The Implementation of Framework for Improvement by Vertical Enhancement
(FIVE) into Energy Exascale Earth System Model (E3SM)
The low cloud bias in global climate models (GCMs) remains an unsolved
problem. Coarse vertical resolution in GCMs has been suggested to be a
significant cause of low cloud bias because planetary boundary layer
parameterizations cannot resolve sharp temperature and moisture
gradients often found at the top of subtropical stratocumulus layers.
This work aims to lessen the low cloud problem by implementing a new
computational method, the Framework for Improvement by Vertical
Enhancement (FIVE), into the Energy Exascale Earth System Model (E3SM).
Three physics schemes representing microphysics, radiation, and
turbulence as well as vertical advection are interfaced to vertically
enhanced physics (VEP), which allows for these processes to be computed
on a higher vertical resolution grid compared to the rest of the E3SM
model. We demonstrate the better representation of subtropical boundary
layer clouds with FIVE while limiting additional computational cost from
the increased number of levels. When the vertical resolution approaches
the LES-like vertical resolution in VEP, the climatological low cloud
amount shows a significant increase of more than 30% in the
southeastern Pacific Ocean. Besides the improvement of low-level cloud
amount, the skill scores of mid- and high-level cloud amounts are not
negatively impacted partly because FIVE can avoid negative consequences
of running deep convection parameterization at high vertical resolution.