Low-Level Marine Tropical Clouds in Six CMIP6 Models Are Too Few, Too Bright but Also Too Compact and Too Homogeneous

Several studies have shown that most climate models underestimate cloud cover and overestimate cloud reflectivity, particularly for the tropical low-level clouds. Here, we analyze the characteristics of low-level tropical marine clouds simulated by six climate models, which provided COSP output within the CMIP6 project. CALIPSO lidar observations and PARASOL mono-directional reflectance are used for model evaluation. It is found that the “too few, too bright” bias is still present for these models. The reflectance is particularly overestimated when cloud cover is low. Models do not simulate any optically thin clouds. They fail to reproduce the increasing cloud optical depth with increasing lower tropospheric stability as observed. These results suggest that most models do not sufficiently account for the effect of the small-scale spatial heterogeneity in cloud properties or the variety of cloud types at the grid scale that is observed.