Quantifying and attributing time step sensitivities in present-day climate simulations conducted with EAMv1

TL;DR

This paper investigates how the choice of time step size affects climate simulation accuracy in the EAMv1 model, revealing significant impacts on climate fidelity and offering insights for reducing numerical errors.

Contribution

It provides a detailed assessment of time step sensitivities in EAMv1 and introduces an experimentation strategy applicable to other climate models.

Findings

Reducing time step size by a factor of 6 significantly alters long-term climate means.

Decreasing time steps degrades agreement with observed climate, indicating a need for retuning.

Separate adjustments of model components' time steps help identify sources of sensitivity.

Abstract

This study assesses the relative importance of time integration error in present-day climate simulations conducted with the atmosphere component of the Energy Exascale Earth System Model version 1 (EAMv1) at 1-degree horizontal resolution. We show that a factor-of-6 reduction of time step size in all major parts of the model leads to significant changes in the long-term mean climate. These changes imply that the reduction of temporal truncation errors leads to a notable although unsurprising degradation of agreement between the simulated and observed present-day climate; the model would require retuning to regain optimal climate fidelity in the absence of those truncation errors. A coarse-grained attribution of the time step sensitivities is carried out by separately shortening time steps used in various components of EAM or by revising the numerical coupling between some processes. The results provide useful clues to help better understand the root causes of time step sensitivities in EAM. The experimentation strategy used here can also provide a pathway for other models to identify and reduce time integration errors.