bluuebunny/crossref_metadata_2025 huggingface dataset:
Martian dust devils: Laboratory simulations of particle threshold
Authors: Greeley Ronald , Balme Matthew R. , Iversen James D. , Metzger Stephen , Mickelson Robert , Phoreman Jim , White Bruce
Published: May 2003

An apparatus has been fabricated to simulate terrestrial and Martian dust devils. Comparisons of surface pressure profiles through the vortex core generated in the apparatus with both those in natural dust devils on Earth and those inferred for Mars are similar and are consistent with theoretical Rankine vortex models. Experiments to determine particle threshold under Earth ambient atmospheric pressures show that sand (particles > 60 μm in diameter) threshold is analogous to normal boundary‐layer shear, in which the rotating winds of the vortex generate surface shear and hence lift. Lower‐pressure experiments down to ∼65 mbar follow this trend for sand‐sized particles. However, smaller particles (i.e., dust) and all particles at very low pressures (∼10–60 mbar) appear to be subjected to an additional lift function interpreted to result from the strong decrease in atmospheric pressure centered beneath the vortex core. Initial results suggest that the wind speeds required for the entrainment of grains ∼2 μm in diameter (i.e., Martian dust sizes) are about half those required for entrainment by boundary layer winds on both Earth and Mars.
Model sensitivity in the effect of Antarctic sea ice and stratification on atmospheric pCO2
Authors: Archer David E. , Martin Pamela A. , Milovich Jose , Brovkin Victor , Plattner Gian‐Kasper , Ashendel Carrie
Published: May 2003

Several recent papers have demonstrated a decrease in atmospheric pCO2 resulting from barriers to communication between the deep sea and the atmosphere in the Southern Ocean. Stephens and Keeling [2000] decreased pCO2 by increasing Antarctic sea ice in a seven‐box model of the world ocean, and Toggweiler [1999] showed a similar response to Southern Ocean stratification. In box models the pCO2 of the atmosphere is controlled by the region of the surface ocean that fills the deep sea [Archer et al., 2000a]. By severing the Southern Ocean link between the deep sea and the atmosphere, atmospheric pCO2 in these models is controlled elsewhere and typically declines, although the models range widely in their responses. “Continuum models,” such as three‐dimensional (3‐D) and 2‐D general circulation models, control pCO2 in a more distributed way and do not exhibit box model sensitivity to high‐latitude sea ice or presumably stratification. There is still uncertainty about the high‐latitude sensitivity of the real ocean. Until these model sensitivities can be resolved, glacial pCO2 hypotheses and interpretations based on Southern Ocean barrier mechanisms (see above mentioned references plus Elderfield and Rickaby [2000], Francois et al. [1998], Gildor and Tziperman [2001], Sigman and Boyle [2000], and Watson et al. [2000]) are walking on thin ice.
Ventilation of the North Atlantic Ocean during the Last Glacial Maximum: A comparison between simulated and observed radiocarbon ages
Authors: Meissner K. J. , Schmittner A. , Weaver A. J. , Adkins J. F.
Published: May 2003

The distribution of radiocarbon during simulations of the Last Glacial Maximum with a coupled ocean‐atmosphere‐sea ice model is compared with sediment core measurements from the equatorial Atlantic Ceara Rise, Blake Ridge, Caribbean Sea, and South China Sea. During these simulations we introduce a perturbation of North Atlantic freshwater fluxes leading to varying strengths of the Atlantic meridional overturning. The best fit with the observations is obtained for an overturning weakened by 40% compared with today. Further, we simulate the phenomenon of an “age reversal” found in deep sea corals, but we suggest that this indicates rather a sudden interruption of deep water formation instead of an increase in ventilation, which was suggested earlier.
A high‐resolution study of Holocene paleoclimatic and paleoceanographic changes in the Nordic Seas
Authors: Risebrobakken Bjørg , Jansen Eystein , Andersson Carin , Mjelde Eirik , Hevrøy Kjersti
Published: May 2003

High‐resolution records from IMAGES core MD95‐2011 in the eastern Norwegian Sea provide evidence for relatively large‐ and small‐scale high‐latitude climate variability throughout the Holocene. During the early and mid‐Holocene a situation possibly driven by consistent stronger westerlies increased the eastward influence of Arctic intermediate and near‐surface waters. For the late Holocene a relaxation of the atmospheric forcing resulted in increased influence of Atlantic water. The main changes in Holocene climate show no obvious connection to changing solar irradiance, and spectral analysis reveals no consistent signature for any periodic behavior of Holocene climate at millennial or centennial timescales. There are, however, indications of consistent multidecadal variability.
High‐resolution climatic evolution of coastal northern California during the past 16,000 years
Authors: Barron John A. , Heusser Linda , Herbert Timothy , Lyle Mitch
Published: May 2003

