bluuebunny/crossref_metadata_2025 huggingface dataset:
Diffuse porous melt flow and melt‐rock reaction in the mantle lithosphere at a slow‐spreading ridge: A structural petrology and LA‐ICP‐MS study of the Othris Peridotite Massif (Greece)
Authors: Dijkstra Arjan H. , Barth Matthias G. , Drury Martyn R. , Mason Paul R. D. , Vissers Reinoud L. M.
Published: August 2003

Harzburgites and plagioclase‐peridotites from the Othris Peridotite Massif in Central Greece preserve microstructural and petrological evidence for interaction with a melt which became saturated in orthopyroxene while migrating by km‐scale diffuse porous flow through the thermal boundary layer (TBL) and the base of the thermal lithosphere. The melt precipitated orthopyroxene, and eventually also plagioclase and clinopyroxene within the peridotites. Major and trace element geochemistry suggests that the melt was a depleted melt, i.e., a melt fraction from the melting column underneath a spreading centre produced by shallow melting of refractory peridotites. We see no evidence for the presence of boninitic melts. We argue that the melts in Othris migrated by diffuse porous flow as they crystallised orthopyroxenes and were therefore inherently unable to create their own high‐permeability melt channels. We propose that depleted melt fractions can remain isolated from deeper melt fractions, possibly already aggregated into a MORB‐like magma, because they migrate by different mechanisms through the TBL and the lithosphere.
Irregular recurrence of paleoearthquakes along the central Garlock fault near El Paso Peaks, California
Authors: Dawson Timothy E. , McGill S. F. , Rockwell T. K.
Published: August 2003

New investigations at the El Paso Peaks paleoseismic site have refined and extended the record of paleoearthquakes for the central Garlock fault. Event evidence, in the form of buried fissures, scarps, folds, and upward terminations, has allowed us to identify six well‐resolved earthquakes, designated as events W, U, R, Q, K, and F, within the last ∼7000 years. The exposures that we excavated did not reveal additional evidence for three poorly resolved events reported by McGill and Rockwell [1998]. Evidence for event Y, identified in the previous study, was reexamined and found not to be a paleoearthquake. Radiocarbon dates of detrital charcoal, combined with a method of interpolating the event ages with revised sedimentation rates, provide our preferred constraints on the timing of faulting events. The most recent surface‐rupturing earthquake, event W, occurred between A.D. 1450 and 1640. The penultimate event U occurred between A.D. 675 and 950. Event R, which was unrecognized in the previous study, occurred between A.D. 250 and 475. Event Q occurred relatively shortly before event R, between A.D. 25 and 275. Event K occurred between 3340 and 2930 B.C. The oldest identified paleoearthquake, event F, occurred between 5300 and 4670 B.C. The event ages indicate that earthquake recurrence is highly irregular at the El Paso Peaks site, with individual preferred intervals ranging from as little as 215 years to as many as 3300 years. The irregular recurrence of surface‐rupturing earthquakes along the central Garlock fault may be related to temporal and spatial clustering of earthquakes proposed for faults in the Eastern California Shear Zone.
Tectonic nature and seismic response of top‐basement detachment faults in magma‐poor rifted margins
Authors: Hölker Andreas B. , Manatschal Gianreto , Holliger Klaus , Bernoulli Daniel
Published: August 2003

Top‐basement detachment faults (TBDF) represent exhumed segments of downward‐concave detachments accommodating tens of kilometers of offset. TBDFs are key structures of magma‐poor rifted margins, but are difficult to identify in seismic profiles, because they coincide with top‐basement reflections. We have constructed realistic seismic models of an Alpine example, which were used to simulate full marine seismic surveys and to identify the seismic fingerprint of TBDFs in CMP‐stacked and time‐migrated sections. Minor extensional allochthons, which locally cap TBDFs, give rise to pronounced diffractions, which are in stark contrast to continuous reflections from uncapped segments of TBDFs. In time‐migrated synthetic sections we find a continuous and constant amplitude reflection alternating with a laterally discontinuous reflection of variable amplitude. These diagnostics were also identified in the Lusigal 12 seismic profile imaging the Hobby High detachment, a TBDF in the Iberia Abyssal Plain. Using these diagnostics, small extensional allochthons and high‐angle faults cutting the Hobby High detachment are observed in Lusigal 12 and the entire sedimentary cover can be shown to be postrift.
Upper mantle structure beneath eastern Siberia: Evidence from gravity modeling and mantle petrology
Authors: Poudjom Djomani Yvette H. , O'Reilly Suzanne Y. , Griffin W. L. , Natapov L. M. , Erinchek Y. , Hronsky J.
Published: August 2003

