New developments in WACCM and CAM-chem

WACCM and CAM-chem have publicly released new capabilities as part of the December 2018 CESM2.1.0 release and the June 2019 CESM2.1.1 release. The latest public release now includes configurations that run out-of-the-box to reproduce all of the WACCM6 simulations performed at 0.95° latitude x 1.25° longitude horizontal resolution, with full chemistry (TSMLT1) for CMIP6, as well as future scenarios that have thus far been run for ScenarioMIP, AerChemMIP and GeoMIP (still in progress). Output from these simulations were also released publicly, and forcings needed to run CESM2 without interactive chemistry were created from this WACCM6 output.The latest public release of CESM also includes simpler configurations of WACCM6 with reduced chemistry (specified chemistry, middle atmosphere chemistry, and middle atmosphere plus D-region chemistry) and reduced horizontal resolution (1.9° latitude x 2.5° longitude).

A complete description of WACCM6 (Gettelman et al., 2019) was published in the Journal of Geophysical Research on October 13, 2019. The figure below, from that paper, compares the development of the Antarctic ozone hole in WACCM6 (blue, green, purple) to observations (black). WACCM6 reproduces well the observed drop in total column ozone over the polar cap in October between 1980 and 2000. WACCM6 simulations nudged to winds and temperatures from MERRA2 (purple) do particularly well in reproducing the observed inter-annual variability in Antarctic ozone. This shows that WACCM6 has a very complete representation of stratospheric chemistry, including interactive aerosols from volcanic eruptions.

October 90o-60oS total column ozone
Figure 1. October 90o-60oS total column ozone (in Dobson Units: DU) from CESM2(WACCM6) simulations (blue symbols), fixed SST (AMIP-type) simulations (green symbols), Specified Dynamics (SD) simulations (purple line and symbols), and observations (black line and symbols).

The model includes new aerosol descriptions in both the stratosphere and troposphere, including prognostic stratospheric aerosols from eruptive volcanoes, as well as an extended description of secondary organic aerosols using the Volatility Basis Set approach (Tilmes et al, 2019, in revision). Additional improvements are in development including simulations with different dynamical cores, different vertical resolution (in WACCM6), and updated aerosols schemes, including a Model for Simulating Aerosol Interactions and Chemistry (MOSAIC), inclusions of radiative forcing of brown carbon, updated DMS ocean emissions, etc.

Figure 2 below, from Gettelman et al. (2019), shows that WACCM6 reproduces well the variability in stratospheric aerosol from volcanic eruptions as observed by lidars at multiple latitudes since the mid-1980s. Stratospheric aerosol optical depth (SAOD) is shown from an ensemble of 3 fully coupled WACCM6 simulations (blue dots), and an ensemble of 3 WACCM6 AMIP simulations (red dots). Black and green dots show SAOD from lidar observations using a backscatter-to-extinction ratio of 50. 

5-day averaged SAOD
Figure 2. 5-day averaged SAOD (above model tropopause) from coupled (blue) and AMIP (red) ensembles compared to lidar observations (green, black, yellow). (a) Ny Alesund (78.9◦N, 11.9◦W) above 10 km (black, Ridley et al. [2014]) (b) Tomsk (56.5◦N, 85.0◦E) 15-30 km 10-day averages from Jan 1986 to Dec 2014 (black, Zuev et al. [2017]), and 12-30 km 10-day averages from Jan 2006 to Dec 2013 (green) (c) Geesthacht (53.4◦N,10.4◦E) above the tropopause (black with 1-σ error bars, Ansmann et al. [1997]) (d) Tsukuba (36.1◦N, 140.1◦E) 15-30 km monthly averages from Apr 1982 to Dec 2014 (yellow circles), above the tropopause monthly averages from Nov 1988 to Dec 2014 (black, Sakai et al. [2016]), and above the tropopause daily from Jan 2008 to Jul 2013 (green) (e) Mauna Loa (19.5◦N, 155.6◦W) above the tropopause (black), Hofmann et al. [2009]] (f) Lauder (45.0◦S, 169.7◦E) monthly averages from Nov 1992 to Dec 2014, 16.5-33 km (yellow) and above the tropopause (black) Sakai et al. [2016]. Click for larger image.

WACCM6 and CAM-chem can be further run with an extended description of secondary organic aerosol, which allows to separate SOA source contributions.  Figure 3 below shows the evolution of source contributions of different regions. 

Annually averaged total SOA burden
Figure 3. Annually averaged total SOA burden (black), and SOA burden from separate  precursor sources, including biogenic (green), fossil fuel (brown), and biomass burning (red), averaged over six regions between 1960 and 2014 derived using WACCM6.


Gettelman, A. et al. (2019), The Whole Atmosphere Community Climate Model Version 6 (WACCM6), J Geophys Res-Atmos, 2019JD030943, doi:10.1029/2019JD030943.

Tilmes, S  et al. (2019) Climate forcing and trends of organic aerosols in the Community Earth System Model (CESM2), JAMES, accepted.