Radiative Convective Equilibrium and Organized Convection: An Observational Perspective

Abstract

Radiative convective equilibrium (RCE) describes a balance between the cooling of the atmosphere by radiation and the heating through latent heat release and surface heat fluxes. While RCE is known to provide an energetic constraint on the atmosphere at the global scale, little is known about the proximity of the atmosphere to RCE at smaller spatial and temporal scales, despite the common use of RCE in idealized modeling studies. Here we provide the first observational evaluation of the scales at which the atmosphere is near RCE. We further use observations of cloud characteristics to investigate the role played by organized convection in the RCE state. While the tropical atmosphere as a whole is near RCE on daily time scales and longer, this is not the case for any given location. Rather, areas in excess of 5,000 × 5,000 km2 must be considered to ensure the atmosphere remains near RCE at least 80% of the time, even for monthly averaged conditions. We confirm that RCE is established through the interplay of regions of active deep convection with high precipitation and weak radiative cooling and regions of subsiding motions leading to shallow cloud states that allow strong radiative cooling with no precipitation. The asymmetry in the maximum amount of radiative cooling and latent heating leads to the well-known ratio of small areas of precipitation and large regions of subsidence observed in the tropics. Finally, we show that organized deep convection does not occur when regions smaller than 1,000 × 1,000 km2 are near RCE.