The spatial distribution of large‐scale lithospheric domains and the boundaries between them may control the emplacement of large ore bodies, and as such, regional mapping of the lithosphere is relevant to mineral exploration. In this study we combine potential‐field geophysical data and mantle petrology to map major lithospheric structures on the eastern part of the Siberian platform. The platform consists of several Archean and Proterozoic terranes that have been mapped from regional magnetic data and basement exposures in the Anabar shield. We use garnet and chromite concentrates from a chain of Paleozoic to Mesozoic kimberlites across the platform to construct mantle sections, which show significant lateral variation in rock type distribution within the lithospheric mantle. These lateral variations correspond to the terranes mapped at the surface and indicate that the terrane boundaries are translithospheric. Archean terranes are underlain by depleted Archean lithosphere more than 200 km thick, while the Proterozoic terranes are underlain by thinner and less depleted lithosphere. Geophysical data show more strongly negative Bouguer anomalies and a more heterogeneous magnetic anomaly pattern over the Archean terranes than on the Proterozoic terranes. The pattern of the gravity data reflects the lateral variation in mantle composition beneath the terranes, as shown by mantle‐petrology studies. We invert gravity and topography data to estimate the flexural strength, or elastic thickness (Te), of the lithosphere across the area. Although on a stable Precambrian craton, the Te is relatively low (<30 km) across most of the area, suggesting a relatively weak lithosphere comparable to that of tectonically much younger areas around the world. A 150‐km‐wide zone of very weak lithosphere (Te < 10 km) runs N‐S across the western part of the study area. This weak zone coincides with a zone of thickened lower crust, and abnormally high sub‐Moho P wave velocities which suggest anisotropy in the upper mantle. The kimberlite fields in the Archean part of the platform are localized on the flanks of this zone of weak lithosphere. We suggest that the low‐Te zone may be a mantle shear zone which has been a preferred conduit for the emplacement of magmas into the lower crust and later has controlled the emplacement of kimberlites in the study area.
Lognormal distribution of precipitable water in Hawaii
Authors: Foster James , Bevis Michael
Published: August 2003

We use four‐year time series of precipitable water (PW) and zenith neutral delay (ZND) derived from a GPS network in Hawaii to show that the statistical distributions of these quantities are closely approximated by the lognormal distribution. The long term average and median values of precipitable water decline exponentially with height, or very nearly so. The arithmetic standard deviation of PW declines nearly linearly with height, whereas the geometric standard deviation increases nearly linearly (but relatively weakly). Our finding that PW has, to a good approximation, a lognormal distribution between sea level and 4 km elevation is confirmed by an analysis of radiosonde profiles. Lognormality is a common property of other meteorological quantities such as precipitation, aerosol optical and cloud distributions. This is the first time, as far as we know, that PW has also been shown to have a lognormal distribution.
Electromagnetic wave structures within subauroral polarization streams
Authors: Mishin E. V. , Burke W. J. , Huang C. Y. , Rich F. J.
Published: August 2003

We report on oscillations in electric (δEY) and magnetic (δBZ) fields and plasma density (δNi) observed by Defense Meteorological Satellite Program (DMSP) satellites within fast subauroral convection streams in the evening sector during the magnetic storm of 6 November 2001. There are two types of wave phenomena. The first and more common is characterized by electromagnetic and plasma density variations that have the same frequency range of ∼0.15 Hz in the spacecraft frame of reference. The second is characterized by large‐amplitude plasma and field oscillations over a broader range of frequencies ∼0.1 to 0.3 Hz. In this case the perturbation densities and fields appear to have different frequency responses. In this and other magnetic storms, strong waves are associated with the precipitation of ∼30 keV ions. Ratios of δEY/δBZ indicate encounters with mixtures of electromagnetic (in part Alfvénic) and electrostatic modes. Poynting vectors associated with the oscillations can be directed either into or out of the ionosphere. The density perturbations appear to be extended east‐west corrugations in the plasma flow streams with north‐south wavelengths of ∼50 km. The δEY and δNi variations were anticorrelated, as required for current conservation. Our analysis shows that Alfvénic perturbations are consistent with expected effects of irregular potential distribution around ionospheric density irregularities mapped to the magnetosphere. Inertial currents act to generate mesoscale field‐aligned currents carried by Alfvén waves, as was previously discussed with regards to auroral arcs formation. We suggest that δNi irregularities observed by DMSP satellites in the evening sector began as striated plasma patches in the polar cap that convected to subauroral latitudes.
A physical appraisal of a new aspect of bradyseism: The miniuplifts
Authors: Gaeta F. S. , Peluso F. , Arienzo I. , Castagnolo D. , De Natale G. , Milano G. , Albanese C. , Mita D. G.
Published: August 2003

The Campi Flegrei caldera is characterized by well known episodes of fast uplift, called “bradyseism,” the last of which produced ∼1.8 m of maximum vertical displacement in the period 1982–1984. Less known is a particular aspect of Campi Flegrei unrests, detected only in the last 20 years, namely, the occurrence of “miniuplift” episodes, characterized by relatively small positive vertical displacements (in the range 0.03–0.11 m). Miniuplift episodes occurred in 1989, 1994, and 2000 and, probably, one or two times in the period 1972–1982. These small ground uplifts were also accompanied by moderate seismicity. Each of them was followed by a short phase of relatively quick subsidence, after which the previous rate of subsidence of the period was resumed. An order‐of‐magnitude analysis of the forces, energy, and power that can cause the onset and determine the evolution of these events is derived on the basis of a thermal‐fluid‐dynamical approach. It allows the quantitative correlation of the power input from the energy source with the rate of energy dissipation in the geologic system during the miniuplifts. In order to assess the likelihood of our fluid‐dynamical model for these episodes, theoretical ground displacements have been computed. These displacements result from the changes in pressure as a function of depth predicted by the fluid‐dynamical model for reasonable perturbations of the geothermal system, described as changes of the Péclet number. Theoretical results agree extremely well with the observed amounts of the miniuplifts.
Structure of the Lesser Antilles subduction zone backstop and its role in a large accretionary system
Authors: Bangs Nathan L. , Christeson Gail L. , Shipley Thomas H.
Published: August 2003

