bluuebunny/crossref_metadata_2025 huggingface dataset:
Comment on “Shear‐wave splitting to test mantle deformation models around Hawaii” by Kristoffer T. Walker, Götz H. R. Bokelmann, and Simon L. Klemperer
Authors: Vinnik L. , Montagner J.‐P. , Girardin N. , Dricker I. , Saul J.
Published: July 2003

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Seismic anisotropy in the mantle transition zone beneath Fiji‐Tonga
Authors: Chen Wang‐Ping , Brudzinski Michael R.
Published: July 2003

We demonstrate a rare, unequivocal case of anisotropy in the transition zone (TZ), evident from many horizontal‐polarized shear‐waves (SH) that arrive up to 3 s earlier than vertical‐polarized shear‐waves (SV). The anisotropic region correlates with deep earthquakes away from the active subduction zone and known boundaries of lateral heterogeneities in compression‐ and shear‐wave speeds. We interpret the anisotropy as strong fabric associated with a large petrologic anomaly in the core of remnant lithosphere which is buoyant and stagnates in the TZ. Fast subduction of cold slab actually favors such a configuration, so slab penetration into the lower mantle does not uniformly occur.
Solar variations in extremely low frequency propagation parameters: 1. A two‐dimensional telegraph equation (TDTE) model of ELF propagation and fundamental parameters of Schumann resonances
Authors: Kulak A. , Zięba S. , Micek S. , Nieckarz Z.
Published: July 2003

The primary purpose of the research presented is to study solar variations in extremely low frequency (ELF) propagation parameters using Schumann resonance (SR) data from our irregular observations carried out in the East Carpathian mountains. The natural electromagnetic ELF fields in the Earth‐ionosphere cavity (the global resonator) are strongly dependent on its resonance and propagation properties as well as on the global thunderstorm activity. Thus a determination of the global resonator parameters from Schumann resonances needs information about such signal properties, which depend only on physical conditions of the Earth‐ionosphere cavity. To seek such signal properties, we consider the two‐dimensional telegraph equation (TDTE) approach for describing propagation of ELF electromagnetic waves in a two‐dimensional spherical transmission line model of the Earth‐ionosphere cavity. We have set up a model consisting of a network of finite, homogeneous, one‐dimensional transmission lines covering the sphere; with the square root dependence of the attenuation rate from frequency as a sufficient model for the study of the solar influence on global properties of the Earth‐ionosphere cavity. Using this model, we have constructed useful formulae and algorithms connecting the observed parameters of Schumann resonances with the attenuation rate. This enables us to study the solar influence on the attenuation rate from diurnal observations of the NS‐magnetic component of the ELF field measured at one station. As the measurements were carried out during both the minimum and the maximum of the solar cycle 23, we present in the work of Kuak et al. [2003] evidence for a change in the ELF attenuation rate in the Earth‐ionosphere cavity with solar activity.
Earth's bow shock and magnetopause in the case of a field‐aligned upstream flow: Observation and model comparison
Authors: Merka J. , Szabo A. , Šafránková J. , Němeček Z.
Published: July 2003

On 5 May 1996 the Interball‐1 and IMP 8 spacecraft crossed the bow shock boundary. The upstream conditions were special in two factors: (1) the interplanetary magnetic field was anti‐parallel to the solar wind flow within 15° and (2) the conditions were stable for a prolonged period (∼9 hours). At the nose of the magnetosphere, the Interball‐1 data revealed that the magnetopause was farther outward by ∼2 RE than model predictions and the subsolar magnetosheath was unusually thin, at most 10% of the magnetopause standoff distance. Both results stand in contrast to predictions of existing magnetopause/bow shock models. Assuming a hyperboloidal (paraboloidal) shock wave, the calculated shock's standoff distance was 13.7 (13.6) RE, and the focus was located on the x axis at 4.5 (4.2) RE. On the basis of the IMP 8 observation, the bow shock flares significantly less than MHD simulations predict for a field‐aligned bow shock at the magnetospheric flanks. This study discusses differences between the observations and existing MHD bow shock simulations for field‐aligned upstream flow. Furthermore, it is suggested that the flow‐aligned IMF orientation causes a significant change of the magnetopause shape into a bullet‐like obstacle.
Modeling variability of plasma conditions in the Io torus
Authors: Delamere P. A. , Bagenal F.
Published: July 2003

