Development of CESM2

The development of the Community Earth System Model (CESM) is an ongoing enterprise, with new model versions assembled and released at approximately five to seven year intervals. CESM1 was released in June 2010, CESM2 will be publically released in late 2017 or early 2018 and CESM3 is expected to be released in approximately 7 years. Continued development is enabled by new scientific understanding and new research areas combined with increasing computational capability and capacity. The development of CESM2 represents the culmination of an on-going effort spanning the last 3-4 years, and is bringing together major new versions of all components but the ocean (Figure 1).

Figure 1: CESM2 Configuration
Figure 1: CESM2 configuration with all active components.

In the process of developing the Community Atmosphere Model (CAM) version 6, it was deemed important that the choice between two available cloud representations (CLUBB and UNICON) was documented as well as possible. For that purpose, a panel of internal and external scientists was put in place, and specific simulations were requested to enable a comprehensive evaluation. The results of the panel evaluation (the complete documentation is available here) was the selection of CLUBB, although UNICON will be included in the CESM2 release, as an unsupported option.

In order to provide the community with a scientifically-supported released version (see here for more details), an extensive model evaluation (with a focus on the behavior of the coupled model under constant 1850 forcings), has been undertaken. This was performed with the workhorse version of the atmosphere and ocean at 1° resolution. In particular, in the process of developing CESM2, an emphasis was placed on performing coupled simulations to test the model. The choice of a 1° resolution was dictated by the preference for focusing our model development on new physics and bias reduction instead of resolution increase. While the model is not fully finalized, it is clear that many aspects of the model are significant improvements over CESM1. In particular, the land model evaluation is showing improvements over many observationally-based metrics (Figure 2).

Figure 2: Comparison of CLM results
Figure 2: Comparison of CLM results against a variety of observationally-based diagnostics, showing the improvements between CESM1 and CESM2.