Cloud Resolving and General Circulation Model Simulations of an
Idealized Walker Circulation
Abstract
The overturning tropical Pacific circulation known as the Walker
circulation embodies complex interactions between large-scale
circulations, deep and shallow convection, stratocumulus clouds, and
microphysical cloud processes. The large and multi-scale nature of the
Walker circulation has made high resolution modeling costly, while
disentangling the relevant circulations and processes in a global model
with more parameterizations is often challenging. This work uses the
framework of the Walker Circulation as a unifying experiment for both
high-resolution and global models with the goal of identifying how deep
tropical convective heating and low-level clouds interact with and are
influenced by the circulations in which they are embedded. A high
resolution model with explicit convection (1km and 2km grid-spacing) is
used to examine the system free of the complications inherent in
convective parameterizations. The same model is also used at GCM-like
resolutions with parameterized convection (25km and 100km grid-spacing)
to gain insight into how the clouds and circulations interact in a GCM
configuration. We define the idealized Walker circulation with a
prescribed sea surface temperature dipole pattern, no rotation, uniform
insolation, fully interactive radiation, and a channel domain (100km x
4000km). All simulations use the the same nonhydrostatic dynamical core
(FV3) with the physics based on those in the AM4 GFDL atmospheric model.
We find large differences in the total condensate between the
high-resolution model and the GCM with the high-resolution model tending
to have less low-level condensate but more condensate in the deep
convective regions. This is reflected in the relative humidity fields as
well. The parameterized entrainment of deep convection and the feedbacks
of low-level tropical clouds are both leading factors contributing to
the large spread of the climate sensitivity. With this in mind
experiments are performed with the GCM in which the lateral mixing rate
of deep convective plumes is varied. In addition, the detailed
representation of cloud fraction between the two models is investigated.
Our goal is to determine to what extent deep tropical convection can
influence remote low-level clouds in regions with a subsiding free
troposphere.