  The sensitivity of cloud feedbacks to atmospheric model parameters is
evaluated using a CAM6 perturbed parameter ensemble (PPE). The CAM6 PPE
perturbs 45 parameters across 262 simulations, 206 of which are used here. The
spread in total cloud feedback and its six components across the CAM6 PPE are
comparable to the spread across the CMIP6 and AMIP ensembles, indicating that
parametric uncertainty mirrors structural uncertainty. However, the high-cloud
altitude feedback is generally larger in the CAM6 PPE than WCRP assessment,
CMIP6, and AMIP values. We evaluate the influence of each of the 45 parameters
on the total cloud feedback and each of the six cloud feedback components. We
also explore whether the CAM6 PPE can be used to constrain the total cloud
feedback, with inconclusive results. Further, we find that despite the large
parametric sensitivity of cloud feedbacks in CAM6, a substantial increase in
cloud feedbacks from CAM5 to CAM6 is not a result of changes in parameter
values. Notably, the CAM6 PPE is run with a more recent version of CAM6
(CAM6.3) than was used for AMIP (CAM6.0), and has a smaller total cloud
feedback (0.56 W m$^{-2}$ K$^{-1}$) as compared to CAM6.0 (0.81 W m$^{-2}$
K$^{-1}$) owing primarily to reductions in low clouds over the tropics and
middle latitudes. The work highlights the large sensitivity of cloud feedbacks
to both parameter values and structural details in CAM6.
