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Numerically Relevant Timescales in the MG2 Microphysics Model
  • Sean Patrick Santos,
  • Christopher S. Bretherton,
  • Peter Martin Caldwell
Sean Patrick Santos
University of Washington

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Christopher S. Bretherton
University of Washington
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Peter Martin Caldwell
Lawrence Livermore National Laboratory (DOE)
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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.
Apr 2020Published in Journal of Advances in Modeling Earth Systems volume 12 issue 4. 10.1029/2019MS001972