| 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. |
- 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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| 5. |
- 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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| 6. |
- 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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| 7. |
- 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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| 8. |
- Kuma, Peter, et al.
(författare)
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Machine learning of cloud types in satellite observations and climate models
- 2023
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 23:1, s. 523-549
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Tidskriftsartikel (refereegranskat)abstract
- Uncertainty in cloud feedbacks in climate models is a major limitation in projections of future climate. Therefore, evaluation and improvement of cloud simulation are essential to ensure the accuracy of climate models. We analyse cloud biases and cloud change with respect to global mean near-surface temperature (GMST) in climate models relative to satellite observations and relate them to equilibrium climate sensitivity, transient climate response and cloud feedback. For this purpose, we develop a supervised deep convolutional artificial neural network for determination of cloud types from low-resolution (2.5∘×2.5∘) daily mean top-of-atmosphere shortwave and longwave radiation fields, corresponding to the World Meteorological Organization (WMO) cloud genera recorded by human observers in the Global Telecommunication System (GTS). We train this network on top-of-atmosphere radiation retrieved by the Clouds and the Earth’s Radiant Energy System (CERES) and GTS and apply it to the Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5 and CMIP6) model output and the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis version 5 (ERA5) and the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalyses. We compare the cloud types between models and satellite observations. We link biases to climate sensitivity and identify a negative linear relationship between the root mean square error of cloud type occurrence derived from the neural network and model equilibrium climate sensitivity (ECS), transient climate response (TCR) and cloud feedback. This statistical relationship in the model ensemble favours models with higher ECS, TCR and cloud feedback. However, this relationship could be due to the relatively small size of the ensemble used or decoupling between present-day biases and future projected cloud change. Using the abrupt-4×CO2 CMIP5 and CMIP6 experiments, we show that models simulating decreasing stratiform and increasing cumuliform clouds tend to have higher ECS than models simulating increasing stratiform and decreasing cumuliform clouds, and this could also partially explain the association between the model cloud type occurrence error and model ECS.
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| 9. |
- McCoy, Isabel L., et al.
(författare)
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The Role of Mesoscale Cloud Morphology in the Shortwave Cloud Feedback
- 2023
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 50:2
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Tidskriftsartikel (refereegranskat)abstract
- A supervised neural network algorithm is used to categorize near-global satellite retrievals into three mesoscale cellular convective (MCC) cloud morphology patterns. At constant cloud amount, morphology patterns differ in brightness associated with the amount of optically thin cloud features. Environmentally driven transitions from closed MCC to other morphology patterns, typically accompanied by more optically thin cloud features, are used as a framework to quantify the morphology contribution to the optical depth component of the shortwave cloud feedback. A marine heat wave is used as an out-of-sample test of closed MCC occurrence predictions. Morphology shifts in optical depth between 65°S and 65°N under projected environmental changes (i.e., from an abrupt quadrupling of CO2) assuming constant cloud cover contributes between 0.04 and 0.07 W m−2 K−1 (aggregate of 0.06) to the global mean cloud feedback.
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| 10. |
- Pulkkinen, Karoliina, et al.
(författare)
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The value of values in climate science
- 2022
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Ingår i: Nature Climate Change. - : Springer Nature. - 1758-678X .- 1758-6798. ; 12:1, s. 4-6
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Tidskriftsartikel (refereegranskat)
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