Holocene and latest Pleistocene oceanographic conditions and the coastal climate of northern California have varied greatly, based upon high‐resolution studies (ca. every 100 years) of diatoms, alkenones, pollen, CaCO3%, and total organic carbon at Ocean Drilling Program (ODP) Site 1019 (41.682°N, 124.930°W, 980 m water depth). Marine climate proxies (alkenone sea surface temperatures [SSTs] and CaCO3%) behaved remarkably like the Greenland Ice Sheet Project (GISP)‐2 oxygen isotope record during the Bølling‐Allerod, Younger Dryas (YD), and early part of the Holocene. During the YD, alkenone SSTs decreased by >3°C below mean Bølling‐Allerod and Holocene SSTs. The early Holocene (ca. 11.6 to 8.2 ka) was a time of generally warm conditions and moderate CaCO3 content (generally >4%). The middle part of the Holocene (ca. 8.2 to 3.2 ka) was marked by alkenone SSTs that were consistently 1–2°C cooler than either the earlier or later parts of the Holocene, by greatly reduced numbers of the gyre‐diatom Pseudoeunotia doliolus (<10%), and by a permanent drop in CaCO3% to <3%. Starting at ca. 5.2 ka, coastal redwood and alder began a steady rise, arguing for increasing effective moisture and the development of the north coast temperate rain forest. At ca. 3.2 ka, a permanent ca. 1°C increase in alkenone SST and a threefold increase in P. doliolus signaled a warming of fall and winter SSTs. Intensified (higher amplitude and more frequent) cycles of pine pollen alternating with increased alder and redwood pollen are evidence that rapid changes in effective moisture and seasonal temperature (enhanced El Niño–Southern Oscillation [ENSO] cycles) have characterized the Site 1019 record since about 3.5 ka.
Overview of Glacial Atlantic Ocean Mapping (GLAMAP 2000)
Authors: Sarnthein M. , Gersonde R. , Niebler S. , Pflaumann U. , Spielhagen R. , Thiede J. , Wefer G. , Weinelt M.
Published: May 2003

GLAMAP 2000 presents new reconstructions of the Atlantic's sea surface temperatures (SST) at the Last Glacial Maximum (LGM), defined at both 21,500–18,000 years B.P. (“Last Isotope Maximum”) and 23,000–19,000 years B.P. (maximum glacial sea level low stand and orbital minimum of solar insolation; EPILOG working group; see Mix et al. [2001]). These reconstructions use 275 sediment cores between the North Pole and 60°S with carefully defined chronostratigraphies. Four categories of core quality are distinguished. More than 100 core sections provide a glacial record with subcentennial‐ to multicentennial‐scale resolution. SST estimates are based on a new set of almost 1000 reference samples of modern planktic foraminifera and on improved transfer‐function techniques to deduce SST from census counts of microfossils, including radiolarians and diatoms. New proxies also serve to deduce sea ice boundaries. The GLAMAP 2000 SST patterns differ significantly in crucial regions from the CLIMAP [1981] reconstruction and thus are important in providing updated boundary conditions to initiate and validate computational models for climate prediction.
The 41 kyr world: Milankovitch's other unsolved mystery
Authors: Raymo Maureen E. , Nisancioglu Kerim H.
Published: May 2003

For most of the Northern Hemisphere Ice Ages, from ∼3.0 to 0.8 m.y., global ice volume varied predominantly at the 41,000 year period of Earth's orbital obliquity. However, summer (or summer caloric half year) insolation at high latitudes, which is widely believed to be the major influence on high‐latitude climate and ice volume, is dominated by the 23,000 year precessional period. Thus the geologic record poses a challenge to our understanding of climate dynamics. Here we propose that variations in the insolation gradient between high and low latitudes control high‐latitude climate and ice volume during the late Pliocene and early Pleistocene. The differential heating between high and low latitudes, driven by obliquity, controls the atmospheric meridional flux of heat, moisture, and latent energy, which may exert the dominant control on high‐latitude climate on Milankovitch timescales. In the two‐dimensional zonal energy balance models typically used to study the long‐term evolution of climate, the meridional atmospheric moisture flux is usually kept fixed. The hypothesis that insolation gradients control the poleward energy fluxes, precipitation, and ice volume at high latitudes has never been directly examined within the context of an ice sheet model. In light of what we know about modern energy fluxes and their relative influence on high‐latitude climate, this possibility should be examined.
A comparison between excess barium and barite as indicators of carbon export
Authors: Eagle Meagan , Paytan Adina , Arrigo Kevin R. , van Dijken Gert , Murray Richard W.
Published: May 2003

Since Dymond et al. [1992] proposed the paleoproductivity algorithm based on “Bio‐Ba,” which relies on a strong correlation between Ba and organic carbon fluxes in sediment traps, this proxy has been applied in many paleoproductivity studies. Barite, the main carrier of particulate barium in the water column and the phase associated with carbon export, has also been suggested as a reliable paleoproductivity proxy in some locations. We demonstrate that Baexcess (total barium minus the fraction associated with terrigenous material) frequently overestimates Babarite (barium associated with the mineral barite), most likely due to the inclusion of barium from phases other than barite and terrigenous silicates (e.g., carbonate, organic matter, opal, Fe‐Mn oxides, and hydroxides). A comparison between overlying oceanic carbon export and carbon export derived from Baexcess shows that the Dymond et al. [1992] algorithm frequently underestimates carbon export but is still a useful carbon export indicator if all caveats are considered before the algorithm is applied. Babarite accumulation rates from a wide range of core top sediments from different oceanic settings are highly correlated to surface ocean 14C and Chlorophyll a measurements of primary production. This relationship varies by ocean basin, but with the application of the appropriate f ratio to 14C and Chlorophyll a primary production estimates, the plot of Babarite accumulation and carbon export for the equatorial Pacific, Atlantic, and Southern Ocean converges to a global relationship that can be used to reconstruct paleo carbon export.
Multispecies approach to reconstructing eastern equatorial Pacific thermocline hydrography during the past 360 kyr
Authors: Spero Howard J. , Mielke Koreen M. , Kalve Erica M. , Lea David W. , Pak Dorothy K.
Published: May 2003

