Sensitivity of Cloud Radiative Effects to Water Vapor Perturbations in Radiative-Convective Equilibrium

The interaction between convection and large-scale dynamics is a primary source of uncertainty in numerical simulations of the atmosphere, impeding our understanding of the climate. Recent work has shown that cloud radiative effects amplify the organization of moist convection, including cyclogenesis and convective self-aggregation. However, we still lack a simple theory of how cloud radiative effects evolve as moist convection organizes, motivating the question:

How do cloud radiative effects, water vapor and convection interact?

We study the effect of water vapor perturbations on cloud radiative effects in the framework of radiative-convective equilibrium. First, we simulate three-dimensional, statistically-homogeneous radiative-convective equilibrium (a.k.a. pop-corn convection), in a small square domain of size 288km x 288km with 3km horizontal resolution. Water vapor, radiation and clouds interact at every time step. We add horizontally-homogeneous water vapor perturbations to snapshots of radiative-convective equilibrium, and repeat the experiment for every vertical level of the model. Finally, we compute the linear sensitivity of cloud water, cloud fraction and cloud radiative heating rates to water vapor perturbations. The experiment is repeated for two different radiation schemes and several different fixed sea surface temperatures to quantify the spread of the cloud radiative response. We discuss how changes in cloud radiative effect modify the linear sensitivity of radiative heating rates to water vapor perturbations, and the consequences for the initial stages of convective self-aggregation.