Low‐Level Marine Tropical Clouds in Six CMIP6 Models Are Too Few, Too Bright but Also Too Compact and Too Homogeneous. Issue 11 (31st May 2022)

• Record Type: Journal Article
• Authors: Konsta, Dimitra; Dufresne, Jean‐Louis; Chepfer, Hélène; Vial, Jessica; Koshiro, Tsuyoshi; Kawai, Hideaki; Bodas‐Salcedo, Alejandro; Roehrig, Romain; Watanabe, Masahiro; Ogura, Tomoo
• Is Part Of: Geophysical research letters. Volume 49:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-05-31
• Subjects: Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL097593
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: 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. Plain Language Summary: Low‐level clouds are ubiquitous in the tropics and play an important role in Earth's radiative balance. Climate models do not explicitly resolve the main low‐level cloud formation processes, which must therefore be parameterized. This modeling work is difficult and in the previous generation of models low‐level clouds had a systematically too low fraction and too large brightness. This models' deficiency is known as the "too few too bright bias." Here, we use six climate models of the latest generation that are compared to lidar and reflectance observations allowing for a detailed characterization of cloud properties. It is found that the too few too bright bias is still present for these models. Other common deficiencies in cloud simulation are revealed. At the daily time scale and models' grid scale, the lower the cloud cover, the greater the overestimation of the cloud brightness. Models do not simulate any thin clouds. They fail to reproduce the increasing cloud brightness with increasing stability of the lower troposphere as observed. The study suggests that most models do not sufficiently account for the variety of cloud properties and cloud types at the models' grid scale that is observed. Key Points: The "too few too bright" bias is still present in six CMIP6 models for low‐level clouds The overestimation of the low‐level cloud brightness gets higher as their cover is low Models fail to reproduce the increasing cloud optical depth with increasing lower tropospheric stability as observed