The role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies; however, few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction. A seismic reflection and refraction survey acquired in 1998 with the R/V Maurice Ewing reveals the geometry of the forearc igneous crust, accretionary wedge, and forearc basin structure of the northern Guadeloupe area of the Lesser Antilles forearc. An accreted block of buoyant crust, accreted in the late Miocene, forms the toe of the overriding arc crust and forms the backstop. We imaged the top of this surface, beneath the forearc basin, to its seaward edge where it meets the subducting oceanic crust. The toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge. The steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop, and a narrow <10 km transition between accreted and forearc basin sediment. Seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe. Separating the decollement and the subducting crust is an interval, usually between 500 and 750 m, of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop. We speculate that the upturned geometry of the toe of the backstop and a weak fluid‐rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle.
Thermal diffusivity of upper mantle rocks: Influence of temperature, pressure, and the deformation fabric
Authors: Gibert Benoit , Seipold Ulfert , Tommasi Andréa , Mainprice David
Published: August 2003

Thermal diffusivity measurements of seven naturally deformed upper mantle rocks were made as a function of pressure (up to 1 GPa), temperature (up to 1250 K), and the deformation fabric of the samples. For each sample the strain‐induced crystal preferred orientations of olivine and pyroxenes were measured, and petrophysical models, based on the thermal diffusivity tensors of the olivine and enstatite crystals, were used to evaluate the three‐dimensional distribution of the thermal diffusivity. Both model predictions and measurements show that the anisotropy of thermal diffusivity remains large at the rock scale: 15–28%, depending on the strength of the olivine crystallographic fabric. The direction of maximum thermal diffusivity is parallel to the lineation (flow direction), and the minimum of thermal diffusivity is normal to the foliation plane (flow plane). This anisotropy is preserved at high temperature and pressure. However, measured thermal diffusivities are 20–30% lower than model predictions. This discrepancy between measurements and model predictions cannot be explained by the presence of cracks in the samples because the closure of these void spaces, evaluated through the high‐pressure experiments, is found to have a negligible effect on measured thermal diffusivities. Thermal diffusivity for all samples displays a weak linear dependence on pressure of ∼10% GPa−1. Thermal diffusivities observed in the high‐temperature experiments (1000–1250 K) are compatible with a weak radiative contribution to the total heat diffusion.
Scandinavia's North Atlantic passive margin
Authors: Mosar Jon
Published: August 2003

The tectonics of the eastern passive margin of the North Atlantic are reexamined. The Scandinavian North Atlantic passive margin includes not only the offshore exploration and basin domain but also large portions of the onshore domains of the Scandinavian Caledonides. Combined information from structural geology, potential field data, regional geology, basin development, and geomorphology made it possible to propose a new definition of the passive margin. The rift shoulder is formed by linked faults defining the present rift flank. These faults are the extensional structures located furthest to the east onshore Norway/Sweden that can be linked to the rifting that led to the formation of the North Atlantic. This innermost boundary fault system (IBF) is formed by a set of normal west dipping crustal faults. It extends over a distance exceeding 2000 km from the North Sea, across the Caledonian mountain belt to the Barents Sea. The passive margin width, between the continent‐ocean boundary and the IBF, ranges from 550 km in the south to over 700 km in mid‐Norway to 165 km north of Lofoten. Rifting and faulting on the IBF started in Permo‐Carboniferous, and a succession of rift phases eventually culminated in continental breakup and the formation of the North Atlantic. Basin development between Permian and Cretaceous was toward the future breakaway fault, but faulting was also active on land from Mesozoic to Present. Onshore‐offshore crustal‐scale cross sections show the geometry of the passive margin, the dip orientation of the major faults and the changes in crustal thickness.
Extensional collapse of the Tibetan Plateau: Results of three‐dimensional finite element modeling
Authors: Liu Mian , Yang Youqing
Published: August 2003

