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The influence of clouds on Earth's radiation budget in global climate models
Stockholm University, Faculty of Science, Department of Meteorology .
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: urn:nbn:se:su:diva-29276ISBN: 978-91-7155-924-1 (print)OAI: oai:DiVA.org:su-29276DiVA: diva2:232096
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
List of papers
1. The simulation of Arctic clouds and their influence on thewinter surface temperature in present-day climate in theCMIP3 multi-model dataset
Open this publication in new window or tab >>The simulation of Arctic clouds and their influence on thewinter surface temperature in present-day climate in theCMIP3 multi-model dataset
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:su:diva-29274 (URN)
Available from: 2009-08-19 Created: 2009-08-19 Last updated: 2010-01-14Bibliographically approved
2. Subtropical cloud regime transitions: boundary layerdepth and cloud-top height evolution
Open this publication in new window or tab >>Subtropical cloud regime transitions: boundary layerdepth and cloud-top height evolution
2010 (English)In: Journal of Applied Meteorology and Climatology, ISSN 1558-8424, E-ISSN 1558-8432, Vol. 49, no 9, 1845-1858 p.Article in journal (Refereed) Published
Abstract [en]

In this study, the mean and variability of boundary layer height (BLH) are analyzed along a transect in the eastern Pacific Ocean for the summer of 2003 using BLH estimates based on the height of the main relative humidity (RH) inversion and the height of low cloud tops (CTH). The observations and the regional and global model data have been prepared in the context of the Global Energy and Water Cycle Experiment (GEWEX) Cloud System Study (GCSS) Pacific Cross-Section Intercomparison (GPCI). The GPCI transect covers the transition from a stratocumulus-topped marine boundary layer (MBL) off the coast of California to a trade cumulus-topped, less-well-defined, MBL, and finally to the deep-convection regions in the intertropical convergence zone (ITCZ). The Atmospheric Infrared Sounder (AIRS) and the Multiangle Imaging Spectroradiometer (MISR) have been used to derive observational records of the two BLH estimates. Analyses from the ECMWF are also used in the study. Both BLH estimates in the models, the ECMWF analysis, and the observations agree on a southward vertical growth of the MBL along the GPCI transect in the stratocumulus region. Away from the region typically associated with extensive cloud cover, the two BLH estimates depict different evolutions of the MBL. In most models, the height of the main RH inversion decreases southward from; similar to 18 degrees N, reaching a minimum at the ITCZ, whereas the height of the RH inversion in the ECMWF analysis and a few of the models is fairly constant all the way to the ITCZ. As a result of insufficient vertical resolution of the gridded dataset, the AIRS data only manage to reproduce the initial growth of the BLH. The median-model CTH increases from the stratocumulus-topped MBL to the ITCZ. In contrast, the observed MISR CTHs decrease southward from 20 degrees N to the ITCZ, possibly indicative of the fact that in these regions MISR manages to capture a variety of cloud tops with a mean that is below the subsidence inversion while the models and the ECMWF analysis mainly simulate CTHs corresponding to the height of the subsidence inversion. In most models and in the ECMWF analysis, the height of the main RH inversion and the CTH tend to coincide in the northern part of the GPCI transect. In the regions associated with trade cumuli and deep convection there is a more ambiguous relation between the two BLH estimates. In this region, most of the models place the CTH above the main RH inversion. The ECMWF analysis shows a good agreement between the BLH estimates throughout the transect.

Keyword
Clouds, Boundary layer, Pacific Ocean, Marine boundary layer, Profilers
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-29273 (URN)10.1175/2010JAMC2338.1 (DOI)000282251100004 ()
Available from: 2009-08-19 Created: 2009-08-19 Last updated: 2017-12-13Bibliographically approved
3. Cloud radiative forcing of subtropical low level clouds in global models
Open this publication in new window or tab >>Cloud radiative forcing of subtropical low level clouds in global models
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.

National Category
Meteorology and Atmospheric Sciences Climate Research
Identifiers
urn:nbn:se:su:diva-15719 (URN)10.1007/s00382-007-0322-1 (DOI)000255090500007 ()
Available from: 2009-01-14 Created: 2009-01-14 Last updated: 2009-08-20Bibliographically approved
4. On the processes determining the Arctic wintertime climate in global climate models
Open this publication in new window or tab >>On the processes determining the Arctic wintertime climate in global climate models
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:su:diva-29275 (URN)
Available from: 2009-08-19 Created: 2009-08-19 Last updated: 2010-01-14Bibliographically approved

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