Controls on Subtropical Cloud Reflectivity during a Waterbelt Scenario for the Cryogenian Glaciations

Abstract

Abstract Waterbelt climate states with an ice-free tropical ocean provide a straightforward explanation for the survival of advanced marine species during the Cryogenian glaciations (720–635 million years ago). Previous work revealed that stable waterbelt states require the presence of highly reflective low-level mixed-phase clouds with a high abundance of supercooled liquid in the subtropics. However, the high uncertainty associated with representing mixed-phase clouds in coarse-scale general circulation models (GCMs) that parameterize atmospheric convection has prohibited assessment of whether waterbelt states are a robust feature of Earth’s climate. Here we investigate whether resolving convective-scale motion at length scales of hectometers helps us to assess the plausibility of a waterbelt scenario. First, we show that substantial differences in cloud reflectivity among GCMs do not arise from the resolved atmospheric circulation. Second, we conduct a hierarchy of simulations using the Icosahedral Nonhydrostatic (ICON) modeling framework, ranging from coarse-scale GCM simulations with parameterized convection to large-eddy simulations that explicitly resolve atmospheric convective-scale motions. Our hierarchy of simulations supports the existence of highly reflective subtropical clouds if we apply moderate ice nucleating particle (INP) concentrations. Third, we test the sensitivity of cloud reflectivity to the INP concentration. In the presence of high but justifiable INP concentrations, cloud reflectivity is strongly reduced. Hence, the existence of stable waterbelt states is controlled by the abundance of INPs. We conclude that explicitly resolving convection can help to constrain Cryogenian cloud reflectivity, but limited knowledge concerning Cryogenian aerosol conditions hampers strong constraints. Thus, waterbelt states remain an uncertain feature of Earth’s climate. Significance Statement The purpose of this study is to assess the impact of atmospheric convection and small airborne ice nucleating particles on the reflectivity of mixed-phase clouds over a subtropical ice margin. This is important as these clouds can determine whether the Cryogenian Earth (720–635 million years ago) was in a hard “snowball” state with a fully ice-covered ocean or a habitable waterbelt state with an ice-free tropical ocean. Our results indicate a clear impact of convection but neither confirm nor deny the existence of a waterbelt state since cloud reflectivity depends critically on the abundance of ice nucleating particles. Therefore, a Cryogenian waterbelt scenario remains uncertain, which calls for more comprehensive Earth system modeling approaches in future studies.