Following their initial collision 50–70 Myr ago, the Indian and Eurasian plates have been continuously converging toward each other. Whereas the regional stress field is predominately compressive, the Late Cenozoic tectonics within the Tibetan Plateau features widespread crustal extension. Numerous causes of the extension have been proposed, but their relative roles remain in debate. We have investigated the major factors contributing to the Tibetan extension in a three‐dimensional viscoelastic model that includes both lateral and vertical variations of lithospheric rheology and relevant boundary conditions. Constrained by the present topography and GPS velocity field, the model predicted predominately extensional stress states within the plateau crust, resulting from mechanical balance between the gravitational buoyancy force of the plateau and the tectonic compressive stresses. The predicted stress pattern is consistent with the earthquake data that indicate roughly E‐W extension in most of Tibet and nearly N‐S extension near the eastern margin of the Tibetan Plateau. We explored the parameter space and boundary conditions to examine the stress evolution during the uplift of the Tibetan Plateau. When the plateau was lower than 50% of its present elevation, strike‐slip and reverse faults were predominate over the entire plateau, and no E‐W crustal extension was predicted. Significant crustal extension occurs only when the plateau has reached ∼75% of its present elevation. Basal shear associated with underthrusting of the Indian plate beneath Tibet may have enhanced crustal extension in southern Tibet and the Himalayas, and a stronger basal shear during the Miocene may help to explain the development of the South Tibetan Detachment System.
Shear velocity structure and azimuthal anisotropy beneath eastern North America from Rayleigh wave inversion
Authors: Li Aibing , Forsyth Donald W. , Fischer Karen M.
Published: August 2003

We have obtained shear velocity structure beneath the northeastern United States and southeastern Canada using Rayleigh wave phases and amplitudes. Thin crust (36–42 km) is observed along the Atlantic coast and in the eastern Appalachian orogen, and thick crust (42–46 km) is imaged in the western Appalachians and in the western New York portion of the Grenville Province. The variation of crustal thickness correlates well with the observed Bouguer gravity anomalies. In the upper mantle, the high‐velocity continental keel of cratonic North America is present in the western part of the study area, while a broad low‐velocity region is imaged in New England from the Hudson River valley to the White Mountains. This low‐velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the Monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel. In addition to lateral variations in velocity, we estimate the azimuthal dependence of phase velocity. Strong and relatively uniform shear wave splitting is observed in the study region, but at periods of 100 s or less, the average azimuthal anisotropy of Rayleigh waves is less than 1% and is not significantly different from zero at any individual period. This small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere. Much of the source of the shear wave splitting must lie deeper than 200 km.
Interannual variability and trend of CH4 lifetime as a measure for OH changes in the 1979–1993 time period
Authors: Dentener Frank , Peters Wouter , Krol Maarten , van Weele Michiel , Bergamaschi Peter , Lelieveld Jos
Published: August 2003

The interannual variability and trend in the CH4 lifetime, as a measure for global mean OH concentration, have been analyzed systematically with three‐dimensional (3‐D) chemistry‐transport model simulations. It is shown that the global mean OH concentration is highly variable from year to year due to changes in meteorology, changes in tropospheric UV radiation intensities, and changes in chemical concentrations owing to variable emissions of photochemical precursor gases (CH4‐CO‐NMVOC‐NOx). The meteorological variability is taken into account with the ECMWF‐ERA15 1979–1993 reanalysis. Satellite observations provide the observed changes in the stratospheric ozone concentrations. Emission inventories are used to account for trends in anthropogenic emissions and their patterns. For the period 1979–1993, our simulations indicate a decrease of the calculated global tropospheric methane lifetime from 9.2 to 8.9 years, corresponding to a positive OH trend of 0.24 ± 0.06% yr−1. The modeled trend is mostly determined by changes in the tropical tropospheric water vapor content, while the changes in photolysis rates and in surface emissions of reactive trace gases compensate in their effect on the calculated OH trend over the analyzed time period.
Modeling study of strong acids formation and partitioning in a polluted cloud during wintertime
Authors: Leriche Maud , Deguillaume Laurent , Chaumerliac Nadine
Published: August 2003

A multiphase chemistry model coupled with a quasi‐spectral microphysical model has been applied to measurements from the European Cloud Ice Mountain Experiment campaign to quantify the formation of the strong acids nitrate and sulfate and to evaluate the role of microphysical processes in redistributing reactive species among the different phases (gas versus cloud and/or rain). Significant formation of nitrate and sulfate are found to be due to the reaction of pernitric acid with the sulfite ion. Moreover, pernitric acid, because of its equilibrium in the gas phase and its high solubility, is always available both in cloud water and in rainwater via mass transfer from the gas phase. The sulfite ion comes from the mass transfer from the gas phase of sulfur dioxide in cloud water. When rain formation begins, it is efficiently transferred to the rainwater by collision/coalescence processes. This leads to an enhancement in strong acid production when microphysics is activated in the model. Modeled results have been compared with experimental data in an effort to retrieve a behavior law related to the partitioning between the gas and aqueous phases of the cloud. In particular, when collision/coalescence processes are considered, an improvement in retrieving the partitioning of soluble species and especially nitrate is observed. A higher production in sulfate could help interpret the discrepancy of global model calculations with observed sulfate concentrations in Europe in wintertime.
Relationships between soil moisture and summer precipitation over the Great Plains and the Southwest
Authors: Mo Kingtse C. , Juang Hann‐Ming Henry
Published: August 2003