Stable isotope data from eastern equatorial Pacific (EEP) core TR163‐19 (2°15′N, 90°57′W, 2348 m) are presented for the surface‐dwelling foraminifers Globigerinoides ruber and G. sacculifer and thermocline‐dwelling Globorotalia menardii and Neogloboquadrina dutertrei. Using species‐specific normalization factors derived from experimental and plankton tow data, we reconstruct a 360 kyr record of water column hydrography across the past three glacial cycles. We demonstrate that G. ruber maintains a mixed layer habitat throughout the entire record, while G. sacculifer records a mixture of thermocline and mixed layer conditions and G. menardii and N. dutertrei record thermocline properties. We conclude that G. sacculifer is not appropriate for paleoceanographic applications in regions with steep vertical hydrographic gradients. Results suggest that this region of the EEP had a thicker mixed layer and deeper δ13CDIC boundary between the surface and equatorial undercurrent during the last two glacial periods. A shift in N. dutertrei and G. sacculifer geochemistry prior to ∼185 kyr suggests water column structure and chemocline gradients changed, possibly due to a shift in the position of the undercurrent relative to this site. The timing and magnitude of glacial‐interglacial δ13C variations between species indicates that near‐surface carbon chemistry is controlled by changes in productivity, atmospheric circulation, and advected intermediate water sources north of the Antarctic polar front. These results demonstrate that when properly calibrated for species differences, multispecies geochemical data sets can be invaluable for reconstructing water column structure and properties in the past.
Global ocean response to orbital forcing in the Holocene
Authors: Liu Z. , Brady E. , Lynch‐Stieglitz J.
Published: May 2003

Global upper ocean evolution in the Holocene is studied in two coupled ocean‐atmosphere models under orbital forcing conditions at 3, 6, 8, and 11 ka. The annual mean sea surface temperature (SST) changes in the early to mid‐Holocene are found to be forced mainly by the annual mean insolation forcing with an overall symmetric response of colder equator (<0.5°C)/warmer high latitudes (<0.4°C in the Southern Ocean and >1°C in the Arctic). This SST change is consistent with a synthesis of mid‐Holocene paleo‐SST records. In contrast, the temperature response in the thermocline is dominated by an antisymmetric pattern with a cooling (warming) in the Northern (Southern) Hemisphere midlatitudes. The thermocline response is determined predominantly by surface water subduction, and ultimately, the insolation forcing in local late winter.
Thermal evolution of Cretaceous Tethyan marine waters inferred from oxygen isotope composition of fish tooth enamels
Authors: Pucéat Emmanuelle , Lécuyer Christophe , Sheppard Simon M. F. , Dromart Gilles , Reboulet Stéphane , Grandjean Patricia
Published: May 2003

The evolution of subtropical (30–35°N) upper ocean temperatures through the Cretaceous is inferred from the oxygen isotope compositions of 64 fish teeth (enamel) coming from the western Tethyan platform. Mean δ18O values of 22‰ at the Berriasian‐Valanginian boundary decrease, with oscillations to 18.5‰ around the Cenomanian‐Turonian boundary, and progressively increase to 21.5‰ by the end of the Cretaceous. The similarity of this oxygen isotope curve for bioapatites from platform environments with those for foraminifera and bulk carbonates that were deposited in deeper waters and at other paleolatitudes indicates that they record global climatic signals. Major cooling events at the million‐year scale can be distinguished: (1) at the Berriasian‐Valanginian boundary and (2) during the earliest Late Valanginian. A third cooling event is detected during the earliest Aptian. These events, already proposed as icehouse interludes during the lower Cretaceous, are also recorded at subtropical latitudes. A progressive warming is identified from the Aptian to the Cenomanian‐Turonian interval that corresponds to a thermal optimum, and then upper ocean temperatures decreased to the Maastrichtian. Minimum isotopic temperatures range from 15°C to 28°C, assuming a δ18Oseawater of −1‰, for an ice‐free world. Taking more realistic δ18Oseawater values of ∼0‰ for tropical waters, during glacial periods (within the Berriasian‐Valanginian interval, and earliest Aptian) or with above average salinities (possibly the Maastrichtian), temperatures are increased by 4–5°C. Temperature differences between climatic extremes of the Valanginian and Cenomanian‐Turonian are estimated to have been 10°C. Latitudinal thermal gradients for the Albian‐Cenomanian, Turonian, and Maastrichtian were 0.2–0.3°C/° latitude and thus weaker than modern oceanic values at about 0.4°C/° latitude.
Timing of ice‐age terminations determined by wavelet methods
Authors: Hargreaves Julia C. , Abe‐Ouchi Ayako
Published: May 2003

