bluuebunny/crossref_metadata_2025 huggingface dataset:
Warm rain processes over tropical oceans and climate implications
Authors: Lau K. M. , Wu H. T.
Published: December 2003

From analysis of TRMM data, we find that warm rain accounts for 31% of the total rain amount and 72% of the total rain area in the tropics, and plays an important role in regulating the moisture content of the tropical atmosphere. There is a substantial increase in precipitation efficiency of light warm rain as the sea surface temperature increases, but precipitation efficiency of heavy rain associated with deep convection is independent of sea surface temperature. This implies that in a warmer climate, there may be more warm rain, at the expense of less cloud water available for middle and high level clouds.
Crustal anisotropy in southern California from local earthquake data
Authors: Paulssen Hanneke
Published: January 2004

Shear wave splitting measurements were made for local earthquakes in southern California at incidence angles larger than the critical incidence angle. For the region of San Bernardino Mountains, the fast polarization directions vary strongly, but are consistently fast for directions roughly perpendicular to the ray paths. This observation is most readily explained by transverse isotropy with a vertical symmetry axis, and is probably associated with horizontally foliated gneisses or schists in the upper crust. Other measurements show a predominance of north‐south fast directions, while some data have fast directions that are parallel to the San Andreas fault. These observations are related to azimuthal anisotropy as found in other local shear wave splitting and SKS splitting studies. The data show that, apart from the free surface effect, shear wave splitting measurements for shear waves at shallow incidence angles can have an imprint of transverse isotropy as well as azimuthal anisotropy.
The deepening of the Atlantic water in the Lofoten Basin of the Norwegian Sea, demonstrated by using an active reduced gravity model
Authors: Orvik Kjell Arild
Published: January 2004

The Norwegian Atlantic Current (NwAC) as a poleward eastern boundary current is to be considered as the northern limb of the Meridional Overturning Circulation in the North Atlantic (MOC). It transports warm and saline Atlantic water (AW) northward toward the Arctic Ocean, before cooling and mixing with cold and low saline water masses return it to the North Atlantic to contribute in the MOC. In this study we focus on the AW in the Lofoten Basin (LB) of the Norwegian Sea (NS), where it occupies a wider and deeper domain than farther south and north. It comprises the major heat‐reservoir in the NS with contact with the atmosphere, and as such is an important area for cooling and buoyancy loss. We show that the deepening of the AW is mainly caused by the reduced northward flow in the LB, manifested in the deep counter‐current encountered by the Polar frontal jet. We demonstrate this effect by using an active reduced gravity model with a topographically‐steered deeper flow. To achieve maintenance of volume transport, the model shows that observed differences must be balanced by an oppositely‐directed deeper flow in the LB. The ocean appears to achieve maintenance of volume transport of AW due to buoyancy loss and varying deeper currents, by adjusting the vertical extension of AW, resulting in a substantial deepening in the LB. Based on the good fit between the model and observations, the suggested mechanism appears to be credible and feasible.
Temperature derivatives of elastic moduli of MgSiO3 perovskite
Authors: Aizawa Yoshitaka , Yoneda Akira , Katsura Tomoo , Ito Eiji , Saito Toshiaki , Suzuki Isao
Published: January 2004

Elastic properties of polycrystalline MgSiO3 perovskite were determined by means of resonance sphere technique at temperatures from 258 to 318 K under atmospheric pressure. The temperature derivatives of adiabatic bulk (∂KS/∂T)P and shear moduli (∂G/∂T)P are −0.029(2) GPa/K and −0.024(1) GPa/K, respectively. Applying the present results in interpretation of seismological observations, we suggest that the chemical composition of the earth's lower mantle is pyrolitic at the potential temperature of 1600 K.
Electric fields deduced from plasmapause motion in IMAGE EUV images
Authors: Goldstein J. , Wolf R. A. , Sandel B. R. , Reiff P. H.
Published: January 2004

The IMAGE extreme ultraviolet (EUV) imager obtained global images of the plasmasphere erosion of 10 July 2000, in which the nightside plasmapause moved inward by about 2 RE during 5–8 UT. We use plasmapause motion to infer the electric (E) field component tangent to the moving plasmapause; in general we lack knowledge of the perpendicular E‐field component. In the midnight‐to‐dawn quadrant where the plasmapause shape is very nearly circular, the tangential E‐field component is equal to the azimuthal electric field Eφ. Peak westward E‐fields of 0.6–1.3 mV/m were found at the plasmapause between L ≈ 4–6, an inner magnetospheric E‐field that was 25% of the solar wind E‐field. The MLT‐concentration of the inferred E‐field suggests that convective flows may produce partial indentation of the nightside plasmapause (especially in the midnight‐to‐dawn sector) that widens as the edges of the indentation propagate azimuthally.
Global oceanic chlorofluorocarbon inventory
Authors: Willey Debra A. , Fine Rana A. , Sonnerup Rolf E. , Bullister John L. , Smethie William M. , Warner Mark J.
Published: January 2004

