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.

The Choice of Baseline Period Influences the Assessments of the Outcomes of Stratospheric Aerosol Injection

“…In this study we use the Community Earth System Model Version 2 (CESM, Danabasoglu et al. (2020)) in its Whole Atmosphere Community Climate Model Version 6 (WACCM6) configuration with simplified tropospheric chemistry (Davis et al., 2022), hereafter CESM2‐WACCM6. This model version has a horizontal resolution of 1.25° longitude by 0.9° latitude with 70 vertical levels that extend up to about 140 km.…”

“…This model version has a horizontal resolution of 1.25° longitude by 0.9° latitude with 70 vertical levels that extend up to about 140 km. The version we use has comprehensive stratospheric and upper-atmospheric chemistry, as well as an interactive aerosol microphysics scheme termed the Modal Aerosol Module (MAM4) (Liu et al, 2016), but has simplified tropospheric chemistry that only includes the most relevant processes and does not have detailed Secondary Organic Aerosol chemistry; in Davis et al (2022), this has been shown to not produce relevant changes in stratospheric chemistry and surface climate.…”

Potential Non‐Linearities in the High Latitude Circulation and Ozone Response to Stratospheric Aerosol Injection

“…We use the CESM2(WACCM6) earth system model (Danabasoglu et al., 2020; Gettelman et al., 2019) with interactive modal aerosol microphysics (MAM4, Liu et al., 2016) and interactive middle atmosphere chemistry (Davis et al., 2023). The horizontal resolution is 1.25° longitude by 0.9° latitude, with 70 vertical levels in hybrid‐pressure coordinates up to ∼140 km.…”

Stratospheric Aerosol Injection Can Reduce Risks to Antarctic Ice Loss Depending on Injection Location and Amount

“…The atmospheric horizontal resolution is 1.25° longitude by 0.95° latitude, with 70 vertical layers extending from the surface to about 140 km. The simulations use the Middle Atmosphere chemistry configuration (Davis et al., 2022) that includes an interactive stratospheric and upper atmospheric chemistry in addition to aerosol microphysics from the Modal Aerosol Module (MAM4; Liu et al., 2016). The ocean model (Parallel Ocean Program Version 2) horizontal resolution is 1.125° in the zonal direction and ranges between about 0.27° and 0.64° in the meridional direction and includes 60 vertical levels (Danabasoglu et al., 2012; R. Smith et al., 2010).…”