Radiative‐convective instability

Abstract

Radiative-moist-convective equilibrium (RCE) is a simple paradigm for the statistical equilibrium the earth's climate would exhibit in the absence of lateral energy transport. It has generally been assumed that for a given solar forcing and long-lived greenhouse gas concentration, such a state would be unique, but recent work suggests that more than one stable equilibrium may be possible. Here we show that above a critical specified sea surface temperature, the ordinary RCE state becomes linearly unstable to large-scale overturning circulations. The instability migrates the RCE state toward one of the two stable equilibria first found by Raymond and Zeng (2000). It occurs when the clear-sky infrared opacity of the lower troposphere becomes so large, owing to high water vapor concentration, that variations of the radiative cooling of the lower troposphere are governed principally by variations in upper tropospheric water vapor. We show that the instability represents a subcritical bifurcation of the ordinary RCE state, leading to either a dry state with large-scale descent, or to a moist state with mean ascent; these states may be accessed by finite amplitude perturbations to ordinary RCE in the subcritical state, or spontaneously in the supercritical state. As first suggested by Raymond (2000) and Sobel et al. (2007), the latter corresponds to the phenomenon of selfaggregation of moist convection, taking the form of cloud clusters or tropical cyclones. We argue that the nonrobustness of self-aggregation in cloud system resolving models may be an artifact of running such models close to the critical temperature for instability.

Thermodynamic control of anvil cloud amount

“…It shows that high surface temperatures not only facilitate the initiation of convective aggregation; they also contribute to the maintenance of a stronger aggregation of convection and therefore to the increasing concentration of rainy areas. Such a process may contribute to the tendency of models to narrow large-scale convergence zones in a warmer climate (6) and may explain why cloud-radiative effects in the free troposphere are found to be so critical, both for the growth and maintenance of convective aggregation (21,24,27,31,32) and for large-scale circulations (3,5,35,36).…”

“…Because the initiation of self-aggregation depends on surface temperature (21,24,27,31), and the clumping of convection is known to be associated with a reduction of the upper-level cloud amount (21,25), the question arises whether the reduction of the anvil cloud amount with SST shown in Fig. 2 simply reflects the evolution of the anvil cloud fraction with convective aggregation or whether a more fundamental process is at play.…”

“…Numerical (21,23,24) and observational studies (25,26) show that when convection is more clustered, the free atmosphere is drier and the area covered by anvil clouds is smaller. Because this clustering, or aggregation, is expected to occur more easily at high surface temperatures (27), such a process might affect cloud feedbacks and climate sensitivity (28,29). However, here again it has remained unclear as to how convective aggregation influences the Significance Assessing the response of clouds to global warming remains a challenge of climate science.…”

Radiative convective equilibrium as a framework for studying the interaction between convection and its large‐scale environment

“…It has also become clear that even with the relative simplicity of RCE, differences in convective parameterization schemes play a decisive role on the overall statistics of the simulation [Popke et al, 2013;Coppin and Bony, 2015;Reed et al 2015]. Due to the multiplicity of solutions that previous RCE simulations have produced [Held et al, 2007;Jeevanjee and Romps, 2013;Emanuel et al, 2014;Wing and Cronin, 2016;Zhou, 2015] which depend on domain details, grid-spacing, surface temperature, and parameterizations, we cannot per se assume that solutions of GCM-RCE simulations will be independent of domain size. Examining these domain size effects using simulations with parameterized convection, extending to near global scales, is the aim of this paper.…”

Scaling of the entropy budget with surface temperature in radiative‐convective equilibrium

“…Furthermore, when rotation is included, such aggregation may result in tropical cyclone formation [Wing et al, 2016], potentially leading to an increase in work output and a decrease in inefficiencies associated with moist processes [Pauluis, 2011]. Analyzing the entropy budget of organized convection in both rotating and nonrotating frameworks may be useful in order to understand the self-aggregation process, particularly in light of the hypothesis that aggregation itself is temperature dependent [Wing and Emanuel, 2013;Emanuel et al, 2014]. The entropy budget of moist convection in an extremely warm atmosphere may also be of interest for future work given the failure of the scaling relations derived in section 4 to predict a closed entropy budget under these conditions.…”