 peak altitudes are significantly higher than what is specified by IRI‐90. Theoretically derived ionospheric parameters such as F2 peak density (NmF2), F2 peak altitude (hmF2), and profile shape, which are contained in the parameterized ionospheric model (PIM), have been shown in some cases to be in better agreement with Jicamarca observations. This paper describes a new low‐latitude option for IRI‐90 that uses five ionospheric parameters derived from PIM: the bottomside profile half thickness, NmF2 hmF2, and two parameters of a topside Chapman profile. The generation of electron density profiles using these five parameters is presented, as well as a description of how these parameters can be implemented into the IRI‐90 model.