Isolating the Radiative Effects of Clouds on Tropical Subseasonal Variability in CESM2

Abstract:

Interactions among radiation, clouds, and circulation are thought to play an important role in the organization and variability of tropical convection. Studies using column-integrated moist static energy budget analyses applied to global climate model (GCM) or cloud resolving model simulations indicate that cloud-radiation interactions (CRI), particularly those involving longwave radiation, strongly impact convective organization. Although these findings are qualitatively aligned with observational estimates, much uncertainty remains due in part to model differences and idealizations used in the simulation design. For example, GCM studies that artificially disable cloud-radiative feedbacks show weakening of the Madden-Julian oscillation (MJO) and strengthening of Kelvin waves, but interpretation of the results can be complicated by concurrent changes in the mean state or the use of idealized boundary conditions unlike those observed.

Here, key results from a project that examines CRI impacts on subseasonal tropical convective variability within Community Earth System Model version 2 are reported. CRI are disabled using cloud locking, a method in which cloud properties taken from a control simulation (CRI active) are prescribed within an experimental simulation (CRI disabled) for radiative tendency calculations. The approach minimizes mean state differences between the two runs, facilitating interpretation of the results. Disabling CRI weakens amplitudes of the MJO and equatorial Rossby waves by roughly 10–35% yet strengthens Kelvin waves by 10–40%. MJO weakening results from suppressed radiation-convection positive feedbacks and increased gross moist stability. Kelvin waves strengthen from reduced convective inhibition and reduced radiative damping on temperature variance. MJO survival, when its primary maintenance mechanism (CRI) is eliminated, stresses the importance of advection and surface flux processes.