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  • Articles  (12)
  • Wiley  (12)
  • Cell Press
  • Frontiers Media
  • National Academy of Sciences
  • 2015-2019  (12)
  • 1995-1999
  • 1980-1984
  • 1955-1959
  • Journal of Geophysical Research JGR - Atmospheres  (4)
  • Journal of Geophysical Research: Atmospheres  (4)
  • Geochemistry Geophysics Geosystems (G3)  (3)
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  • Articles  (12)
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  • 2015-2019  (12)
  • 1995-1999
  • 1980-1984
  • 1955-1959
  • 2010-2014  (12)
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  • 1
    Publication Date: 2019
    Description: Abstract The width of the tropical Hadley circulation (HC) has garnered intense interest in recent decades, owing to the emerging evidence for its expansion in observations and models and to the anticipated impacts on surface climate in its descending branches. To better clarify the causes and impacts of tropical widening, this work generalizes the zonal mean HC to the regional level by defining meridional overturning cells (RC) using the horizontally divergent wind. The edges of the RC are more closely connected to surface hydroclimate than more traditional metrics of regional tropical width (such as the sea level pressure ridge) or even than the zonal mean HC. Simulations reveal a robust weakening of the RC in response to greenhouse gas increases, along with a widening of the RC in some regions. For example, simulated widening of the zonal mean HC in the Southern Hemisphere appears to arise in large part from regional overturning anomalies over the Eastern Pacific, where there is no clear RC. Unforced interannual variability in the position of the zonal mean HC edge is associated with a more general regional widening. These distinct regional signatures suggest that the RCs may be well suited for the attribution of observed circulation trends. The spatial pattern of regional meridional overturning trends in reanalyses corresponds more closely to the pattern associated with unforced interannual variability than to the pattern associated with CO2 forcing, suggesting a large contribution of natural variability to the recent observed tropical widening trends.
    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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  • 2
    Publication Date: 2016-06-23
    Description: Till from moraines at the heads of six major outlet glaciers in the Transantarctic Mountains (TAM) and from till beneath three West Antarctic ice streams have a ubiquitous zircon U-Pb age population spanning the time of the Ross/Pan-African Orogenies (610-475 Ma). Geo- and thermochronology of detrital minerals in these Antarctic glacial tills reveal two different thermal histories for the central and southern TAM. Double-dating of the zircons reveals a geographically widespread (U-Th)/He (ZHe) population of 180-130 Ma in most of the till samples. Sandstone outcrops at Shackleton Glacier, and three Beacon Supergroup sandstone clasts from three moraines, have ZHe ages that fall entirely within this range. The similar population and proximity of many of the till samples to Beacon outcrops lead us to suggest that this extensive ZHe population in the tills is derived from Beacon Supergroup rocks and reflects the thermal response of the Beacon Basin to the breakup of Gondwana. A second population of older (〉200 Ma) ZHe ages in tills at the head of Byrd, Nimrod and Reedy Glaciers. For the tills at the head of the Nimrod and Byrd Glacier, integrating the double-dated zircon results with 40 Ar/ 39 Ar of hornblende, muscovite and biotite, and U-Pb and (U-Th-Sm)/He double-dates on apatite yields a typical pattern of early rapid orogenic cooling (∼4-10˚C/my) 590-475 Ma after the emplacement of the Granite Harbour Intrusives. Low temperature thermochronometers at these sites yield variable but quite old ages (ZHe 480-70 Ma and AHe 200-70 Ma) that require a long history at low temperature. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
    Publication Date: 2016-04-08
    Description: Based on a uniquely dense network of surface towers measuring continuously the atmospheric concentrations of Greenhouse Gases (GHG's), we developed the first comprehensive monitoring systems of CO 2 emissions at high resolution over the city of Indianapolis. The urban inversion evaluated over the 2012-2013 dormant season, showed a statistically significant increase of about 20% (from 4.5 to 5.7 MtC ±0.23 MtC) compared to the Hestia CO 2 emission estimate, a state-of-the-art building-level emission product. Spatial structures in prior emission errors, mostly undetermined, appeared to affect the spatial pattern in the inverse solution and the total carbon budget over the entire area by up to 15%, while the inverse solution remains fairly insensitive to the CO 2 boundary inflow and to the different prior emissions ( i.e. ODIAC). Preceding the surface emission optimization, we improved the atmospheric simulations using a meteorological data assimilation system also informing our Bayesian inversion system through updated observations error variances. Finally, we estimated the uncertainties associated with undetermined parameters using an ensemble of inversions. The total CO 2 emissions based on the ensemble mean and quartiles (5.26 - 5.91 MtC) were statistically different compared to the prior total emissions (4.1 to 4.5 MtC). Considering the relatively small sensitivity to the different parameters, we conclude that atmospheric inversions are potentially able to constrain the carbon budget of the city, assuming sufficient data to measure the inflow of GHG over the city, but additional information on prior emission error structures are required to determine the spatial structures of urban emissions at high resolution.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
    Publication Date: 2019
    Description: Abstract Production and transport of NOx by convection is critical as it serves as a precursor to tropospheric ozone, an important greenhouse gas. Lightning serves as the largest source of nitrogen oxides (NOx = NO + NO2) to the upper troposphere (UT) and is one of the largest natural sources of NOx. Interest is placed on the vertical advection of NOx because its lifetime increases to several days in the UT compared to roughly three hours in the lower troposphere and boundary layer. Thus, lightning can play an important role in ozone production within the UT. However, the amount of NOx produced per flash and NOx advection in storms remain uncertain. This study investigates lightning NOx (LNOx) production and transport processes in anomalous (mid‐level positive charge) and normal polarity (mid‐level negative charge) thunderstorms by advecting parcels containing LNOx from the flash channels of over 5600 lightning flashes observed during the Deep Convective Clouds and Chemistry (DC3) field campaign. Results reveal most flash channels occur near 6‐8 km in the normal polarity thunderstorms and 5‐6 km within anomalous polarity thunderstorms. Larger flash rates and stronger updrafts in anomalous storms result in considerably larger LNOx mixing ratios (peaks of 0.75‐1.75 ppb) in the UT compared to normal polarity storms (peaks 〈 0.5 ppb). A slightly lower mean flash LNOx production was also found among all five storms in this study (storm mean values of 72‐158 moles per flash) compared to previous estimates, which generally parameterize LNOx by flash rate rather than flash rate.
