Climate, variability, and climate sensitivity of “Middle Atmosphere”
chemistry configurations of the Community Earth System Model Version 2,
Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6))
Abstract
Simulating whole atmosphere dynamics, chemistry, and physics is
computationally expensive. It can require high vertical resolution
throughout the middle and upper atmosphere, as well as a comprehensive
chemistry and aerosol scheme coupled to radiation physics. An
unintentional outcome of the development of one of the most
sophisticated and hence computationally expensive model configurations
is that it often excludes a broad community of users with limited
computational resources. Here, we analyze two configurations of the
Community Earth System Model Version 2, Whole Atmosphere Community
Climate Model Version 6 (CESM2(WACCM6)) with simplified “middle
atmosphere” chemistry at nominal 1 and 2 degree horizontal resolutions.
Using observations, a reanalysis, and direct model comparisons, we find
that these configurations generally reproduce the climate, variability,
and climate sensitivity of the 1 degree nominal horizontal resolution
configuration with comprehensive chemistry. While the background
stratospheric aerosol optical depth is elevated in the middle atmosphere
configurations as compared to the comprehensive chemistry configuration,
it is comparable between all configurations during volcanic eruptions.
For any purposes other than those needing an accurate representation of
tropospheric organic chemistry and secondary organic aerosols, these
simplified chemistry configurations deliver reliable simulations of the
whole atmosphere that require 35% to 86% fewer computational resources
at nominal 1 and 2 degree horizontal resolution, respectively.