The relationships between evaporation (E) and summer precipitation (P) over the United States are examined using the observed precipitation data set, the National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) reanalysis and the NCEP regional spectral model (RSM) simulations. The composites of P anomalies based on soil moisture and E anomalies indicate that the relationships between E and P are regionally dependent. The E anomalies over the Great Plains are associated with large P anomalies located north of the E anomalies and areas downstream along the path of the Great Plains low‐level jet (GPLLJ). The impact of E anomalies on P over the Southwest is small and is only recognized during the strong moisture surge events from the Gulf of California. The vertically integrated meridional moisture fluxes [qv] associated with the GPLLJ are stronger and more persistent than the fluxes associated with the low‐level jet from the Gulf of California to the Southwest (GCLLJ). The E anomalies over the Great Plains are stronger and more persistent than the E anomalies over the Southwest. Therefore the E anomalies over the Great Plains have a better chance for the local feedback and for the changes in moisture flux convergence to take effect. The impact of E anomalies on P over the Southwest is small and only recognized during strong moisture surge events from the Gulf of California. Both the surge events and E anomalies over the Southwest persist for less than one week. The time window for the large E anomalies to occur during surge events is narrow. Therefore the net impact is small.
Post‐Mount Pinatubo eruption ground‐based infrared stratospheric column measurements of HNO3, NO, and NO2 and their comparison with model calculations
Authors: Rinsland Curtis P. , Weisenstein D. K. , Ko M. K. W. , Scott C. J. , Chiou L. S. , Mahieu E. , Zander R. , Demoulin P.
Published: August 2003

Infrared solar spectra recorded between July 1991 to March 1992 and November 2002 with the Fourier transform spectrometer on Kitt Peak (31.9°N latitude, 111.6°W longitude, 2.09 km altitude) have been analyzed to retrieve stratospheric columns of HNO3, NO, and NO2. The measurements cover a decade time span following the June 1991 Mount Pinatubo volcanic eruption and were recorded typically at 0.01 cm−1 spectral resolution. The measured HNO3 stratospheric column shows a 20% decline from 9.16 × 1015 molecules cm−2 from the first observation in March 1992 to 7.40 × 1015 molecules cm−2 at the start of 1996 reaching a broad minimum of 6.95 × 1015 molecules cm−2 thereafter. Normalized daytime NO and NO2 stratospheric column trends for the full post‐Pinatubo eruption time period equal (+1.56 ± 0.45)% yr−1, 1 sigma, and (+0.52 ± 0.32)% yr−1, 1 sigma, respectively. The long‐term trends are superimposed on seasonal cycles with ∼10% relative amplitudes with respect to mean values, winter maxima for HNO3 and summer maxima for NO and NO2. The measurements have been compared with two‐dimensional model calculations utilizing version 6.1 Stratospheric Aerosol and Gas Experiment (SAGE) II sulfate aerosol surface area density measurements through 1999 and extended to the end of the time series by repeating the 1999 values. The model‐calculated HNO3, NO, and NO2 stratospheric column time series agree with the measurements to within ∼8% after taking into account the vertical sensitivity of the ground‐based measurements. The consistency between the measured and model‐calculated stratospheric time series confirms the decreased impact on stratospheric reactive nitrogen chemistry of the key heterogeneous reaction that converts reactive nitrogen to its less active reservoir form as the lower‐stratospheric aerosol surface area density declined by a factor of ∼20 after the eruption maximum.
The University of Bern Atmospheric Ion Model: Time‐dependent modeling of the ions in the mesosphere and lower thermosphere
Authors: Kazil Jan , Kopp Ernest , Chabrillat Simon , Bishop James
Published: August 2003

In this paper the first time‐dependent model of ion chemistry in the mesosphere/lower thermosphere (MLT) region acting within a global, time‐dependent, two‐dimensional neutral atmosphere model is described. Selected diurnal results are presented for undisturbed solar minimum conditions. The University of Bern Atmospheric Ion Model (UBAIM) is a time‐dependent, pseudo‐two‐dimensional model of the ion chemistry in the Earth atmosphere. It covers latitudes from 85°S to 85°N and (log‐pressure) altitudes from 20 to 120 km. On this grid a system of differential equations describing the ion chemistry is integrated numerically until a periodical solution, governed by the diurnal changes in the incident radiation, is reached; this solution constitutes a model for quiet or undisturbed conditions. The basic ion chemistry of the UBAIM contains 311 reactions for 71 charged species. Ionization sources are solar X‐ray and EUV radiation, resonantly scattered Lyman α and β photons, and galactic cosmic rays. Densities of main and trace neutral atmospheric constituents are taken from a new version of the bidimensional NCAR model SOCRATES, which has been specifically optimized for mesospheric and lower thermospheric processes with upper boundary conditions set using the empirical MSIS thermosphere model. Direct solar flux inputs are computed by the SOLAR2000 model; scattered Lyman α and β fluxes are calculated using geocoronal hydrogen density profiles consistent with the adopted MSIS density distributions.
Exposing the Sunda shelf: Tropical responses to eustatic sea level change
Authors: Bush Andrew B. G. , Fairbanks Richard G.
Published: August 2003

