| 1. |
- Artaxo, Paulo, et al.
(författare)
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Tropical and Boreal Forest – Atmosphere Interactions : A Review
- 2022
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 74:1, s. 24-163
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Forskningsöversikt (refereegranskat)abstract
- This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiäla in Finland. The review is complemented by short-term observations from networks and large experiments.The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction.Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink.It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.
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| 2. |
- Baró Pérez, Alejandro, et al.
(författare)
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Comparing the simulated influence of biomass burning plumes on low-level clouds over the southeastern Atlantic under varying smoke conditions
- 2024
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Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 24:8, s. 4591-4610
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Tidskriftsartikel (refereegranskat)abstract
- Biomass burning plumes are frequently transported over the southeast Atlantic (SEA) stratocumulus deck during the southern African fire season (June-October). The plumes bring large amounts of absorbing aerosols and enhanced moisture, which can trigger a rich set of aerosol-cloud-radiation interactions with climatic consequences that are still poorly understood. We use large-eddy simulation (LES) to explore and disentangle the individual impacts of aerosols and moisture on the underlying stratocumulus clouds, the marine boundary layer (MBL) evolution, and the stratocumulus-to-cumulus transition (SCT) for three different meteorological situations over the southeast Atlantic during August 2017. For all three cases, our LES shows that the SCT is driven by increased sea surface temperatures and cloud-top entrainment as the air is advected towards the Equator. In the LES model, aerosol indirect effects, including impacts on drizzle production, have a small influence on the modeled cloud evolution and SCT, even when aerosol concentrations are lowered to background concentrations. In contrast, local semi-direct effects, i.e., aerosol absorption of solar radiation in the MBL, cause a reduction in cloud cover that can lead to a speed-up of the SCT, in particular during the daytime and during broken cloud conditions, especially in highly polluted situations. The largest impact on the radiative budget comes from aerosol impacts on cloud albedo: the plume with absorbing aerosols produces a total average 3 d of simulations. We find that the moisture accompanying the aerosol plume produces an additional cooling effect that is about as large as the total aerosol radiative effect. Overall, there is still a large uncertainty associated with the radiative and cloud evolution effects of biomass burning aerosols. A comparison between different models in a common framework, combined with constraints from in situ observations, could help to reduce the uncertainty.
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| 3. |
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A Beginner's Guide to Swedish Academia
- 2022
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Samlingsverk (redaktörskap) (övrigt vetenskapligt/konstnärligt)abstract
- As new to the Swedish research system, one is faced with a series of questions, about what applies to qualifications, what the networks look like, but also practical issues. To make things easier, YAS has developed a guide for international researchers, to help navigate Swedish academia and remove time-consuming obstacles.
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| 4. |
- Bender, Frida A-M, 1978-
(författare)
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Earth's albedo in a changing climate
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The albedo is a key parameter in the radiative budget of the Earth and a primary determinant of the planetary temperature and is therefore also central to questions regarding climate stability, climate change and climate sensitivity. Climate models and satellite observations are essential for studying the albedo, and the parameters determining it, on large spatial and temporal scales. Although climate models are able to capture the large-scale characteristics of the albedo, a bias is found between modelled and observed global albedo estimates, and on a regional scale particular problematic regions can be identified. Cloud parameters, that are of great importance for determining the albedo, vary widely among models, but lack of observations makes constraining models, and even evaluating models, difficult. The freedom of variability for cloud parameters can be used to make models agree with observations of the better constrained radiative budget. It is shown that tuning a model to different radiative budget estimates by altering cloud parameters can influence the climate sensitivity of the model, but the effect seen is small, compared to the range of climate sensitivities estimated by different models. Despite their different parameterizations of clouds, aerosols etc., models do have fundamental features in common, which can further the understanding of the real climate system. For instance it is found that sensitivity to volcanic forcing is related to climate sensitivity in an ensemble of models. If this relation is valid for the real climate as well, observations of the volcanic sensitivity can help restrict the climate sensitivity. The range of climate sensitivity estimates in models can largely be attributed to variations in cloud response to forcing. It is found that in models with high climate sensitivity changes in cloud cover and cloud reflectivity enhance a positive radiative forcing due to increased CO2 concentrations, feeding back on the warming and in models with low climate sensitivity, cloud response counteracts the positive radiative forcing and warming induced by the same forcing. As a consequence the total albedo response to increased CO2 forcing is found to be stronger (more negative) in high sensitivity models and vice versa. Cloud albedo and its variation between different cloud regimes, is important in this regard, yet not well known. A method based on the relation between cloud fraction and albedo is presented, giving a way to estimate regional cloud albedo, primarily for homogeneous cloud regimes, but possibly also extended to a global scale.
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| 5. |
- Jönsson, Aiden, 1991-, et al.
