Improved Representation of Clouds in the Atmospheric Component LMDZ6A of the IPSL‐CM6A Earth System Model

Abstract

Abstract The cloud parameterizations of the LMDZ6A climate model (the atmospheric component of the IPSL‐CM6 Earth system model) are entirely described, and the global cloud distribution and cloud radiative effects are evaluated against the CALIPSO‐CloudSat and CERES observations. The cloud parameterizations in recent versions of LMDZ favor an object‐oriented approach for convection, with two distinct parameterizations for shallow and deep convection and a coupling between convection and cloud description through the specification of the subgrid‐scale distribution of water. Compared to the previous version of the model (LMDZ5A), LMDZ6A better represents the low‐level cloud distribution in the tropical belt, and low‐level cloud reflectance and cover are closer to the PARASOL and CALIPSO‐GOCCP observations. Mid‐level clouds, which were mostly missing in LMDZ5A, are now better represented globally. The distribution of cloud liquid and ice in mixed‐phase clouds is also in better agreement with the observations. Among identified deficiencies, low‐level cloud covers are too high in mid‐latitude to high‐latitude regions, and high‐level cloud covers are biased low globally. However, the cloud global distribution is significantly improved, and progress has been made in the tuning of the model, resulting in a radiative balance in close agreement with the CERES observations. Improved tuning also revealed structural biases in LMDZ6A, which are currently being addressed through a series of new physical and radiative parameterizations for the next version of LMDZ.

Keywords

• global climate model
• subgrid‐scale parameterization
• climate model tuning
• cloud radiative effect
• mixed‐phase clouds
• CMIP6