Telescopic observations and in situ measurements of the Io plasma torus show the density, temperature and composition to vary over time, sometimes up to a factor of 2. While previous models of the physical and chemical processes in the Io plasma torus have reasonably modeled the conditions of the Voyager 1 era, their authors have not addressed the observed variability nor explored the sensitivity of torus conditions to input parameters. In this paper we present a homogeneous torus model parameterized by five variables (transport timescale, neutral source strength, ratio of oxygen to sulfur atoms in the source, fraction of superthermal electrons, temperature of these hot electrons). The model incorporates the most recent data for ionization, recombination, charge exchange and radiative energy losses for the major torus species (S, S+, S++, S+++, O, O+, O++). We solve equations of conservation of mass and energy to find equilibrium conditions for a set of input parameters. We compare model plasma conditions with those observed by Voyager 1, Voyager 2 and Cassini. Furthermore, we explore the sensitivity of torus conditions to each parameter. We find that (1) torus conditions are distinctly different for the Voyager 1, 2 and Cassini eras, (2) unique torus input parameters for any given era are poorly constrained given the wide range of solution space that is consistent with the range of observed torus conditions, (3) ion composition is highly sensitive to the specification of a non‐thermal electron distribution, (4) neutral O/S source ratio is highly variable with model values ranging between 1.7 for Cassini to 4.0 for Voyager conditions, (5) transport times range between 23 days for Voyager 2 to 50 days for Voyager 1 and Cassini, (6) neutral source strengths range between 7 to 30 × 10−4 cm−3 s−1 which corresponds to a net production of 0.4 to 1.3 tons/s for a torus volume of 1.4 × 1031 cm3, or 38 RJ3.
Using an MHD simulation to interpret the global context of a coronal mass ejection observed by two spacecraft
Authors: Riley Pete , Linker J. A. , Mikić Z. , Odstrcil D. , Zurbuchen T. H. , Lario D. , Lepping R. P.
Published: July 2003

In late February 1999 the ACE spacecraft observed a coronal mass ejection (CME) at 1 AU, in the ecliptic plane. Thirteen days later, Ulysses observed a CME at 5 AU and 22°S. We present a detailed analysis of the plasma, magnetic field, and composition signatures of these two events. On the basis of this comparison alone, it is not clear that the two spacecraft observed the same solar event. However, using a generic MHD simulation of a fast CME initiated at the Sun by magnetic flux cancellation and propagated out into the solar wind, together with additional evidence, we argue that indeed the same CME was observed by both spacecraft. Although force‐free models appear to fit the observed events well, our simulation results suggest that the ejecta underwent significant distortion during its passage through the solar wind, indicating that care should be taken when interpreting the results of force‐free models. Comparison of composition measurements at the two spacecraft suggests that significant spatial inhomogeneities can exist within a single CME.
Short large‐amplitude magnetic structures and whistler wave precursors in a full‐particle quasi‐parallel shock simulation
Authors: Scholer Manfred , Kucharek Harald , Shinohara Iku
Published: July 2003

A one‐dimensional (1‐D) full‐particle electromagnetic simulation code (PIC) is used to investigate the role of upstream whistler and low‐frequency upstream waves during cyclic reformation of a medium Alfvén Mach number quasi‐parallel collisionless shock (magnetic field ‐ shock normal angle = 30°). The ion to electron mass ratio is assumed to be 100. Compared with previous PIC simulations, the upstream region is large enough to allow for the emergence of low‐frequency upstream waves by the interaction of backstreaming ions with the solar wind via an ion/ion beam instability in the far upstream region. It is shown that the low‐frequency upstream waves steepen up into pulsations (or short large‐amplitude magnetic structures (SLAMS)), as has been shown earlier by hybrid simulations. As these pulsations are added to the shock and thus comprise the shock, the upstream edge radiates a phase standing whistler train. This whistler train propagates partway into the newly arriving pulsation. The nonlinear interaction of reflected ions and incoming solar wind ions in the electrostatic potential of the whistler leads to ion trapping and rapid whistler damping. This results in SLAMS consisting of two regions with different ion temperatures. The cyclic reformation is essentially due to the SLAMS being added to the shock and is of larger scale (∼10 ion inertial lengths) as compared with the whistler scale.
Seismic imaging of a convergent continental margin and plateau in the central Andes (Andean Continental Research Project 1996 (ANCORP'96))
Authors: Oncken O. , Asch G. , Haberland C. , Metchie J. , Sobolev S. , Stiller M. , Yuan X. , Brasse H. , Buske S. , Giese P. , Görze H.‐J. , Lueth S. , Scheuber E. , Shapiro S. , Wigger P. , Yoon M.‐K. , Bravo P. , Vieytes H. , Chong G. , Gonzales G. , Wilke H.‐G. , Lüschen E. , Martinez E. , Rössling R. , Ricaldi E. , Rietbrock A.
Published: July 2003

A 400‐km‐long seismic reflection profile (Andean Continental Research Project 1996 (ANCORP'96)) and integrated geophysical experiments (wide‐angle seismology, passive seismology, gravity, and magnetotelluric depth sounding) across the central Andes (21°S) observed subduction of the Nazca plate under the South American continent. An east dipping reflector (Nazca Reflector) is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km. We interpret parts of the Nazca Reflector as a fluid trap located at the front of recent hydration and shearing of the mantle, the fluids being supplied by dehydration of the oceanic plate. Patches of bright (Quebrada Blanca Bright Spot) to more diffuse reflectivity underlie the plateau domain at 15–30 km depth. This reflectivity is associated with a low‐velocity zone, P to S wave conversions, the upper limits of high conductivity and high Vp/Vs ratios, and to the occurrence of Neogene volcanic rocks at surface. We interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during Neogene shortening and plateau growth. The imaging properties of the continental Moho beneath the Andes indicate a broad transitional character of the crust‐mantle boundary owing to active processes like hydration of mantle rocks (in the cooler parts of the plate margin system), magmatic underplating and intraplating under and into the lowermost crust, mechanical instability at Moho, etc. Hence all first‐order features appear to be related to fluid‐assisted processes in a subduction setting.
Deep structure of an island arc backstop, Lesser Antilles subduction zone
Authors: Christeson Gail L. , Bangs Nathan L. , Shipley Thomas H.
Published: July 2003