(författare)
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A recipe for simulating the observed interhemispheric albedo symmetry and constraining cloud radiative feedbacks
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Earth’s albedo has remained symmetric between the northern and southern hemispheres over the satellite record, a feature that climate models have difficulty capturing. We investigate causes of these biases using a perturbed parameter ensemble of atmospheric simulations to probe the sensitivity of the albedo symmetry to cloud properties and the processes that control them. We find that the most significant parameters to simulated albedo symmetry impact precipitation, turbulent dissipation, and sea salt aerosol emissions. Constraining shortwave cloud feedbacks using the observed albedo symmetry leads to a range of +0.61±0.24 W m-2 K-1 (66% confidence). These are stronger than the model’s control settings due to greater loss of subtropical low clouds and weaker negative cloud phase feedback. Comparing the constrained and control parameter settings shows a preference towards settings that would reduce the control simulation’s biases, indicating that the constraint can select for representations that capture the observed cloud cover.
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| 6. |
- Jönsson, Aiden, 1991-, et al.
(författare)
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Net northward ocean heat transport modulates mean hemispheric cloud asymmetries
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Clouds in the Southern Hemisphere (SH) extratropics make up for the Northern Hemisphere (NH)’s greater tropical cloud cover and clear-sky albedo, making Earth’s planetary albedo hemispherically symmetric over the satellite record. Knowledge of a mechanism for maintaining hemispheric albedo symmetry would prove valuable for understanding cloud responses to external forcings. Using simulations of an Earth-like aquaplanet, we investigate the role of ocean heat transport (OHT) in determining hemispheric differences in cloud cover. With increasing northward cross-equatorial OHT, the SH becomes dominant in low cloud cover at all latitudes, while NH increases in high clouds are negated by reductions in low clouds. We describe a dynamical link between the increasing SH extratropical cloud cover and increasing NH tropical cloud cover with more northward cross-equatorial OHT. We investigate the effects of clouds and condensation on AHT responses, which increase southward AHT through latent heating in the extratropics and radiative effects in lower latitudes, aiding in reducing the hemispheric energy contrast. Because SH cloud increases are greater than NH cloud reductions, increasing cloud asymmetry with more northward cross-equatorial OHT leads to net increases in global cloud cover and cooling.
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| 7. |
- Jönsson, Aiden Robert, 1991-
(författare)
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Clouds and Earth's hemispheric albedo symmetry : How do clouds affect hemispheric contrasts in heat and energy flows?
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Earth's Northern and Southern Hemispheres (NH and SH, respectively) have significantly different properties: the NH has a higher concentration of bright land surface area and aerosol emissions than the SH, making the Earth's clear-sky albedo hemispherically asymmetric. However, satellite observations have shown that higher cloud amount and reflectivity in the SH exactly compensate for this, making Earth's planetary albedo hemispherically symmetric. A physical explanation for this symmetry has not yet been found, but because it would give constraints for global cloud cover and its features, discovery of one may be a powerful tool in predicting the behavior of clouds in a changing climate.The first chapter of this thesis investigates the hemispheric albedo symmetry in observations, and finds that its variability primarily stems from the tropics. General circulation models (GCMs) exhibit a large spread in albedo asymmetry biases; comparing these with observations reveals that the extratropics control mean-state modeled albedo asymmetry.The second chapter compares the evolution of albedo asymmetries in GCMs when forced with increased CO2 concentrations. Models agree on an initial asymmetry response due to Arctic warming and albedo reductions, but diverge thereafter, with some models recovering their pre-industrial asymmetry. Those that recover their asymmetry do so via SH extratropical cloud loss and thus have stronger positive cloud feedbacks, illustrating that an albedo symmetry-maintaining mechanism could have implications for climate sensitivity.Sources of modeled albedo asymmetry biases are investigated in a single atmospheric GCM using a perturbed parameter ensemble in the third chapter. The most significant parameters to simulated albedo asymmetry are those controlling warm rain formation, turbulent dissipation, and sea salt aerosol emissions. Parameters controlling warm rain formation and turbulent dissipation primarily affect extratropical low cloud cover, and those affecting ice particle formation disproportionately affects SH midlatitude albedo. Parameter settings that reproduce the observed albedo symmetry tend towards more strongly positive shortwave cloud feedbacks.The link between hemispheric asymmetries in clouds and large-scale circulation is investigated with idealized atmospheric GCM experiments in the fourth chapter. Introducing hemispheric asymmetry in ocean heat fluxes that emulate heat divergence (convergence) in the SH (NH) drives an atmospheric response that qualitatively reproduces the observed cloud distribution. We conclude that the hemispheric albedo symmetry is not possible without implicating surface forcing from ocean circulation and heat transport.
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| 8. |
- Jönsson, Aiden R., et al.
