GCM clouds and actual clouds as seen from different space lidars: towards a long-term assessment of cloud representation in GCMs using lidar simulator

Abstract. In Earth's radiative budget, clouds play a central role but their representation in General Circulation Models (GCMs) remains a major source of uncertainty for climate projection. Here, we used spaceborne lidar observations to assess cloud distribution in the IPSL-CM6-LR model using the CFMIP Observation Simulator Package (COSP). We focused on the lidars onboard CALIPSO and AEOLUS satellites during 2006–2023 and 2018–2023. While CALIPSO has been widely used for GCMs evaluation, ALADIN was originally designed for wind profiling. However, studies have demonstrated its potential to retrieve reliable cloud profiles. A new module was developed to simulate AEOLUS observations within COSP-lidar, extending original implementations made for CALIPSO, including wavelength change (532 nm to 355 nm), viewing geometry (35° off-nadir) and specific parameters adjustments related to sensivity and resolution. We compared our simulations to 1-year observations for both instruments. Results show that AEOLUS observations can effectively evaluate clouds in GCMs, as it shows similar cloud fraction biases in IPSL-CM6-LR to those obtained with CALIPSO. Significant underestimations of low (up to 20 %) and high clouds in certain regions (e.g. warm pool) were re-assessed for this model. Sensitivity analyses highlighted the small role of instrument-specific parameters in COSP-lidar: viewing geometry, multiple scattering coefficient and cloud detection threshold (associated with wavelength and sensivity). This work lays the foundation for a consistent multi-decades evaluation of cloud representation using different lidar missions, and supports the integration of EarthCARE/ATLID in COSP-lidar for further model evaluation.