2025-04-10 All-Hands Presentation Meeting Notes

Paths to numerically stable and momentum-conserving atmosphere-surface coupling in E3SM

Sean Santos
Pacific Northwest National Laboratory

Abstract: 

E3SM’s near-land-surface winds are often subject to unphysical high-frequency oscillations with period twice the model time step, particularly in stable boundary layers. At high latitudes (especially over coastal Antarctica and Greenland), these unphysical oscillations are extremely common and one of the primary sources of variability in surface winds (and thus fluxes). The oscillations also affect rough (mountainous and forested) regions globally, particularly at night. Using a simplified model of the atmosphere, we can reproduce these oscillations, and attribute them to numerical instability caused by E3SM’s use of explicit flux coupling between the atmosphere and other components. To address this issue, we implemented an implicit flux coupling scheme to the ocean and land components of E3SM, which is stable for a 30-minute time step. We present a few different sets of modifications as possible candidates for inclusion in future model versions to address this instability.

Furthermore, we note that CLUBB(/SHOC) does not directly apply surface stresses to the atmosphere but rather approximates the effect of the stress by adding a diffusion coefficient to its semi-implicit solvers. This approximate method is currently needed for model stability (reverting to the original, direct application of surface stresses causes the model to crash!), but it does not conserve momentum and also does not correctly account for the movement of the ocean or sea ice with respect to the atmospheric grid. This produces a systematic (though likely small) bias toward weaker winds and surface currents. Some possible changes to alleviate this bias are proposed.

Date Apr 10, 2025 

Time

• PT: 8:30 am

• ET: 11:30 am

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