| 1. |
- Andrews, Timothy, et al.
(author)
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On the Effect of Historical SST Patterns on Radiative Feedback
- 2022
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In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 127:18
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Journal article (peer-reviewed)abstract
- We investigate the dependence of radiative feedback on the pattern of sea-surface temperature (SST) change in 14 Atmospheric General Circulation Models (AGCMs) forced with observed variations in SST and sea-ice over the historical record from 1871 to near-present. We find that over 1871–1980, the Earth warmed with feedbacks largely consistent and strongly correlated with long-term climate sensitivity feedbacks (diagnosed from corresponding atmosphere-ocean GCM abrupt-4xCO2 simulations). Post 1980, however, the Earth warmed with unusual trends in tropical Pacific SSTs (enhanced warming in the west, cooling in the east) and cooling in the Southern Ocean that drove climate feedback to be uncorrelated with—and indicating much lower climate sensitivity than—that expected for long-term CO2 increase. We show that these conclusions are not strongly dependent on the Atmospheric Model Intercomparison Project (AMIP) II SST data set used to force the AGCMs, though the magnitude of feedback post 1980 is generally smaller in nine AGCMs forced with alternative HadISST1 SST boundary conditions. We quantify a “pattern effect” (defined as the difference between historical and long-term CO2 feedback) equal to 0.48 ± 0.47 [5%–95%] W m−2 K−1 for the time-period 1871–2010 when the AGCMs are forced with HadISST1 SSTs, or 0.70 ± 0.47 [5%–95%] W m−2 K−1 when forced with AMIP II SSTs. Assessed changes in the Earth's historical energy budget agree with the AGCM feedback estimates. Furthermore satellite observations of changes in top-of-atmosphere radiative fluxes since 1985 suggest that the pattern effect was particularly strong over recent decades but may be waning post 2014.
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| 2. |
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| 3. |
- Breton, Simon-Philippe, et al.
(author)
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Study Of The Influence Of Atmospheric turbulence On The Asymptotic wake Deficit In A very Long Line Of Wind Turbines
- 2013
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In: Proceedings of the 2013 International Conference on Aerodynamics of Offshore Wind Energy Systems and Wakes (ICOWES 2013). - Denmark. ; , s. 420-434
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Conference paper (peer-reviewed)abstract
- The influence of atmospheric turbulence on the development of the flow along a long row of wind turbines is studied, in search for an asymptotic wake deficit state. Calculations are performed using EllipSys3D, a CFD code that solves the Navier-Stokes equations in their incompressible form using a finite volume approach. In this code, the Large Eddy Simulation technique is used for modelling turbulence, and the wind turbine rotors are represented as actuator disks whose loading is determined through the use of tabulated airfoil data by applying the blade-element method.Ten turbines are located along a row and separated from each other by seven rotor diameters, which is representative of the distance used in today’s offshore wind farms. Turbulence is pregenerated with the Mann model, with imposed turbulent levels of 4.5 and 8.9%. The turbines are in this study isolated from their environment, as no effect from the ground is modeled. This makes the proposed study of the asymptotic wake state behavior easier. Analysis of the characteristics of the flow as a function of the position along the row of turbines is performed in terms of turbulence intensity, mean velocity, and power spectra of the velocity fluctuations. Power production along the row of turbines is also used as an indicator.Calculations are performed below rated power, where a generator torque controller implemented in EllipSys3D renders it possible for the turbines to adapt to the inlet conditions in which they operate.The results obtained for the turbulence intensity, power and mean velocity as a function of downstream distance show that an asymptotic wake state seems close to be reached near the end of the 10 turbine row. They also show a certain dependency on the imposed level of turbulence. Uncertainties obtained in the power spectra of the velocity fluctuations suggest that further investigation is necessary.
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| 4. |
- Loeb, Norman G., et al.
(author)
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New Generation of Climate Models Track Recent Unprecedented Changes in Earth's Radiation Budget Observed by CERES
- 2020
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 47:5
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Journal article (peer-reviewed)abstract
- We compare top-of-atmosphere (TOA) radiative fluxes observed by the Clouds and the Earth's Radiant Energy System (CERES) and simulated by seven general circulation models forced with observed sea-surface temperature (SST) and sea-ice boundary conditions. In response to increased SSTs along the equator and over the eastern Pacific (EP) following the so-called global warming hiatus of the early 21st century, simulated TOA flux changes are remarkably similar to CERES. Both show outgoing shortwave and longwave TOA flux changes that largely cancel over the west and central tropical Pacific, and large reductions in shortwave flux for EP low-cloud regions. A model's ability to represent changes in the relationship between global mean net TOA flux and surface temperature depends upon how well it represents shortwave flux changes in low-cloud regions, with most showing too little sensitivity to EP SST changes, suggesting a pattern effect that may be too weak compared to observations.
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| 5. |
- Olivares-Espinosa, Hugo, et al.
