Abstract
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 timestep
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 significantly 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.