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  • Articles  (15,635)
  • 2010-2014  (15,635)
  • Geophysical Research Letters  (4,699)
  • Journal of Geophysical Research JGR - Atmospheres  (3,523)
  • 4905
  • 7528
  • Geosciences  (15,635)
  • 1
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    EXPLORING THE FAINT YOUNG SUN PROBLEM AND THE POSSIBLE CLIMATES OF THE ARCHEAN EARTH WITH A 3D GCM (2013)
    Wiley
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    Publication Date: 2013-09-08
    Description: [1]  Different solutions have been proposed to solve the ‘faint young Sun problem’, defined by the fact that the Earth was not fully frozen during the Archean despite the fainter Sun. Most previous studies were performed with simple 1D radiative convective models and did not account well for the clouds and ice-albedo feedback or the atmospheric and oceanic transport of energy. We apply a Global Climate Model (GCM) to test the different solutions to the faint young Sun problem. We explore the effect of greenhouse gases (CO 2 and CH 4 ), atmospheric pressure, cloud droplet size, land distribution and Earth's rotation rate. We show that, neglecting organic haze, 100 mbars of CO 2 with 2 mbars of CH 4 at 3.8 Ga and 10 mbars of CO 2 with 2 mbars of CH 4 at 2.5 Ga allow a temperate climate (mean surface temperature between 10 °C and 20 °C). Such amounts of greenhouse gases remain consistent with the geological data. Removing continents produces a warming lower than +4 °C. The effect of rotation rate is even more limited. Larger droplets (radii of 17  µm versus 12  µm ) and a doubling of the atmospheric pressure produce a similar warming of around +7 °C. In our model, ice-free waterbelts can be maintained up to 25° N/S with less than 1 mbar of CO 2 and no methane. An interesting cloud feedback appears above cold oceans, stopping the glaciation. Such a resistance against full glaciation tends to strongly mitigate the faint young Sun problem.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Suomi-NPP CrIS Radiometric Calibration Uncertainty (2013)
    Wiley
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    Publication Date: 2013-09-08
    Description: [1]  The Cross-track Infrared Sounder (CrIS) is the high spectral resolution spectroradiometer on the Suomi National Polar-Orbiting Partnership (NPP) satellite, providing operational observations of top-of-atmosphere thermal infrared radiance spectra for weather and climate applications. This paper describes the CrIS radiometric calibration uncertainty based on pre-launch and on-orbit efforts to estimate calibration parameter uncertainties, and provides example results of recent post-launch validation efforts to assess the predicted uncertainty. Pre-launch Radiometric Uncertainty (RU) estimates computed for the laboratory test environment are less than ~0.2 K 3-sigma for blackbody scene temperatures above 250 K, with primary uncertainty contributions from the calibration blackbody temperature, calibration blackbody reflected radiance terms and detector nonlinearity. Variability of the pre-launch RU among the longwave band detectors and midwave band detectors is due to different levels of detector nonlinearity. A methodology for on-orbit adjustment of nonlinearity correction parameters to reduce the overall contribution to RU and to reduce FOV-to-FOV variability is described. The resulting on-orbit RU estimates for Earth view spectra are less than 0.2 K 3-sigma in the midwave and shortwave bands, and less than 0.3 K 3-sigma in the longwave band. Post-launch validation efforts to assess the radiometric calibration of CrIS are underway; validation results to date indicate that the on-orbit RU estimates are representative. CrIS radiance products are expected to reach “Validated” status in early 2014.
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  • 3
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    Assimilation of T-TREC-Retrieved wind data with WRF 3DVAR for the Short-term forecasting of typhoon meranti (2010) near landfall (2013)
    Wiley
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    Publication Date: 2013-09-08
    Description: [1]  An extended Tracking Radar Echo by Correlation (TREC) technique, called T-TREC technique, has been developed recently to retrieve horizontal circulations within tropical cyclones (TCs) from single Doppler radar reflectivity ( Z ) and radial velocity ( V r , when available) data. This study explores, for the first time, the assimilation of T-TREC-retrieved winds for a landfalling typhoon, Meranti (2010), into a convection-resolving model, the WRF (Weather Research and Forecasting). The T-TREC winds or the original V r data from a single coastal Doppler radar are assimilated at the single time using the WRF 3DVAR, at 8, 6, 4 and 2 hours before the landfall of typhoon Meranti. In general, assimilating T-TREC winds results in better structure and intensity analysis of Meranti than directly assimilating V r data. The subsequent forecasts for the track, intensity, structure and precipitation are also better, although the differences becomes smaller as the V r data coverage improves when the typhoon gets closer to the radar. The ability of the T-TREC retrieval in capturing more accurate and complete vortex circulations in the inner-core region of TC is believed to be the primary reason for its superior performance over direct assimilation of V r data; for the latter, the data coverage is much smaller when the TC is far away and the cross-beam wind component is difficult to analyze accurately with 3DVAR method.
