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Negative Shortwave Cloud Feedback in Middle to High Latitudes

Paper Reviewed
Ceppi, P., Hartmann, D.L. and Webb, M.J. 2016. Mechanisms of the Negative Shortwave Cloud Feedback in Middle to High Latitudes. Journal of Climate 29: 139-157.

Noting that despite continuing model improvement efforts, "cloud feedback remains the largest source of uncertainty in climate sensitivity estimates" -- citing Soden et al. (2008), Boucher et al. (2013) and Vial et al. (2013) -- Ceppi et al. (2016) write that "accurately portraying the cloud response to warming constitutes a major challenge in the development of future generations of climate models." And in taking up this challenge, the three researchers investigated "the processes involved in the liquid water path (LWP) response by perturbing temperature in the cloud microphysics schemes of two climate models in aqua-planet configuration, GFDL AM2.1 and CESM-CAM5, each of which models have separate prognostic equations for liquid water and ice." And what did they thereby learn?

Ceppi et al. report that "although models and observations all agree on LWP increasing with warming in mixed-phase cloud regions, that (1) "most models appear to overestimate the LWP sensitivity to temperature compared with satellite observations," that (2) the "models overestimate the efficiency of ice-phase microphysical processes" and that they (3) "do not maintain enough super-cooled liquid in the historical climate." Thus, they conclude that (4,5) "additional work will therefore be necessary to confirm the relevance of cloud microphysics to the forced LWP response and the associated shortwave cloud feedback in the real world," while further noting that (6) "most models appear to overestimate the importance of microphysical processes in the LWP response to warming."

Last of all, Ceppi et al. write that (7,8) "an improved representation of ice-phase microphysical processes appears to be crucial to reduce the large model errors in both the present-day climatology and future response of condensed cloud water," citing Choi et al. (2014) and Komurcu et al. (2014)." We wish them and others the best of results in their efforts to achieve these worthy goals.

References
Boucher, O. and Coauthors. 2013. Clouds and aerosols. Climate Change 2013: The Physical Science Basis. T.R. Stocker et al., Eds. Cambridge University Press, 571-657.

Choi, Y.-S., Ho, C.-H., Park, C.-E., Storelvmo, T. and Tan, I. 2014. Influence of cloud phase composition on climate feedbacks. Journal of Geophysical Research: Atmospheres 119: 3687-3700.

Komurcu, M. and Coauthors. 2014. Intercomparison of the cloud water phase among global climate models. Journal of Geophysical Research: Atmospheres 119: 3372-3400.

Soden, B.J., Held, I.M., Colman, R., Shell, K.M., Kiehl, J.T. and Shields, C.A. 2008. Quantifying climate feedbacks using radiative kernels. Journal of Climate 21: 3504-3520.

Vial, J., Dufresne, J.-L. and Bony, S. 2013. On the interpretation of inter-model spread in CMIP5 climate sensitivity estimates. Climate Dynamics 41: 3339-3362.

Posted 4 May 2016