Chlorofluorocarbons (CFCs) dissolve in the oceans, but the total quantity and spatial distribution in the oceans was not previously known. The first estimate of the global oceanic CFC‐11 uptake using field measurements is calculated from WOCE (World Ocean Circulation Experiment) CFC‐11 concentrations. Here we find the total oceanic uptake of 5.5 (±1.2) × 108 moles was about 1% of total emissions through 1994. Eighty‐two percent of the CFC‐11 inventory is in the upper 1000 meters. The CFC inventory distribution implies that the dominant physical air‐sea exchange of gases on decadal time scales occurs due to a combination of high gas solubility in cold high latitude waters and effectiveness of the wind‐driven circulation. The global inventory provides a benchmark for models simulating climate change.
Impact of temperature inversions on SST evolution in the South‐Eastern Arabian Sea during the pre‐summer monsoon season
Authors: Durand F. , Shetye S. R. , Vialard J. , Shankar D. , Shenoi S. S. C. , Ethe C. , Madec G.
Published: January 2004

Temperature inversions are known to occur in the near‐surface ocean regime where salinity stratification is large enough to influence the density field. However, they have not been known as features that alter near‐surface processes significantly to influence the sea surface temperature (SST). From the analysis of new observed datasets as well as of state‐of‐the‐art numerical model outputs, this paper shows that heat trapped within a temperature inversion makes significant contribution to warming of the SST in the South‐Eastern Arabian Sea during the pre‐southwest monsoon season.
Vienna mapping functions in VLBI analyses
Authors: Boehm Johannes , Schuh Harald
Published: January 2004

In the past few years, numerical weather models (NWM) have been investigated to improve mapping functions which are used for tropospheric delay modeling in VLBI and GPS data analyses. The Vienna Mapping Functions VMF are based on direct raytracing through NWM, and so they are able to exploit the full information provided in the NWM. On the other hand, the Isobaric Mapping Functions IMF are using intermediate parameters calculated from the NWM. In this study, pressure level data from ECMWF (European Centre for Medium‐Range Weather Forecasts) are applied to determine the coefficients of the VMF and the IMF. Used for the analyses of IVS‐R1 and IVS‐R4 VLBI sessions, both mapping functions improve the repeatability of baseline lengths (by ∼10% for IVS‐R1 and ∼5% for IVS‐R4) compared to the Niell Mapping Functions NMF.
Numerical investigation of induced cracking and seismic velocity changes in brittle rock
Authors: Hazzard James F. , Young R. Paul
Published: January 2004

A numerical technique is presented to investigate seismic velocity changes in brittle rock due to stress and damage. A rock sample is simulated by an assembly of bonded particles, which is then subjected to hydrostatic and deviatoric loading regimes. During hydrostatic loading, cracks close through formation of new contacts and velocities increase. During deviatoric loading, new cracks form by the breaking of bonds and velocities decrease in directions perpendicular to the opening cracks. The nature of the velocity changes mimics closely what is observed in the laboratory. The model offers the unique ability to examine directly the microprocesses leading to observed velocity changes.
Limitations of the force field equation to describe cosmic ray modulation
Authors: Caballero‐Lopez R. A. , Moraal H.
Published: January 2004

The force field approximation to the transport equation which describes cosmic ray modulation in the heliosphere is a widely used tool. It is popular because it provides an easy to use, quasi‐analytical method to describe the level of modulation with a single parameter. A simple numerical solution of the one‐dimensional cosmic ray transport equation is used to show that this is a good approximation for galactic cosmic rays in the inner heliosphere but that its accuracy decreases toward the outer heliosphere. On the other hand, the even simpler convection‐diffusion approximation improves with radial distance. The reason for the complementary behavior of these two approximations is that energy losses are relatively important in the inner heliosphere but not in the outer heliosphere. The force field approximation is worse for anomalous cosmic rays at all radial distances due to the exponential cutoff of these spectra at high energies. The ranges of validity are quantified. Since both approximations have their limitations, a simple numerical solution of the one‐dimensional transport equation is provided for general use.
Stress field variations in the Swiss Alps and the northern Alpine foreland derived from inversion of fault plane solutions
Authors: Kastrup Ulrike , Zoback Mary Lou , Deichmann Nicholas , Evans Keith F. , Giardini Domenico , Michael Andrew J.
Published: January 2004

