11 February 2019

Cloud Feedbacks to Surface Warming in the World’s First Global Climate Model to Include Explicit Boundary-Layer Turbulence.

A computationally ambitious quest to refine the resolution of standard superparameterization produces remarkably familiar cloud feedbacks to warming.

Science

Most earth system models still rely on imperfect parameterizations of unresolved cloud processes. Cloud SuperParameterization (SP) – embedding thousands of limited-domain cloud-resolving arrays in a global climate model – has proved a promising alternative for deep convection, but its computational cost has been too prohibitive to include the sub-km scales of motion that form low clouds, which are especially critical to climate. Here researchers from a SciDac project spanning UC Irvine, the University of Washington, Stony Brook University and the Pacific Northwest National Laboratory, show results from a new form of SP calculations that avoid parameterizing even sub-km scales by using near-LES resolution, i.e. “ultraparameterization (UP).” Surprisingly, the results show remarkably familiar zonal mean cloud feedbacks to surface warming compared to both standard SP simulations, as well as SP augmented with higher-order closures of the sub-km scale.

Impact

It is simultaneously reassuring, surprising and troubling to discover an insensitivity of overall cloud radiative feedback across global simulations that make radically different grey-zone resolution choices. The good news is that this may imply strong constraints on circulation and thermodynamics dominate irksome grey zone sensitivities; this may suggest promise for reducing error bars on cloud feedbacks to warming in the convection-permitting era. The bad news is that subjective microphysical tuning choices prove more impactful than intentional turbulence-permitting grid resolution choices. This points to the urgency of a broad problem facing our community – that cloud microphysical parameterizations continue to be unsatisfyingly hard to constrain.

Summary

Global cloud feedbacks to surface warming are analyzed for the first time using UltraParameterization (UP), a new form of superparameterization (SP) that uses near-LES resolution to explicitly resolve even the boundary layer turbulence that forms low clouds. Comparing UP’s response to +4K surface warming against standard SP reveals a remarkably similar cloud radiative response. Some muting of high latitude phase change feedback strength happens with UP but this is due to microphysical tuning choices, not grey zone grid resolution refinement.

Contact
Mike Pritchard
University of California at Irvine
Publications
Parishani, H, M Pritchard, C Bretherton, C Terai, M Wyant, M Khairoutdinov, and B Singh.  2019.  "Insensitivity of the Cloud Response to Surface Warming Under Radical Changes to Boundary Layer Turbulence and Cloud Microphysics: Results From the Ultraparameterized CAM."  Journal of Advances in Modeling Earth Systems 10(12): 3139-3158, doi:10.1029/2018ms001409.