General Circulation Model Errors are Variable across Exoclimate Parameter Spaces

TL;DR

This study evaluates the variability of general circulation model errors across different exoclimate parameters, revealing that model accuracy varies significantly with planetary rotation periods and that certain numerical choices impact error levels.

Contribution

It demonstrates that model errors are highly variable across exoclimate parameter spaces and highlights the importance of numerical schemes and parameter tuning for accurate simulations.

Findings

Model errors vary greatly across rotation periods.

Higher grid resolution reduces errors, but higher-order diffusion schemes can increase errors.

Proper surface friction timescale improves model accuracy.

Abstract

General circulation models are often used to explore exoclimate parameter spaces and classify atmospheric circulation regimes. Models are tuned to give reasonable climate states for standard test cases, such as the Held-Suarez test, and then used to simulate diverse exoclimates by varying input parameters such as rotation rates, instellation, atmospheric optical properties, frictional timescales and so on. In such studies, there is an implicit assumption that the model which works reasonably well for the standard test case will be credible at all points in an arbitrarily wide parameter space. Here, we test this assumption using the open-source general circulation model THOR to simulate atmospheric circulation on tidally locked Earth-like planets with rotation periods of 0.1 to 100 days. We find that the model error, as quantified by the ratio between physical and spurious numerical contributions to the angular momentum balance, is extremely variable across this range of rotation periods with some cases where numerical errors are the dominant component. Increasing model grid resolution does improve errors but using a higher-order numerical diffusion scheme can sometimes magnify errors for finite-volume dynamical solvers. We further show that to minimize error and make the angular momentum balance more physical within our model, the surface friction timescale must be smaller than the rotational timescale.