We present the results from a coincident seismic reflection/refraction grid conducted at the Lesser Antilles subduction zone near 16°N. This paper focuses on the seismic refraction data and constraints these data place on the three‐dimensional structure of the island arc backstop. We find that the backstop in this region contains considerable topography in both the strike and dip directions. Two ridges, each 25–35 km in length and ∼10 km in width, rise 1–6 km above the adjacent basement. The eastern edge of one of the ridges deepens by ∼4–6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop. In contrast to the complex nature of the backstop, the adjacent accretionary wedge displays little lateral variability at large scales. This may be a consequence of the spatial scales involved: the backstop topography is ∼10–35 km in width, while the accretionary wedge extends ∼125 km from the deformation front to the backstop. The top of the subducting oceanic crust, as identified by an increase in velocities to 6–6.5 km/s, intersects the backstop at a depth of ∼14–15 km. The updip limit of plate boundary seismicity is located 75–100 km west and downdip of the backstop. However, two earthquake clusters are observed at the intersection of the subducted Barracuda Ridge and Tiburon Ridge with the backstop, suggesting active deformation associated with the backstop edge at these locations.
Quasi‐three‐dimensional modeling of rip current systems
Authors: Haas Kevin A. , Svendsen I. A. , Haller Merrick C. , Zhao Qun
Published: July 2003

The focus of the paper is the analysis of the flow in rip current systems generated by channels in longshore bars on a beach. The horizontal variations of rip current systems are described through the use of the quasi‐three‐dimensional nearshore circulation model SHORECIRC. Model predictions are compared to laboratory measurements of waves and current velocities throughout the entire rip current system and show reasonable agreement. The rips in the two channels are found to behave differently because of the depth variation across the basin. It is found that higher bottom stress leads to more stable flow where the rip current meanders less and fewer eddies are generated. The wave current interaction creates forcing which reduces the distance rip currents flow offshore and can lead to a slow pulsation of the rip current. This pulsation is in addition to the instabilities of a jet which can also be present in rip currents. The three dimensionality of the rip current system is found to have a significant effect on the overall circulation patterns.
Spatial and temporal variability in flow at the mouth of Narragansett Bay
Authors: Kincaid Chris , Pockalny Robert A. , Huzzey Linda M.
Published: July 2003

Results are presented from field measurements of the spatial and temporal variability of circulation and exchange between Narragansett Bay and Rhode Island Sound (RIS) at both tidal and seasonal timescales. Data were collected during 13 acoustic Doppler current profiler (ADCP) surveys between April 1998 and July 1999. Significant horizontal and vertical structure was observed in the instantaneous, tidally driven flow and the residual, nontidal flow. This was especially evident through the deeper East Passage (EP), where persistent inflow was observed along the eastern side, and outflow along the western side of the channel. The residual volume transport of water to RIS is greater through the EP than through the West Passage entrance to the Bay, particularly in summer. Data from the offshore, or coastline‐normal survey lines reveal a strong seasonal variability in RIS circulation. A residual (5–15 cm s−1) cyclonic coastal current is recorded within RIS during summer, stratified conditions. This residual current, which was not present during the winter surveys, enhances Narragansett Bay‐RIS exchange by efficiently advecting the bay outflow to the southwest, away from the entrance. Results also indicate the resupply pathways for deep water to the entrance region vary in summer versus winter. Observed differences in RIS circulation, and related estuary‐shelf exchange, have significant implications for physical, chemical, and biological models of Narragansett Bay.
Large‐scale mass transports, water mass formation, and diffusivities estimated from World Ocean Circulation Experiment (WOCE) hydrographic data
Authors: Ganachaud Alexandre
Published: July 2003

A global inversion is used to estimate the oceanic mass transports and vertical diffusivities based on a subset of hydrographic data from the World Ocean Circulation Experiment (WOCE) and Java Australia Dynamic Experiment. The analysis is based on the linear inverse “box” model of Ganachaud and Wunsch [2000] that consistently combines the transoceanic sections. A globally consistent solution is obtained for a depth‐independent adjustment to the thermal wind field, the freshwater flux divergences, the Ekman transport, and the advective and diffusive dianeutral fluxes between layers. A detailed error budget permits calculation of statistical uncertainties, taking into account both the nonresolved part of the solution and the systematic errors due to the temporal oceanic variability. The horizontal structure of the transports for different density classes is analyzed regionally. During the WOCE period (1985–1996), net meridional transports are estimated, with 16 ± 2 Sv (106 m3/s) of North Atlantic Deep Water (NADW) being produced in the northern North Atlantic, moving southward, entraining Antarctic Bottom Water (AABW), and Antarctic Intermediate Water increasing in volume transport to 23 ± 3 Sv at 30°S. In the Southern Ocean, 21 ± 6 Sv of bottom water was formed from lower Circumpolar Deep Water, which corresponds approximately to the lower NADW density range. Bottom water inflows (NADW + AABW mixture) to the Atlantic, Indian, and Pacific Oceans are estimated at 6 ± 1.3, 11 ± 4, and 7 ± 2 Sv, respectively. In the Indian and Pacific Oceans this water returns southward at deep and intermediate levels. Property anomaly conservation constraints permit estimation of dianeutral diffusivities in deep layers, with a global average of 3.7 ± 0.7 cm2s−1 north of 30°S.
U.S. regional climate simulations and seasonal forecasts
Authors: Roads J. , Chen S.‐C. , Kanamitsu M.
Published: July 2003