This paper presents a new analysis of four time series of paleoclimate data, using wavelet methods to determine the timings and characteristics of the ice‐age terminations. We use an antisymmetric wavelet which detects locations in the time series where there are large changes. The results are consistent with previous work, but we have, for the first time, been able to explore the features of the individual terminations in an objective way. We find that at the terminations the interval between atmospheric carbon dioxide changes and sea level is about 7 kyr, compared with 15 kyr obtained using Fourier methods, which average the phase over the whole 100 kyr signal. The sea level change is the fastest but occurs after the other three components.
Lower Miocene to present stratigraphy of the equatorial Pacific sediment bulge and carbonate dissolution anomalies
Authors: Mitchell Neil C. , Lyle Mitchell W. , Knappenberger Marie B. , Liberty Lee M.
Published: May 2003

The bulge is a 600‐m‐thick regional deposit of pelagic sediment accumulated around the equator. Its stratigraphy reflects a number of factors: how accumulation rates have varied over time, how accumulation has been spatially focused around the equator, how much carbonate dissolution and reworking or nondeposition by bottom currents have occurred, and how much the deposits have been translated northward by motion of the Pacific tectonic plate on which they have accumulated. In order to fully explore the effects of these processes, a spatially continuous stratigraphic database is desirable, as existing cores provide information at only discrete points, and they tend to be sited in locally thicker accumulations. We illustrate the utility of seismic data tied to drill cores by comparing the carbonate dissolution effect in two north‐south lines crossing the sediment bulge. Bathymetry over our easterly seismic line deepens relatively little going north, whereas our westerly line deepens by 750 m from 4°N to 7°N. At a common latitude the primary pelagic input to both these lines will have been similar, so the difference of their sediment mass should reflect mostly their relative carbonate loss due to dissolution. We use this fact to calculate the slope of the carbonate dissolution curve and find a remarkably small slope (little relative carbonate dissolution) compared to that expected from previous studies. The techniques here may suggest a possible way forward for studying the equatorial carbonate stratigraphy more generally.
Response of planktic foraminiferal size to late Quaternary climate change
Authors: Schmidt Daniela N. , Renaud Sabrina , Bollmann Jörg
Published: May 2003

Geographical size distribution within entire Holocene foraminiferal assemblages is related to global environmental gradients such as temperature, primary productivity, and environmental variability. This study demonstrates that these correlations are also recognizable in late Quaternary assemblages from three locations in the South Atlantic on temporal and latitudinal scales. The size response to temporal paleoenvironmental changes during glacial‐interglacial cycles mimics the geographic Holocene size variability. The amplitude of size variability is directly related to the amplitude of the climatic fluctuations as shown by the stable size‐temperature relationship over time. The documented changes in the assemblage size are caused by species replacement and intraspecific size variability. The relative importance of these processes depends on the environmental setting. Species have been shown to reach their maximum size and abundance under certain optimum conditions and decrease in size if environmental conditions differ from these optima. We confirm that late Quaternary species sizes were largest at paleotemperatures identical to Holocene ones.
Planktonic foraminiferal flux seasonality in Subantarctic sediment traps: A test for paleoclimate reconstructions
Authors: King Alexandra L. , Howard William R.
Published: May 2003

Sediment trap moorings deployed during 1997 and 1998 in the Subantarctic to Polar Frontal regions of the Southern Ocean reveal distinct seasonality in foraminiferal flux. Foraminiferal assemblages vary between each site, yet major species exhibit very similar patterns of seasonal succession which can be associated with changes in mixed layer depth. Enhanced foraminiferal productivity is also associated with periods of high biogenic silica and particulate organic carbon flux. On a broader scale, foraminiferal assemblages are strongly delineated by temperature. Temperature estimates derived from the assemblages using the modern analog technique (MAT) are mostly within 2.5°C of the satellite advanced very high resolution radiometer temperatures observed during the deployment period. This indicates that core top sediments included in the MAT database do reflect modern observed conditions at the sea surface, providing a robust technique for estimating past temperature change from foraminiferal assemblages in Southern Ocean environments.
Extreme polar warmth during the Cretaceous greenhouse? Paradox of the late Turonian δ18O record at Deep Sea Drilling Project Site 511
Authors: Bice Karen L. , Huber Brian T. , Norris Richard D.
Published: May 2003

Oxygen isotope data for upper Turonian planktonic foraminifera at Deep Sea Drilling Project Site 511 (Falkland Plateau, 60°S paleolatitude) exhibit an ∼2‰ excursion to values as low as −4.66‰ (Vienna Peedee belemnite standard; PDB) coincident with the warmest tropical temperature estimates yet obtained for the open ocean. The lowest planktonic foraminifer δ18O values suggest that the upper ocean was as warm as 30–32°C. This is an extraordinary temperature for 60°S latitude but is consistent with temperatures estimated from apparently coeval mollusc δ18O from nearby James Ross Island (65°S paleolatitude). Glassy textural preservation, a well‐defined depth distribution in Site 511 planktonics, low sediment burial temperature (∼32°C), and lack of evidence of highly depleted pore waters argue against diagenesis (even solid state diffusion) as the cause of the very depleted planktonic values. The lack of change in benthic foraminifer δ18O suggests brackish water capping as the mechanism for the low planktonic δ18O values. However, mixing ratio calculations show that the amount of freshwater required to produce a 2‰ shift in ambient water would drive a 7 psu decrease in salinity. The abundance and diversity of planktonic foraminifera and nannofossils, high planktonic:benthic ratios, and the appearance of keeled foraminifera argue against lower‐than‐normal marine salinities. Isotope calculations and climate models indicate that we cannot call upon more depleted freshwater δ18O to explain this record. Without more late Turonian data, especially from outside the South Atlantic basin, we can currently only speculate on possible causes of this paradoxical record from the core of the Cretaceous greenhouse.
La Niña‐like conditions in the eastern equatorial Pacific and a stronger Choco jet in the northern Andes during the last glaciation
Authors: Martínez Ignacio , Keigwin Lloyd , Barrows Timothy T. , Yokoyama Yusuke , Southon John
Published: May 2003

