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Cloud radiative forcing of subtropical low level clouds in global models
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
Stockholm University, Faculty of Science, Department of Meteorology .
2008 (English)In: Climate Dynamics, Vol. 30, no 7-8, 779-788 p.Article in journal (Refereed) Published
Abstract [en]

Simulations of subtropical marine low clouds and their radiative properties by nine coupled oceanatmosphere climate models participating in the fourth assesment report (AR4) of the intergovernmental panel on climate change (IPCC) are analyzed. Satellite observations of cloudiness and radiative fluxes at the top of the atmosphere (TOA) are utilized for comparison. The analysis is confined to the marine subtropics in an attempt to isolate low cloudiness from tropical convective systems. All analyzed models have a negative bias in the low cloud fraction (model mean bias of –15%). On the other hand, the models show an excess of cloud radiative cooling in the region (model mean excess of 13 W m–2). The latter bias is shown to mainly originate from too much shortwave reflection by the models clouds rather than biases in the clear-sky fluxes. These results confirm earlier studies, thus no major progress in simulating the marine subtropical clouds is noted. As a consequence of the combination of these two biases, this study suggests that all investigated models are likely to overestimate the radiative response to changes in low level subtropical cloudiness.

Place, publisher, year, edition, pages
2008. Vol. 30, no 7-8, 779-788 p.
National Category
Meteorology and Atmospheric Sciences Climate Research
Identifiers
URN: urn:nbn:se:su:diva-15719DOI: 10.1007/s00382-007-0322-1ISI: 000255090500007OAI: oai:DiVA.org:su-15719DiVA: diva2:182239
Available from: 2009-01-14 Created: 2009-01-14 Last updated: 2009-08-20Bibliographically approved
In thesis
1. The influence of clouds on Earth's radiation budget in global climate models
Open this publication in new window or tab >>The influence of clouds on Earth's radiation budget in global climate models
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Considering the high sensitivity of climate to changes in cloudiness, the way clouds might change in a perturbed climate is important for the total temperature response. In fact, the cloud feedback has been identified as the main uncertainty in future projections, as simulated by global climate models.

To increase the confidence of future scenario simulations it is necessary, although not sufficient, that the models manage to represent the present-day climate in a realistic manner. We compare the simulations of cloudiness to available observations in the marine subtropics and the Arctic, two regions in which changes in cloudiness are believed to have large impact on the climate.

In terms of the annual cycle of Arctic cloud properties, climate models show large disagreement with each other and with observations. There exists a tentative across-model relationship, such that models with higher amounts of clouds in the winter are also associated with larger surface cloud forcing. However, across-model differences in the wintertime surface cloud forcing cannot explain differences in the simulated surface temperatures. Rather, we identify across-model differences in temperature and moisture properties of the air entering the Arctic region to be of greater importance.

We find that climate models in general underestimate the amount of low clouds in the marine subtropics but still overestimate the regional averaged cloud radiative cooling. As a consequence we suggest that models are likely to overestimate the radiative response to changes in the cloudiness. We also demonstrate the potential of satellite derived cloud top heights to be used as model diagnostics in the climatologically important transition from stratus-topped to cumulus-topped marine boundary layers in the subtropics.

 

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2009. 51 p.
National Category
Meteorology and Atmospheric Sciences Meteorology and Atmospheric Sciences Oceanography, Hydrology, Water Resources
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-29276 (URN)978-91-7155-924-1 (ISBN)
Public defence
2009-09-23, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, 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 1: Submitted. Paper 2: Submitted. Paper 4: Manuscript.Available from: 2009-09-01 Created: 2009-08-19 Last updated: 2009-09-02Bibliographically approved

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