The Life Cycle and Net Radiative Effect of Extended Anvil Clouds

Abstract

We hypothesize that the neutral top-of-atmosphere net radiative effect (NCRE) of convective clouds in the deep tropics results in large measure from the life cycle of anvil clouds, starting as thick and highly reflective cloud to rise and thin into subvisable cirrus remnants. We construct an idealized, horizontally homogeneous anvil cloud in a 2D framework using the System for Atmospheric Modeling (SAM) cloud resolving model. A suite of simulations is performed by varying the initial cloud thickness. Clouds that start sufficiently thick will produce an initial negative NCRE, which is later canceled by a positive NCRE as the cloud thins, rises, and eventually dissipates. Turning off interactive cloud radiation reveals that in-cloud convection is fundamental in driving net radiative neutrality. In-cloud convection acts to 1) thin initially thick anvil clouds and 2) loft and maintain thin cirrus. The maintenance of anvil clouds is tied to a cycling of water vapor and cloud ice through sublimation and deposition as air parcels circulate vertically within the cloud layer. Without interactive radiation, the cloud sediments and sublimates away, producing a large negative NCRE. We test the sensitivity of our results for various cloud and model parameters. The parameterization of cloud microphysics substantially influences the cloud's behavior and life cycle. Our study shows that in the Tropical West Pacific, small-scale processes influence anvil cloud evolution in a way that fundamentally influences NCRE over a cloud’s lifetime. Since climate models are still too coarse to represent these processes, higher resolutions may be required to accurately predict radiation balance changes in the Tropical Western Pacific in future climates.