This study is devoted to a systematic analysis of the state of stress of the central European Alps and northern Alpine foreland in Switzerland based on focal mechanisms of 138 earthquakes with magnitudes between 1 and 5. The most robust feature of the results is that the azimuth of the minimum compressive stress, S3, is generally well constrained for all data subsets and always lies in the NE quadrant. However, within this quadrant, the orientation of S3 changes systematically both along the structural strike of the Alpine chain and across it. The variation in stress along the mountain belt from NE to SW involves a progressive, counterclockwise rotation of S3 and is most clear in the foreland, where it amounts to 45°–50°. This pattern of rotation is compatible with the disturbance to the stress field expected from the indentation of the Adriatic Block into the central European Plate, possibly together with buoyancy forces arising from the strongly arcuate structure of the Moho to the immediate west of our study area. Across the Alps, the variation in azimuth of S3 is defined by a progressive, counterclockwise rotation of about 45° from the foreland in the north across the Helvetic domain to the Penninic nappes in the south and is accompanied by a change from a slight predominance of strike‐slip mechanisms in the foreland to a strong predominance of normal faulting in the high parts of the Alps. The observed rotation can be explained by the perturbation of the large‐scale regional stress by a local uniaxial deviatoric tension with a magnitude similar to that of the regional differential stress and with an orientation perpendicular to the strike of the Alpine belt. The tensile nature and orientation of this stress is consistent with the “spreading” stress expected from lateral density changes due to a crustal root beneath the Alps.
Transient volcano deformation sources imaged with interferometric synthetic aperture radar: Application to Seguam Island, Alaska
Authors: Masterlark Timothy , Lu Zhong
Published: January 2004

Thirty interferometric synthetic aperture radar (InSAR) images, spanning various intervals during 1992–2000, document coeruptive and posteruptive deformation of the 1992–1993 eruption on Seguam Island, Alaska. A procedure that combines standard damped least squares inverse methods and collective surfaces, identifies three dominant amorphous clusters of deformation point sources. Predictions generated from these three point source clusters account for both the spatial and temporal complexity of the deformation patterns of the InSAR data. Regularized time series of source strength attribute a distinctive transient behavior to each of the three source clusters. A model that combines magma influx, thermoelastic relaxation, poroelastic effects, and petrologic data accounts for the transient, interrelated behavior of the source clusters and the observed deformation. Basaltic magma pulses, which flow into a storage chamber residing in the lower crust, drive this deformational system. A portion of a magma pulse is injected into the upper crust and remains in storage during both coeruption and posteruption intervals. This injected magma degasses and the volatile products accumulate in a shallow poroelastic storage chamber. During the eruption, another portion of the magma pulse is transported directly to the surface via a conduit roughly centered beneath Pyre Peak on the west side of the island. A small amount of this magma remains in storage during the eruption, and posteruption thermoelastic contraction ensues. This model, made possible by the excellent spatial and temporal coverage of the InSAR data, reveals a relatively simple system of interrelated predictable processes driven by magma dynamics.
Gulf of Mexico circulation within a high‐resolution numerical simulation of the North Atlantic Ocean
Authors: Romanou Anastasia , Chassignet Eric P. , Sturges Wilton
Published: January 2004

The Gulf of Mexico circulation is examined from the results of a high‐resolution (1/12°) North Atlantic simulation using the Miami Isopycnic Coordinate Ocean Model. The motivation for this paper is twofold: first, we validate the model's performance in the Gulf of Mexico by comparing the model fields to past and recent observations, and second, given the good agreement with the observed Gulf of Mexico surface circulation and Loop Current variability, we expand the discussion and analysis of the model circulation to areas that have not been extensively observed/analyzed, such as the vertical structure of the Loop Current and associated eddies, especially the deep circulation below 1500 m. The interval between successive model eddy sheddings is 3 to 15 months, the eddy diameters range between 140 and 500 km, the life span is about 1 year, and the translational speeds are 2–3 km d−1, in good agreement with observations. Areas of high cyclonic eddy occurrence in the model are southwest of Florida, the Loop Current boundary, and the western Campeche Bay area. The cyclonic eddy diameters range between 50 and 375 km, the orbital speeds range between 1 and 55 cm s−1, the translational speeds range between 0.5 and 14 km d−1, and the eddy life spans range between 1 and 3 months. The vertical structure of the temperature and salinity of each modeled eddy, from the moment it is shed until it disintegrates in the western Gulf of Mexico, is in agreement with the few available observations. Below 1500 m, deep cyclonic eddies are associated with the surface Loop Current anticyclones. The eddy variability is consistent with Rossby waves propagating westward, and there is bottom intensification of the flow close to steep topography. Overall, we show that this very high horizontal resolution isopycnic coordinate ocean model, which is able to produce a quite realistic surface circulation for the North and equatorial Atlantic, is also able to reproduce well the smaller‐scale, basin‐wide intricate dynamics such as the Gulf of Mexico variability.
Seasonal and spatial evolution of albedo in a snow‐ice‐land‐ocean environment
Authors: Grenfell Thomas C. , Perovich Donald K.
Published: January 2004

