Abstract:
This study evaluates the representation of cloud fraction, cloud liquid and ice water and cloud radiative effect biases in five Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6) climate models, both globally and over the Southern Ocean. The models are compared with satellite observation data from the GCM-Oriented CALIPSO Cloud Product CALIPSO-GOCCP, as well as data from the CERES dataset. We also show that cloud water content has a comparable effect as that of cloud fraction to top of the atmosphere radiative flux sensitivity in the CMIP6 climate models studied. Many CMIP5 models had difficulty simulating these features both globally and over the Southern Ocean in particular, which resulted in significant radiation budget biases over this region compared with observations. Previous studies showed two main biases in CMIP5 GCMs in general: too few low clouds and too infrequent high clouds. In general, the CMIP5 models do not produce enough clouds, and compensate by making clouds optically too thick, in order to get correct fluxes at the top of the atmosphere. The results show that from CMIP5 to CMIP6: low cloud fractions have generally increased; however, most models studied are still underestimating this; cloud liquid water fraction has increased, especially over the Southern Ocean but this still seems to be underestimated in general. Out of the models examined, the IPSL-6A-LR has the best overall representation of cloud cover and water content, as well at TOA radiative fluxes when compared with observations. Using radiative transfer code (RRTMG) we also show that the shortwave cloud radiative effect is highly sensitive to the cloud liquid water content, particularly over the Southern Ocean, and of a similar size to that of cloud fraction alone. Liquid droplet particle size is also shown to have a significant effect on sensitivity. This highlights a need for modelling groups to pay careful attention to these parameters.
