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  • 1
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    Aerosol-cloud interactions in climate [Physical Sciences] (2016)
    National Academy of Sciences
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    Publication Date: 2016-05-25
    Description: The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth’s clouds is the most uncertain component of the overall global radiative forcing from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and...
    Keywords: Sackler Colloquium on Improving Our Fundamental Understanding of the Role of Aerosol&ndash ; Cloud Int
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
    Published by National Academy of Sciences
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  • 2
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    Quantifying the Drivers of the Clear Sky Greenhouse Effect, 2000–2016 (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract The clear sky greenhouse effect (G) is defined as the trapping of infrared radiation by the atmosphere in the absence of clouds. The magnitude and variability of G is an important element in the understanding of Earth's energy balance; yet the quantification of the governing factors of G is poor. The global mean G averaged over 2000 to 2016 is 130–133 W m−2 across data sets. We use satellite observations from Clouds and the Earth's Radiant Energy System Energy Balance and Filled (CERES EBAF) to calculate the monthly anomalies in the clear sky greenhouse effect (ΔG). We quantify the contributions to ΔG due to changes in surface temperature, atmospheric temperature, and water vapor by performing partial radiation perturbation experiments using ERA‐Interim and Geophysical Fluid Dynamics Laboratory's Atmospheric Model 4.0 climatological data. Water vapor in the middle troposphere and upper troposphere is found to contribute equally to the global mean and tropical mean ΔG. Holding relative humidity (RH) fixed in the radiative transfer calculations captures the temporal variability of global mean ΔG while variations in RH control the regional ΔG signal. The variations in RH are found to help generate the clear sky super greenhouse effect (SGE). Thirty‐six percent of Earth's area exhibits SGE, and this disproportionately contributes to 70% of the globally averaged magnitude of ΔG. In the global mean, G's sensitivity to surface temperature is 3.1–4.0 W m−2 K−1, and the clear sky longwave feedback parameter is 1.5–2.0 W m−2 K−1. Observations from CERES EBAF lie at the more sensitive ends of these ranges and the spread arises from its cloud removal treatment, suggesting that it is difficult to constrain clear sky feedbacks.
    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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  • 3
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    Tropical Climate Change Control of the Lower Stratospheric Circulation (2015)
    Wiley
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    Publication Date: 2015-01-28
    Description: The behavior of the Brewer-Dobson circulation is investigated using a suite of global climate model simulations with different forcing agents, in conjunction with observation-based analysis. We find that the variations in the Brewer-Dobson circulationare strongly correlated with those in the tropical-mean surface temperature through changes in the upper tropospheric temperature and zonal winds. This correlation is seen on both interannual and multi-decadal timescales, and holds for natural and forced variations alike. The circulation change is relatively insensitive to the spatial pattern of the forcings. Consistent changes in the Brewer-Dobson circulation with respect to those in the tropical-mean surface temperature prevail across timescales and forcings, and constitute an important attribution element of the atmospheric adjustment to global climate change.
    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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  • 4
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    Variations in water vapor continuum radiative transfer with atmospheric conditions (2012)
    Wiley
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    Publication Date: 2012-08-29
    Description: A newly formulated empirical water vapor continuum (the “BPS continuum”) is employed, in conjunction with ERA-40 data, to advance the understanding of how variations in the water vapor profile can alter the impact of the continuum on the Earth's clear-sky radiation budget. Three metrics are investigated: outgoing longwave radiation (OLR), Longwave surface downwelling radiation (SDR) and shortwave absorption (SWA). We have also performed a detailed geographical analysis on the impact of the BPS continuum upon these metrics and compared the results to those predicted by the commonly used MT CKD model. The globally averaged differences in these metrics when calculated with MT CKD 2.5 versus BPS were found to be 0.1%, 0.4% and 0.8% for OLR, SDR and SWA respectively. Furthermore, the impact of uncertainty upon these calculations is explored using the uncertainty estimates of the BPS model. The radiative response of the continuum to global changes in atmospheric temperature and water vapor content are also investigated. For the latter, the continuum accounts for up to 35% of the change in OLR and 65% of the change in SDR, brought about by an increase in water vapor in the tropics.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    An assessment of recent water vapor continuum measurements upon longwave and shortwave radiative transfer (2011)