In the course of a program studying Arctic coastal processes we have investigated the albedos for the range of surfaces in the region and their response to seasonal changes in a land‐ice‐ocean regime near Barrow, Alaska. From April through June in the years 2000, 2001, and 2002, spectral and wavelength‐integrated albedos were measured along 200‐m survey lines. These lines were installed at four sites and included sea ice, lagoon ice, fresh ice, and tundra. Initially, all sites were completely snow‐covered, and the albedo was high (0.8–0.85) and spatially uniform. The concentration of absorptive contaminants in the snow was enough to produce a slight reduction below the pure snow values. As the melt season progressed, albedos decreased at all sites. The decrease was greatest and most rapid at the tundra site, where the albedo dropped from 0.8 to 0.15 in only 2 weeks. The spectral signature also changed as the wavelength of maximum albedo at the tundra site shifted from 500 nm for snow to 1100 nm for vegetation. As the snow cover melted, there was rapid and extensive ponding, resulting in a decrease of the spatially averaged, wavelength‐integrated albedo from 0.6 to 0.2 in only 5 days. Extensive pond drainage and below‐freezing temperatures caused the albedo to rebound briefly to 0.55 before resuming a steady decrease. Comparison of these results with data collected in the central Arctic show that albedos of fast ice in the coastal regime evolve significantly faster than pack ice albedos. Interannual seasonal variations showed additional melting phases that might be expected during protracted warming. The albedo evolution in the coastal lagoon was different each of the 3 years because of variations in sediment loading of the ice during freeze‐up and in the melt/freeze pattern the following spring. The record early melt in May 2002 followed by a pronounced freezing event 2 weeks later produced a distinct melt regime that gave rise to reduced melt pond coverage and higher albedo. Such events provide insight into processes associated with the melt ponds that will be important for modeling sea ice in a warming climate.
Bioluminescence imaging of wave‐induced turbulence
Authors: Stokes M. Dale , Deane Grant B. , Latz Michael I. , Rohr Jim
Published: January 2004

The ability to measure turbulent processes on small spatial and temporal scales is a long standing problem in physical oceanography. Here we explore a novel means of measuring fluid shear stress using the cell flashing behavior of bioluminescent dinoflagellates. To illustrate this technique, we present estimates of the heterogeneous, time‐varying shear stress inside a breaking wave crest. These results have implications for a better understanding of upper ocean wave physics, air‐sea gas transfer, and the biology of planktonic near‐surface organisms as well as providing a new quantitative fluid visualization tool.
Mixing of the Storfjorden overflow (Svalbard Archipelago) inferred from density overturns
Authors: Fer Ilker , Skogseth Ragnheid , Haugan Peter M.
Published: January 2004

Observations were made of the dense overflow from Storfjorden from a survey conducted at closely spaced stations in August 2002. The field data set consists of conventional conductivity‐temperature‐depth profiles and short‐term moored current meters and thermistor strings. Finestructure estimates were made by calculating Thorpe scales over identified overturns using 0.1‐dbar vertically averaged density profiles. Dissipation rate of turbulent kinetic energy per unit mass, ε, is estimated assuming proportionality between Thorpe and Ozmidov length scales. Vertical eddy diffusivity Kz is estimated using Osborn's model assuming a constant mixing efficiency. Survey‐averaged profiles suggest enhanced mixing near the bottom with values of Kz and ε, when averaged within the overflow, equal to 10 × 10−4 m2 s−1 and 3 × 10−8 W kg−1, respectively. Kz is found to decrease with increasing buoyancy frequency as N−1.2 (±0.3), albeit values of N covered only 0.5–8 cph (1 cph = 2π/3600 s−1). Values of heat flux obtained using Kz suggest that the plume gains a considerable amount of heat, 45 ± 25 W m−2, when averaged over the thickness of the plume, from overlying waters of Atlantic origin. This value is lower than but, considering the errors in estimates of Kz, comparable with 100 W m−2, the rate of change of heat in the overflow derived from sections across the sill and 80 km downstream.
Prediction skill of nearshore profile evolution models
Authors: Plant Nathaniel G. , Holland K. Todd , Puleo Jack A. , Gallagher Edith L.
Published: January 2004

The hindcast prediction skill of a beach profile evolution model has been evaluated using bathymetric observations obtained at Duck, North Carolina. The model included coupling and feedback between evolving bathymetry, wave‐averaged hydrodynamics, and parameterized cross‐shore sediment transport. Statistically optimum predictions were obtained by tuning free model parameters using rigorous inverse methods. When compared to persistence predictions (i.e., substitution of the initial, observed profile at all prediction times), significant prediction skill was found for prediction periods longer than 3 days and shorter than 17 days. The average skill (defined as 1 minus the ratio of prediction to persistence error variances) for a typical 5‐day prediction was 0.4, and the maximum skill was 0.8. In contrast to several previously published comparisons with field data, the present approach yielded significant predictive skill during conditions dominated by onshore sediment transport. To make significant predictions, it was necessary to vary model parameters values, which showed a dependence on wave conditions at the seaward boundary of the model domain. Finally, interpretation of the estimated parameter values and prediction skills suggests that (1) errors in existing hydrodynamic models contributed significantly to profile prediction error; (2) a transport model using only second‐order statistics of hydrodynamics was as accurate as (and sometimes more accurate than) a typically used energetics model; (3) existing sediment transport models contained significant errors in the formulations describing downslope and bed load transport; and (4) when driving models with observed flow fields, sampling limitations contribute to profile prediction error.
An adjoint sensitivity study of chlorofluorocarbons in the North Atlantic
Authors: Li Xingwen , Wunsch Carl
Published: January 2004