Six deep sea cores from the eastern equatorial Pacific (EEP) were analyzed for planktonic foraminifera and stable isotopes in order to reconstruct sea surface temperatures (SST) for the last 40 ka. South of the Equatorial Front the abundance of Globorotalia inflata increased, and SST decreased by >5°C (core ODP846B), creating a stronger SST meridional gradient and advection of the Peru Current than present for the ∼16–35 ka interval. A sharper SST meridional gradient forced stronger Choco jet events and a moisture increase in western Colombia, which supplied, through the San Juan River and the south‐flowing equatorial and the Peru‐Chile countercurrents, abundant hemipelagic quartz over the northern Peru basin (core TR163‐31B). The Choco jet, and its associated mesoscale convective cells, provoked an increase in snow precipitation over the Central Cordillera of Colombia and the advance of the Murillo glacier. In synchrony with the intensified Choco jet events, the “dry island” effect over the Eastern Cordillera of Colombia intensified, and the level of Fuquene Lake dropped.
A study of the transformation bandwidth and the thickness sensitivity of the anisotropic‐slab LP to CP polarizer with several media
Authors: Huang Y. C. , Lin K. H.
Published: May 2003

In this paper, we investigate the transformation bandwidth and the thickness sensitivity of the anisotropic‐slab linearly polarized (LP) to circularly polarized (CP) polarizer. The effect is represented graphically on the complex polarization ratio plane. We define a transformation bandwidth and the thickness sensitivity by using a specified axial ratio, e.g., 3 dB, as a basis. It is shown that the polarization locus for a given axial ratio leads to a circle in the polarization state diagram. When combined with the graphical description of the change in the polarization state, the transformation bandwidth and the thickness sensitivity from an initial LP wave to a desired CP wave can be obtained easily. Under the small reflection approximation, i.e., only the forward waves are considered, this method can be applied to the design of the anisotropic‐slab LP to CP polarizer. This method eliminates the lengthy derivation and gives deeper physical insight to the problem. The transformation bandwidths for the anisotropic‐slab polarizer with a lossless and nondispersive biaxial medium are studied. The thickness sensitivities for the anisotropic‐slab polarizer with several lossless media are also investigated. The results are discussed and illustrated.
Modeling of apparent radar reflectivity due to convective clouds at attenuating wavelengths
Authors: Marzano Frank S. , Roberti Laura , Di Michele Sabatino , Mugnai Alberto , Tassa Alessandra
Published: May 2003

Spaceborne precipitation radars are usually designed to operate at attenuating wavelengths, mostly at X, Ku and Ka band. At these frequencies and above, convective rainfall can cause severe attenuation. Moreover, raindrops and precipitating ice can give rise to appreciable multiple scattered radiation which apparently tends to enhance the nominal attenuated reflectivity. In order to properly describe radar observations in such conditions, apparent reflectivity has to be modeled taking into account both path attenuation and incoherent effects. To this aim, a general definition of volume radar reflectivity is introduced, and a Monte Carlo model of backscattered specific intensity is implemented. The numerical model is applied to synthetic profiles, extracted from a mesoscale cloud‐resolving model simulation and representing intense and heavy convective precipitation at a developing and mature stage. Realistic appearance of these average profiles is argued by resorting to radar measurements available in literature. Spaceborne apparent reflectivity due to multiple scattering is shown to be significantly different from the attenuated one for the near‐surface layers of mature convection at Ku band and even for growing convection at Ka band. A discussion about this discrepancy is carried out at Ku band showing its possible impact for estimated rain rate profiles. If precipitation incoherent effects are formally treated as perturbation factors of the specific attenuation model, constrained single‐frequency inversion techniques are shown to be suitable to minimize rain rate retrieval errors due to multiple scattering phenomenon.
Retrieval of two‐dimensional absorption coefficient structure from a scanning radiometer at 23.8 GHz
Authors: Bosisio Ada Vittoria , Drufuca Giuseppe
Published: May 2003

