Numerically Relevant Timescales in the MG2 Microphysics Model.

Climate models rely on parameterizations of a variety of processes in the atmospheric physics, but a common concern is that the temporal resolution is too coarse to consistently resolve the behavior that individual parameterizations are designed to capture. This study examines timescales numerically derived from the Morrison‐Gettelman (MG2) microphysics as implemented within the Energy Exascale Earth System Model, version 1 (E3SMv1). Numerically relevant timescales in MG2 are derived by computing the eigenspectrum of its Jacobian. These timescales are found to often be smaller than the default 5 min time step used for MG2. The fast timescales are then heuristically connected to individual microphysics processes. By substepping a few particular rain processes within MG2, the time discretization error for those processes was considerably reduced with minimal additional expense to the overall microphysics. While this improvement has a substantial effect on the target processes and on the vertical distribution of stratiform‐derived rain within E3SMv1, the overall model climate is found to not be sensitive to MG2 time step. We hypothesize that this is because the surface climate does not depend strongly on certain process rates, especially MG2's rain evaporation rate. Plain Language Summary: The atmospheric components of climate models contain a number of physics parameterizations, subcomponents that are designed to capture particular aspects of the atmospheric physics. Cloud microphysics models are parameterizations designed to represent very small scale cloud processes, including phase changes and the formation of precipitation. The accuracy of these parameterizations depends on the model time step: A shorter time step typically requires more computational resources, but also improves the model's accuracy. This paper examines a particular microphysics model, MG2, used with a time step of 5 min in the E3SMv1 climate model. By linearizing MG2, we can find characteristic timescales associated with this model, which are often much shorter than 5 min. This suggests that the usual time step is too large to fully capture the physics that MG2 represents. We also experiment with using a shorter time step for parts of the rain physics and find that in many cases the rain mass is strongly affected. However, reducing the MG2 time step does not have much effect on the overall climate of E3SMv1. Key Points: MG2 contains several processes that are poorly resolved at a time step of 5 minSubstepping MG2's rain‐related processes changes their rates considerablyHowever, reducing MG2's time step within E3SM has little to no impact on the climate [ABSTRACT FROM AUTHOR]