  By reflecting solar radiation and reducing longwave emissions, clouds
regulate the earth's radiation budget, impacting atmospheric circulation and
cloud dynamics. Given the diurnal fluctuation of shortwave and longwave
radiation, a shift in the cloud cycle phase (CCP) may lead to substantial
feedbacks to the climate system. While most efforts have focused on the overall
cloud feedback, the response of CCP to climate change has received much less
attention. Here we analyze the variations of CCP using both long-term global
satellite records and general circulation models (GCM) simulations to evaluate
their impacts on the earth's energy budget. Satellite records show that in warm
periods the CCP shifts earlier in the morning over the oceans and later in the
afternoon over the land. Although less marked and with large inter-model
spread, similar shifting patterns also occur in GCMs over the oceans with
non-negligible CCP feedbacks. Over the land, where the GCM results are not
conclusive, our findings are supported by atmospheric boundary layer models. A
simplified radiative model further suggests that such shifts may in turn cause
reduced reflection of solar radiation, thus inducing a positive feedback on
climate. The crucial role of the cloud cycle calls for increased attention to
the temporal evolution of the cloud diurnal cycle in climate models.
