Evaluation of Marine Boundary Layer Clouds over the Northeast Pacific
during the CSET Campaign in E3SM version 2
Abstract
It is still challenging to reproduce marine boundary layer (MBL) clouds
well in large-scale models despite their importance to the Earth’s
radiation budget and hydrological cycle. This study evaluates MBL and
clouds in the Energy Exascale Earth System Model (E3SM) version 2. The
E3SM simulation results are compared with remote sensing and reanalysis
data during the Cloud System Evolution in the Trades (CSET) field
campaign to better understand stratocumulus to cumulus cloud transition
(SCT) over the northeast Pacific. E3SM results are extracted along the
CSET Lagrangian trajectories. The comparison shows that the E3SM
simulation applying horizontal wind nudging performs well in reproducing
thermodynamic variables of the MBL and evolution trends of cloud
variables along the trajectories. However, substantial overestimations
of aerosol and cloud drop number ($N_d$) are observed, which is
explained as an issue with version 2 of the model. Cloud fraction (CF)
does decrease from the Californian coast to Hawaii in the E3SM
simulation, but most CF values indicate an overcast or almost clear sky,
which differ with satellite and reanalysis data. The effect of $N_d$
overestimation on CF evolution is assessed via prescribed $N_d$
simulations. Those simulations with $N_d$ modifications show
negligible CF changes. A comparison of estimated inversion strength
(EIS) also shows that the simulated EIS values are similar to those of
reanalysis data. Our study suggests that cloud macrophysics and boundary
layer processes are more important in improving the simulation rather
than improving the model’s dynamics or cloud microphysics to capture SCT
better in the model.