(författare)
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Persistence and Variability of Earth's Interhemispheric Albedo Symmetry in 19 Years of CERES EBAF Observations
- 2022
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 35:1, s. 249-268
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Tidskriftsartikel (refereegranskat)abstract
- Despite the unequal partitioning of land and aerosol sources between the hemispheres, Earth’s albedo is observed to be persistently symmetric about the equator. This symmetry is determined by the compensation of clouds to the clear-sky albedo. Here, the variability of this interhemispheric albedo symmetry is explored by decomposing observed radiative fluxes in the CERES EBAF satellite data record into components reflected by the atmosphere, clouds, and the surface. We find that the degree of interhemispheric albedo symmetry has not changed significantly throughout the observational record. The variability of the interhemispheric difference in reflected solar radiation (asymmetry) is strongly determined by tropical and subtropical cloud cover, particularly those related to nonneutral phases of El Niño–Southern Oscillation (ENSO). As ENSO is the most significant source of interannual variability in reflected radiation on a global scale, this underscores the interhemispheric albedo symmetry as a robust feature of Earth’s current annual mean climate. Comparing this feature in observations with simulations from coupled models reveals that the degree of modeled albedo symmetry is mostly dependent on biases in reflected radiation in the midlatitudes, and that models that overestimate its variability the most have larger biases in reflected radiation in the tropics. The degree of model albedo symmetry is improved when driven with historical sea surface temperatures, indicating that the degree of symmetry in Earth’s albedo is dependent on the representation of cloud responses to coupled ocean–atmosphere processes.
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| 9. |
- Jönsson, Aiden R., et al.
(författare)
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The implications of maintaining Earth's hemispheric albedo symmetry for shortwave radiative feedbacks
- 2023
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Ingår i: Earth System Dynamics. - : Copernicus GmbH. - 2190-4979 .- 2190-4987. ; 14:2, s. 345-365
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Tidskriftsartikel (refereegranskat)abstract
- The Earth's albedo is observed to be symmetric between the hemispheres on the annual mean timescale, despite the clear-sky albedo being asymmetrically higher in the Northern Hemisphere due to more land area and aerosol sources; this is because the mean cloud distribution currently compensates for the clear-sky asymmetry almost exactly. We investigate the evolution of the hemispheric difference in albedo in the Coupled Model Intercomparison Project Phase 6 (CMIP6) coupled model simulations following an abrupt quadrupling of CO2 concentrations, to which all models respond with an initial decrease of albedo in the Northern Hemisphere (NH) due to loss of Arctic sea ice. Models disagree over whether the net effect of NH cloud responses is to reduce or amplify initial NH albedo reductions. After the initial response, the evolution of the hemispheric albedo difference diverges among models, with some models remaining stably at their new hemispheric albedo difference and others returning towards their pre-industrial difference primarily through a reduction in SH cloud cover. Whereas local increases in cloud cover contribute to negative shortwave cloud feedback, the cross-hemispheric communicating mechanism found to be primarily responsible for restoring hemispheric symmetry in the models studied implies positive shortwave cloud feedback.
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| 10. |
- Kuma, Peter, 1987-, et al.
(författare)
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Climate Model Code Genealogy and Its Relation to Climate Feedbacks and Sensitivity
- 2023
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Ingår i: Journal of Advances in Modeling Earth Systems. - 1942-2466. ; 15:7
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Tidskriftsartikel (refereegranskat)abstract
- Contemporary general circulation models (GCMs) and Earth system models (ESMs) are developed by a large number of modeling groups globally. They use a wide range of representations of physical processes, allowing for structural (code) uncertainty to be partially quantified with multi-model ensembles (MMEs). Many models in the MMEs of the Coupled Model Intercomparison Project (CMIP) have a common development history due to sharing of code and schemes. This makes their projections statistically dependent and introduces biases in MME statistics. Previous research has focused on model output and code dependence, and model code genealogy of CMIP models has not been fully analyzed. We present a full reconstruction of CMIP3, CMIP5, and CMIP6 code genealogy of 167 atmospheric models, GCMs, and ESMs (of which 114 participated in CMIP) based on the available literature, with a focus on the atmospheric component and atmospheric physics. We identify 12 main model families. We propose family and ancestry weighting methods designed to reduce the effect of model structural dependence in MMEs. We analyze weighted effective climate sensitivity (ECS), climate feedbacks, forcing, and global mean near-surface air temperature, and how they differ by model family. Models in the same family often have similar climate properties. We show that weighting can partially reconcile differences in ECS and cloud feedbacks between CMIP5 and CMIP6. The results can help in understanding structural dependence between CMIP models, and the proposed ancestry and family weighting methods can be used in MME assessments to ameliorate model structural sampling biases.
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