Rethinking how to characterize cloud biases in coarse resolution models: a regime-based approach

Large-scale models struggle to accurately represent maritime low-level boundary layer clouds, leading to uncertainties in projecting a future climate. This study uses a regime-based approach to assess global climate model representation of warm-phase microphysical processes over the Southern Ocean (SO) and northeast Pacific, regions frequently characterized by the stratocumulus-to-cumulus transition (SCT) regime. In situ aircraft observations collected during the Southern Ocean Clouds, Radiation and Aerosol Transport Experimental Study (SOCRATES) and the Cloud Systems Evolution in the Trades (CSET) campaigns were used to evaluate simulated low-level cloud microphysical properties. Using environmental variables and airborne remote sensing, we investigate methods for compositing aircraft observations into stratocumulus, open-cell, and undetermined cloud sampling regimes. This approach aims to mitigate issues with scaling aircraft observations to model grid resolutions and discrepancies with collocating aircraft observations with simulated clouds. We assessed simulated warm-phase cloud processes in the Community Earth System Model (CESM) using new model diagnostics tools to improve our representation of the SCT regime, and results from both CESM2 and CESM3 will be presented.