    Print ISSN: 2169-897X
    Electronic ISSN: 2169-8996
    Topics: Geosciences , Physics
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  • 5
    Publication Date: 2019
    Description: Abstract This study systematically examines the regional uncertainties and biases in carbon dioxide (CO2) mole fractions from two of the state‐of‐the‐art global CO2 analysis products, namely the Copernicus Atmosphere Monitoring Service (CAMS) real‐time atmospheric analysis from the European Centre for Medium‐Range Weather Forecasts and the CarbonTracker Near‐Real Time (CT‐NRT) reanalysis from the National Oceanic and Atmospheric Administration, by evaluation against hundreds of hours of airborne in situ measurements from the summer 2016 and winter 2017 Atmospheric Carbon and Transport (ACT) ‐‐ America field campaigns. Both the CAMS and CT‐NRT analyses agree reasonably well with the independent ACT‐America airborne CO2 measurements in the free troposphere, with root‐mean‐square deviations (RMSDs) between analyses and observations generally between 1‐‐2 ppm, but show considerably larger uncertainties in the atmospheric boundary layer where the RMSDs exceed 8 ppm in the lowermost 1 km of the troposphere in summer. There are strong variations in accuracy and bias between seasons, and across three different subregions in the United States (Mid‐Atlantic, Midwest and South), with the largest uncertainties in the Mid‐Atlantic region in summer. Overall, the RMSDs of the CAMS and CT‐NRT analyses against airborne data are comparable to each other, and largely consistent with the differences between the two analyses. The current study provides uncertainty estimates for both analysis products over North America and suggests that these two independent estimates can be used to approximate regional CO2 analysis uncertainties. Both statistics are important in future studies in quantifying the uncertainties in regional CO2 mole fraction and flux estimates.
    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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  • 6
    Publication Date: 2019
    Description: Abstract Hazard analysis at caldera volcanoes is challenging due to the wide range of eruptive and environmental conditions that can plausibly occur during renewed activity. Taupo volcano, New Zealand, is a frequently active and productive rhyolitic caldera volcano that has hosted the world's youngest known supereruption and numerous smaller explosive events. To assess ashfall hazard from future eruptions, we have simulated atmospheric ash dispersal using the Ash3d model. We consider five eruption scenarios spanning magma volumes of 0.1–500 km3 and investigate the main factors governing ash dispersal in modern atmospheric conditions. Our results are examined in the context of regional synoptic weather patterns (Kidson types) that provide a framework for assessing the variability of ashfall distribution in different wind fields. For the smallest eruptions (~0.1 km3 magma), ashfall thicknesses 〉1 cm are largely confined to the central North Island, with dispersal controlled by day‐to‐day weather and the dominance of westerly winds. With increasing eruptive volume (1–5 km3 magma), ashfall thicknesses 〉1 cm would likely reach major population centers throughout the North Island. Dispersal is less dependent on weather patterns as the formation of a radially‐expanding umbrella cloud forces ash upwind or crosswind, although strong stratospheric winds significantly restrict umbrella spreading. For large eruptions (50–500 km3 magma), powerful expansion of the umbrella cloud results in widespread ashfall at damaging thicknesses (〉10 cm) across most of the North Island and top of the South Island. Synoptic climatology may prove a useful additional technique for long‐term hazard planning at caldera volcanoes.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 7
    Publication Date: 2018-02-18
    Description: An optimization algorithm is developed to retrieve liquid water cloud properties including cloud optical depth (COD), droplet size distribution and cloud top height (CTH), and aerosol above cloud properties including aerosol optical depth (AOD), single scattering albedo and microphysical properties from sweep-mode observations by JPL's AirMSPI instrument. The retrieval is composed of three major steps: (1) retrieval of an initial estimate of the mean droplet size distribution across the entire image of 80-100 km along-track by 10-25 km across-track from polarimetric cloudbow observations; (2) coupled retrieval of image-scale cloud and above-cloud aerosol properties by fitting the polarimetric data at all observation angles; and (3) iterative retrieval of 1D-RT based COD and droplet size distribution at pixel-scale (25 m) by establishing relationships between COD and droplet size and fitting the total radiance measurements. Our retrieval is tested using 134 AirMSPI datasets acquired during the NASA ORACLES field campaign. The retrieved above-cloud AOD and CTH are compared to coincident HSRL-2 (NASA LaRC) data, and COD and droplet size distribution parameters (effective radius r eff and effective variance v eff ) are compared to coincident RSP (NASA GISS) data. Mean absolute differences (MADs) between AirMSPI and HSRL-2 retrievals of above-cloud AOD at 532 nm and CTH are 0.03 and 〈 0.5 km, respectively. At RSP's footprint scale (323 m), MADs between RSP and AirMSPI retrievals of COD, r eff and v eff in the cloudbow area are 2.33, 0.69 μm and 0.020, respectively. Neglect of smoke aerosols above cloud leads to an underestimate of image-averaged COD by ~15%.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 8