In this work the retrieval of the structure of the atmospheric specific absorption coefficient through a tomographic acquisition is discussed. Brightness temperatures have been collected daily at 23.8 GHz using a radiometer in tipping curve mode regardless of the sky condition, except for rainy days, for about 60 elevation scans. Each scan has been processed to retrieve the two‐dimensional structure of the underlying atmosphere through the (x‐z) values of the specific absorption coefficient. The forward model is inverted after binning in space and linearization around a reference model. The scanned vertical plan is modeled by allocating M bins spanning in both the vertical and the horizontal direction, whereas the measurements have been collected at 12 elevation angles between 23° and 90°. Singular value decomposition allows identifying an orthogonal basis for the data space (12 elevation angles) and for the model space (M bins). It indicates also that only five eigenvalues are such that the condition number of the matrix representing the linearized problem is acceptable (≈20), so that only five components of the model space may be resolved. A higher number would imply unacceptable noise contamination. In spite of these limitations, the retrieved panel of specific absorption coefficient reveals the horizontal structure of the atmosphere. The scale of the experiment is of the order of kilometers. Therefore, because the vertical structure of the atmosphere dominates over the horizontal, a vertical detrending is needed to reveal horizontal structures. An example shows the typical problems encountered in the inversion and the remedies that have been adopted.
Radiometer‐based calibration of the relation between radar reflectivity and microwave attenuation
Authors: Capsoni Carlo , Caboni V.
Published: May 2003

The use of higher and higher frequencies in telecommunication systems requires the investigation of the effects of troposphere on electromagnetic wave propagation, and particular attention must be paid to the attenuation induced by raindrops along the radio path. For this purpose the direct measurements approach, in many cases, is not feasible or convenient, and the use of indirect measuring technique is now a common practice. In this respect, meteorological radar operating at nonattenuating frequencies can be of great help owing to their ability in collecting data over wide areas in a short time. The drawback is the incomplete knowledge of the relation between radar reflectivity (Z) and the quantity of direct interest so that an external source of “calibration” is required. In this paper we show that radiometers are proper instruments to calibrate single parameter radar and specifically to derive, on event basis, the relation to convert Z into specific attenuation (α). For this purpose, brightness temperatures at 13 GHz collected by a radiometer along the 30.6° slant path toward the Olympus satellite, direct attenuation measurements, and radar reflectivity profiles have been jointly worked out. First, the choice of the average effective medium temperature to estimate attenuation from brightness measurements has been addressed. Second, the best α ‐Z relation on an event basis has been evaluated through a best fit procedure that forced the radar predicted attenuation to agree with radiometer “measured” ones. The results are very encouraging and confirm the possibility to make use of radiometers as an external source of radar calibration.
Inversion for refractivity parameters from radar sea clutter
Authors: Gerstoft Peter , Rogers L. Ted , Krolik Jeffrey L. , Hodgkiss William S.
Published: May 2003

This paper describes estimation of low‐altitude atmospheric refractivity from radar sea clutter observations. The vertical structure of the refractive environment is modeled using five parameters, and the horizontal structure is modeled using six parameters. The refractivity model is implemented with and without an a priori constraint on the duct strength, as might be derived from soundings or numerical weather‐prediction models. An electromagnetic propagation model maps the refractivity structure into a replica field. Replica fields are compared to the observed clutter using a squared‐error objective function. A global search for the 11 environmental parameters is performed using genetic algorithms. The inversion algorithm is implemented on S‐band radar sea‐clutter data from Wallops Island, Virginia. Reference data are from range‐dependent refractivity profiles obtained with a helicopter. The inversion is assessed (1) by comparing the propagation predicted from the radar‐inferred refractivity profiles and from the helicopter profiles, (2) by comparing the refractivity parameters from the helicopter soundings to those estimated, and (3) by examining the fit between observed clutter and optimal replica field. This technique could provide near‐real‐time estimation of ducting effects. In practical implementations it is unlikely that range‐dependent soundings would be available. A single sounding is used for evaluating the radar‐inferred environmental parameters. When the unconstrained environmental model is used, the “refractivity‐from‐clutter,” the propagation loss generated and the loss from this single sounding, is close within the duct; however, above the duct they differ. Use of the constraint on the duct strength leads to a better match also above the duct.
Investigation of atmospheric boundary layer temperature, turbulence, and wind parameters on the basis of passive microwave remote sensing
Authors: Kadygrov Evgeny N. , Shur Genrih N. , Viazankin Anton S.
Published: May 2003

The MTP‐5, a microwave temperature profiler, has been widely used since 1991 for investigation of the atmospheric boundary layer (ABL). The MTP‐5 is an angular scanning single‐channel instrument with a central frequency of about 60 GHz, designed to provide continuous, unattended observations. It can measure the thermal emission of the atmosphere with high sensitivity (0.03 K at 1 s integration time) from different zenith angles. On the basis of this measurement, it is possible to retrieve temperature profiles at the altitude range up to 600 m, to calculate wind speed and wind direction at the lowest 250 m, and to get information about some parameters of atmospheric turbulence. This report presents some applications of the MTP‐5 instrument data collected in 1998–2001 within a number of international field projects: the dynamics of ABL temperature inversion in a mountain valley (Mesoscale Alpine Program (MAP)), as well as along an island coast (north part of Sakhalin Island, Russia‐Japan Project); continuous measurements of the ABL temperature profile provided from a special scientific train that crossed the territory of Russia (the Transcontinental Observations of the Chemistry of the Atmosphere Project (TROICA)); and simultaneous measurements of the ABL temperature profile provided over the central and northern part of Moscow in a continuous mode (the Global Urban Research Meteorology and Environment Project (GURME)). In 1999, two MTP‐5 instruments were installed on a platform that was rotating in the azimuth direction at the 310 m Obninsk Meteorological Research Tower (Meteo Tower) to validate the method and microwave equipment for measurement of wind speed and wind direction and investigation of atmospheric turbulence. Spectral analyses of the integrated signal provided an opportunity to estimate the inertial subrange low‐frequency limit and its height dependence for thermal turbulence at the lowest 200 m layer. Wavelet analysis of the signal made it possible to determine the convective thermic and other coherent structures; also to estimate energy and to provide visualization of the transformation processes of those structures during changing ABL stability. For a wide spectrum of ABL models the integrated character of radiometric data is more representative than in situ data. The measurement cycle for main wind is about 45 min, with an accuracy of about 1–2 m/s for wind speed and 10–20° for wind direction. Measurements at different levels of the Meteo Tower of vertical wind speed were in good agreement with in situ data. The possibility of manufacturing the microwave instrument for simultaneous measurements of the temperature profile and wind parameters in the ABL will be also discussed in this report.
Two‐dimensional synthetic aperture millimeter‐wave radiometric interferometer for measuring full‐component Stokes vector of emission from hydrometeors
Authors: Kutuza B. G. , Zagorin G. K.
Published: May 2003

