Quoted from: Gettelman, A., M. J. Mills, D. E. Kinnison, R. R. Garcia, A. K. Smith, D. R. Marsh, S. Tilmes et al. "The whole atmosphere community climate model version 6 (WACCM6)." Journal of Geophysical Research: Atmospheres 124, no. 23 (2019): 12380-12403.  https://doi.org/10.1029/2019JD030943 

WACCM6 is the whole atmosphere version of the Community Atmosphere Model version 6 (CAM6). Unlike in previous versions of CESM, WACCM6 is identical to CAM6 in the range of processes that are parameterized. The only exception is the representation of parameterized gravity waves (see section 2.2). A summary of the current parameterizations for CAM6 and WACCM6 clouds and aerosols is contained in Table 1. These are detailed further below as applied to WACCM6.

Table 1 describes the different versions of WACCM starting with WACCM4, described by Marsh et al. (2013). WACCM‐CCMI was developed for the Chemistry Climate Modeling Iniative (CCMI) and features updated tropospheric and stratospheric chemistry. WACCM5 (Mills et al., 2017) is based on WACCM4 chemistry, but with updated physical parameterizations following Neale et al. (2010) and features adjustments to the physical parameterizations and gravity wave schemes (section 2.2), as well as updated stratospheric prognostic aerosols (Mills et al., 2016). WACCM5 also transitions to higher horizontal and vertical resolution. WACCM‐CCMI emphasized the evolution of chemistry and WACCM5 the evolution of dynamics. WACCM6 combines the new aspects of both models with additional updates for CESM2.

WACCM6 was also designed to better match CAM6 in CESM2 and uses all the same physical parameterizations as CAM6 (plus some additional gravity waves detailed in section 2.2). WACCM6 also moved to higher horizontal resolution. One of the major reasons to move to a 1° horizontal resolution for WACCM is to match the resolution of the low top model CAM6: this removes one of the major differences between CAM and WACCM. A lower resolution (2°) version of WACCM6 was released in CESM2.1.1.