Adjoint sensitivities of CFC‐11 concentrations and CFC‐11/CFC‐12 ratio ages in a North Atlantic general circulation model are analyzed. These sensitivities are compared with those of spiciness, T − (β/α) S, where α, β are the thermal and haline expansion coefficients, respectively. High‐sensitivity fields are candidates for providing the most powerful constraints in the corresponding inverse problems. In the dual (adjoint) solutions all three variables exhibit the major ventilation pathways and define the associated timescales in the model. Overall, however, spiciness shows the highest sensitivity to the flow field. In the North Atlantic Deep Water, sensitivities of CFC properties and spiciness to the isopycnal mixing and thickness diffusion are of the same order of magnitude. In the lower subtropical thermocline, sensitivities of CFC properties to the isopycnal mixing and thickness diffusion are higher. The utility of this sensitivity is undermined by the need to reconstruct their boundary conditions. Given the influence of T, S measurements on the density field, they produce the most powerful constraints on the model on the large scale. It still remains possible, however, that transient tracers can provide a larger relative information content concerning the mixing process between the near‐surface boundary layer and the thermocline but dependent upon the ability to reconstruct accurate initial and boundary conditions.
Spectral properties of backscattered UV radiation in cloudy atmospheres
Authors: Ahmad Z. , Bhartia P. K. , Krotkov N.
Published: January 2004

We use a Plane‐Parallel Cloud (PPC) model to illustrate how Mie scattering from cloud particles interacts with Rayleigh scattering in the atmosphere and produces a complex wavelength dependence in the top‐of‐the‐atmosphere (TOA) reflectances measured by satellite instruments that operate in the ultraviolet (UV) part of the spectrum. Comparisons of the PPC‐model‐derived spectral dependence of reflectances with the Total Ozone Mapping Spectrometer (TOMS) measurements show surprisingly good agreement over a wide range of observational conditions. The PPC model results also are compared with the results of two other cloud models: Lambert Equivalent Reflectivity (LER) and Modified LER (MLER) that have been used to analyze satellite data in the UV. These models assume that clouds are opaque Lambertian reflectors rather than Mie scattering particles. Although one of these models (MLER) can be adjusted to agree reasonably well with the TOMS data, the adjustments are somewhat arbitrary and may not be suitable for interpreting satellite data if one desires high accuracy. We also use the PPC model to illustrate how clouds can perturb tropospheric O3 absorption in complex ways that cannot be explained by models that treat them as reflecting surfaces rather than as volume scatterers.
Lower stratospheric densities from solar occultation measurements of continuum absorption near 2400 cm−1
Authors: Rinsland Curtis P. , McHugh Martin J. , Irion Fredrick W.
Published: January 2004

Continuous absorption in the atmospheric window near 2400 cm−1 has been analyzed from infrared solar occultation spectra recorded by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer at 0.01 cm−1 spectral resolution during its 4 shuttle flights between 1985 and 1994. The spring and autumn measurements were recorded over a wide range of temperatures and sulfate aerosol loadings. We used the ratio of the transmissions in microwindows at 2415 cm−1 and 2501 cm−1 as a measure of the underlying continuum absorption in the spectra. This ratio decreases from 1.0 to 0.0 as the tangent point density increases from 5 × 1017 to 100 × 1017 molecules cm−3 (altitudes ∼10–30 km). Attenuation in both microwindows is due to overlapping absorptions by the fundamental N2 collision‐induced band and distant sub‐Lorentzian wings of CO2 lines from the strong ν3 fundamental band. Optical depths for both N2 and CO2 absorption are proportional to the square of the air density at constant temperature. These microwindows provide high sensitivity to density and low sensitivity to temperature in the lower stratosphere. Stratospheric transmission ratios from all 4 missions are fitted with a single parameter as a function of tangent point density with a root‐mean square residual of ∼1%. Corresponding densities from this fit agree with ATMOS version 3 densities to ∼2.3% in the region of good sensitivity at 2 × 1018 to 8 × 1018 molecules cm−3 (∼12–20 km altitude). The measured transmission ratio versus density relation has been compared with calculations from temperature‐dependent laboratory measurements for both the N2 and CO2 continua. Simulation of the ATMOS spectra based on these laboratory measurements also predict a globally compact relation between the transmission ratio and density but differs from the empirically‐determined relation by up to 7% in the lower stratosphere.
Equatorial Kelvin wave signatures in ozone column measurements from the Global Ozone Monitoring Experiment (GOME)
Authors: Timmermans R. M. A. , van Oss R. F. , Kelder H. M.
Published: January 2004

