Clouds and Radiation in a Mock‐Walker Circulation

Abstract

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.

Intermodel spread in Walker circulation responses linked to spread in moist stability and radiation responses

“…These results suggest that changes in clouds associated with the WC dominate the spread in the radiation contribution across models and, in general, amplify the weakening of the WC with warming in models. That the radiation and circulation influence one another is consistent with the findings of Peters and Bretherton (2005) and Silvers and Robinson (2021).…”

Using Radiative Convective Equilibrium to Explore Clouds and Climate in the Community Atmosphere Model

“…The RCEMIP framework provides a firm foundation from which to incrementally add complexity for exploration of the links between convection and climate. Examples include the introduction of rotation (e.g., Chavas & Reed, 2019; Merlis & Held, 2019; Wing et al., 2016), simplifying physical parameterizations (Frierson et al., 2006; Reed & Jablonowski, 2012), or using a warm patch of SST to simulate a mock‐Walker circulation (Silvers & Robinson, 2021). When combined with a community modeling framework such as CESM, the standardized RCEMIP baseline offers a unique toolkit for the science community to advance our understanding of some of the most important topics in climate science and inform model development efforts.…”

“…The influence of atmospheric cooling by low‐level clouds on $$\bar{\mathrm{P}}$$ was recently discussed more thoroughly in the context of a mock‐Walker circulation by Silvers and Robinson (2021). Silvers and Robinson (2021) used experiments with the longwave cloud radiative effect to show that low‐level clouds are a source of atmospheric cooling because they increase the downwelling longwave radiation. Because CAM5 has more low‐level clouds relative to CAM6, the additional cooling from the low‐level clouds will likely contribute to more compensating $$\bar{\mathrm{L}\mathrm{H}}$$ and $$\bar{\mathrm{P}}$$ in CAM5.…”

“…In particular, the larger low‐level cloud fraction in CAM5 (see Figure 3) leads to additional atmospheric cooling. The influence of atmospheric cooling by low‐level clouds on $$\bar{\mathrm{P}}$$ was recently discussed more thoroughly in the context of a mock‐Walker circulation by Silvers and Robinson (2021). Silvers and Robinson (2021) used experiments with the longwave cloud radiative effect to show that low‐level clouds are a source of atmospheric cooling because they increase the downwelling longwave radiation.…”

Simulating Dropsondes to Assess Moist Static Energy Variability in Tropical Cyclones

“…Organized convective systems like tropical cyclones (TCs) (Emanuel, 2003) produce much of the extreme weather in the tropics, and influence large‐scale patterns of circulation, moisture, and radiation (Khairoutdinov & Emanuel, 2010; Silvers & Robinson, 2021; Wing et al., 2020). Idealized modeling studies have improved our understanding of convective organization, including the spontaneous self‐aggregation that arises due to interactions between clouds, water vapor, radiation, and circulations (e.g., Held et al.…”