Does a regional climate model provide a more useful regional climatology than can be obtained from larger‐scale global analyses or a better regional forecast than can be obtained by a large‐scale seasonal prediction model? To examine these questions, U.S. regional spectral model (RSM) climate simulations forced by 1‐day global spectral model (GSM) forecasts initialized from the National Centers for Environmental Prediction (NCEP) operational analysis are compared with regional RSM simulations forced by the NASA Seasonal to Interannual Prediction Project general circulation model (GCM). The GCM was continuously forced by observed sea surface temperature variations, but since sea surface temperatures (SSTs) are so persistent, these forced GCM simulations were equivalent to seasonal GCM predictions, which are mainly based upon persistent SSTs anyhow. RSM simulations forced by these two global modes were compared at the same spatial resolution as the global models (200 km) and at higher resolution (50 km). Resolution was important for producing better geographic pictures but does not currently produce significantly more skillful regional climate simulations or forecasts of the temporal variability, which already have significant skill from the global models. However, regional climate simulations and forecasts better depict the precipitation intensity, especially over the U.S. West. Finally, re‐initialized (from the large‐scale analysis) 1‐day RSM forecasts were compared with continuous RSM simulations, and this increased the overall regional skill back to the original GSM skill, which was somewhat degraded in continuous RSM simulations.
The UARS Microwave Limb Sounder version 5 data set: Theory, characterization, and validation
Authors: Livesey N. J. , Read W. G. , Froidevaux L. , Waters J. W. , Santee M. L. , Pumphrey H. C. , Wu D. L. , Shippony Z. , Jarnot R. F.
Published: July 2003

This paper describes the latest and (for most products) definitive data set from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS). MLS data have formed the basis of numerous studies, and the version 5 data, produced using more advanced algorithms than earlier versions, represent a significant improvement in quality and scientific applicability for most of the MLS data products. The version 5 data include midstratospheric to lower mesospheric temperature and geopotential height (the latter is a new product from MLS), water vapor from the upper troposphere to the mesosphere, stratospheric and mesospheric ozone, and stratospheric nitric acid, chlorine monoxide, and methyl cyanide (also a new product). The vertical retrieval grid over the stratosphere and lower mesosphere has been doubled, to six surfaces per decade change in pressure (∼2.5 km), compared to three surfaces per decade in previous versions. The accuracy and precision of lower stratospheric ozone, chlorine monoxide, and nitric acid have been improved. For each product, a description of relevant changes to the algorithms is given, along with an update on its validation, a description of the accuracy, precision, and vertical resolution of the data, and information on what quality control methods to apply when using the data.
Airborne Sun photometer measurements of aerosol optical depth and columnar water vapor during the Puerto Rico Dust Experiment and comparison with land, aircraft, and satellite measurements
Authors: Livingston John M. , Russell Philip B. , Reid Jeffrey S. , Redemann Jens , Schmid Beat , Allen Duane A. , Torres Omar , Levy Robert C. , Remer Lorraine A. , Holben Brent N. , Smirnov Alexander , Dubovik Oleg , Welton Ellsworth J. , Campbell James R. , Wang Jun , Christopher Sundar A.
Published: July 2003

Analyses of aerosol optical depth (AOD) and columnar water vapor (CWV) measurements obtained with the six‐channel NASA Ames Airborne Tracking Sunphotometer (AATS‐6) mounted on a twin‐engine aircraft during the summer 2000 Puerto Rico Dust Experiment are presented. In general, aerosol extinction values calculated from AATS‐6 AOD measurements acquired during aircraft profiles up to 5 km above sea level (asl) reproduce the vertical structure measured by coincident aircraft in situ measurements of total aerosol number concentration. AATS‐6 extinction retrievals also agree with corresponding values derived from ground‐based lidar measurements for altitudes above the trade inversion. The spectral behavior of AOD within specific layers beneath the top of the aircraft profile is consistent with attenuation of incoming solar radiation by large dust particles or by dust plus sea salt, with mean Ångström wavelength exponents of ∼0.20. Values of CWV calculated from profile measurements by AATS‐6 at 941.9 nm and from aircraft in situ measurements agree to within ∼4% (0.13 g/cm2). AATS‐6 AOD values measured on the ground at Roosevelt Roads Naval Air Station and during low‐altitude aircraft runs over the adjacent Cabras Island aerosol/radiation ground site agree to within 0.004–0.030 with coincident data obtained with an AERONET Sun/sky radiometer located on Cabras Island. For the same observation times, AERONET retrievals of CWV exceed AATS‐6 values by ∼21%. AATS‐6 AOD values measured during low‐altitude aircraft traverses over the ocean are compared with corresponding AOD values retrieved over water from upwelling radiance measurements by the Moderate‐Resolution Imaging Spectroradiometer (MODIS), Total Ozone Mapping Spectrometer (TOMS), and GOES 8 Imager satellite sensors, with mixed results.
Predictability of seasonal runoff in the Mississippi River basin
Authors: Maurer Edwin P. , Lettenmaier Dennis P.
Published: July 2003