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  • 4
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    Near-term acceleration of hydroclimatic change in the western U.S. (2013)
    Wiley
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    Publication Date: 2013-09-08
    Description: [1]  Given its large population, vigorous and water-intensive agricultural industry, and important ecological resources, the western United States presents a valuable case study for examining potential near-term changes in regional hydroclimate. Using a high-resolution, hierarchical, five-member ensemble modeling experiment that includes a global climate model (CCSM), a regional climate model (RegCM), and a hydrological model (VIC), we find that increases in greenhouse forcing over the next three decades result in an acceleration of decreases in spring snowpack and a transition to a substantially more liquid-dominated water resources regime. These hydroclimatic changes are associated with increases in cold-season days above freezing and decreases in the cold-season snow-to-precipitation ratio. The changes in the temperature and precipitation regime in turn result in shifts toward earlier snowmelt, baseflow, and runoff dates throughout the region, as well as reduced annual and warm-season snowmelt and runoff. The simulated hydrologic response is dominated by changes in temperature, with the ensemble members exhibiting varying trends in cold-season precipitation over the next three decades, but consistent negative trends in cold-season freeze days, cold-season snow-to-precipitation ratio, and April 1 st snow water equivalent. Given the observed impacts of recent trends in snowpack and snowmelt runoff, the projected acceleration of hydroclimatic change in the western U.S. has important implications for the availability of water for agriculture, hydropower and human consumption, as well as for the risk of wildfire, forest die-off, and loss of riparian habitat.
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  • 5
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    Projections of global changes in precipitation extremes from CMIP5 models (2013)
    Wiley
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    Publication Date: 2013-09-10
    Description: [1]  Precipitation extremes are expected to increase in a warming climate, thus it is essential to characterise their potential future changes. Here we evaluate eight high-resolution Global Climate Model simulations in the twentieth century and provide new evidence on projected global precipitation extremes for the 21 st century. A significant intensification of daily extremes for all seasons is projected for the mid and high latitudes of both hemispheres at the end of the present century. For the subtropics and tropics, the lack of reliable and consistent estimations found for both the historical and future simulations might be connected with model deficiencies in the representation of organised convective systems. Low inter-model variability and good agreement with high-resolution regional observations are found for the twentieth century winter over the Northern Hemisphere mid and high latitudes.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 6
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    Mid-depth mixing linked to north atlantic current variability (2013)
    Wiley
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    Publication Date: 2013-09-10
    Description: [1]  Repeated observations at the subpolar front west of the Mid-Atlantic Ridge show a link between the position of the North Atlantic Current (NAC), and the spatial distribution of finescale variance, diapycnal diffusivity, and integrated energy dissipation. Observational data were collected during three cruises. A single branch of the NAC was found in 2008, approximately at 50 ∘ 30Õ N, and two branches were observed in 2010 and 2011, with alternating intensities. Shear variance was elevatedbelow the core of the NAC in all cases, resulting in an average diapycnal diffusivity that is higher by a factor of 3 compared to the averages north and south of the NAC. The integrated energy dissipation has maxima at or south of the fronts; background dissipation is highest during 2008, also the year with the highest surface eddy kinetic energy.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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  • 7
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    The role of synoptic eddies in the tropospheric response to stratospheric variability (2013)
    Wiley
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    Publication Date: 2013-09-10
    Description: [1]  The tropospheric response to sudden stratospheric warmings (SSWs) is analyzed in an idealized model setup regarding the respective roles of planetary-scale and synoptic-scale waves. The control model run includes a full interactive wave spectrum, while a second run includes interactive planetary-scale waves but only the time-mean synoptic-scale wave forcing from the control run. In both runs the tropospheric response is characterized by the negative phase of the respective tropospheric annular mode. But given their different latitudinal structure, the control run shows the expected response, i.e. an equatorward shift of the tropospheric jet, whereas the response in the absence of interactive synoptic eddies is characterized by a poleward jet shift. This opposite jet shift is associated with a different planetary wave variability that couples with the zonal flow between the stratosphere and the surface. These results indicate that the synoptic eddy feedback is necessary for the observed tropospheric response to SSWs.
