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Aerosol-cloud-radiation interactions in global climate models
Stockholm University, Faculty of Science, Department of Meteorology .
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Clouds can reflect, absorb and re-emit radiation, thereby inducing a cooling or warming effect on the climate. However, the response of clouds to a changing climate is highly uncertain and the representation of clouds in state-of-the-art climate models remains a key challenge for future climate projections. Factors contributing to this uncertainty include processes on the microphysical scale involving aerosol particles with the size of just a few nanometers to micrometers. This thesis focuses on the representation of aerosol-cloud-radiation interactions in global climate models. Using idealized experiments from a model-intercomparison project with different anthropogenic aerosol forcings, it was found that both sulfate and non-sulfate aerosols yield an increase in cloud albedo in five regions of subtropical marine stratocumulus clouds. The changes in cloud albedo in the models were driven by changes in the cloud droplet number concentration and liquid water content. Further, it was found that the microphysical coupling of underlying aerosol-cloud interactions in models seems to dominate on the monthly timescale in subtropical marine stratocumulus regions, which can not be confirmed in observations. Quantifying the effect of aerosols on cloud properties in observations remains challenging. In addition, comparisons with satellite retrievals and the global climate model NorESM showed that this model is not able to capture elevated aerosol above cloud, seen in observations in two regions of marine stratocumulus clouds. Sensitivity experiments revealed that the model is most sensitive to the aerosol emissions, convection and wet scavenging in terms of the vertical aerosol distribution. Finally, the representation of aerosol absorption in global climate models was investigated. It was found that most of the models underestimate absorption by aerosols in a focus domain in Asia. Sensitivity studies with NorESM give rise to variations that lie within the large inter-model diversity.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2019. , p. 58
Keywords [en]
Aerosols, clouds, global climate models
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-168266ISBN: 978-91-7797-612-7 (print)ISBN: 978-91-7797-613-4 (electronic)OAI: oai:DiVA.org:su-168266DiVA, id: diva2:1307747
Public defence
2019-06-14, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2019-05-22 Created: 2019-04-29 Last updated: 2019-05-23Bibliographically approved
List of papers
1. Cloud albedo changes in response to anthropogenic sulfate and non-sulfate forcings in CMIP5 models
Open this publication in new window or tab >>Cloud albedo changes in response to anthropogenic sulfate and non-sulfate forcings in CMIP5 models
2017 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 14, p. 9145-9162Article in journal (Refereed) Published
Abstract [en]

The effects of different aerosol types on cloud albedo are analysed using the linear relation between total albedo and cloud fraction found on a monthly mean scale in regions of subtropical marine stratocumulus clouds and the influence of simulated aerosol variations on this relation. Model experiments from the Coupled Model Intercomparison Project phase 5 (CMIP5) are used to separately study the responses to increases in sulfate, non-sulfate and all anthropogenic aerosols. A cloud brightening on the month-to-month scale due to variability in the background aerosol is found to dominate even in the cases where anthropogenic aerosols are added. The aerosol composition is of importance for this cloud brightening, that is thereby region dependent. There is indication that absorbing aerosols to some extent counteract the cloud brightening but scene darkening with increasing aerosol burden is generally not supported, even in regions where absorbing aerosols dominate. Month-to-month cloud albedo variability also confirms the importance of liquid water content for cloud albedo. Regional, monthly mean cloud albedo is found to increase with the addition of anthropogenic aerosols and more so with sulfate than non-sulfate. Changes in cloud albedo between experiments are related to changes in cloud water content as well as droplet size distribution changes, so that models with large increases in liquid water path and/or cloud droplet number show large cloud albedo increases with increasing aerosol. However, no clear relation between model sensitivities to aerosol variations on the month-to-month scale and changes in cloud albedo due to changed aerosol burden is found.

National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-150506 (URN)10.5194/acp-17-9145-2017 (DOI)000437939600001 ()
Available from: 2017-12-21 Created: 2017-12-21 Last updated: 2019-04-30Bibliographically approved
2. Assessment of aerosol-cloud-radiation correlations in satellite observations, climate models and reanalysis
Open this publication in new window or tab >>Assessment of aerosol-cloud-radiation correlations in satellite observations, climate models and reanalysis
Show others...
2019 (English)In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 52, no 7-8, p. 4371-4392Article in journal (Refereed) Published
Abstract [en]

Representing large-scale co-variability between variables related to aerosols, clouds and radiation is one of many aspects of agreement with observations desirable for a climate model. In this study such relations are investigated in terms of temporal correlations on monthly mean scale, to identify points of agreement and disagreement with observations. Ten regions with different meteorological characteristics and aerosol signatures are studied and correlation matrices for the selected regions offer an overview of model ability to represent present day climate variability. Global climate models with different levels of detail and sophistication in their representation of aerosols and clouds are compared with satellite observations and reanalysis assimilating meteorological fields as well as aerosol optical depth from observations. One example of how the correlation comparison can guide model evaluation and development is the often studied relation between cloud droplet number and water content. Reanalysis, with no parameterized aerosol–cloud coupling, shows weaker correlations than observations, indicating that microphysical couplings between cloud droplet number and water content are not negligible for the co-variations emerging on larger scale. These observed correlations are, however, not in agreement with those expected from dominance of the underlying microphysical aerosol–cloud couplings. For instance, negative correlations in subtropical stratocumulus regions show that suppression of precipitation and subsequent increase in water content due to aerosol is not a dominating process on this scale. Only in one of the studied models are cloud dynamics able to overcome the parameterized dependence of rain formation on droplet number concentration, and negative correlations in the stratocumulus regions are reproduced.

Keywords
Aerosol–cloud–radiation interaction, GCM-evaluation, Satellite observations, Reanalysis, Volcanic aerosol
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-168045 (URN)10.1007/s00382-018-4384-z (DOI)000467187600032 ()
Available from: 2019-04-16 Created: 2019-04-16 Last updated: 2019-05-27Bibliographically approved
3. Investigating processes that control the vertical distribution of aerosol in five subtropical marine stratocumulus regions: A sensitivity study using the climate model NorESM1-M
Open this publication in new window or tab >>Investigating processes that control the vertical distribution of aerosol in five subtropical marine stratocumulus regions: A sensitivity study using the climate model NorESM1-M
(English)Manuscript (preprint) (Other academic)
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-168049 (URN)
Available from: 2019-04-28 Created: 2019-04-28 Last updated: 2019-04-29Bibliographically approved
4. Absorbing aerosols over Asia: An inter-model and model-observation comparison study using CAM5.3-Oslo
Open this publication in new window or tab >>Absorbing aerosols over Asia: An inter-model and model-observation comparison study using CAM5.3-Oslo
(English)Manuscript (preprint) (Other academic)
National Category
Earth and Related Environmental Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-168050 (URN)
Available from: 2019-04-28 Created: 2019-04-28 Last updated: 2019-08-28Bibliographically approved

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