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Improving the prediction of clouds in shallow-cumulus regimes via turbulence parameterization in the planetary boundary layer (PBL) will likely increase the global skill of global climate models (GCMs) because this cloud regime is common over tropical oceans where low-cloud fraction has a large impact on Earth's radiative budget. This study attempts to improve the prediction of PBL structure in tropical trade wind regimes in the Community Atmosphere Model (CAM) by updating its formulation of momentum flux in CLUBB (Cloud Layers Unified by Binormals), which currently does not by default allow for upgradient momentum fluxes. Hindcast CAM output from custom CLUBB configurations which permit countergradient momentum fluxes are compared to in situ observations from weather balloons collected during the ElUcidating the RolE of Cloud–Circulation Coupling in ClimAte and Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign (EUREC4A/ATOMIC) field campaign in the tropical Atlantic in early 2020. Comparing a version with CAM–CLUBB with a prognostic treatment of momentum fluxes results in vertical profiles that better match large-eddy simulation results. Countergradient fluxes are frequently simulated between 950 and 850 hPa over the EUREC4A/ATOMIC period in CAM–CLUBB. Further modification to the planetary boundary layer (PBL) parameterization by implementing a more generalized calculation of the turbulent length scale reduces model bias and root mean squared error (RMSE) relative to sounding data when coupled with the prognostic momentum configuration. Benefits are also seen in the diurnal cycle, although more systematic model errors persist. A cursory budget analysis suggests the buoyant production of momentum fluxes, both above and below the jet maximum, significantly contributes to the frequency and depth of countergradient vertical momentum fluxes in the study region. This paper provides evidence that higher-order turbulence parameterizations may offer pathways for improving the simulation of trade wind regimes in global models, particularly when evaluated in a process study framework.
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An Anti-Anti-Turbulence Conjecture
Sandstrom's theorem suggested that buoyancy fluxes can not drive the overturning circulation, an idea that physical oceanographers continue to debate. This paper attempts to provide the most fundamental examination of Sandstrom's theorem that is possible. We conclude that the fundamental role of buoyant forcing in the modern ocean is to damp the circulation, that buoyance fluxes drive significant levels of turbulence, that buoyant turbulent generation is not solely proportional to molecular diffusivity and that some of the main pathways to turbulence are non-Boussineq. We conjecture that the ocean might not be constrained by the `Anti-Turbulence' theorem.
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- Award ID(s):
- 1941963
- PAR ID:
- 10269828
- Date Published:
- Journal Name:
- Ocean and Coastal Research
- ISSN:
- 2675-2824
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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