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The parameterization of subgrid‐scale processes such as boundary layer (PBL) turbulence introduces uncertainty in Earth System Model (ESM) results. This uncertainty can contribute to or exacerbate existing biases in representing key physical processes. This study analyzes the influence of tunable parameters in an experimental version of the Cloud Layers Unified by Binormals (CLUBBX) scheme. CLUBB is the operational PBL parameterization in the Community Atmosphere Model version 6 (CAM6), the atmospheric component of the Community ESM version 2 (CESM2). We perform the Morris one‐at‐a‐time (MOAT) parameter sensitivity analysis using short‐term (3‐day), initialized hindcasts of CAM6‐CLUBBX with 24 unique initial conditions. Several input parameters modulating vertical momentum flux appear most influential for various regionally‐averaged quantities, namely surface stress and shortwave cloud forcing (SWCF). These parameter sensitivities have a spatial dependence, with parameters governing momentum flux most influential in regions of high vertical wind shear (e.g., the mid‐latitude storm tracks). We next evaluate several experimental 20‐year simulations of CAM6‐CLUBBX with targeted parameter perturbations. We find that parameter perturbations produce similar physical mechanisms in both short‐term and long‐term simulations, but these physical responses can be muted due to nonlinear feedbacks manifesting over time scales longer than 3 days, thus causing differences in how output metrics respond in the long‐term simulations. Analysis of turbulent fluxes in CLUBBX indicates that the influential parameters affect vertical fluxes of heat, moisture, and momentum, providing physical pathways for the sensitivities identified in this study.
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Absence of Aerosol Indirect Effect Dependence on Background Climate State in NCAR CESM2
Abstract The aerosol indirect effect (AIE) dominates uncertainty in total anthropogenic aerosol forcing in phase 6 of the Coupled Model Intercomparison Project (CMIP6) models. AIE strength depends on meteorological conditions that have been shown to change between preindustrial (PI) and present-day (PD) climates, such as cloud cover and atmospheric moisture. Hence, AIE strength may depend on background climate state, impacting the dependence of model-based AIE estimates on experiment design or the evolution of AIE strength with intensifying climate change, which has not previously been explicitly evaluated. Using atmosphere-only simulations with prescribed observed sea surface temperatures (SSTs) and sea ice in the National Center for Atmospheric Research (NCAR) Community Earth System Model 2, version 2.1.3 (CESM2), Community Atmosphere Model, version 6.0 (CAM6), model, we impose a PD (2000) aerosol perturbation onto a PI (1850), PD, and PD with a uniform 4 K increase in the SST (PD + 4 K) background climate to assess the dependence of the total aerosol effective radiative forcing (ERF) and AIE on background climate. We find statistically insignificant increases in aerosol ERF when estimated in the different background climates, almost entirely from increases in direct ERF but with some regionally significant compensating signals in PD + 4 K. The absence of an AIE dependence on background climate in our PD simulation may be tied to documented differences in cloud responses to the observed SSTs used in our simulations versus SSTs produced by the fully coupled models from which most cloud feedback studies are derived, known as the “pattern effect.” Our findings indicate that AIE and aerosol forcing overall may not have a strong dependence on the background climate state in the near future but could regionally under extreme climate change. Significance StatementDiverse model representations of aerosol–cloud interactions strongly contribute to uncertainty in historical anthropogenic aerosol forcing and are associated with uncertainty in climate sensitivity. This study aims to highlight the dependence of aerosol indirect effects on the background climate state in Community Earth System Model 2, version 2.1.3 (CESM2), Community Atmosphere Model, version 6.0 (CAM6), by identifying microphysical and meteorological changes between aerosol-driven atmospheric responses in present-day and preindustrial climate states to understand anthropogenic aerosol-driven forcing more thoroughly.
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- Award ID(s):
- 2235177
- PAR ID:
- 10558853
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 38
- Issue:
- 1
- ISSN:
- 0894-8755
- Format(s):
- Medium: X Size: p. 147-163
- Size(s):
- p. 147-163
- Sponsoring Org:
- National Science Foundation
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