The conservation of total water is an important numerical feature for global Earth system models, as the lack thereof might affect the prediction of sea level rise and other aspects of the global water cycle. This study quantifies and reduces various sources of water conservation error in the atmosphere component of the Energy Exascale Earth System Model (E3SM).
In the V1a version of the model, the largest source of water conservation error results from the numerical coupling between the resolved dynamics and the parameterized sub-grid physics. A hybrid coupling using different methods for fluid dynamics and tracer transport provides a reduction of water conservation error by a factor of 50 at one-degree horizontal resolution as well as consistent improvements at other resolutions. The second largest error source is associated with the use of an overly simplified relationship between the surface moisture flux and latent heat flux at the interface between the host model and the turbulence parameterization. This error can be prevented by applying the same (correct) relationship throughout the entire model. Two additional types of conservation error that result from clipping of negative water concentrations can be avoided by using mass-conserving fixers. With all four error sources addressed, the water conservation error in the V1 model can be reduced to a negligible level.
Sensitivity analysis are carried out to show that the magnitudes of the conservation errors increase with horizontal resolution but decrease with temporal resolution. Empirically chosen time step sizes for the physics parameterizations that are not proportional to the dynamics time steps can lead to non-monotonic changes of total error when resolution is varied. We also note that for some of the error sources identified for V1a, the proposed fixers are remedies rather than solutions of the problem at its roots. Future improvements in time integration would be beneficial for this model.
Revised: September 30, 2020 |
Published: June 1, 2018
Citation
Zhang K., P.J. Rasch, M. Taylor, H. Wan, L. Leung, P. Ma, and J. Golaz, et al. 2018.Impact of numerical choices on water conservation in the E3SM Atmosphere Model Version 1 (EAM V1).Geoscientific Model Development 11, no. 5:1971-1988.PNNL-SA-129738.doi:10.5194/gmd-11-1971-2018