The third phase of the Cloud Feedback Model Intercomparison Project requested that modeling centers perform a pair of simulations where the climate system is subjected to an abrupt change of the solar constant by +/- 4%. The forcing is designed to loosely match the amount of radiative forcing incurred by quadrupling atmospheric CO2 concentrations. Using these simulations, we examine how clouds respond to changes in solar forcing and act as a feedback on global surface temperature. Specifically, in this paper, we study the temperature mediated cloud changes that occur following an abrupt increase and decrease of the solar constant and compare with temperature mediated cloud changes that occur following quadrupling and halving of CO2. We seek to answer two primary questions: 1) How do cloud feedbacks differ in response to abrupt changes in CO2 and solar forcing? And 2) Are there symmetrical (equal and opposite) cloud feedbacks to an increase and a decrease in solar forcing? We find that temperature mediated cloud changes are similar from increasing solar and CO2 forcing, with the only robust difference being that there is a larger reduction of low cloud amount following solar forcing; and we find that cloud responses to warming and cooling are not symmetric, due primarily to non-linearity introduced by phase changes in mid-to-high latitude low clouds, and sea ice loss/formation.

In this paper we examine differences in cloud adjustments (often called rapid adjustments) that occur as a direct result of abruptly increasing the solar constant by 4% or abruptly quadrupling of atmospheric CO2. In doing so, we devised a novel method for calculating the cloud adjustments for the abrupt solar forcing experiment that uses differences between coupled model simulations with abrupt solar and CO2 forcing, in combination with uncoupled, atmosphere-only, abrupt CO2 forced experiments that have prescribed sea-surface temperature. Our main findings are that 1) there are substantial differences in the response of stratocumulus and cumulus clouds to solar and CO2 forcing, which follow the differences in the direct radiative effect that solar and CO2 forcing have at cloud top, and 2) there are differences in the adjustment of the average optical depth of high clouds to solar and CO2 forcing that we speculate are driven by the differences in the vertical profile of radiative heating, and differences in the pattern of sea-surface temperature change (for a fixed global mean temperature). Such adjustments do contribute significantly to the total net cloud radiative effect, even after 150 years of simulation.

Cloud droplet number concentration (Nd) is a key microphysical property that is largely controlled by the balance between sources and sinks of aerosols that serve as cloud condensation nuclei (CCN). Despite being a key sink of CCN, the impact of coalescence scavenging on Southern Ocean (SO) cloud is poorly known. We apply a simple source-and-sink budget model based on parameterizations to austral summer aircraft observations to test model behavior and examine the relative influence of processes that determine Nd in SO stratocumulus clouds. The model predicts Nd with little bias and a correlation coefficient of ~0.7 compared with observations. Coalescence scavenging is found to be an important sink of CCN in both liquid and mixed-phase precipitating stratocumulus and reduces the predicted Nd by as much as 90% depending on the precipitation rate. The free tropospheric aerosol source controls Nd more strongly than the surface aerosol source during austral summer.