Southern Ocean cloud and shortwave radiation biases in a nudged climate model simulation: does the model ever get it right?

• Published in: Atmospheric chemistry and physics Vol. 22; no. 22; pp. 14603 - 14630
• Main Authors: Fiddes, Sonya L, Protat, Alain, Mallet, Marc D, Alexander, Simon P, Woodhouse, Matthew T
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 17.11.2022; Copernicus Publications
• Subjects: Aerosols; Atmosphere; Atmospheric models; Atmospheric physics; Bias; Climate; Climate and weather; Climate models; Cloud microphysics; Cloud properties; Cloud types; Clouds; Clustering; Decomposition; Histograms; Microphysics; Modelling; Ocean; Ocean models; Oceans; Radiation; Radiation-cloud interactions; Rain; Satellites; Short wave radiation; Simulation; Simulation methods; Simulators; Weather; Southern Ocean
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-22-14603-2022

The Southern Ocean radiative bias continues to impact climate and weather models, including the Australian Community Climate and Earth System Simulator (ACCESS). The radiative bias, characterised by too much shortwave radiation reaching the surface, is attributed to the incorrect simulation of cloud properties, including frequency and phase. To identify cloud regimes important to the Southern Ocean, we use k-means cloud histogram clustering, applied to a satellite product and then fitted to nudged simulations of the latest-generation ACCESS atmosphere model. We identify instances when the model correctly or incorrectly simulates the same cloud type as the satellite product for any point in time or space. We then evaluate the cloud and radiation biases in these instances. We find that when the ACCESS model correctly simulates the cloud type, cloud property and radiation biases of equivalent, or in some cases greater, magnitude remain compared to when cloud types are incorrectly simulated. Furthermore, we find that even when radiative biases appear small on average, cloud property biases, such as liquid or ice water paths or cloud fractions, remain large. Our results suggest that simply getting the right cloud type (or the cloud macrophysics) is not enough to reduce the Southern Ocean radiative bias. Furthermore, in instances where the radiative bias is small, it may be so for the wrong reasons. Considerable effort is still required to improve cloud microphysics, with a particular focus on cloud phase.