    Publication Date: 2016-11-12
    Description: We investigate the impact of cirrus cloud heterogeneity on the direct emission by cloud or surface and on the scattering by ice particles in the thermal infrared (TIR). Realistic 3D cirri are modeled with the 3DCLOUD code and top-of-atmosphere radiances are simulated by the 3D Monte Carlo radiative transfer (RT) algorithm 3DMCPOL for two (8.65  μ m and 12.05  μ m) channels of the Imaging Infrared Radiometer (IIR) on CALIPSO. At nadir, comparisons of 1D and 3D RT show that 3D radiances are larger than their 1D counterparts for direct emission but smaller for scattered radiation. For our cirrus cases, 99% of the 3D total radiance is computed by the third scattering order, which corresponds to 90% of the total computational effort, but larger optical thicknesses need more scattering orders. To radically accelerate the 3D RT computations (using only few percent of 3D RT time with a Monte Carlo code), even in presence of large optical depths, we develop a hybrid model based on exact 3D direct emission, the first scattering order from 1D in each homogenized column, and an empirical adjustment linearly dependent on the optical thickness to account for higher scattering orders. Good agreement is found between the hybrid model and the exact 3D radiances for two very different cirrus models without changing the empirical parameters. We anticipate that a future deterministic implementation of the hybrid model will be fast enough to process multi-angle thermal imagery in a practical tomographic reconstruction of 3D cirrus fields.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 9
    Publication Date: 2018-01-09
    Description: Face centered cubic ( fcc ) FeH X was synthesized at pressures of 18 to 68 GPa and temperatures exceeding 1500 K. Thermally quenched samples were evaluated using synchrotron X-ray diffraction (XRD) and nuclear resonant inelastic X-ray scattering (NRIXS) to determine sample composition and sound velocities to 82 GPa. To aid in the interpretation of non-ideal (X≠1) stoichiometries, two equations of state for fcc FeH X were developed, combining an empirical equation of state for iron with two distinct synthetic compression curves for interstitial hydrogen. Matching the density deficit of the Earth's core using these equations of state requires 0.8-1.1 wt.% hydrogen at the core-mantle boundary and 0.2-0.3 wt.% hydrogen at the interface of the inner and outer cores. Furthermore, a comparison of Preliminary Reference Earth Model (PREM) [Dziewonski and Anderson; 1981] to a Birch's law extrapolation of our experimental results suggests that an iron alloy containing ∼0.8 to 1.3 wt.% hydrogen could reproduce both the density and compressional velocity ( V P ) of the Earth's outer core.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 10
    Publication Date: 2017-03-21
    Description: The Explorer segment of northernmost Cascadia is an end-member “warm” subduction zone with very young incoming plate and slow convergence rate. Understanding the megathrust earthquake potential of this type of subduction zone is of both geodynamic and societal importance. Available geodetic observations indicate that the subduction megathrust of the Explorer segment is currently locked to some degree, but the downdip extent of the fault area that is potentially seismogenic is not known. Here we construct finite element models to estimate the thermally allowed megathrust seismogenic zone, using available knowledge of regional plate kinematics, structural data, and heat flow observations as constraints. Despite ambiguities in plate interface geometry constrained by hypocenter locations of low-frequency earthquakes beneath Vancouver Island, the thermal models suggest a potential rupture zone of ∼60 km downdip width located fully offshore. Using dislocation modeling, we further illustrate that a rupture zone of this size, even with a conservative assumption of ∼100 km strike length, can cause significant tsunami-genic deformation. Future seismic and tsunami hazard assessment in northern Cascadia must take the Explorer segment into account. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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