D4.3: Report with priority list of sub-grid scale processes to address and improve in phase two
Creators: name: Dziekan, Piotr
name: Nowak, Jakub
name: Dragaud, Ian
name: Kuma, Peter
name: Malinowski, Szymon
name: Pawlowska, Hanna
name: Waclawczyk, Marta
Publisher: name: Zenodo
Year: 2023

description: About this document

Report providing a listing of sub-grid scale processes related to turbulent mixing and cloud microphysics that are not realistically represented and require further improvement.

Work package in charge

WP4

Executive summary

In climate models, processes that occur on scales smaller than model resolution are accounted for using so-called parameterizations. The type of parameterization used can have a large effect on model results. As the resolution of models is increased, some processes are resolved on a model grid and no longer need to be parameterized. The others, operating at smaller scales, will still need to be parameterized even in future, high-resolution models.

In the second phase of the project, we recommend addressing and improving two sub-grid schemes: 



turbulent mixing scheme, with particular focus on vertical mixing in stable stratification,

microphysics scheme, with a particular focus on the partitioning of hydrometeors (cloud water, rain, snow, ice, and graupel).


The work described in this report shows that there is significant uncertainty associated with the choice of turbulence and microphysics parameterization. We observed high sensitivity of the amount of low-level clouds over the ocean to the type of turbulence parameterization. The Smagorinsky parameterization is beneficial in that it adapts to the model resolution, but it is found to give too little vertical mixing in regions with strong temperature inversion. In microphysics parameterizations, condensed and frozen water is arbitrarily divided into categories, which leads to discrepancies between models, e.g., in the amount of ice in clouds. We started to explore alternative sub-grid schemes that could potentially replace the parameterizations used today.

Conclusion & Results

Parameterization of sub-grid scale (SGS) turbulence has large impact on low-level clouds over the ocean. In IFS-FESOM turbulence in the mixed layer is modeled with the eddy diffusivitymass flux (EDMF) scheme, and a shallow convection parameterization is used. This approach gives good agreement with observations in the trade-wind regime, as we show by comparing profiles of Cycle 2 simulations with soundings from the Barbados Cloud Observatory (BCO).

In most of the nextGEMS ICON simulations, turbulence is modeled using the Smagorinsky scheme and there is no parameterization of shallow convection. The Smagorinsky model is typically used in large-eddy simulations (LES). The rationale for its use in global ICON simulations is that it adapts to changes in resolution, it has fewer tuning parameters and the future ESM resolution will be comparable to that of coarse LES. However, comparison with observations over subtropical oceans suggests that this approach provides too little vertical mixing in the case of stable stratification. On the other hand, comparison with observations over the Southern Ocean suggests the simulated mixing is too strong in the conditions of extremely stable stratification over the surface. This effect is related to stability correction function in combination with vertical resolution of the model grid. In consequence, liquid water path (LWP) in stratocumulus and cumulus regions is around two times larger in ICON than in IFS-FESOM, as we show based on Cycle 3 simulations. Moreover, the occurrence of low-level clouds over the Southern Ocean is underestimated while the mixed-layer depth overestimated with respect to observations.

An attempt to fix this issue was one of the Cycle 3 ICON simulations that used a Smagorinsky scheme with a modified stability correction function in order to increase the amount of vertical mixing in regions with high stability. This modification was successful in decreasing the amount of stratocumuli, but failed in decreasing the amount of cumuli. In consequence, the amount of liquid water no longer linearly depended on the lower tropospheric stability, as it is expected based on long-term observations. It shows that low-level clouds are in fact strongly dependent on the SGS turbulence model and that a more physically-based modification of the Smagorinsky scheme is required.

Another parameterization that contributes to model uncertainty is cloud microphysics. Both ICON and IFS-FESOM use single-moment bulk microphysics, but with different details. In particular, ICON simulates snow, ice and graupel while IFS-FESOM only snow and ice. In consequence, in ICON 15 % and 20 % of frozen water is considered ice and graupel, respectively, while in IFS-FESOM 35 % is ice. Since graupel and ice sediment with very different velocities, this can affect the lifetime of clouds.