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  • Articles  (71)
  • Wiley  (71)
  • 2015-2019  (43)
  • 2000-2004  (21)
  • 1975-1979  (7)
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  • Articles  (71)
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  • 1
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    Data‐driven inference of thermosphere composition during solar minimum conditions (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Mass density variations can deviate from the expected behaviour caused by temperature due to changes in the composition. Such deviations can be especially significant during solar minimum conditions. Model‐data differences are typically resolved through temperature corrections while overlooking the role of errors in lower boundary composition. In this work, we use a data‐driven methodology to simultaneously estimate thermosphere composition and temperature contributions to model‐data differences. The methodology uses modal decomposition to extract high‐dimensional, reduced order basis functions for the covariance of the neutral thermospheric species and temperature. The extracted basis functions are combined with CHAMP and GRACE mass density measurements using a non‐linear least squares solver. We demonstrate the methodology using the Naval Research Laboratory's empirical MSIS (Mass Spectrometer and Incoherent Scatter) model to derive high‐dimensional basis functions. We characterize and quantify the contribution of temperature and lower boundary effects with oxygen and helium since the two species have a direct impact on drag and orbit prediction through gas‐surface interactions and mass density. We analyze the month of December in 2008, based on the work of Thayer et al. [2012], and estimate that lower boundary composition errors contribute approximately 50% of the model‐data differences.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    The Record-Breaking 2015 Hurricane Season in the eastern North Pacific: An Analysis of Environmental Conditions (2016)
    Wiley
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    Publication Date: 2016-08-18
    Description: The presence of a near-record El Niño and a positive Pacific Meridional Mode provided an extraordinarily warm background state that fueled the 2015 eastern North Pacific hurricane season to near-record levels. We find that the western portion of the eastern North Pacific, referred to as the Western Development Region (WDR; 10°–20°N, 116°W–180°), set records for named storms, hurricane days and Accumulated Cyclone Energy in 2015. When analyzing large-scale environmental conditions, we show that record-warm sea surface temperatures, high mid-level relative humidity, high low-level relative vorticity, and record-low vertical wind shear were among the environmental forcing factors contributing to the observed tropical cyclone activity. We assess how intraseasonal atmospheric variability may have contributed to active and inactive periods observed during the 2015 hurricane season. We document that, historically, active seasons are associated with May-June El Niño conditions, potentially allowing for predictability of future active WDR seasons.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Quantifying the Storm Time Thermospheric Neutral Density Variations Using Model and Observations (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Accurate determination of thermospheric neutral density holds crucial importance for satellite drag calculations. The problem is twofold and involves the correct estimation of the quiet time climatology and storm time variations. In this work, neutral density estimations from two empirical and three physics‐based models of the ionosphere‐thermosphere are compared with the neutral densities along the Challenging Micro‐Satellite Payload satellite track for six geomagnetic storms. Storm time variations are extracted from neutral density by (1) subtracting the mean difference between model and observation (bias), (2) setting climatological variations to zero, and (3) multiplying model data with the quiet time ratio between the model and observation. Several metrics are employed to evaluate the model performances. We find that the removal of bias or climatology reveals actual performance of the model in simulating the storm time variations. When bias is removed, depending on event and model, storm time errors in neutral density can decrease by an amount of 113% or can increase by an amount of 12% with respect to error in models with quiet time bias. It is shown that using only average and maximum values of neutral density to determine the model performances can be misleading since a model can estimate the averages fairly well but may not capture the maximum value or vice versa. Since each of the metrics used for determining model performances provides different aspects of the error, among these, we suggest employing mean absolute error, prediction efficiency, and normalized root mean square error together as a standard set of metrics for the neutral density.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
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  • 4
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    Reverse Engineering Ancient Greek Ceramics: Morphological and Spectral Characterization of Replicates (2016)
    Wiley
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    Publication Date: 2016-02-16
    Description: Athenian pottery, the ceramics produced in the Attica region of Greece between the 6th and 4th centuries B.C., is considered a benchmark technological achievement of the preindustrial world. This work advances our understanding of the firing protocols employed by the ancient Greeks to produce their black-on-red designs by characterizing replicates painted with a refined illite clay and fired under oxidizing/reducing/reoxidizing conditions (three-stage firing). Systematically varying the temperature, atmosphere, and duration of each firing stage within the three-stage firing scheme allowed the conditions necessary to obtain black and red gloss, both of which are observed on ancient vessels, to be determined. The morphology and elemental distribution of particles formed within the gloss thus formed were characterized using transmission electron microscopy and spectrocolorimetric measurements. Comparison of the results obtained from ancient sherds with those obtained from the replicate samples provides a means of estimating the firing conditions used to create the ancient vessels.
