Journal cover Journal topic
Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
Geosci. Model Dev., 11, 1971-1988, 2018
https://doi.org/10.5194/gmd-11-1971-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Development and technical paper
01 Jun 2018
Impact of numerical choices on water conservation in the E3SM Atmosphere Model version 1 (EAMv1)
Kai Zhang1, Philip J. Rasch1, Mark A. Taylor2, Hui Wan1, Ruby Leung1, Po-Lun Ma1, Jean-Christophe Golaz3, Jon Wolfe4, Wuyin Lin5, Balwinder Singh1, Susannah Burrows1, Jin-Ho Yoon1,a, Hailong Wang1, Yun Qian1, Qi Tang3, Peter Caldwell3, and Shaocheng Xie3 1Pacific Northwest National Laboratory, Richland, WA, USA
2Sandia National Laboratories, Albuquerque, NM, USA
3Lawrence Livermore National Laboratory, Livermore, CA, USA
4Los Alamos National Laboratory, Los Alamos, NM, USA
5Brookhaven National Laboratory, Upton, NY, USA
anow at: Gwangju Institute of Science and Technology, Gwangju, South Korea
Abstract. The conservation of total water is an important numerical feature for global Earth system models. Even small conservation problems in the water budget can lead to systematic errors in century-long simulations. This study quantifies and reduces various sources of water conservation error in the atmosphere component of the Energy Exascale Earth System Model.

Several sources of water conservation error have been identified during the development of the version 1 (V1) model. The largest errors result 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 1° horizontal resolution as well as consistent improvements at other resolutions. The second largest error source is 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 correcting the surface moisture flux and clipping 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 becomes negligible and insensitive to the horizontal resolution. The associated changes in the long-term statistics of the main atmospheric features are small.

A sensitivity analysis is carried out to show that the magnitudes of the conservation errors in early V1 versions decrease strongly with temporal resolution but increase with horizontal resolution. The increased vertical resolution in V1 results in a very thin model layer at the Earth's surface, which amplifies the conservation error associated with the surface moisture flux correction. We note that for some of the identified error sources, the proposed fixers are remedies rather than solutions to the problems at their roots. Future improvements in time integration would be beneficial for V1.

Citation: Zhang, K., Rasch, P. J., Taylor, M. A., Wan, H., Leung, R., Ma, P.-L., Golaz, J.-C., Wolfe, J., Lin, W., Singh, B., Burrows, S., Yoon, J.-H., Wang, H., Qian, Y., Tang, Q., Caldwell, P., and Xie, S.: Impact of numerical choices on water conservation in the E3SM Atmosphere Model version 1 (EAMv1), Geosci. Model Dev., 11, 1971-1988, https://doi.org/10.5194/gmd-11-1971-2018, 2018.
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Short summary
The conservation of total water is an important numerical feature for global Earth system models. Even small conservation problems in the water budget can lead to systematic errors in century-long simulations for sea level rise projection. This study quantifies and reduces various sources of water conservation error in the atmosphere component of the Energy Exascale Earth System Model.
The conservation of total water is an important numerical feature for global Earth system...
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