    Wiley
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    Publication Date: 2011-10-18
    Description: Recent measurements of the water vapor continuum have been combined to form an empirical continuum termed the BPS continuum model. This covers the 800 to 7500 cm−1 spectral region for the self continuum and most of the major absorbing spectral regions between 240 and 7300 cm−1 for the foreign continuum. Longwave (i.e., absorption/emission of terrestrial radiation between 1 and 3000 cm−1) and shortwave (i.e., using solar radiation as a source and considering atmospheric absorption between 1000 and 17000 cm−1) line by line (LBL) radiative transfer calculations have been performed for clear-sky conditions in three standard test atmospheres using line data from the HITRAN database. This has allowed BPS to be compared to the commonly used CKD and MT CKD continuum models, in addition to conducting a more detailed investigation of the separate roles of the self and foreign continua than previously provided in the literature. Using uncertainties obtained from multiple experimental studies it has been possible to estimate the upper and lower limits of the effects due to the continuum in many spectral regions. The outgoing longwave radiation in a midlatitude-summer (MLS) atmosphere calculated by all three continuum models agree to within 0.6 Wm−2 with a ±1.1 Wm−2 estimated uncertainty. The corresponding values for surface downwelling radiation are 1.3 Wm−2 ± 2.5 Wm−2. For shortwave absorption, the different models agree within 1.0%, with an estimated uncertainty of ±1.7%. However, the three models differ in the amount by which the self and foreign continua contribute to shortwave absorption.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Analysis of the biases in the downward shortwave surface flux in the GFDL CM2.1 general circulation model (2011)
    Wiley
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    Publication Date: 2011-04-22
    Description: Simulations of downward shortwave surface fluxes by the coupled Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 general circulation model are compared against climatology derived from the Baseline Surface Radiation Network (BSRN), Global Energy Balance Archive, and International Satellite Cloud Climatology Project ISCCP-FD data sets. The spatial pattern of the model's biases is evaluated. An investigation is made of how these relate to accompanying biases in total cloud amount and aerosol optical depth and how they affect the surface temperature simulation. Comparing CM2.1's clear-sky fluxes against BSRN site values, for European, Asian, and North American locations, there are underestimates in the direct and overestimates in the diffuse, resulting in underestimates in the total flux. These are related to overestimates of sulfate aerosol optical depth, arising owing to the behavior of the parameterization function for hygroscopic growth of these aerosols at very high relative humidity. Contrastingly, flux overestimate biases at lower latitude locations are associated with underestimates in sea-salt and carbonaceous aerosol amounts. All-sky flux biases consist of underestimates for North America, Eurasia, southern Africa, and northern oceanic regions and overestimates for the Amazon region, equatorial Africa, off the west coast of the Americas, and southern oceanic regions. These biases show strong correlations with cloud amount biases. There are modest correlations of the flux biases with cool surface temperature biases over North America and Eurasia, warm biases over the Amazon region, and cool (warm) biases over the northern (southern) oceanic regions. Analyses assuming nonhygroscopicity illustrate that there is a reduction of surface temperature biases accompanying a reduction of sulfate aerosol optical depth biases, whereas a more significant improvement in the temperature simulation requires refining the model's simulation of cloudiness.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    The Role of Aerosol Absorption in Driving Clear-Sky Solar Dimming over East Asia (2014)
    Wiley
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    Publication Date: 2014-08-15
    Description: . Surface-based observations indicate a significant decreasing trend in clear-sky downward surface solar radiation (SSR) over East Asia since the 1960s. This “dimming" is thought to be driven by the region's long-term increase in aerosol emissions, butlittle work has been done to quantify the underlying physical mechanisms or the contribution from aerosol absorption within the atmospheric column. Given the distinct climate impacts that absorption-driven dimming may produce, this constitutes an important, but thus far rather neglected, line of inquiry. We examine experiments conducted in the Geophysical Fluid Dynamics Laboratory's Atmospheric General Circulation Models, AM2.1 and AM3, in order to analyze the model-simulated East Asian clear-sky SSR trends. We also use the models’ standalone radiation module to examine the contribution from various aerosol characteristics in the two models (such as burden, mixing state, hygroscopicity, and seasonal distribution) to the trends. Both models produce trends in clear-sky SSR that are comparable tothat observed, but via disparate mechanisms. Despite their different aerosol characteristics, the models produce nearly identical increases in aerosol absorption since the 1960s, constituting as much as half of the modeled clear-sky dimming. This is due to a compensation between the differences in aerosol column burden and mixing state assumed in the two models, i.e. plausible clear-sky SSR simulations can be achieved via drastically different aerosol parameterizations. Our novel results indicatethat trends in aerosol absorption drive a large portion of East Asian clear-sky solar dimming in the models presented here and for the time periods analyzed, and that mechanistic analysis of the factors involved in aerosol absorption is an important diagnostic in evaluating modeled clear-sky solar dimming trends.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Late Holocene primary productivity and sea surface temperature variations in the northeastern Arabian Sea: implications for winter monsoon variability (2014)