    Print ISSN: 0002-7820
    Electronic ISSN: 1551-2916
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 5
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    Observations of near-surface current shear help describe oceanic oil and plastic transport (2017)
    Wiley
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    Publication Date: 2017-12-06
    Description: Plastics and spilled oil pose a critical threat to marine life and human health. As a result of wind forcing and wave motions, theoretical and laboratory studies predict very strong velocity variation with depth over the upper few centimeters of the water column, an observational blind spot in the real ocean. Here we present the first-ever ocean measurements of the current vector profile defined to within 1 cm of the free surface. In our illustrative example, the current magnitude averaged over the upper 1 cm of the ocean is shown to be nearly four times the average over the upper 10 m, even for mild forcing. Our findings indicate that this shear will rapidly separate pieces of marine debris which vary in size or buoyancy, making consideration of these dynamics essential to an improved understanding of the pathways along which marine plastics and oil are transported.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Antibody opsonization enhances MAIT cell responsiveness to bacteria via a TNF ‐dependent mechanism (2019)
    Wiley
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    Publication Date: 2019-02-25
    Print ISSN: 0818-9641
    Electronic ISSN: 1440-1711
    Topics: Biology , Medicine
    Published by Wiley
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  • 7
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    Beyond receptors and signaling: epigenetic factors in the regulation of innate immunity (2015)
    Wiley
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    Publication Date: 2015-01-06
    Print ISSN: 0818-9641
    Electronic ISSN: 1440-1711
    Topics: Biology , Medicine
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  • 8
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    New density estimates derived using accelerometers on-board the CHAMP and GRACE satellites (2017)
    Wiley
    Details
    Publication Date: 2017-03-25
    Description: Atmospheric mass density estimates derived from accelerometers on-board satellites such as CHAMP and GRACE are crucial in gaining insight into open science questions about the dynamic coupling between space weather events and the upper atmosphere. Recent advances in physics-based satellite drag coefficient modeling allow derivation of new density data sets. This paper uses physics-based satellite drag coefficient models for CHAMP and GRACE to derive new estimates for the neutral atmospheric density. Results show an average difference of 14-18% for CHAMP and 10-24% for GRACE between the new and existing data sets depending on the space weather conditions (i.e. solar and geomagnetic activity levels). The newly derived densities are also compared with existing models and results are presented. These densities are expected to be useful to the wider scientific community for validating the development of physics-based models and helping to answer open scientific questions regarding our understanding of upper atmosphere dynamics such as the sensitivity of temporal and global density variations to solar and geomagnetic forcing.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
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  • 9
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    A methodology for reduced order modeling and calibration of the upper atmosphere (2017)
    Wiley
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    Publication Date: 2017-07-22
    Description: Atmospheric drag is the largest source of uncertainty in accurately predicting the orbit of satellites in low Earth orbit (LEO). Accurately predicting drag for objects that traverse LEO is critical to Space Situational Awareness. Atmospheric models used for orbital drag calculations can be characterized either as empirical or physics-based (first principles based). Empirical models are fast to evaluate but offer limited real-time predictive/forecasting ability, while physics-based models offer greater predictive/forecasting ability but require dedicated parallel computational resources. Also, calibration with accurate data is required for either type of models. This paper presents a new methodology based on proper orthrogonal decomposition (POD) towards development of a quasi-physical, predictive, reduced order model that combines the speed of empirical and the predictive/forecasting capabilities of physics-based models. The methodology is developed to reduce the high-dimensionality of physics-based models while maintaining its capabilities. We develop the methodology using the Naval Research Lab's MSIS model and show that the diurnal and seasonal variations can be captured using a small number of modes and parameters. We also present calibration of the reduced order model using the CHAMP and GRACE accelerometer-derived densities. Results show that the method performs well for modeling and calibration of the upper atmosphere.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
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  • 10
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    A Laboratory Investigation of Spume Generation in High Winds for Fresh and Seawater (2019)
    Wiley
    Details
    Publication Date: 2019
    Description: Abstract Given spume's role in mediating air‐sea exchange at the base of tropical cyclones or other storm events, the focus of studies on spray dynamics has been within the marine environment. In contrast, spume production in non‐seawater bodies has been under‐explored and potential differences between sea and freshwater are neglected. The laboratory remains the primary means for directly observing spray processes near the surface, because of the challenges to making robust field measurements. There is no standardization on the water type used for these experiments and the effect this has on the generation process is unknown. This adds uncertainty in our ability to make physically realistic spume generation functions that are ultimately applied to the geophysical domain. We have conducted a laboratory experiment that aims to address this simple, yet over‐looked, question of whether water type impacts the spume droplet concentration entrained in the air flow above actively breaking waves. We compared directly imaged concentrations for fresh and seawater droplets produced in 10‐m equivalent winds from 36‐54 m/s. Substantially higher concentrations of seawater spume were observed, as compared to freshwater across all particle sizes and wind speeds. The seawater particles’ vertical distribution was concentrated near the surface, whereas the freshwater droplets were more uniformly distributed. Our statistical analysis of these findings suggest significant differences in the size‐ and height‐dependent distribution response to increased wind forcing between fresh and seawater. These unexpected findings suggest an unanticipated role of the source water physiochemical properties on the spume generation mechanism.
    Print ISSN: 2169-897X
    Electronic ISSN: 2169-8996
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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