Recent advances in climate prediction and remote sensing offer the potential to improve long‐lead streamflow forecasts and to provide better land surface state estimates at the time of forecast. We characterize predictability of runoff at seasonal timescales in the Mississippi River basin due to climatic persistence (represented by El Niño‐Southern Oscillation and the Arctic Oscillation) and persistence related to the initial land surface state (soil moisture and snow). These climate and land surface state indicators, at varying lead times, are then used in a multiple linear regression to explain the variance of seasonal average runoff. Soil moisture dominates runoff predictability for lead times of 1 1/2 months, except in summer in the western part of the basin, where snow dominates. For the western part of the basin, the land surface state has a stronger predictive capability than climate indicators through leads of two seasons; climate indicators are more important in the east at lead times of one season or greater. Modest winter runoff predictability exists at a lead time of 3 seasons due to both climate and soil moisture, but this is in areas producing little runoff and is therefore of lessened importance. Local summer runoff predictability is limited to the western mountainous areas (generating high runoff) through a lead of 2 seasons. This could be useful to water managers in the western portion of the Mississippi River basin, because it suggests the potential to provide skillful forecast information earlier in the water year than currently used in operational forecasts.
Increasing reflectivity of the Antarctic ocean‐atmosphere system: Analysis of Total Ozone Mapping Spectrometer (TOMS) and passive microwave data for 1979–1994
Authors: Lubin Dan , Lynch Stephen , Clarke Rory , Morrow Esther , Hart Steven
Published: July 2003

Measurements of Lambert equivalent reflectance at 380 nm from the Total Ozone Mapping Spectrometer (TOMS) instrument have shown increases in reflectivity between 1979 and 1994 over much of the Southern Ocean, encompassing 280° in longitude. These trends represent a possible change in the state of the Antarctic ocean‐atmosphere system related to recent climate warming. To determine if these reflectivity trends are due to changes in cloud cover or sea ice, or both, the TOMS data were collocated with a contemporaneous passive microwave satellite data set from the scanning multichannel microwave radiometer and the Special Sensor Microwave Imager. The passive microwave data sets specify total sea ice concentration, retrieved by a uniform method for all years using the NASA Team algorithm. To first order the locations of TOMS reflectivity increases coincide with regions where sea ice concentration has increased over the past 2 decades, signifying that the TOMS trends are the result of trends in underlying sea ice and not cloud cover. However, when the TOMS reflectivity measurements are sorted into fixed sea ice concentration bins of 0.1 width, the TOMS data also show increasing reflectivity trends in regions where sea ice extent has been decreasing (Amundsen and Bellingshausen Seas and the Western Antarctic Peninsula). Over open water, TOMS reflectivity trends are less convincing and may be artifacts related to uncertainties in passive microwave sea ice identification. These results suggest that a significant component of the Southern Ocean TOMS reflectivity trends may be a gradual increase in the albedo of the underlying sea ice. This could be caused by a gradual lengthening of the sea ice season, with a concomitant increase in the persistence of dry snow on the sea ice cover.
Performance of a single‐monochromator diode array spectroradiometer for the determination of actinic flux and atmospheric photolysis frequencies
Authors: Edwards Gavin D. , Monks Paul S.
Published: July 2003

The performance of a new spectroradiometer instrument based on a diode array technique during its first field deployment at the International Modeling and Measurement Intercomparison (IPMMI) has been evaluated. The instrument is a single‐monochromator diode array spectroradiometer that was used to measure both actinic flux and derived values of j(O1D) and j(NO2). Spectra may be collected over the wavelength region λ = 280–450 nm at variable integration times (typically 1 s for normal operation), and data may be converted to actinic flux/photolysis frequencies using National Institute of Standards and Technology traceable calibration standards. The relative accuracy of these measurements is compared to photolysis frequencies derived using similar and contrasting experimental techniques. The instrument performed adequately in the comparison, but the problem of poor stray light rejection in the single monochromator has the potential to propagate significant errors in the derived photolysis frequencies. A full error analysis together with instrument characterization has been performed and is reported. The feasibility of using such instruments in the field for the measurement of atmospheric photolysis frequencies is also discussed.
On particle formation prediction in continental boreal forest using micrometeorological parameters
Authors: Buzorius G. , Rannik Ü. , Aalto P. , dal Maso M. , Nilsson E. D. , Lehtinen K. E. J. , Kulmala M.
Published: July 2003