During the last glacial period, eustatic lowering of sea level exposed large areas of continental shelves that are presently submerged. By a series of direct numerical simulations we investigate the sensitivity of the atmosphere‐ocean system to exposure of the extensive Sunda shelf in the western tropical Pacific. We demonstrate that this specific forcing factor triggers increased convection and upper level divergence over the shelf itself, an increased Walker circulation over the far western Pacific, and subsidence over the central and eastern Pacific that weakens the surface easterlies. A comparison of atmosphere‐only with coupled atmosphere‐ocean integrations demonstrates that dynamic coupling of the ocean enhances the equatorial response, with cooling and drying of the tropics north and south of the Intertropical Convergence Zone. Exposure of the shelf therefore represents one mechanism by which glacial climates are affected by eustatic sea level changes. These changes constitute a fundamental shift in the El Niño background state.
Modeling the formation of secondary organic aerosol in coastal areas: Role of the sea‐salt aerosol organic layer
Authors: Cai Xuyi , Griffin Robert J.
Published: August 2003

In recent models the formation of secondary organic aerosol has been described by absorption of semivolatile oxidation products to an organic aerosol mixture and/or dissolution into an aerosol aqueous phase. In coastal areas, freshly formed sea‐salt aerosol absorbs secondary semivolatile organic oxidation products into its surface organic layer, which consequently allows for the partitioning of the secondary organics to the sea‐salt aerosol brine core. This process is modeled in the present study. Equilibrium between the gas phase and a continental primary organic aerosol phase, a sea‐salt organic layer, and a continental aqueous aerosol is maintained, as is equilibrium between the organic layer and the brine core. Within aqueous phases, dissociation of acidic species is considered, and charge balance determines pH. Six semivolatile organic oxidation products that have been observed as secondary organic aerosol constituents, eight primary organic aerosol constituents representative of urban aerosols, five sea‐salt organic layer species, and four organic components of the brine core are described molecularly. Numerical iteration techniques are used to solve the relevant nonlinear governing equations for the concentrations of each product in each aerosol phase and the gas phase, as well as for the pH of the relevant aqueous phases. Variables of interest include ambient temperature and species concentrations. Results indicate that the aqueous aerosols and the sea‐salt aerosol brine core are the most efficient media for absorption under the conditions described, that organic layers enhance the partitioning to the brine core, and that secondary organic acids contribute to the acidification of the sea‐salt aerosol brine core.
Extreme NAT supersaturations in mountain wave ice PSCs: A clue to NAT formation
Authors: Luo B. P. , Voigt C. , Fueglistaler S. , Peter T.
Published: August 2003

There is evidence for the existence of nitric acid trihydrate (NAT) in polar stratospheric clouds (PSCs) and for its role in sedimentary denitrification enhancing chlorine‐catalyzed ozone reduction. However, the scientific understanding of NAT formation mechanisms is vague. We show that high cooling rates in mountain waves, though causing only moderate NAT saturation ratios (SNAT ≲ 30) within supercooled ternary solution (STS) droplets, lead to SNAT ≳ 500 within the gas phase. Evidence from previous laboratory measurements suggests that such extreme supersaturations promote NAT deposition nucleation on ice or other solid surfaces, as long as they are not coated by STS. On the basis of airborne lidar PSC observations in three Arctic winters and a combined microphysical/optical model, we derive a simple parameterization for a NAT deposition nucleation rate which depends only on NAT supersaturation and temperature. The results indicate that deposition nucleation of NAT on ice surfaces exposed directly to the gas phase driven by extreme NAT supersaturations is able to explain the formation of NAT particle number densities up to ∼10 cm−3.
Sensitivity of single column model simulations of Arctic springtime clouds to different cloud cover and mixed phase cloud parameterizations
Authors: Zhang Junhua , Lohmann Ulrike
Published: August 2003

The single column model of the Canadian Centre for Climate Modeling and Analysis (CCCma) climate model is used to simulate Arctic spring cloud properties observed during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment. The model is driven by the rawinsonde observations constrained European Center for Medium‐Range Weather Forecasts (ECMWF) reanalysis data. Five cloud parameterizations, including three statistical and two explicit schemes, are compared and the sensitivity to mixed phase cloud parameterizations is studied. Using the original mixed phase cloud parameterization of the model, the statistical cloud schemes produce more cloud cover, cloud water, and precipitation than the explicit schemes and in general agree better with observations. The mixed phase cloud parameterization from ECMWF decreases the initial saturation specific humidity threshold of cloud formation. This improves the simulated cloud cover in the explicit schemes and reduces the difference between the different cloud schemes. On the other hand, because the ECMWF mixed phase cloud scheme does not consider the Bergeron‐Findeisen process, less ice crystals are formed. This leads to a higher liquid water path and less precipitation than what was observed.
Ozone production efficiency and NOx depletion in an urban plume: Interpretation of field observations and implications for evaluating O3‐NOx‐VOC sensitivity
Authors: Zaveri Rahul A. , Berkowitz Carl M. , Kleinman Lawrence I. , Springston Stephen R. , Doskey Paul V. , Lonneman William A. , Spicer Chester W.
Published: August 2003

