Held-Suarez simulations with the Community Atmosphere Model Spectral Element (CAM-SE) dynamical core: a detailed global axial angular momentum analysis using Eulerian and Floating Lagrangian vertical coordinates

Conservation of Axial Angular Momentum (AAM) is a critical component of global general circulation models. For planets such as Venus and Titan super-rotation comes about through small imbalances in surface sources/sinks of AAM and are therefore susceptible to non-conservation errors in the dynamical core. In Earth's atmosphere the physical sources/sinks of AAM (e.g., resolved and parameterized mountain torques) are large so the lack of AAM may be less apparent. Nonetheless AAM conservation affects important phenomena such as the Quasi-Biennial Oscillation (QBO). In this paper a detailed analysis of the global AAM conservation properties of NCAR's Community Atmosphere Model Spectral Element (CAM-SE) dynamical core under Held-Suarez forcing is presented. It is shown that the spurious sources/sinks of AAM in CAM-SE are three orders of magnitude smaller than the parameterized (physical) sources/sinks. The effect on AAM conservation by changing various numerical aspects of the dynamical core (e.g., different vertical coordinates, reduced formal order of accuracy, increased dissipation and decreased divergence damping) is investigated. In particular it is noted that changing from Eulerian to Lagrangian vertical coordinates does not alter the excellent global AAM conservation properties of CAM-SE.