Cloudier, Warmer, and Rainier: A Look at the Arctic Mean State in the Updated Community Earth System Model

The Arctic is undergoing rapid change, and global climate models (GCMs) are invaluable tools for understanding both the drivers of that change as well as what the Arctic of the future could look like. GCMs are improving, however they continue to have difficulty accurately representing Arctic cloud processes. The Community Earth System Model (CESM), a widely used GCM in the Earth science community, recently released a new version (CESM2) with updates to all components, including the Community Atmosphere Model version 6 (CAM6). In this work, we evaluate the representation of Arctic clouds and precipitation in CESM2 with CAM6 relative to its predecessor (CESM1 with CAM5). We isolate changes in the mean state by using pre-industrial control runs for the two versions. We include modern era observations from satellites (CloudSat and CALIPSO) to provide an independent frame of reference for changes in select model variables.

We find that the mean state CESM2 Arctic atmosphere has ~5 times more cloud liquid than CESM1, greatly increasing the low cloud amount. This increase in cloudiness results in large changes to the surface downwelling radiation. The Arctic surface temperature in CESM2 is 3 K warmer in the annual mean and 6 K warmer in winter. While the mean Arctic snowfall rate has decreased slightly in the new model version, rainfall has increased, occurring year-round across the whole region. In summary, the Arctic in CESM2 with CAM6 is cloudier, warmer, and rainier than CESM1 with CAM5. In this presentation, we will present our findings, give context on whether these changes can be seen as improvements, and discuss the broader implications of cloud process changes in the Arctic system.