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  • 1
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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
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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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
    Topics: Geosciences , Physics
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  • 3
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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.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 4
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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.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 5
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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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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 6
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    Deep moist atmospheric convection in a sub-kilometer global simulation (2013)
    Wiley
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    Publication Date: 2013-09-11
    Description: [1]  Deep moist atmospheric convection is a key element of the weather and climate system for transporting mass, momentum, and thermal energy. It has been challenging to simulate convection realistically in global atmospheric models, because of the large gap in spatial scales between convection (10 0  km) and global motions (10 4  km). We conducted the first ever sub-kilometer global simulation and described the features of convection. Through a series of grid-refinement resolution testing, we found that an essential change for convection statistics occurred around 2-km grid spacing. The convection structure, number of convective cells, and distance to the nearest convective cell dramatically changed at this resolution. The convection core was resolved using multiple grids in simulations with grid spacings less than 2.0 km.
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  • 7
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    El Nino, the 2006 Indonesian Peat Fires, and the distribution of atmospheric methane (2013)
    Wiley
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    Publication Date: 2013-09-11
    Description: [1]  Dry conditions from a moderate El Nino during the fall of 2006 resulted in enhanced burning in Indonesia with fire emissions of CO approximately 4–6 times larger than the prior year. Here we use new tropospheric methane and CO data from the Aura Tropospheric Emission Spectrometer (TES) and new CO profile measurements from the Terra Measurements of Pollution in the Troposphere (MOPITT) satellite instruments with the GEOS-Chem model to estimate methane emissions of 4.25 +/− 0.75 Tg for October-November 2006 from these fires. Errors in convective parameterization in GEOS-Chem, evaluated by comparing MOPITT and GEOS-Chem CO profiles, are the primary uncertainty of the emissions estimate. The El Nino related Indonesian fires increased the tropical distribution of atmospheric methane relative to 2005, indicating that tropical biomass burning can compensate for expected decreases in tropical wetland methane emissions from reduced rainfall during El Nino as found in previous studies.
    Print ISSN: 0094-8276
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    Topics: Geosciences , Physics
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  • 8
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    Genesis of hurricane sandy (2012) simulated with a global mesoscale model (2013)
    Wiley
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    Publication Date: 2013-09-11
    Description: [1]  In this study, we investigate the formation predictability of Hurricane Sandy (2012) with a global mesoscale model. We first present five track and intensity forecasts of Sandy initialized at 00Z October 22-26, 2012, realistically producing its movement with a northwestward turn prior to its landfall. We then show that three experiments initialized at 00Z Oct. 16-18 captured the genesis of Sandy with a lead time of up to six days and simulated reasonable evolution of Sandy's track and intensity in the next two-day period of 18Z Oct. 21-23. Results suggest that the extended lead time of formation prediction is achieved by realistic simulations of multi-scale processes, including (1) the interaction between an easterly wave and a low-level westerly wind belt (WWB); (2) the appearance of the upper-level trough at 200-hPa to Sandy's northwest. The low-level WWB and upper-level trough are likely associated with a Madden-Julian Oscillation.
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    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
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  • 9
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    The influence of water vapor absorption in the 290-350 nm region on solar radiance: Laboratory studies and model simulation (2013)
    Wiley
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    Publication Date: 2013-09-11
    Description: [1]  Water vapor is an important greenhouse gas in the earth's atmosphere. Absorption of the solar radiation by water vapor in the near UV region may partially account for the up to 30% discrepancy between the modeled and the observed solar energy absorbed by the atmosphere. But the magnitude of water vapor absorption in the near UV region at wavelengths shorter than 384 nm is not known. We have determined absorption cross sections of water vapor at 5 nm intervals in the 290-350 nm region, by using cavity ring-down spectroscopy. Water vapor cross section values range from 2.94 × 10 -24 to 2.13 × 10 -25  cm 2 /molecule in the wavelength region studied. The effect of the water vapor absorption in the 290-350 nm region on the modeledradiation flux at the ground level has been evaluated using radiative transfer model.
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    Topics: Geosciences , Physics
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  • 10
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    Atmosphere drives recent interannual variability of the Atlantic meridional overturning circulation at 26.5°N (2013)
    Wiley
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    Publication Date: 2013-09-12
    Description: [1]  The RAPID-MOCHA array has observed the Atlantic Meridional overturning circulation (AMOC) at 26.5°N since 2004. During 2009/2010, there was a transient 30% weakening of the AMOC driven by anomalies in geostrophic and Ekman transports. Here, we use simulations based on the Met Office Forecast Ocean Assimilation Model (FOAM) to diagnose the relative importance of atmospheric forcings and internal ocean dynamics in driving the anomalous geostrophic circulation of 2009/10. Data assimilating experiments with FOAM accurately reproduce the mean strength and depth of the AMOC at 26.5°N. In addition, agreement between simulated and observed stream functions in the deep ocean is improved when we calculate the AMOC using a method that approximates the RAPID observations. The main features of the geostrophic circulation anomaly are captured by an ensemble of simulations without data-assimilation. These model results suggest that the atmosphere played a dominant role in driving recent interannual variability of the AMOC.
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    Topics: Geosciences , Physics
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