This study investigates tropical Kelvin wave signatures in the total ozone column data from the Global Ozone Monitoring Experiment (GOME) instrument. A new approach for spectral analysis is introduced by generalizing an unequally spaced data technique from one to two dimensions. This enables the handling of satellite data containing gaps. The simple statistical behavior of the method furthermore allows an easy determinination of the statistical significance of any observed spectral features. Seven years of GOME data (1995–2002) have been analyzed in which we have identified three periods of high Kelvin wave activity in 1996, 1998, and 2000. The periods are in conjunction with westward equatorial zonal winds at 30 hPa and show eastward propagating waves 1–2 with periods of ∼12–15 days. The induced Kelvin wave signatures in the ozone concentrations are around 2–4 DU peak‐to‐peak and can be attributed to “slow” Kelvin waves. The results are shown to be significant. Our study provides an important contribution to the study of Kelvin waves by introducing the bidimensional unequally spaced data spectral analysis and is the first to demonstrate the potential of the GOME ozone data set to contribute to a global description of equatorial Kelvin wave activity.
Development of an atmosphere‐ocean coupled model and its application over the Adriatic Sea during a severe weather event of Bora wind
Authors: Loglisci N. , Qian M. W. , Rachev N. , Cassardo C. , Longhetto A. , Purini R. , Trivero P. , Ferrarese S. , Giraud C.
Published: January 2004

This study presents the results of an application to a Bora severe weather episode (January 1995) in the northern Adriatic Sea of the regional two‐way atmosphere‐ocean coupled model (RAMS‐DieCAST), developed jointly by the Università di Torino and the Istituto Sperimentale Talassografico del CNR, Trieste. RAMS‐DieCAST showed significantly better ability to predict the sea surface temperature (SST) and its time evolution during the above mentioned episode using a full two‐way coupling as opposed to simpler one‐way forcing of the ocean. In this context, we found out that even in the high‐frequency variability conditions that are typical of Bora events, heat fluxes from the sea must be taken into account for a better description of air‐sea interaction processes in a dynamical framework. The SST evolution has been chosen as a validation parameter, owing to its availability and relevance for the characterization of the marine environment and local weather and climate studies. The simulations carried out with RAMS‐DieCAST present a small systematic error in calculating the SST evolution; however, a sensitivity analysis of the model to the preparation of initial conditions of the simulation suggested that climatological initialization could be partly responsible for this error, which might be reduced by assimilating satellite‐derived SSTs into the preparation of the initial conditions of the model.
Temperature inversions, thermal tides, and water ice clouds in the Martian tropics
Authors: Hinson D. P. , Wilson R. J.
Published: January 2004

We report new results on the structure and dynamics of the tropical atmosphere of Mars derived from a combination of radio occultation measurements by Mars Global Surveyor and simulations by a Mars general circulation model (MGCM). Radio occultation experiments sounded the equatorial atmosphere at latitudes of 36°N to 30°S during midsummer of the Northern Hemisphere (Ls = 134°–162°), sampling the predawn thermal structure at a local time of ∼0412. Elevated temperature inversions are a conspicuous feature of these observations. They appear at pressures between 30 and 200 Pa, well above the surface, and their magnitude exceeds 6 K in 34% of the temperature profiles in this latitude band. The properties and spatial distribution of these elevated inversions are organized across the tropics on planetary scales. Inversions are strongest and occur most frequently above elevated terrain, achieving a peak magnitude of ∼30 K near Tharsis, and their altitude generally increases toward the south. According to MGCM simulations, which closely resemble the observations, these temperature inversions arise from zonally modulated thermal tides. The best simulation includes an interactive hydrologic cycle, which results in strong coupling between the thermal tides and radiatively active water ice clouds. Prominent clouds form in response to wave‐induced adiabatic cooling and evolve in a pattern closely correlated with the thermal structure of the tides. The tides in turn are intensified by radiative forcing from the clouds. This tide‐cloud coupling imposes strong diurnal modulation on the properties of clouds in the tropics.
Modeling volcanic deformation in a regional stress field: Implications for the formation of graben structures on Alba Patera, Mars
Authors: Cailleau Beatrice , Walter Thomas R. , Janle Peter , Hauber Ernst
Published: December 2003

Abundant grabens transect the volcano Alba Patera. Their complex geometry and formation mechanisms are still poorly understood. Tectonic processes and magmatic intrusions are responsible for these long surface features. Cross‐cutting relationships of the grabens show radial fractures that were formed during early stages and were progressively overprinted by concentric fractures on the mid and upper flanks of the volcano. Two modeling methods are used to understand the formation of the observed structures and to evaluate their implications for hidden subvolcanic processes. Surface deformation and fault arrangements predicted in finite element models are compared to the graben systems observed in Viking images. The orientation and position of the concentric grabens are found to be best reproduced by local crustal subsidence, superimposed on a regional NW‐SE oriented extension with decreasing magnitude from south to north. In analogue sandbox models we also simulate surface structures of arrangements that almost perfectly mimic the observed lineaments on Alba Patera. Formation of the grabens spans a period on the order of a billion years, suggesting long‐term geodynamic processes to be responsible for the subsidence of the central Alba Patera area. The progressive change toward higher concentricity is likely resultant from an increase in density in the crust by accumulation of intrusive material and cooling, thus causing subsidence of the region above this volcanic root.
Intensities of atomic lines and molecular bands observed in impact‐induced luminescence
Authors: Sugita Seiji , Schultz Peter H. , Hasegawa Sunao
Published: December 2003

