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Assessment of aerosol-cloud-radiation correlations in satellite observations, climate models and reanalysis
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
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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.

Place, publisher, year, edition, pages
2019. Vol. 52, no 7-8, p. 4371-4392
Keywords [en]
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: urn:nbn:se:su:diva-168045DOI: 10.1007/s00382-018-4384-zISI: 000467187600032OAI: oai:DiVA.org:su-168045DiVA, id: diva2:1305466
Available from: 2019-04-16 Created: 2019-04-16 Last updated: 2019-05-27Bibliographically approved
In thesis
1. Aerosol-cloud-radiation interactions in global climate models
Open this publication in new window or tab >>Aerosol-cloud-radiation interactions in global climate models
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
Aerosols, clouds, global climate models
National Category
Meteorology and Atmospheric Sciences
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-168266 (URN)978-91-7797-612-7 (ISBN)978-91-7797-613-4 (ISBN)
Public defence
2019-06-14, Ahlmannsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 10:00 (English)
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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

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