(author)
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Assessment of Turbulence Modelling in the Wake of an Actuator Disk with a Decaying Turbulence Inflow
- 2018
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In: Applied Sciences. - : MDPI. - 2076-3417. ; 8:9
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Journal article (peer-reviewed)abstract
- The characteristics of the turbulence field in the wake produced by a wind turbine model are studied. To this aim, a methodology is developed and applied to replicate wake measurements obtained in a decaying homogeneous turbulence inflow produced by a wind tunnel. In this method, a synthetic turbulence field is generated to be employed as an inflow of Large-Eddy Simulations performed to model the flow development of the decaying turbulence as well as the wake flow behind an actuator disk. The implementation is carried out on the OpenFOAM platform, resembling a well-documented procedure used for wake flow simulations. The proposed methodology is validated by comparing with experimental results, for two levels of turbulence at inflow and disks with two different porosities. It is found that mean velocities and turbulent kinetic energy behind the disk are well estimated. The development of turbulence lengthscales behind the disk resembles what is observed in the free flow, predicting the ambient turbulence lengthscales to dominate across the wake, with little effect of shear from the wake envelope. However, observations of the power spectra confirm that shear yields a boost to the turbulence energy within the wake noticeable only in the low turbulence case. The results obtained show that the present implementation can successfully be used in the modelling and analysis of turbulence in wake flows.
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| 6. |
- Olivares-Espinosa, Hugo, et al.
(author)
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Characterization of wind turbine wakes in a non-sheared turbulent flow
- 2013
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In: Proceedings of the 2013 International Conference on Aerodynamics of Offshore Wind Energy Systems and Wakes (ICOWES2013). - Denmark. ; , s. 435-449
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Conference paper (peer-reviewed)abstract
- The characteristics of the turbulence field in wakes produced by horizontal-axis wind turbines under a uniform inflow are studied. To this aim, LES simulations of a rotor subjected to different types of inflow are performed with OpenFOAM. Two different Actuator Disk (AD) models for the rotor are implemented with the goal of assessing their capabilities to reproduce the turbulence characteristics of the wake: a uniformly loaded rotor and an AD model based on the blade element theory that employs tabulated airfoil data to calculate the distribution of forces over the disk as well as the geometry and other physical parameters from a conceptual 5 MW offshore wind turbine. The latter AD implementation makes use of a control system to adjust the rotational velocity of the rotor to the conditions of the wind flow.The turbulent wakes produced by the two AD models are analyzed over three different inflow conditions: laminar and two turbulent inflows of intensities 3.2 % and 6.7 % measured at the location of the rotor when the turbine is not present. The turbulence is pre-generated using the Mann model, that produces turbulent fields with the same second order statistics of the atmospheric turbulence. The turbulence is introduced in the computational domain at a position ahead of the rotor instead of at the inlet, to minimize its decay as it is convected downstream in the domain.A study of the turbulence evolution in the absence of the rotor reveals a discrepancy between the values of the turbulence intensity of the pre-generated field when it is measured before and after it has been introduced in the computational domain. This in turn suggests a numerical issue caused by imposing a turbulent velocity field in the flow. However, once the turbulence is introduced, it is found to exhibit a very small decay as the flow moves downstream.The simulations of the turbulent wake show differences in the turbulence characteristics of the near wake of each AD model. We also observe a dissimilar behaviour of the turbulence intensity along the wake for the different inflow conditions which difficult the task of determining a conclusive trend. Nonetheless, the results show that the turbulence characteristics in the far wake are nearly independent of the AD model or the inflow turbulence characteristics.This new implementation of the AD techniques and the use of the Mann method in OpenFOAM can be also proved useful in the future when comparing with the results of similar studies for which other CFD platforms are used.
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| 7. |
- Rugenstein, Maria, et al.
(author)
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Equilibrium Climate Sensitivity Estimated by Equilibrating Climate Models
- 2020
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 47:4
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Journal article (peer-reviewed)abstract
- The methods to quantify equilibrium climate sensitivity are still debated. We collect millennial-length simulations of coupled climate models and show that the global mean equilibrium warming is higher than those obtained using extrapolation methods from shorter simulations. Specifically, 27 simulations with 15 climate models forced with a range of CO2 concentrations show a median 17% larger equilibrium warming than estimated from the first 150 years of the simulations. The spatial patterns of radiative feedbacks change continuously, in most regions reducing their tendency to stabilizing the climate. In the equatorial Pacific, however, feedbacks become more stabilizing with time. The global feedback evolution is initially dominated by the tropics, with eventual substantial contributions from the mid-latitudes. Time-dependent feedbacks underscore the need of a measure of climate sensitivity that accounts for the degree of equilibration, so that models, observations, and paleo proxies can be adequately compared and aggregated to estimate future warming.
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| 8. |
- Rugenstein, Maria, et al.