A series of hypervelocity impact experiments were conducted at NASA Ames Vertical Range, using spherical copper projectiles and polycrystalline dolomite targets. The intensities of atomic lines and molecular bands in the first 2 microseconds after each impact were measured using a high‐speed spectrometer system and assessed as functions of impact velocity (2–5.5 km/s) at a fixed impact angle (45° from the horizontal). Although the measured emission intensities follow power law relations well, the power law exponents of individual emission lines and bands are very different, ranging from 2.1 to 9.1. In contrast, the exponent for wavelength‐integrated visible light intensity (435–650 nm) is about 5. Such a large variation in exponents suggests a complex nature for the power law relations. In order to understand the physical processes controlling the power law relations, a theoretical model is developed considering chemical equilibrium of both molecular and atomic species, the electronic excitation of atoms, and ionization. This model accounts for the observed variety of the power law exponents well. Such a model will provide a physical framework for predicting the emission intensities from both artificial and natural impacts on planetary surfaces and for estimating size of an impactor from the emission intensity.
Dynamics of rifting and modes of extension on icy satellites
Authors: Nimmo F.
Published: January 2004

A simple numerical model of extension in icy satellite shells is developed. Thinning of the ice weakens the shell, promoting further extension. If lateral flow in the lower part of the shell is unimportant, extension is opposed and wide rifts are generated; if lateral flow is rapid, localized extension is favored and narrow rifts are produced. Thick shells or high strain rates favor the development of narrow rifts; low strain rates favor wide rifting. It is proposed that bands, extensional features on Europa, are narrow rifts, while groove lanes on Ganymede are wide rifts. The existence of wide rifting on Ganymede is consistent with previous estimates of a conductive shell thickness at the time of rifting of 4–8 km and a strain rate of <10−14 s−1. Narrow rifting on Europa can be produced at sufficiently high strain rates for a wide range of shell thicknesses. Assuming a prerift brittle‐ductile transition depth of 0.5–3 km restricts the solutions to relatively thin (<15 km) shells and high (>10−15 s−1) strain rates at the time of rifting. Whether this shell thickness applies to present‐day Europa depends on the age of band formation, which is poorly known. The difference between rifting behavior on Ganymede and Europa is due to either higher strain rates or higher shell thicknesses on Europa during rifting. The mean stresses required to cause the observed rifting are ≈0.2 MPa for Ganymede and ≈0.3 MPa for Europa. These values are comparable to estimates derived from flexural features.
Adaptive tracking of narrowband HF channel response
Authors: Arikan F. , Arikan O.
Published: December 2003

Estimation of channel impulse response constitutes a first step in computation of scattering function, channel equalization, elimination of multipath, and optimum detection and identification of transmitted signals through the HF channel. Due to spatial and temporal variations, HF channel impulse response has to be estimated adaptively. Based on developed state‐space and measurement models, an adaptive Kalman filter is proposed to track the HF channel variation in time. Robust methods of initialization and adaptively adjusting the noise covariance in the system dynamics are proposed. In simulated examples under good, moderate and poor ionospheric conditions, it is observed that the adaptive Kalman filter based channel estimator provides reliable channel estimates and can track the variation of the channel in time with high accuracy.
Performance comparison of cross correlation and filled aperture imaging riometers
Authors: Hagfors T. , Grill M. , Honary F.
Published: December 2003

In striving for ever greater angular resolution in riometer (relative ionospheric opacity meter) observations it is tempting to try to use a nonfilled array to reduce the cost of antenna elements. We analyze as an example the Mills Cross configuration, compare its performance as a riometer with that of a filled square array of the same angular resolution and inquire about the penalty of using a sparsely filled array. We find that the susceptibility to interference from sources in the sidelobes is greater than in the filled array, that strong point sources in the sidelobes can cause both positive and negative excursions in the output and that the noise performance is inferior and requires longer integration times for a given rms error than the filled array.
Storm sudden commencement events and the associated geomagnetically induced current risks to ground‐based systems at low‐latitude and midlatitude locations
Authors: Kappenman John G.
Published: December 2003

Large impulsive geomagnetic field disturbances from auroral current systems have always been well understood as a concern for power grids in close proximity to these disturbance regions, predominantly at high‐latitude locations. Magnetospheric shocks (SSCs) due to large‐scale interplanetary pressure pulses are familiar from a geomagnetic disturbance perspective but have not been understood in the context as a potential driver for large geomagnetically induced currents (GICs). Observational evidence and analysis contained in this paper illustrate such events are capable of producing large geoelectric fields and associated GIC risks at any latitude, even equatorial locations. A large SSC disturbance on 24 March 1991 produced some of the largest GICs ever measured in the United States at midlatitude locations. The analysis methods and understanding of electromagnetic coupling processes at that time were unable to fully explain these observations. Electrojet‐driven disturbances common at high‐latitude locations during geomagnetic substorms cause large amplitude variations in locally observed B field, while SSC events are characterized as low‐amplitude B field disturbance events. Disturbance amplitude only accounts for part of the electromagnetic coupling process. The attribute of spectral content of the disturbance is equally important and heretofore had not been well understood and was not well measured unless high‐cadence observations were conducted. The deep‐earth ground conductivity also provides an important enabling role at higher frequencies. Deep‐earth ground response to geomagnetic field disturbances is highly frequency‐dependent. Therefore for nearly all ground conditions the higher the spectral content of the incident magnetic field disturbance, the higher the relative geoelectric field response.
Kinematic dispersion effects of hillslope velocities
Authors: Saco P. M. , Kumar P.
Published: January 2004

