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Modeling Cloud Processes and Their Effects on Earth's Climate

Paper Reviewed
Park, S., Bretherton, C.S. and Rasch, P.J. 2014. Integrating cloud processes in the Community Atmosphere Model, Version 5. Journal of Climate 27: 6821-6856.

In introducing the subject of their work, Park et al. (2014) write that "clouds cool the Earth-atmosphere system by reflecting incoming shortwave (SW) radiation and warm it by absorbing outgoing longwave (LW) radiation from the surface." And they say that "satellite observations reveal that the net radiative effect of clouds on the Earth-atmosphere system is a cooling of 20-24 Wm-2 in the global average," which they note is "about six times larger than the radiative forcing associated with doubled CO2," citing Ramanathan et al. (1989) and Loeb et al. (2009). And, hence, it can be appreciated that properly modeling cloud processes is an important aspect of ongoing efforts to predict the future course of Earth's climate.

As for their contribution to this important task, the three US researchers devised several unique adjustments to previous versions of the Community Atmosphere Models (CAMs) versions 3 and 4, which are now found in the new-and-improved CAM5 set of models. And "compared with the previous versions," as they write, "the cloud parameterizations in CAM5 are more consistent and physically based, due to inclusion of more realistic and complex parameterizations and much attention given to the interactions among them within a more consistent framework."

However, they go on to acknowledge that even with these improvements, "several systematic biases were also identified in the simulated cloud fields in CAM5," which they grouped into three different categories: deficient regional tuning, inconsistency between various physics parameterizations, and incomplete modeled physics. And in the case of the latter category, they list the following problems: (1) "underestimation of LW CRF [cloud radiative forcing] due to the horizontal heterogeneity assumption of water vapor within each grid layer in the radiation scheme," (2) "overly strong SW CRF and LW CRF in the tropics due to the use of a single-type cloud within the radiation scheme," and (3) "under-frequent shallow convective activity over summer continents due to the neglect of forced convection."

In light of these several remaining problems, therefore, they ultimately conclude that "while substantially improved from its predecessors [CAM3/CAM4], many aspects of CAM5 can and should be improved in the future," which they describe as something "upon which we are continuously working with collaborators." And until such improvements are made, model treatment of clouds should be treated with a healthy dose of skepticism.

References
Loeb, N., Wielicki, D., Doelling, D., Smith, G., Keyes, D., Kato, S., Manalo-Smith, N. and Wong, T. 2009. Toward optimal closure of the earth's top-of-atmosphere radiation budget. Journal of Climate 22: 748-766.

Ramanathan, V., Cess, R., Harrison, E.,Minnis, P., Barkstrom, B., Ahmad, E. and Hartmann, D. 1989. Cloud-radiative forcing and climate: Results from the Earth Radiation Budget Experiment. Science 243: 57-63.

Posted 20 January 2015