Insights on Tropical High-Cloud Radiative Effect From a New Conceptual Model

The new capabilities of global storm-resolving models to resolve individual clouds allow for a more physical perspective on the tropical high-cloud radiative effect and how it might change with warming. In this study, we develop a conceptual model of the high-cloud radiative effect as a function of cloud thickness measured by ice water path. We use atmospheric profiles from a global ICON simulation with $5km$ horizontal grid spacing to calculate the radiation offline with the ARTS line-by-line radiative transfer model. The conceptual model of the high-cloud radiative effect reveals that it is sufficient to approximate high clouds as a single layer characterized by an albedo, emissivity and temperature, which vary with ice water path. The increase of the short-wave high-cloud radiative effect with ice water path is solely explained by the high-cloud albedo. The increase of the long-wave high-cloud radiative effect with ice water path is governed by an increase of emissivity for ice water path below $10^{-1}kg{m}^{-2}$, and by a decrease of high-cloud temperature with increasing ice water path above this threshold. The mean high-cloud radiative effect from the ARTS simulations for the chosen day of this ICON model run is $1.25W{m}^{-2}$, which is closely matched by our conceptual model with $1.26W{m}^{-}2$. Because the high-cloud radiative effect depends on the assumed radiative alternative, assumptions on low clouds make a substantial difference. The conceptual model predicts that doubling the fraction of low clouds roughly doubles the positive high-cloud radiative effect.