A First Look at the Global Climatology of Low-Level Clouds in Storm Resolving Models

Abstract

The representation of subtropical stratocumulus and trade-wind cumulus clouds by preliminary versions of Integrated Forecasting System (IFS) and ICON km-scale global coupled climate models is explored. These models differ profoundly in their strategy to represent subgrid-scale processes. The IFS employs complex parameterizations, including eddy-diffusivity mass-flux and convection schemes. ICON applies a minimal set of paramaterizations, including the Smagorinsky-Lilly closure. Five-year simulations are performed and evaluated for their representation of cloud albedo, its variability with environmental parameters and the vertical structure of the atmospheric boundary layer in eight regions: four corresponding to canonical Atlantic and Pacific stratocumulus and four in their downstream trades. For stratocumulus, both models capture the albedo's mean, annual cycle, and its relationship with the parameters relevant for low cloudiness, including lower tropospheric stability. They simulate an expected thermodynamic vertical structure of a stratocumulus-topped boundary layer. ICON largely exhibits a lower cloud base and inversion height than IFS. We speculate the disagreement can be attributed to the contrasting treatment of subgrid mixing and cloud top entrainment. In the case of trade-wind cumulus, both models well differentiate the cloud amount, the character of annual cycles and parameter correlations, and the vertical structure from the upstream stratocumulus. The annual cycles and parameter correlations reflect the dry and wet periods. Both models overestimate mean albedo and underestimate the strength of trade-wind inversion. With an additional ICON run, we demonstrate the strong sensitivity of stratocumulus and the weaker response of trade-wind cumulus to the treatment of subgrid mixing.