It has been hypothesized that biogenic aerosol formation in the southern Finland troposphere can be predicted using micrometeorological parameters and knowing pre‐existing aerosol surface area in the previous publication. There the concurrence of nucleation beginning with the rapid growth of convective boundary layer was demonstrated. In this work a nucleation probability term is introduced using statistical correlation between the particle formation and micrometeorological parameters. More refined analysis presented here demonstrates the increasing probability of nucleation mode aerosol presence with the increasing heat flux, temperature standard deviation, and vertical wind speed variance. Diurnal maximal values of the parameters mentioned are larger on days with nucleation event than on days without although these values are not significantly different during the convective layer growth. What distinguishes days with nucleation events is the size of condensation sink, which is significantly smaller on days with events than on days without.
Propagation and initiation mechanisms of the Madden‐Julian oscillation
Authors: Seo Kyong‐Hwan , Kim Kwang‐Yul
Published: July 2003

An observational study of the propagation and onset mechanisms of the Madden‐Julian oscillation (MJO) is performed using cyclostationary empirical orthogonal function analysis and a Kelvin‐Rossby wave decomposition method for the NOAA outgoing longwave radiation and the NCEP/NCAR Reanalysis data. In these analyses, two different regions of surface convergence are observed ahead of MJO convection by different wind components as previously found in a SST‐coupled GCM simulation. At the leading edge of enhanced convection, the friction‐induced meridional convergence appears during the developing phase when the enhanced convection is located over the Indian Ocean and the Maritime Continents. Another region of surface convergence is identified just east of an enhanced convection core, and this tends to pull the convective core to the east. The surface convergence is formed by zonal wind convergence, and Kelvin and Rossby waves both play a comparable role in the formation of the convergence zone. Therefore the frictional Kelvin‐Rossby wave‐CISK (conditional instability of the second kind) is regarded as a primary factor for the eastward propagation of the MJO. Over the western Indian Ocean, moistening in the boundary layer occurs ∼2 weeks earlier than the beginning of a new MJO cycle. This moistening accompanies low‐level convergence by encircling Kelvin waves which propagated from the region of enhanced convection during the previous cycle, as well as by Rossby waves generated from the region of reduced convection over the Indian Ocean. This earlier formation of the boundary‐layer moisture convergence provides a favorable environment for triggering new convection. This interaction of Kelvin and Rossby waves also accounts for the suppression of convection from the western side of the convection core. For example, over the Indian Ocean, anomalous surface divergence appears at the western edge of the enhanced convection due to both circumnavigating Kelvin waves from the region of reduced convection of the previous cycle and Rossby waves from the region of the enhanced convection itself. Thus the MJO is a self‐maintaining and self‐generating form of tropical variability through the interaction between convection and large‐scale circulation in the presence of boundary‐layer dynamics.
Simulated changes due to global warming in daily precipitation means and extremes and their interpretation using the gamma distribution
Authors: Watterson I. G. , Dix M. R.
Published: July 2003

The potential change in precipitation due to global warming is studied using five‐member ensembles of climate simulations by the CSIRO Mark 2 atmosphere‐ocean model for the period 1871–1990 and forward to 2100 under both the Special Report on Emission Scenarios (SRES) A2 (rapid CO2increase) and B2 (moderate increase) forcing scenarios. The mean surface warming for the period 1961–1990 is 0.3 K. The warming from 1961–1990 to 2071–2100 is 3.5 K under A2, 29% more than for B2, and with a very similar spatial pattern. The daily precipitation (P) frequency distributions for January and July days in these periods are presented, focusing on the A2 case. The distributions for wet days at each point are approximated by the gamma distribution. The global meanPincrease of around 6%, in both months, is related to a mean increase in the gamma's scale parameter of 18%, offset by small decreases in the shape parameter and wet day frequency. However, local changes of opposite signs also occur, especially in the tropics. Ensemble averages of 30‐year extreme daily precipitation for January and July, and other months, are generally greater for 2071–2100 than for 1961–1990, with an average increase of 14%. Extreme value theory based on the monthly gamma distributions provides a good match to these values. The theory is extended to the annual case. In general, the 1961–1990 extremes peak in the subtropical rainbands in the model, where increases of 10 to 30% are common. Larger relative increases occur in polar regions, and also over northern land in January.
A stratospheric aerosol climatology from SAGE II and CLAES measurements: 1. Methodology
Authors: Bauman J. J. , Russell P. B. , Geller M. A. , Hamill Patrick
Published: July 2003

This paper presents the methods used to produce a global climatology of the stratospheric aerosol using data from two satellite instruments: the Stratospheric Aerosol and Gas Experiment (SAGE II) and the Cryogenic Limb Array Etalon Spectrometer (CLAES). The climatology, which spans from December 1984 to August 1999, includes values and uncertainties of measured extinction and optical depth and of retrieved particle effective radius Reff, distribution width σg, surface area S, and volume V. As a basis for aerosol retrievals, a multiwavelength look‐up table (LUT) algorithm was developed that matches the satellite‐measured extinction ratios to precomputed ratios that are based on a range of unimodal lognormal size distributions. For cases in which the LUT does not find an acceptable match between measured and precomputed extinction spectra, a different technique called the parameter search technique is utilized. The combination of these two techniques and data from both satellites allows us to retrieve values of Reff, σg, S, and V over a wider range of conditions and from a wider range of wavelengths than used by other methods. This greater wavelength range helps constrain retrieved results, especially in postvolcanic conditions when particle sizes are greatly increased and SAGE II extinction spectra become essentially independent of wavelength. Our method includes an altitude‐ and time‐dependent procedure that uses bimodal size distributions from in situ measurements to estimate bias and uncertainty introduced by assuming a unimodal functional form. Correcting for this bias reduces uncertainty in retrievals of Reff, S, and V by about 7%, 5%, and 1% (averaged over all altitude bands), leaving remaining uncertainties from the unimodal assumption of about ±18%, ±20%, and ±21%, respectively. Additional uncertainties, resulting from measurement error and spatiotemporal variability, are evaluated by propagating input uncertainties through the retrieval algorithm. In an accompanying paper we report on a climatology of Reff, S, and V and consider uncertainties in our retrieved values of these parameters. In this paper we examine the sensitivity of our retrievals to refractive index and measurement wavelength. We find, for example, that changing refractive index from a value for the stratospheric temperature of 215 K to that for 300 K can increase retrieved Reff by ∼7.5%, owing largely to effects at the CLAES 12.82 μm wavelength. When only SAGE II wavelengths are used, corresponding changes in Reff are much smaller.
Annual variations of physical properties of desert dust over Israel
Authors: Israelevich P. L. , Ganor E. , Levin Z. , Joseph J. H.
Published: July 2003