Calculations of attainable values for space resolution and radiometric sensibility of a spaceborne two‐dimensional synthetic aperture microwave polarimetric interferometer are carried out. It is shown that for the most preferable Y‐shaped interferometer configuration, it is possible to achieve (within the transparency windows of waveband 3–22.5 mm) a space resolution of 0.5–1 km and a radiometric resolution close to 1 K at the receiver bandwidth of 100 MHz. For cross‐ and T‐shaped resolutions, values close to the above can be reached.
A study of the equatorial anomaly ionosphere using tomographic images
Authors: Franke S. J. , Yeh K. C. , Andreeva E. S. , Kunitsyn V. E.
Published: May 2003

Tomographic techniques are applied to relative total electron content (TEC) data obtained using radio signals transmitted by the polar orbiting Navy Navigation Satellite System (NNSS) and received at a chain of six stations located at approximately 121°E longitude. Images reconstructed for each satellite passage provide a picture of electron density over a 25° latitude range and to an altitude of about 1000 km at a longitude of 121°E. Over 350 ionospheric images of the northern equatorial anomaly region have been reconstructed for October/November 1994. The daytime equatorial anomaly crest is a prominent feature of the reconstructed images. The rms difference between foF2 computed from tomographic images and foF2 measured by two ionosondes is found to be less than 12%. Causes for the differences are discussed. We show how the crest develops on average days in October and November and discuss the nature of seasonal variations. The fully developed anomaly core is aligned approximately along geomagnetic field lines, resulting in the existence of strong and directional gradients in the anomaly region. As a consequence, the crest latitude and the maximum vertical TEC (VTEC) latitude are noncoincident. The tilt of the anomaly core also causes the slant TEC integrated through the crest to be highly directional. In fact, the use of spherical stratification in predicting slant TEC based on the knowledge of the vertical TEC may give an error as large as 30 to 50% in some directions. Implications for GPS ranging measurements are discussed.
Peculiarities of vertical atmosphere absorption in the millimeter wave band
Authors: Ruzhentsev Nikolay V.
Published: May 2003

One complete year of continuous observations of vertical radiation of the atmosphere in two points of millimeter wave band (94 and 38 GHz) was conducted. The total vertical absorption, liquid and vapor water content of atmosphere, and average (effective) temperature of clouds are restored on the basis of these two‐frequency radiometric data. Numerous cases of abnormal differences between experimental and theoretical values of cloud absorption at the two frequencies were observed. An analysis of the collection of direct and indirect features that accompany the abnormal events is carried out. It is shown that the main sources of observed abnormal differences between theoretical and experimental data are snow and ice particles in clouds and, probably, water droplets of large size. The possibility of using two‐frequency radiometric data not only for retrieving of liquid and vapor water content of atmosphere but also for determining the effective temperature of clouds and for assessing hydrometeor microstructure is noted. For this purpose, polarization measurements are very useful.
Cross‐correlation ratio method to estimate cross‐beam wind and comparison with a full correlation analysis
Authors: Zhang Guifu , Doviak Richard J. , Vivekanandan J. , Brown William O. J. , Cohn Stephen A.
Published: May 2003

Cross‐beam wind is usually estimated using a full correlation analysis (FCA) method applied to signals from spaced antennas. In this paper we present a cross‐correlation ratio (CCR) method for wind measurements. The CCR method is illustrated using theory, and data obtained with the National Center for Atmospheric Research's multiple antenna profiling radar. The standard errors of estimated cross‐beam wind using CCR and a FCA are studied based on a rigorous analysis of the variance of the cross‐correlation estimates. The results of the analysis are compared with previous works. It is shown that the current method is easy to implement and has smaller error for receiving antenna spacing small compared to the transmitting antenna dimensions.
A closed form, physical optics expression for the radar cross section of a perfectly conducting flat plate over a dielectric half‐space
Authors: Anastassiu Hristos T.
Published: May 2003

