Competing Effect of Radiative and Moisture Feedback in Convective Aggregation States in Two CRMs

Abstract

Radiative-convective equilibrium (RCE) of an ensemble of clouds has been used as an example of a statistical equilibrium state of the atmosphere able to mimic the tropical part of the climate system (K. Emanuel et al., 2014). Indeed, given the crucial importance of the balance between radiative cooling and latent heating due to condensation and surface-sensitive heat flux in the Tropics in the Earth's global balance, the RCE state of the atmosphere has become a proxy to study the link between global circulation and convection (Held et al., 1993;Pauluis & Held, 2002a, 2002bRandall et al., 1994;Yano & Plant, 2012). The implicit assumption in this is that the tropical circulation may be larger than the small-scale convective processes of an ensemble of clouds and that the statistical equilibrium implied by the RCE state is based on processes in near-equilibrium on different space and time scales with the large-scale flow. This point was brought to general attention when the increased computing capability available made it possible to run three-dimensional high-resolution simulations (Bretherton et al., 2005;Tompkins & Craig, 1998) and to study the sensitivity of RCE states using models with enhanced dimensions of the grid reaching the dimensions of mesoscale processes, with explicit moist variables and different physics

The Unreasonable Efficiency of Total Rain Evaporation Removal in Triggering Convective Self‐Aggregation

“…It is as yet unclear whether a low‐level circulation—a common feature of the conventional radiatively driven aggregation (RDA)—would also be present in NE‐CSA. A recent study has found that CSA can still develop without a shallow circulation when the subcloud layer is close to saturation (Cerlini et al., 2023), which would prevent precipitation from evaporating, analogous to NE‐CSA. To observe the circulation fields we show the MSE and stream function Ψ ranked by column relative humidity (CRH) averaged over the final 5 days in Figure 2 for selected α cases (see Bretherton et al., 2005, for computation details of Ψ).…”

“…Generally, CSA has been associated with physical processes that generate a low‐level circulation leading to an upgradient transport of moist static energy (MSE) from dry to moist regions (Bretherton et al., 2005; Naumann et al., 2017; Yanase et al., 2022). However, the role of and precise mechanisms producing this circulation remain ambiguous: shallow circulation has been argued to hamper CSA from developing in smaller domains (Jeevanjee & Romps, 2013); its strength and importance have been found to be model dependent (Cerlini et al., 2023); questions also remain whether it is radiatively (Muller & Held, 2012; Naumann et al., 2017) or convectively driven (Holloway & Woolnough, 2016).…”

Numerical Diffusion and Turbulent Mixing in Convective Self‐Aggregation

“…C. Muller and Bony (2015) found similar results even when radiative cooling rates were homogenized across the free troposphere. This "moisture-memory aggregation" can be replicated by the simple coarsening model by Craig and Mack (2013) and has recently been found to occur spontaneously in models with a nearly saturated sub-cloud layer (Cerlini et al, 2023). Additionally, D. Yang (2018) showed that CSA can occur also with minimal ingredients by homogenizing radiative cooling throughout the entire column, switching off rain evaporation over the boundary layer and without interactive surface fluxes.…”

“…Different initial conditions may strongly influence the sub cloud layer properties and CSA as demonstrated by Cerlini et al. (2023). Further details about smaller domain simulations can be found in Text S3 in Supporting Information .…”

“…Similar differences between SAM-CRM and WRF-CRM model were found in RCEMIP project (Wing et al, 2020, see their Figures 7g and 8g). Different initial conditions may strongly influence the sub cloud layer properties and CSA as demonstrated by Cerlini et al (2023). Further details about smaller domain simulations can be found in Text S3 in Supporting Information S1.…”