  Clouds' efficiency at reflecting solar radiation and trapping the terrestrial
one is strongly modulated by their diurnal cycle. Much attention has been paid
to mean cloud properties due to their critical role in climate projections;
however, less research has been devoted to their diurnal cycle. Here, we
quantify the mean, amplitude, and phase of the cloud cycles in current climate
models and compare them with satellite observations and reanalysis data. While
the mean values appear to be reliable, the amplitude and phase of the diurnal
cycles of clouds show marked inconsistencies, especially over land. We show
that, to compensate for the increased net radiation input implied by such
errors, an overestimation of the cloud liquid water path may be introduced
during calibration of climate models to keep temperatures in line with
observations. These discrepancies are likely to be related to cloud
parametrization problems in relation to atmospheric convection.