The physical optics approximation is employed in the derivation of a closed form expression for the radar cross section (RCS) of a flat, perfectly conducting plate of various shapes, located over a dielectric, possibly lossy half‐space. The half‐space is assumed to lie in the far field region of the plate. The well‐known “four‐path model” is invoked in a first‐order approximation of the half‐space contribution to the scattering mechanisms. Numerical results are compared to a reference, Moment Method solution, and the agreement is investigated, to assess the accuracy of the approximations used. The analytical expressions derived can facilitate very fast RCS calculations for realistic scatterers, such as ships in a sea environment, or aircraft flying low over the ground.
Fringe‐washing function calibration in aperture synthesis microwave radiometry
Authors: Butora R. , Martín‐Neira M. , Rivada‐Antich Angel‐Luis
Published: May 2003

This paper describes how the fringe‐washing function of a wide field of view interferometric radiometer can be accurately estimated using a relatively simple method. Fringe washing is an undesired effect that can be avoided in radio‐telescopes due to its narrow‐beam nature, but which is unavoidable in wide field of view interferometers, as those built for Earth Observation applications. The 3‐delay method is based on performing correlations at ‐T, 0 and +T lags of each baseline of the interferometer, when all receiver inputs are connected to a common noise source (centralized correlated noise injection). In addition, a fringe‐washing function closure relationship is presented which allows to extend this concept to aperture synthesis radiometers using distributed correlated noise injection. The results presented in this paper are simulations based on real measurements of the end‐to‐end frequency response of the first prototype receivers of ESA's Microwave Imaging Radiometer with Aperture Synthesis (MIRAS).
A microstrip Yagi antenna using EBG structure
Authors: Padhi Shantanu K. , Bialkowski Marek E.
Published: May 2003

In this paper a detailed design, development and performances of a 5 GHz microstrip Yagi antenna, which uses a two‐dimensional (2‐D) electromagnetic band gap (EBG) structure in the ground plane, are presented. The results indicate that the use of the EBG structure improves the radiation pattern of the antenna. The cross polarization is suppressed by properly choosing the period and dimensions of EBGs. Also, the broadside gain is improved in comparison with the analogous antenna without the EBGs.
Analysis of finite‐length gyrotropic hard‐surface waveguide
Authors: Uusitupa Tero , Viitanen Ari
Published: May 2003

Electromagnetic fields and mode transformation effect are theoretically investigated inside a hard‐surface (HS) circular waveguide. The waveguide section causing a mode transformation is filled with gyrotropic material, e.g., magnetoplasma or ferrite. In this paper, the gyrotropic material is ferrite. The effect of gyrotropy is taken into account exactly, i.e., no approximation of small gyrotropy or small anisotropy is made. There are different ways to implement HS boundary. One can use longitudinal strips at the surface of a dielectric coating or longitudinal corrugation. In principle, if the corrugation is in axial direction and the depth of the corrugation is effectively a quarter wavelength, a boundary condition equal to hard surface is obtained. Inside the gyrotropic waveguide section the eigenwaves are elliptically polarized and are propagating with different propagation factors. The difference in propagation factors causes a mode transformation. Reflection and transmission formulas are derived for a structure of type isotropic‐gyrotropic‐isotropic in a general case. Transformer examples are given.
L‐band scintillation activity and space‐time structure of low‐latitude UHF scintillations
Authors: Bhattacharyya A. , Groves K. M. , Basu S. , Kuenzler H. , Valladares C. E. , Sheehan R.
Published: May 2003

Spatial correlation function of intensity scintillation patterns produced by the propagation of a UHF signal through irregularities in the nighttime low‐latitude ionosphere is deduced from an analysis of spaced receiver records of such scintillations. This analysis requires that random temporal variations of the irregularity drift speed be taken into account. It is seen from the results that the occurrence of strong scintillations on an L‐band signal requires the presence of short (∼20 m) coherence scale lengths in the UHF scintillation pattern obtained in the plane of the receiver. This condition is satisfied near the crest of the equatorial ionization anomaly (EIA) region, but not near the dip equator. In the decay phase of L‐band scintillations recorded near the crest of the EIA region, the maximum strength of these scintillations at any point in time is found to be correlated with the magnetic eastward drift speed of the pattern of intensity scintillations on an UHF signal recorded in this region, which is determined mainly by the magnetic eastward drift velocity of the ionospheric irregularites. Dependence of the corresponding strength of UHF scintillations on the drift speed indicates that toward the end of the decay phase of L‐band scintillations, the irregularity power spectrum steepens, and the large scale irregularities that remain can cause the UHF signal to be focused in the plane of the receiver, yielding UHF S4‐indices greater than one, while focusing of the UHF signal is less evident at earlier times when there is focusing of the L‐band signal.
Quasi‐classic approximation in Markov's parabolic equation for spaced position and frequency coherency
Authors: Bitjukov A. A. , Gherm V. E. , Zernov N. N.
Published: May 2003

An asymptotic technique to solve Markov's parabolic equation for the second‐order spaced position and frequency coherence function is discussed. Rather than employing separation of variables, the technique is based on the quasi‐classic representation in terms of complex trajectories and is also valid in the case of a nonhomogeneous background medium and does not demand the statistical homogeneity of fluctuations. It has no constraints relevant to the initial conditions in the form of an incident plane wave and produces in automatic fashion different known rigorous solutions, in particular, to the case of quadratic structure function.