Computing and Partitioning Cloud Feedbacks using Cloud

7 Cloud radiative kernels and histograms of cloud fraction, both as functions of cloud 8 top pressure and optical depth, are used to quantify cloud amount, cloud height and cloud 9 optical depth feedbacks. The analysis is applied to doubled CO2 slab-ocean simulations 10 from ten global climate models participating in the Cloud Feedback Model Intercomparison 11 Project. In the ensemble mean, total cloud amount decreases, especially between 55◦S 12 and 60◦N, cloud altitude increases, and optical depths increase poleward of about 40◦ and 13 decrease at lower latitudes. Both longwave (LW) and shortwave (SW) cloud feedbacks are 14 positive, with the latter nearly twice as as large as the former. We show that increasing 15 cloud top altitude is the dominant contributor to the positive LW cloud feedback, and that 16 the extra-tropical contribution to the altitude feedback is approximately 70% as large as 17 the tropical contribution. In the ensemble mean, the positive impact of rising clouds is 18 50% larger than the negative impact of reductions in cloud amount on LW cloud feedback, 19 but the degree to which reductions in cloud fraction offset the effect of rising clouds varies 20 considerably across models. In contrast, reductions in cloud fraction make a large and 21 virtually unopposed positive contribution to SW cloud feedback, though the inter-model 22 spread is greater than for any other individual feedback component. In general, models 23 exhibiting greater reductions in subtropical marine boundary layer cloudiness tend to have 24 larger positive SW cloud feedbacks, in agreement with previous studies. Overall reductions 25 in cloud amount have twice as large an impact on SW fluxes as on LW fluxes such that 26 the net cloud amount feedback is moderately positive, with no models analyzed here having 27 a negative net cloud amount feedback. As a consequence of large but partially offsetting 28 effects of cloud amount reductions on LW and SW feedbacks, increasing cloud altitude 29 1 actually makes a greater contribution to the net cloud feedback than does the reduction in 30 cloud amount. Furthermore, the inter-model spread in net cloud altitude feedback is actually 31 larger than that of net cloud amount feedback. Finally, we find that although global mean 32 cloud optical depth feedbacks are generally smaller than the other components, they are 33 the dominant process at high latitudes. This large negative optical depth feedback at high 34 latitudes appears to result from a combination of increased cloud water content and changes 35 in phase from ice to liquid, not from increases in total cloud amount associated with the 36 poleward shift of the storm track, as is commonly assumed. 37