    Print ISSN: 0094-8276
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  • 8
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    Sensitivity of stratospheric dynamics and chemistry to QBO nudging width in the chemistry-climate model WACCM (2013)
    Wiley
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    Publication Date: 2013-09-10
    Description: [1]  The consequences of different Quasi-Biennial Oscillation (QBO) nudging widths on stratospheric dynamics and chemistry are analyzed by comparing two model simulations with NCAR's WACCM model where the width of the QBO is varied between 22° and 8.5°N and S. The sensitivity to the nudging width is strongest in Northern Hemisphere (NH) winter where the Holton-Tan effect in the polar stratosphere, i.e., stronger zonal mean winds during QBO west phases, is enhanced for the wider compared to the narrower nudging case. The differences between QBO west and east conditions for the two model experiments can be explained with differences in wave propagation, wave-mean flow interaction and the residual circulation. In the wider nudging case a divergence anomaly in the mid-latitude upper stratosphere/lower mesosphere occurs together with an equatorward anomaly of the residual circulation. This seems to result in a strengthening of the meridional temperature gradient and hence a significant strengthening of the polar night jet (PNJ). In the narrower nudging case these circulation changes are weaker and not statistically significant, consistent with a weaker and less significant impact on the PNJ. Chemical tracers like ozone, water vapour and methane react accordingly. From a comparison of westerly minus easterly phase composite differences in the model to reanalysis and satellite data we conclude that the standard WACCM configuration (QBO22) generates more realistic QBO effects in stratospheric dynamics and chemistry during NH winter. Our study also confirms the importance of the secondary mean meridional circulation associated with the QBO for the Holton-Tan effect.
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  • 9
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    Density-driven transport in the umbrella region of volcanic clouds: implications for tephra dispersion models (2013)
    Wiley
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    Publication Date: 2013-09-10
    Description: [1]  Large explosive volcanic eruptions can generate ash clouds from rising plumes that spread in the atmosphere around a Neutral Buoyancy Level (NBL). These ash clouds spread as inertial intrusions and are advected by atmospheric winds. For low mass flow rates, tephra transport is mainly dictated by wind advection, because ash cloud spreading due to gravity current effects is negligible (passive transport). For large mass flow rates, gravity driven transport at the NBL can be the dominant transport mechanism. Conditions under which the passive transport assumption is valid have not yet been critically studied. We analyze the conditions when gravity-driven transport is dominant in terms of the cloud Richardson number. Moreover, we couple an analytical model that describes cloud spreading as a gravity current with an advection–diffusion model. This coupled model is used to simulate the evolution of the volcanic cloud during the climatic phase of the 1991 Pinatubo eruption.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
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  • 10
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    Geographical versus dynamically defined boundary layer cloud regimes and their use to evaluate general circulation model cloud parameterizations (2013)
    Wiley
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    Publication Date: 2013-09-10
    Description: [1]  Regimes of tropical low-level clouds are commonly identified according to large-scale subsidence and lower tropospheric stability (LTS). This definition alone is insufficient for the distinction between regimes and limits the comparison of low-level clouds from CloudSat radar observations and the ECHAM5 GCM run with the COSP radar simulator. Comparisons of CloudSat radar cloud altitude-reflectivity histograms for stratocumulus and shallow cumulus regimes, as defined above, show nearly identical reflectivity profiles because the distinction between the two regimes is dependent upon atmospheric stability below 700 hPa and observations above 1.5 km. Regional subsets, near California and Hawaii for example, have large differences in reflectivityprofiles than the dynamically defined domain; indicating different reflectivity profiles exist under a given large-scale environment. Regional subsets are better for the evaluation of low-level clouds in CloudSat and ECHAM5 as there is less contamination between 2.5 km to 7.5 km from precipitating hydrometeors which obscured cloud reflectivities.
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