The Walker circulation connects the regions with deep atmospheric convection in the western tropical Pacific to the shallow‐convection, tropospheric subsidence, and stratocumulus cloud decks of the eastern Pacific. The purpose of this study is to better understand the multi‐scale interactions between the Walker circulation, cloud systems, and interactive radiation. To do this we simulate a mock‐Walker Circulation with a full‐physics general circulation model using idealized boundary conditions. Our experiments use a doubly‐periodic domain with grid‐spacing of 1, 2, 25, and 100 km. We thus span the range from General Circulation Models (GCMs) to Cloud‐system Resolving Models (CRMs). Our model is derived from the Geophysical Fluid Dynamics Laboratory atmospheric GCM (AM4.0). We find substantial differences in the mock‐Walker circulation simulated by our GCM‐like and CRM‐like experiments. The CRM‐like experiments have more upper level clouds, stronger overturning circulations, and less precipitation. The GCM‐like experiments have a low‐level cloud fraction that is up to 20% larger. These differences leads to opposite atmospheric responses to changes in the longwave cloud radiative effect (LWCRE). Active LWCRE leads to increased precipitation for our GCMs, but decreased precipitation for our CRMs. The LWCRE leads to a narrower rising branch of the circulation and substantially increases the fraction of precipitation from the large‐scale cloud parameterization. This work demonstrates that a mock‐Walker circulation is a useful generalization of radiative convective equilibrium that includes a large‐scale circulation.
Previous work has found that as the surface warms the large‐scale tropical circulations weaken, convective anvil cloud fraction decreases, and atmospheric static stability increases. Circulation changes inevitably lead to changes in the humidity and cloud fields which influence the surface energetics. The exchange of mass between the boundary layer (BL) and the midtroposphere has also been shown to weaken in global climate models. What has remained less clear is how robust these changes in the circulation are to different representations of convection, clouds, and microphysics in numerical models. We use simulations from the Radiative‐Convective Equilibrium Model Intercomparison Project to investigate the interaction between overturning circulations, surface temperature, and atmospheric moisture. We analyze the underlying mechanisms of these relationships using a 21‐member model ensemble that includes both General Circulation Models and Cloud‐system Resolving Models. We find a large spread in the change of intensity of the overturning circulation. Both the range of the circulation intensity, and its change with warming can be explained by the range of the mean upward vertical velocity. There is also a consistent decrease in the exchange of mass between the BL and the midtroposphere. However, the magnitude of the decrease varies substantially due to the range of responses in both mean precipitation and mean precipitable water. We hypothesize based on these results that despite well understood thermodynamic constraints, there is still a considerable ability for the cloud fields and the precipitation efficiency to drive a substantial range of tropical convective responses to warming.more » « less
- Award ID(s):
- NSF-PAR ID:
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- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Advances in Modeling Earth Systems
- Medium: X
- Sponsoring Org:
- National Science Foundation
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