(author)
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LongRunMIP : Motivation and Design for a Large Collection of Millennial-Length AOGCM Simulations
- 2019
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In: Bulletin of The American Meteorological Society - (BAMS). - 0003-0007 .- 1520-0477. ; 100:12, s. 2551-2570
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Journal article (peer-reviewed)abstract
- We present a model intercomparison project, LongRunMIP, the first collection of millennial-length (1,000+ years) simulations of complex coupled climate models with a representation of ocean, atmosphere, sea ice, and land surface, and their interactions. Standard model simulations are generally only a few hundred years long. However, modeling the long-term equilibration in response to radiative forcing perturbation is important for understanding many climate phenomena, such as the evolution of ocean circulation, time- and temperature-dependent feedbacks, and the differentiation of forced signal and internal variability. The aim of LongRunMIP is to facilitate research into these questions by serving as an archive for simulations that capture as much of this equilibration as possible. The only requirement to participate in LongRunMIP is to contribute a simulation with elevated, constant CO2 forcing that lasts at least 1,000 years. LongRunMIP is an MIP of opportunity in that the simulations were mostly performed prior to the conception of the archive without an agreed-upon set of experiments. For most models, the archive contains a preindustrial control simulation and simulations with an idealized (typically abrupt) CO2 forcing. We collect 2D surface and top-of-atmosphere fields and 3D ocean temperature and salinity fields. Here, we document the collection of simulations and discuss initial results, including the evolution of surface and deep ocean temperature and cloud radiative effects. As of October 2019, the collection includes 50 simulations of 15 models by 10 modeling centers. The data of LongRunMIP are publicly available. We encourage submissions of more simulations in the future.
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| 9. |
- Seneviratne, Sonia I., et al.
(author)
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Impact of soil moisture-climate feedbacks on CMIP5 projections: First results from the GLACE-CMIP5 experiment
- 2013
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In: Geophysical Research Letters. - : American Geophysical Union (AGU). - 1944-8007 .- 0094-8276. ; 40:19, s. 5212-5217
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Journal article (peer-reviewed)abstract
- The Global Land-Atmosphere Climate Experiment-Coupled Model Intercomparison Project phase 5 (GLACE-CMIP5) is a multimodel experiment investigating the impact of soil moisture-climate feedbacks in CMIP5 projections. We present here first GLACE-CMIP5 results based on five Earth System Models, focusing on impacts of projected changes in regional soil moisture dryness (mostly increases) on late 21st century climate. Projected soil moisture changes substantially impact climate in several regions in both boreal and austral summer. Strong and consistent effects are found on temperature, especially for extremes (about 1-1.5K for mean temperature and 2-2.5K for extreme daytime temperature). In the Northern Hemisphere, effects on mean and heavy precipitation are also found in most models, but the results are less consistent than for temperature. A direct scaling between soil moisture-induced changes in evaporative cooling and resulting changes in temperature mean and extremes is found in the simulations. In the Mediterranean region, the projected soil moisture changes affect about 25% of the projected changes in extreme temperature.
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| 10. |
- Smith, Christopher J., et al.
(author)
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Effective radiative forcing and adjustments in CMIP6 models
- 2020
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In: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 20:16, s. 9591-9618
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Journal article (peer-reviewed)abstract
- The effective radiative forcing, which includes the instantaneous forcing plus adjustments from the atmosphere and surface, has emerged as the key metric of evaluating human and natural influence on the climate. We evaluate effective radiative forcing and adjustments in 17 contemporary climate models that are participating in the Coupled Model Intercomparison Project (CMIP6) and have contributed to the Radiative Forcing Model Intercomparison Project (RFMIP). Present-day (2014) global-mean anthropogenic forcing relative to pre-industrial (1850) levels from climate models stands at 2.00 (+/- 0.23) W m(-2), comprised of 1.81 (+/- 0.09) Wm(-2) from CO2, 1.08 (+/- 0.21) Wm(-2) from other well-mixed greenhouse gases, -1.01 (+/- 0.23) W m(-2) from aerosols and -0.09 (+/- 0.13) W m(-2) from land use change. Quoted uncertainties are 1 standard deviation across model best estimates, and 90 % confidence in the reported forcings, due to internal variability, is typically within 0.1 W m(-2). The majority of the remaining 0.21 W m(-2) is likely to be from ozone. In most cases, the largest contributors to the spread in effective radiative forcing (ERF) is from the instantaneous radiative forcing (IRF) and from cloud responses, particularly aerosol-cloud interactions to aerosol forcing. As determined in previous studies, cancellation of tropospheric and surface adjustments means that the stratospherically adjusted radiative forcing is approximately equal to ERF for greenhouse gas forcing but not for aerosols, and consequentially, not for the anthropogenic total. The spread of aerosol forcing ranges from -0.63 to -1.37 W m(-2), exhibiting a less negative mean and narrower range compared to 10 CMIP5 models. The spread in 4 x CO2 forcing has also narrowed in CMIP6 compared to 13 CMIP5 models. Aerosol forcing is uncorrelated with climate sensitivity. Therefore, there is no evidence to suggest that the increasing spread in climate sensitivity in CMIP6 models, particularly related to high-sensitivity models, is a consequence of a stronger negative present-day aerosol forcing and little evidence that modelling groups are systematically tuning climate sensitivity or aerosol forcing to recreate observed historical warming.
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