The annual variation of the desert dust aerosol loading above the eastern Mediterranean is studied. Three periods are identified, March–May, July–August, and September–November, for which the properties of dust particles are distinctly different. The dust layers are at higher altitudes and consist of larger particles in summer and autumn than in spring. The real part of the refractive index of the particles is the same for summer and autumn periods and exceeds the real part of the refractive index measured during the spring. The imaginary part of the refractive index is negligible both in spring and in summer, whereas the imaginary refractive index becomes significant in September–November, indicating the presence of absorbing aerosols. The difference is attributed to different sources and desert dust trajectories in these periods. In spring the desert aerosol from the source in Chad is transported to the eastern Mediterranean predominantly along the North African coast. This is the motion associated with Sharav cyclones. The aerosols come to the eastern Mediterranean via Egypt from the sources near the Red Sea in July–August. In autumn the dust arrives to the eastern Mediterranean from the Libyan coast.
Rheology of a long lava flow at Pavonis Mons, Mars
Authors: Baloga S. M. , Mouginis‐Mark P. J. , Glaze L. S.
Published: July 2003

Dimensions of lava flows can be used to unravel the relative roles of viscosity changes and concurrent formation of levees, stationary margins, and stagnant zones. This approach is applied to data derived from the Mars Orbiter Laser Altimeter (MOLA) experiment for a long lava flow on the plains north of Pavonis Mons, Mars. We obtain a formula for the relative change in viscosity on the basis of a steady state Newtonian flow rate. Our approach features a new length scale that describes the transfer of lava from the active advancing component to passive components. This length scale can be determined from planetary image and topographic data by estimating the volume fraction of lava contained in flow margins relative to the total flow volume. We find only modest changes in viscosity over the distal 175 km of the Pavonis flow. Allowing the flow to also lose volume through degassing (resulting in a density increase) does little to affect the overall viscosity change. Thickening and widening of the flow with distance are as expected for a single coherent, isothermal, viscous flow. This dynamic regime features a balance between the formation of an outer skin and shedding of lava into stationary zones. Requirements for attaining such a regime include a thick flow, shallow slopes over extended distances, and preexisting surface roughness that is small compared to flow thickness. This style of emplacement may explain why many of the long, thick sheet‐like flows on the plains of Mars often exhibit an unexpected lack of thickening with distance.
Hansteen Alpha: A volcanic construct in the lunar highlands
Authors: Hawke B. Ray , Lawrence D. J. , Blewett D. T. , Lucey P. G. , Smith G. A. , Spudis P. D. , Taylor G. J.
Published: July 2003

We have used data from the Clementine and Lunar Prospector spacecraft in conjunction with near‐IR reflectance spectra collected with Earth‐based telescopes to study the composition and origin of Hansteen Alpha, an arrowhead‐shaped highlands feature located on the southern margin of Oceanus Procellarum. Hansteen Alpha is a member of a class of lunar spectral anomalies (Red Spots) that are characterized by a relatively high albedo and a strong absorption in the UV. It has been suggested that at least some of these spectral anomalies were produced by nonmare volcanism. The stratigraphic relationships among the geologic and compositional units in the region show that Hansteen Alpha was emplaced by extrusive igneous processes. The Imbrian‐aged craters Hansteen and Billy emplaced relatively FeO‐ and TiO2‐rich ejecta. Hansteen Alpha exhibits much lower FeO and TiO2 values. If Hansteen Alpha was present prior to the Billy and Hansteen impact events, it should have been covered with FeO‐ and TiO2‐rich ejecta because it is located within one crater diameter of the rim crest of each crater. Since it is not, Hansteen Alpha was superposed on these ejecta units. Nonmare volcanism is the only viable process for the formation of Hansteen Alpha. The morphology and surface texture of Hansteen Alpha is similar to many terrestrial features of dacitic and rhyolitic composition formed by extrusions of relatively viscous lavas. Such highly evolved compositions should be very enriched in Th, but Hansteen Alpha exhibits Th abundances of ∼6 ppm. Hence Hansteen Alpha is not composed of a highly evolved highlands lithology.
Methane production by large iron meteorite impacts on early Earth
Authors: Sekine Yasuhito , Sugita Seiji , Kadono Toshihiko , Matsui Takafumi
Published: July 2003

