Moisture, Clouds, and Radiation in a Mock-Walker Circulation

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-scale nature of the Walker circulation has made high-resolution modeling costly, while understanding the feedbacks between parameterized clouds and the large-scale circulation have remained challenging in the context of global models.

This study uses an idealized Walker circulation to explore how multiple tropical cloud types interact with a large-scale circulation. 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 configuration similar to a GCM. We use prescribed SSTs, fully interactive radiation, and a channel domain with a width ranging from 2000km to 16000km. All simulations use the same nonhydrostatic dynamical core (FV3) with the physics based on those in the AM4 GFDL atmospheric model.

We find that the experiments with parameterized convection result in large asymmetries in the precipitation and humidity fields with the result that the precipitation maximum is not centered over the regions with the highest SST. This is at least partly due to vertical shear that is generated when the convection is parameterized. The longwave cloud radiative effect plays a major role in establishing these asymmetries in the circulation and the low-level clouds. High-resolution experiments have a much stronger overturning circulation and higher humidity in the ascent region and lower boundary layer humidity over the regions with cold SSTs. The high-resolution models produce almost no stratocumulus clouds. This configuration can also be used as a bridge between experiments of radiative convective equilibrium and those with more complex boundary conditions.