The Neutral Radiative Effect of Anvil Clouds in the Tropical West Pacific

It has long been noted that the clouds in Tropical Western Pacific have a neutral net radiative effect at the top of the atmosphere, meaning the total effect of clouds in the longwave and shortwave spectra sum to a small value. Individual convective clouds can have a strong positive or negative radiative effect. Deep convective clouds, which have a strong negative effect, are balanced by cirrus clouds resulting from the anvil outflow, which have a strong positive effect. This net radiative neutrality thus depends on the amount of non-precipitating anvil cloud that is associated with tropical systems. We hypothesize that the small-scale processes within anvil clouds are important in determining their radiative effects and, thus, the radiative balance of the whole system. We construct an idealized, horizontally homogeneous anvil cloud in a 2D framework using the System for Atmospheric Modeling (SAM) cloud resolving model version 6.10.6 and two-moment microphysics. Varying initial cloud physical and optical depth, we are able investigate the interaction between convection, cloud microphysics, and radiation as the cloud layer evolves. A complimentary set of simulations without interactive radiation is performed. We also vary the background environment’s moisture content to investigate why the Tropical Western Pacific, versus the Tropical Eastern Pacific, is characterized by this radiative net neutrality. We find radiation to initially destabilize the cloud layer causing it to rise through vertical cloud ice advection while reducing the ice water path and optical depth through ice fallout and entrainment. The changing optical depth and vertical rising play major roles in setting the cloud radiative effect (CRE) in the shortwave and longwave, respectively. Integrating the net CRE over a cloud’s lifetime quantifies the full effect a given cloud has on the radiation budget. As the initial cloud depth increases, the integrated net CRE becomes less positive and closer to neutral. Therefore, if we think of tropical convective clouds going through a lifecycle from thick anvil to thin cirrus, then our study shows that the thick anvils observed in the Tropical West Pacific should have an approximately neutral CRE over their lifetime, producing an aggregated neutral radiative effect, as observed. As expected, the simulations without interactive radiation do not exhibit integrated net radiative neutrality. Instead of rising, the cloud layer slowly sediments and evaporates away due to the lack of radiative destabilization. Consequently, the integrated net CRE these clouds would have is largely negative. If we alter the background moisture profile to be significantly drier, the integrated net CRE increases for all simulations, even though the magnitude of both the shortwave and longwave components increase due to reduced particle effective radii and increased cloud vertical motion, respectively. Our results may provide new insight for understanding how the earth’s radiation balance and tropical circulations will respond under climate change. The fundamental physical processes investigated in our study should remain invariant, but we have shown that the background environment can have a profound influence on the overall radiation balance of these systems.