    Wiley
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    Publication Date: 2014-07-19
    Description: Variability in the oceanic environment of the Arabian Sea region is strongly influenced by the seasonal monsoon cycle of alternating wind directions. Prominent and well-studied is the summer monsoon, but much less is known about late Holocene changes in winter monsoon strength with winds from the northeast that drive convective mixing and high surface ocean productivity in the northeastern Arabian Sea. To establish a high-resolution record of winter monsoon variability for the late Holocene, we analyzed alkenone-derived sea surface temperature (SST) variations and proxies of primary productivity (organic carbon and δ 15 N) in a well-laminated sediment core from the Pakistan continental margin. Weak winter monsoon intensities off Pakistan are indicated from 400 B.C. to 250 A.D. by reduced productivity and relatively high SST. At about 250 A.D. the intensity of the winter monsoon increased off Pakistan as indicated by a trend to lower SST. We infer that monsoon conditions were relatively unstable from ~500 to 1300 A.D., because primary production and SST were highly variable. Declining SST and elevated biological production from 1400 to 1900 A.D. suggest invigorated convective winter mixing by strengthening winter monsoon circulation, most likely a regional expression of colder climate conditions during the Little Ice Age on the Northern Hemisphere. The comparison of winter monsoon intensity with records of summer monsoon intensity suggests that an inverse relationship between summer and winter monsoon strength exists in the Asian monsoon system during the late Holocene, effected by shifts in the Intertropical Convergence Zone.
    Print ISSN: 0883-8305
    Electronic ISSN: 1944-9186
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
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    Investigating the impact of the shortwave water vapor continuum upon climate simulations using GFDL global models (2014)
    Wiley
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    Publication Date: 2014-08-28
    Description: We have added the BPS-MTCKD 2.0 parameterization for the shortwave water vapor continuum to the GFDL global model. We find that inclusion of the shortwave continuum in the fixed SST case (AM3) results in a similar increase in shortwave absorption and heating rates to that seen for the ‘benchmark’ line-by-line radiative transfer calculations. The surface energy budget adjusts to the inclusion of the shortwave continuum predominantly through a decrease in both surface latent and sensible heat. This leads to a decrease in tropical convection and a subsequent 1% reduction in tropical rainfall. The inclusion of the shortwave continuum in the fully coupled atmosphere–ocean model (CM3) yields similar results, but a smaller overall reduction of 0.5% in tropical rainfall due to global warming of ~0.1 K linked to enhanced near infrared absorption. We also investigated the impact of adding a stronger version of BPS-MTCKD (version 1.1) to the GCM. In most cases we found that the GCM responds in a similar manner to both continua, but that the strength of the response scales with the level of absorbed shortwave radiation. Global warming experiments were run in both AM3 and CM3. The shortwave continuum was found to cause a 7 to 15% increase in clear-sky global dimming depending upon whether the stronger or weaker continuum version was used. Neither version resulted in a significant change to the climate sensitivity.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
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    Tropospheric Aerosol Climate Forcing in Clear-Sky Satellite Observations over the Oceans (1999)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 1999-02-26
    Description: Tropospheric aerosols affect the radiative forcing of Earth's climate, but their variable concentrations complicate an understanding of their global influence. Model-based estimates of aerosol distributions helped reveal spatial patterns indicative of the presence of tropospheric aerosols in the satellite-observed clear-sky solar radiation budget over the world's oceans. The results show that, although geographical signatures due to both natural and anthropogenic aerosols are manifest in the satellite observations, the naturally occurring sea-salt is the leading aerosol contributor to the global-mean clear-sky radiation balance over oceans.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Haywood -- Ramaswamy V -- Soden -- New York, N.Y. -- Science. 1999 Feb 26;283(5406):1299-1303.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Meteorological Research Flight, United Kingdom Meteorological Office, Farnborough, Hants GU14 0LX, UK. Geophysical Fluid Dynamics Laboratory, Princeton University, Princeton, NJ 08542, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/10037595" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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