Ozone production efficiency (OPE) can be defined as the number of ozone (O3) molecules photochemically produced by a molecule of NOx (NO + NO2) before it is lost from the NOx − O3 cycle. Here we consider observational and modeling techniques to evaluate various operational definitions of OPEs using aircraft and surface measurements taken as part of the 1999 Southern Oxidant Study field campaign in Nashville, Tennessee. A key tool in our analysis is a Lagrangian box model, which is used to quantitatively describe the effects of emissions, dilution, dry deposition, and photochemistry in an urban air parcel as it was advected downwind. After evaluating the model using the observed downwind concentrations of several key species, we show that the modeled NOx oxidation and O3 production rates as well as the associated instantaneous and cumulative OPEs depend on the time of day and the photochemical age of the air parcel. The observation‐based OPEs are found to be consistent with the modeled values with the expected biases. A model sensitivity study suggests that downwind O3 concentrations in the Nashville plume are more sensitive to NOx emissions than anthropogenic VOC emissions. Because the OPE exhibits a nonlinear dependence on emissions and meteorological effects, it would be difficult to rely only on observations to map out the nonlinear response of O3 to a wide span of NOx and VOC emission changes. Properly constrained and well‐evaluated models using a variety of observations are therefore necessary to reliably predict O3‐NOx‐VOC sensitivity for designing effective O3 control strategies.
Monte Carlo uncertainty and sensitivity analysis of the CACM chemical mechanism
Authors: Rodriguez Marco A. , Dabdub Donald
Published: August 2003

This paper presents a global uncertainty and sensitivity analysis of the Caltech Atmospheric Chemistry Mechanism (CACM). Emphasis is placed on the characterization of uncertainties for product concentrations that constitute secondary organic aerosol (SOA) in CACM. The sensitivity analysis of the chemical mechanism is performed using Monte Carlo techniques combined with Latin hypercube sampling. Uncertainties in rate parameters are propagated through box model simulations with CACM for three different summer cases. Cases studied cover a range of initial concentrations of reactive organic gases and nitrogen oxides that represent episodes of high ozone levels in polluted urban areas. In addition to estimated uncertainties of gas‐phase SOA precursor concentrations, similar calculations are performed for O3, HCHO, H2O2, and peroxyacetyl nitrate (PAN). Results indicate that SOA precursor concentrations predicted using nominal CACM rate parameters are similar to estimates from the Monte Carlo simulations. SOA gas‐phase precursors in CACM present relative errors that range from 30% at a VOC:NOx ratio of 8:1 to 39% when the ratio changes to 32:1.
Observed variability of North Atlantic oceanic precipitating systems during winter
Authors: Chaboureau Jean‐Pierre , Claud Chantal
Published: August 2003

The variability of large precipitating weather systems as observed from polar satellites over the North Atlantic Ocean is investigated using a statistical analysis. Nine winters (from 1987 to 1995) have been considered. Cloud systems and troughs over the area 40–60°N, 70–10°W are automatically detected with retrievals of the cloud top pressure, a precipitation index, and the temperature of the lower stratosphere. A classification of the largest precipitating systems as characterized by these variables leads to eight classes, whose occurrence significantly differs over the years. It also shows the systematic presence of a trough upstream of the precipitating area covered by high‐level clouds, as expected in the case of baroclinic interaction. In order to understand the large interannual variability an attempt to identify systematic differences in cyclone structures during different flow regimes is then performed. It shows that the large‐scale (typically 3000 km wide) and zonally elongated cloud systems are observed mainly to the south of 45°N when the phase of the Arctic Oscillation (AO) is negative (which also corresponds to the Greenland Anticyclone regime). Conversely, zonally elongated cloud systems of smaller scale (1000 km wide) associated with frontal waves are favored also to the south but during the positive AO phase (particularly during zonal regimes). The most tilted weather systems, without any preferential AO phase, are found mostly during the blocking regime. These systems are cyclonically tilted in the northeastern branch of the storm track and anticyclonically tilted in the southwestern branch. It must be noted that similar results are obtained when the AO daily index is replaced by the North Atlantic Oscillation index. This analysis gives observational evidence of previous idealized simulations linking the large‐scale circulation to preferential life cycles of weather systems. In addition, a weak response to extreme El Niño Southern Oscillation events has been observed in the location of cyclones but not in their average structure.
Retrieval of sea surface wind vectors from simulated satellite microwave polarimetric measurements
Authors: Liu Quanhua , Weng Fuzhong
Published: August 2003

