Quantifying and attributing time step sensitivities in present-day climate simulations conducted with EAMv1
<p>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<span class="inline-formula"><sup>∘</sup></span>...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2021-04-01
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Series: | Geoscientific Model Development |
Online Access: | https://gmd.copernicus.org/articles/14/1921/2021/gmd-14-1921-2021.pdf |
Summary: | <p>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<span class="inline-formula"><sup>∘</sup></span> 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.
Examples of changes in 10-year mean zonal averages include the following:
</p><ol><li>
<p id="d1e161">up to 0.5 K of warming in the lower troposphere and cooling
in the tropical and subtropical upper troposphere,</p></li><li>
<p id="d1e165">1 %–10 % decreases in relative humidity throughout the troposphere, and</p></li><li>
<p id="d1e169">10 %–20 % decreases in cloud fraction in the upper troposphere and
decreases exceeding 20 % in the subtropical lower troposphere.</p></li></ol><p>
In terms of the 10-year mean geographical distribution, systematic decreases
of 20 %–50 % are seen in total cloud cover and cloud radiative effects
in the subtropics.
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;
to regain optimal climate fidelity
in the absence of those truncation errors,
the model would require retuning.</p>
<p>A coarse-grained attribution of the time step sensitivities is carried out
by shortening time steps used in various components of EAM or
by revising the numerical coupling between some processes.
Our analysis leads to the finding that the
marked decreases in the subtropical low-cloud fraction
and total cloud radiative effect are caused not by the step size used for the collectively subcycled
turbulence, shallow convection, and stratiform cloud macrophysics and microphysics parameterizations
but rather by the step sizes used outside those subcycles.
Further analysis suggests that the coupling frequency between the subcycles
and the rest of EAM significantly affects the subtropical marine stratocumulus decks, while
deep convection has significant impacts on trade cumulus.
The step size of the cloud macrophysics and microphysics subcycle
itself appears to have a primary impact on cloud fraction
in the upper troposphere
and also in the midlatitude near-surface layers.
Impacts of step sizes used by the dynamical core and the radiation parameterization
appear to be relatively small.
These results provide useful clues for future studies aiming at
understanding and addressing the root causes of sensitivities
to time step sizes and process coupling frequencies in EAM.</p>
<p>While this study focuses on EAMv1 and
the conclusions are likely model-specific,
the presented experimentation strategy
has general value for weather and climate model development,
as the methodology can help researchers identify and understand
sources of time integration error in sophisticated multi-component models.</p> |
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ISSN: | 1991-959X 1991-9603 |