Impact of Numerical Choices on Water Conservation in the E3SM Atmosphere Model Version 1

TitleImpact of Numerical Choices on Water Conservation in the E3SM Atmosphere Model Version 1
Publication TypeJournal Article
Year of Publication2017
AuthorsZhang, Kai, Rasch Philip, Taylor Mark, Wan Hui, Leung Ruby, Ma Po-Lun, Golaz Jean-Christophe, Wolfe Jon, Lin Wuyin, Singh Balwinder, Burrows Susannah, Yoon Jin-Ho, Wang Hailong, Qian Yun, Tang Qi, Caldwell Peter, and Xie Shawocheng
JournalGeoscientific Model Development Discussions
Date Published12/2017
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 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.

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 is 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 decrease strongly with temporal resolution but increase with horizontal resolution. The increased vertical resolution in the new model 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 fixes are remedies rather than solutions to the problems at their roots. Future improvements in time integration would be beneficial for this model.

URLhttps://www.geosci-model-dev-discuss.net/gmd-2017-293/gmd-2017-293.pdf
DOI10.5194/gmd-2017-293
Funding Program: 
Journal: Geoscientific Model Development Discussions

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.

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 is 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 decrease strongly with temporal resolution but increase with horizontal resolution. The increased vertical resolution in the new model 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 fixes are remedies rather than solutions to the problems at their roots. Future improvements in time integration would be beneficial for this model.

DOI: 10.5194/gmd-2017-293
Year of Publication: 2017
Citation:
Zhang, K, P Rasch, M Taylor, H Wan, R Leung, P Ma, J Golaz, J Wolfe, W Lin, B Singh, S Burrows, J Yoon, H Wang, Y Qian, Q Tang, P Caldwell, and S Xie.  2017.  "Impact of Numerical Choices on Water Conservation in the E3SM Atmosphere Model Version 1."  Geoscientific Model Development Discussions, doi:10.5194/gmd-2017-293.