An algorithm is developed to retrieve sea surface wind vectors from satellite microwave polarimetry. The radiative transfer model produces simulated measurements under global atmospheric and surface conditions. The simulations for wind speeds less than 5 m s−1 are not utilized in the retrievals because of the unreliable performance of the current surface emissivity model. In the algorithm, the upwelling and downwelling radiative components are treated as one variable since the two components are highly correlated. As a result, under rain‐free conditions, the full polarimtric measurements obtained at a single frequency can form a closure of a set of radiative transfer equations so that both surface and atmospheric column parameters can be simultaneously derived. The first guess to the solutions is normally required because the retrieval is nonlinear. In doing so, the third and fourth Stokes components and their ratio are utilized to estimate the wind direction. It is found that this ratio alone can determine the wind direction with an accuracy of 30 degrees. Without the sensor noises being added to the simulations, the algorithm can produce the wind direction with an RMS error of 6.5 degrees and the wind speed with an error of 0.3 m s−1, respectively. If the realistic random noises are taken into account with 0.1 K for vertical and horizontal polarizations and 0.15 K for the third and fourth Stokes components, the errors increase to about 10 degrees and 0.6 m s−1 for the wind direction and speed, respectively. This wind speed error is much smaller than that from the statistical algorithms (1.4 m s−1) applied for the same data set. Thus the physical retrieval significantly improves the uses of the microwave polarimetric data for remote sensing of ocean wind.
A multichannel radiometric profiler of temperature, humidity, and cloud liquid
Authors: Ware Randolph , Carpenter Richard , Güldner Jürgen , Liljegren James , Nehrkorn Thomas , Solheim Fredrick , Vandenberghe Francois
Published: August 2003

A microwave radiometer is described that provides continuous thermodynamic (temperature, water vapor, and moisture) soundings during clear and cloudy conditions. The radiometric profiler observes radiation intensity at 12 microwave frequencies, along with zenith infrared and surface meteorological measurements. Historical radiosonde and neural network or regression methods are used for profile retrieval. We compare radiometric, radiosonde, and forecast soundings and evaluate the accuracy of radiometric temperature and water vapor soundings on the basis of statistical comparison with radiosonde soundings. We find that radiometric soundings are equivalent in accuracy to radiosonde soundings when used in numerical weather forecasting. A case study is described that demonstrates improved fog forecasting on the basis of variational assimilation of radiometric soundings. The accuracy of radiometric cloud liquid soundings is evaluated by comparison with cloud liquid sensors carried by radiosondes. Accurate high‐resolution three‐dimensional water vapor and wind analysis is described on the basis of assimilation of simulated thermodynamic and wind soundings along with GPS slant delays. Examples of mobile thermodynamic and wind profilers are shown. Thermodynamic profiling, particularly when combined with wind profiling and slant GPS, provides continuous atmospheric soundings for improved weather and dispersion forecasting.
Fourth‐order approximation of fluid flow through rough‐walled rock fracture
Authors: Inoue J. , Sugita H.
Published: August 2003

A stochastic approach for estimating the permeability between rough‐walled rock fracture surfaces is presented. In the present analysis, the Reynolds lubrication equation, which is adopted for analyzing fluid flow through a rough surface of rock fracture, is solved as a stochastic partial differential equation using perturbative expansion method. The results are compared with the numerical deterministic analysis for randomly generated aperture distributions. The analytical results are in a good agreement with numerical results obtained by the Monte Carlo approach. Through the present analysis, a fourth‐order accurate approximation to the effective permeability of a single joint is given in terms of the mean, the standard deviation, and the spatial correlation, so that the effect of higher‐order terms is clarified more accurately than the classical second‐order approximation.
Contaminant migration from an axisymmetric source in a porous medium
Authors: Selvadurai A. P. S.
Published: August 2003

This paper examines the problem of the nonreactive advective transport of a contaminant that is introduced at the boundary of a three‐dimensional cavity contained in a fluid‐saturated nondeformable porous medium of infinite extent. The advective Darcy flow is caused by a hydraulic potential maintained at a constant value at the boundary of the three‐dimensional cavity. In order to develop a generalized solution to the problem the three‐dimensional cavity region is modeled as having either a prolate or an oblate shape. Analytical results are developed for the time‐ and space‐dependent distribution of contaminant concentration in the porous medium, which can also exhibit natural attenuation. The exact closed‐form analytical results are also capable of providing solutions to advective transport problems related to spherical, flat disc‐shaped and elongated needle‐shaped cavities.
A Shuffled Complex Evolution Metropolis algorithm for optimization and uncertainty assessment of hydrologic model parameters
Authors: Vrugt Jasper A. , Gupta Hoshin V. , Bouten Willem , Sorooshian Soroosh
Published: August 2003

Markov Chain Monte Carlo (MCMC) methods have become increasingly popular for estimating the posterior probability distribution of parameters in hydrologic models. However, MCMC methods require the a priori definition of a proposal or sampling distribution, which determines the explorative capabilities and efficiency of the sampler and therefore the statistical properties of the Markov Chain and its rate of convergence. In this paper we present an MCMC sampler entitled the Shuffled Complex Evolution Metropolis algorithm (SCEM‐UA), which is well suited to infer the posterior distribution of hydrologic model parameters. The SCEM‐UA algorithm is a modified version of the original SCE‐UA global optimization algorithm developed by Duan et al. [1992]. The SCEM‐UA algorithm operates by merging the strengths of the Metropolis algorithm, controlled random search, competitive evolution, and complex shuffling in order to continuously update the proposal distribution and evolve the sampler to the posterior target distribution. Three case studies demonstrate that the adaptive capability of the SCEM‐UA algorithm significantly reduces the number of model simulations needed to infer the posterior distribution of the parameters when compared with the traditional Metropolis‐Hastings samplers.