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Modeling Top-of-the-Atmosphere Cloud Radiative Effects

Reference
Wang, H. and Su, W. 2013. Evaluating and understanding top of the atmosphere cloud radiative effects in Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) Coupled Model Intercomparison Project Phase 5 (CMIP5) models using satellite observations. Journal of Geophysical Research: Atmospheres 118: 683-699.
Writing in the Journal of Geophysical Research (Atmospheres), Wang and Su (2013) note that "coupled general circulation models (GCMs) are the major tool to predict future climate change, yet cloud-climate feedback constitutes the largest source of uncertainty in these modeled future climate projections." Thus, they correctly state that "our confidence in the future climate change projections by the coupled GCMs to a large extent depends on how well these models simulate the observed present-day distribution of clouds and their associated radiative fluxes." And in describing how they made this determination, they write that in their particular study, "the annual mean climatology of top of the atmosphere (TOA) shortwave and longwave cloud radiative effects in 12 Atmospheric Model Intercomparison Project (AMIP)-type simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) [was] evaluated and investigated using satellite-based observations, with a focus on the tropics."

So what did they find?

The two researchers report that (1) the CMIP5 AMIPs "produce considerably less cloud amount [than what is observed], particularly in the middle and lower troposphere," that (2) there are "good model simulations in tropical means," but they are "a result of compensating errors over different dynamical regimes," that (3) "over the Maritime Continent, most of the models simulate moderately less high-cloud fraction, leading to weaker shortwave cooling and longwave warming and a larger net cooling," that (4) "over subtropical strong subsidence regimes, most of the CMIP5 models strongly underestimate stratocumulus cloud amount and show considerably weaker local shortwave cloud radiative forcings," that (5) "over the transitional trade cumulus regimes, a notable feature is that while at varying amplitudes, most of the CMIP5 models consistently simulate a deeper and drier boundary layer, more moist free troposphere, and more high clouds and consequently overestimate shortwave cooling and longwave warming effects there," such that, in the final analysis, (6) "representing clouds and their TOA radiative effects remains a challenge in the CMIP5 models."

Additional References
Bindoff, N.L., Willebrand, J., Artale, V., Cazenave, A., Gregory, J., Gulev, S., Hanawa, K., Le Quéré, C., Levitus, S., Nojiri, Y., Shum, C.K., Talley, L.D. and Unnikrishnan, A. 2007: Observations: Oceanic Climate Change and Sea Level. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom.

Archived 1 October 2013