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. The conservationof total water is an important numerical feature for global Earth systemmodels. Even small conservation problems in the water budget can lead tosystematic errors in century-long simulations. This study quantifies andreduces various sources of water conservation error in the atmospherecomponent of the Energy Exascale Earth System Model. Several sources of water conservation error have been identified during thedevelopment of the version 1 (V1) model. The largest errors result from thenumerical coupling between the resolved dynamics and the parameterizedsub-grid physics. A hybrid coupling using different methods for fluiddynamics and tracer transport provides a reduction of water conservationerror by a factor of 50 at 1∘ horizontal resolution as well asconsistent improvements at other resolutions. The second largest error sourceis the use of an overly simplified relationship between the surface moistureflux and latent heat flux at the interface between the host model and theturbulence parameterization. This error can be prevented by applying the same(correct) relationship throughout the entire model. Two additional types ofconservation error that result from correcting the surface moisture flux andclipping negative water concentrations can be avoided by usingmass-conserving fixers. With all four error sources addressed, the waterconservation error in the V1 model becomes negligible and insensitive to thehorizontal resolution. The associated changes in the long-term statistics ofthe main atmospheric features are small. A sensitivity analysis is carried out to show that the magnitudes of theconservation errors in early V1 versions decrease strongly with temporalresolution but increase with horizontal resolution. The increased verticalresolution in V1 results in a very thin model layer at the Earth's surface,which amplifies the conservation error associated with the surface moistureflux correction. We note that for some of the identified error sources, theproposed fixers are remedies rather than solutions to the problems at theirroots. Future improvements in time integration would be beneficial for V1.

Impact of numerical choices on water conservation in the E3SM Atmosphere Model version 1 (EAMv1)