The diurnal cycle of cloud feedback and land surface warming in climate models.

We examine the diurnal cycle of cloud feedback using high frequency outputs in CFMIP-2 idealised uniform +4K SST perturbation experiments from seven CMIP5 models. Most of the inter-model spread in the diurnal mean marine shortwave cloud feedback can be explained by low cloud responses, although these do not explain the model responses at the neutral/weakly negative end of the feedback range, where changes in mid and high level cloud properties are more important. All of the models show reductions in marine low cloud fraction in the warmer climate, and these are in almost all cases largest in the mornings when more cloud is present in the control simulations. This results in shortwave cloud feedbacks being slightly stronger and having the largest inter-model spread at this time of day. The diurnal amplitudes of the responses of marine cloud properties to the warming climate are however small compared to the inter-model differences in their diurnally meaned responses. This indicates that the diurnal cycle of cloud feedback is not strongly relevant to understanding inter-model spread in overall cloud feedback and climate sensitivity. Shortwave cloud feedback over land is positive in all models examined but one, but generally weaker than over the ocean. The diurnal cycles of these responses exhibit more inter-model diversity than those over the ocean. Land surface temperatures warm less during the daytime than at night in spite of reductions in low level cloud fraction. Decreases in clear-sky downwelling shortwave fluxes at the land surface due to increased near-infrared absorption by water vapour are in most models large enough to counter the effects of positive shortwave cloud feedbacks on the diurnal land surface temperature range. A number of unusual behaviours in individual models are highlighted for future investigation.