When the flow parameters, such as celerity and hydrodynamic coefficient, are allowed to vary spatially within a basin, three mechanisms, namely, geomorphologic, kinematic, and hydrodynamic dispersion, contribute to the variance of the instantaneous response function. The relative contributions of the three dispersion mechanisms as a function of scale, or Strahler order of the basin, were studied earlier by Saco and Kumar [2002a, 2002b]. In this study we investigate the main mechanisms that are responsible for the variance when we take into account the hillslope dynamics. We use an approach similar to that derived by Saco and Kumar [2002a, 2002b], but for width functions, to compute the relative contributions of geomorphologic and kinematic dispersion due to hillslope celerities. We find that the kinematic dispersion due to hillslopes does not tend to reinforce the effect of geomorphologic dispersion; rather, it tends to counteract it. This means that the effect of hillslopes tends to “decrease” the variance induced by the geomorphology of the network. For hillslope celerities that are very small compared with channel celerities, the travel times in the hillslopes begin to be of the same order of magnitude as those in the network. If the celerity continues to decrease, all the variance is induced by the travel times at the hillslopes and the impact of geomorphologic dispersion becomes negligible.
Lattice‐Boltzmann simulation of two‐phase flow in porous media
Authors: Pan C. , Hilpert M. , Miller C. T.
Published: January 2004

We simulate two‐fluid‐phase flow at the pore scale using a lattice Boltzmann (LB) approach. Using a parallel processing version of the Shan‐Chen model that we developed, we simulate a set of ideal two‐fluid systems and a model two‐fluid‐phase porous medium system comprised of a synthetic packing with a relatively uniform distribution of spheres. We use the set of ideal two‐phase systems to validate the approach and provide parameter information, which we then use to simulate a sphere‐pack system. The sphere‐pack system is designed to mimic laboratory experiments conducted to evaluate the hysteretic capillary pressure saturation relation for a system consisting of water, tetrachloroethylene, and a glass bead porous medium. Good agreement is achieved between the measured hysteretic capillary pressure saturation relations and the LB simulations when comparing entry pressure, displacement slopes, irreducible saturation, and residual entrapment. Our results further show that while qualitatively similar results are obtained when comparing systems consisting of 1200 spheres and 150 spheres, there is a significant difference between these two levels, suggesting a lower bound on the size of a representative elementary volume.
Influence of spatial correlation of fracture centers on the permeability of two‐dimensional fracture networks following a power law length distribution
Authors: de Dreuzy Jean‐Raynald , Darcel Caroline , Davy Philippe , Bour Olivier
Published: January 2004

Observations of outcrops of fractured media show that fractures are correlated at all scales of the medium and that there are also fractures of sizes ranging from the microscale to the macroscale. Recent studies have shown that the correlation pattern is often a fractal characterized by its dimension Dc and that the fracture length distribution is a power law of characteristic exponent −a. We study the influence of these long‐range heterogeneities on the equivalent medium permeability. Fracture correlation and fracture length are present at all scales of the medium and may thus influence the hydraulic properties and their scaling at all scales. We have numerically investigated the effect of the fractal correlation pattern and of the power law length distribution on the equivalent permeability of fracture networks. The correlation and the length distribution have opposite effects on connectivity and permeability. Increasing correlation lets connectivity and permeability decrease and conversely for the length distribution. In most cases, one of the characteristics (i.e., length or correlation) supersedes the other. There is only a small area defined by a > 2 and a ≤ Dc + 1 where both characteristics have a simultaneous influence on permeability. In this area the influence of the correlation remains low, more precisely a scale increase of at least 5 orders of magnitude is required to produce a permeability increase of 1 order of magnitude. Although the correlation pattern does not have much influence on the equivalent permeability, its influence on the flow pattern increases from negligible for networks at threshold to important for dense networks.
A geostatistical approach to recover the release history of groundwater pollutants
Authors: Butera Ilaria , Tanda Maria Giovanna
Published: December 2003

In this paper the problem of recovering the temporal release history of a pollutant is approached with a geostatistical methodology that analyzes the pollutant concentration measured at a given time in the aquifer. The adopted methodology was developed by Snodgrass and Kitanidis [1997] for one‐dimensional flow and transport. Here it is extended to the case of two‐dimensional transport and additional improvements are carried out, with important consequences on technical applications. A literature numerical case study is used to assess the quality of the results and the performance of the algorithms with regard to (1) the plume sampling scheme, (2) the impact of concentration measurement errors, (3) the impact of errors on the estimated aquifer parameters, and (4) the erroneous identification of the hydraulic gradient direction. The new applications focus on the incorporation of nonpoint and multiple sources in order to quantify the relative legal liability of the different sources. The results of the numerical analysis show that the method provides a reasonable description of the release history and associated estimate error variance.