Abstract
Abstract. Numerical general circulation models of the atmosphere are generally required to conserve mass and energy for their application to climate studies. Here we draw attention to another conserved global integral, viz. the component of angular momentum (AM) along the Earth's axis of rotation, which tends to receive less consideration. We demonstrate the importance of global AM conservation in climate simulations with the example of the Community Atmosphere Model (CAM) with the finite-volume (FV) dynamical core, which produces a noticeable numerical sink of AM. We use a combination of mathematical analysis and numerical diagnostics to pinpoint the main source of AM non-conservation in CAM–FV. We then present a method to enforce global conservation of AM, and we discuss the results in a hierarchy of numerical simulations of the atmosphere of increasing complexity. In line with theoretical expectations, we show that even a crude, non-local enforcement of AM conservation in the simulations consistently results in the mitigation of certain persistent model biases.
Highlights
The atmosphere exchanges angular momentum (AM) with the material bodies at the surface, which are, to a good approximation, in a state of motion consisting of uniform rotation about the planetary axis connecting the poles
In order to minimise the impact of other minor numerical sources and sinks of AM, in all idealised numerical tests presented in this paper we applied the following modifications: (1) the order of the advection scheme is kept the same for all model layers, instead of reducing it to first in the top layer and to second up to the eighth layer; (2) an additional conservation check is applied in the vertical remapping of zonal wind, and column momentum is conserved in the moist-mass adjustment at the end of physics; (3) the surface stress residual resulting from closure of the diffusion operator is applied in full rather than partially
AM conservation in Community Atmosphere Model (CAM)–FV has been substantially improved by means of a correction that reduces the zonalmean numerical sink of Lin and Rood’s (1997) shallow-water scheme and a fixer that ensures the conservation of global angular momentum under advection
Summary
The atmosphere exchanges angular momentum (AM) with the material bodies at the surface, which are, to a good approximation, in a state of motion consisting of uniform rotation about the planetary axis connecting the poles. Toniazzo et al.: Angular momentum conservation in the CAM AGCM spheric AM until air is lifted in cumulus convection within the inter-tropical convergence zone (ITCZ) This circulation is the object of numerical simulations with general circulation models (GCMs) used in meteorological forecasting and in climate modelling. The global total numerical torque due to the FV dynamical core was diagnosed at every time step of the reference FV simulation and averaged in time afterwards This was converted into a solid-body axial rotation tendency that was applied continuously everywhere as a constant sink of AM in a new integration with the spectral dynamical core, resulting in the simulation represented by the red curve.
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