The continuous existence of life on the Earth is thought to have begun around 4 Ga, which is near the end of the heavy bombardment period. Impacts of asteroids and comets may have produced and delivered organic matter on the Hadean Earth. However, the nature of those processes has not been understood well yet. In this paper, we propose a new process of organic synthesis induced by asteroid impacts. We consider the effect of impact vapor condensates reentering the Earth's early atmosphere. Vapor condensates produced by a large‐scale impact are dispersed around the globe and reenter the atmosphere at very high speed. The reentering condensates are heated and decelerated by the friction with the atmosphere and form a “hot condensate layer” around the globe. We calculate the temperature and atmospheric pressure of the hot condensate layer as a function of time. A Fischer‐Tropsch reaction on the surface of the reentering iron and nickel condensates will lead to efficient methane production. The reaction rate of Fischer‐Tropsch catalysis is estimated based on a kinetic model. Calculation results indicate that the amount of methane produced by the impact of an iron meteorite with a diameter of 10 km at 15 km sec−1of velocity reaches ∼1012–1013kg. Some of the methane formed by this process is photodissociated to HCN and organic aerosols. The amount of the resulting HCN is estimated to be ∼1010–1012kg. This is 103–104times the preimpact inventory of HCN on the Earth, which is maintained by UV radiation and electric discharge in a mildly reducing (i.e., CO‐CO2dominant) atmosphere. Such an episodic increase in the organic inventory on the surface of the early Earth may have played an important role in the origin of life.
A particle‐tracking approach to simulating transport in a complex fracture
Authors: Tsang Chin‐Fu , Doughty Christine
Published: July 2003

Nearly all of the studies of flow and transport in fractured rocks have assumed that each fracture can be modeled as an open space between two surfaces with constant or variable separation. Field observations of rock fractures have shown that a fracture in the field can be rather more complex. Recent studies indicate that a complex fracture can be characterized as a thin fracture zone having several interconnected subfractures, which can contain mechanically dislodged and chemically altered materials with an enhanced porosity. This paper proposes a particle‐tracking approach to calculate solute transport in a complex fracture, with structures in the fracture thickness normal to the fracture plane. These structures include subfractures, dead‐end pores, gouge materials, small matrix blocks, and the adjacent rock matrix, all providing material in which solute diffusion and sorption can occur. The method is described; an example is provided to show its feasibility and the reasonableness of its parameter dependence; and, finally, direction for its further development is discussed.
Magnetite dissolution in siliceous sediments
Authors: Florindo Fabio , Roberts Andrew P. , Palmer Martin R.
Published: July 2003

Magnetite dissolution, and consequent loss of magnetization, is widely observed in reducing sedimentary environments, where the decrease in Eh‐pH values with depth is driven by bacterially mediated degradation of organic carbon. We have observed low magnetizations in sediments with elevated pore water silica concentrations that arise from diagenesis of biogenic silica and/or silicic volcanic ash. These depletions in magnetization are greater than can be accounted for by dilution with magnetite‐poor sediments and suggest that postdepositional destruction of magnetite has occurred. Biosiliceous sediments usually also contain elevated concentrations of organic carbon, which makes it difficult to separate organic‐carbon‐related magnetite dissolution from other possible mechanisms for magnetite dissolution. However, the extent of magnetite dissolution in the sedimentary sequences that we have studied is not obviously related to the redox‐state of the environment. This suggests that other mechanisms might have given rise to magnetite dissolution in these siliceous sediments. Thermodynamic calculations indicate that magnetite is unstable under conditions of elevated dissolved silica concentrations (and appropriate Eh‐pH conditions) and predict that magnetite will break down to produce iron‐bearing smectite. A survey of magnetic susceptibility and pore water geochemical data from widely distributed Ocean Drilling Program sites supports this observed link between high dissolved silica concentrations and low magnetic susceptibilities. This observed link also holds for environments with low biogenic silica productivity (and low organic carbon content) but with high interstitial silica concentrations due to dissolution of silicic volcanic ashes. Dissolution of magnetite is therefore predicted to be a common feature of siliceous sedimentary environments.
Critical space scales for aggregation‐mediated carbon export from ocean fertilization
Authors: Waite Anya M. , Johnson David
Published: July 2003

Several experiments have attempted to test Martin's hypothesis that addition of soluble iron to iron‐limited ocean regions will lead to an increase in carbon sequestration via phytoplankton growth and sedimentation. However, artificially iron‐induced blooms do not always trigger increases in vertical carbon export. Here we show that to trigger export, a patch must be larger than a threshold size. Phytoplankton sink en masse only after reaching a critical concentration for aggregation, and concentration is dependent on the competitive balance between growth and horizontal turbulent diffusion, which in turn varies with patch length scale. We summarize this balance using a non‐dimensional parameter, Q, and use a simple 2D model solving a growth‐diffusion‐export equation to show that export flux occurs from a fertilized patch when Q < 1, and that flux is maximized at a value of Q below this critical point. A simple nutrient limitation model generates predictions of particle export from patch fertilization experiments.