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Cloud Resolving and General Circulation Model Simulations of an Idealized Walker Circulation
  • Levi Silvers,
  • Nadir Jeevanjee
Levi Silvers
Princeton University & The Geophysical Fluid Dynamics Laboratory

Corresponding Author:[email protected]

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Nadir Jeevanjee
Princeton University
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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.