Parameterization adaption needed to unlock the benefits of increased resolution for the ITCZ in ICON

The double Inter-Tropical Convergence Zone (ITCZ) feature is a prominent bias in the precipitation distribution simulated by most weather and climate models. Its persistence over several CMIP generations without substantial improvements is one factor motivating the investigation of whether higher resolution and the discard of parameterizations can solve the misrepresentation of large-scale atmospheric circulation. In this work, we analyze uncoupled simulations with the Icosahedral Nonhydrostatic Weather and Climate Model, ICON, in the eXtended Predictions and Projections configuration (XPP), spanning horizontal resolutions from 160 km up to 5 km. Although improvements in the precipitation bias are apparent due to the better representation of orography starting at 40 km horizontal resolution, we demonstrate that the double ITCZ feature persists over the entire resolution spectrum – even if deep convective parameterization is discarded at 5 km. In a subsequent sensitivity study, we can tie the emergence of the double ITCZ to biases in the near surface humidity arising from the turbulence scheme. We perform a physically motivated parameter optimization to correct for the bias in near surface specific humidity and can reduce tropcial precipitation biases over the entire resolution hierarchy. Our findings not only showcase the benefits of models supporting various resolutions but also underline the importance of further developing indispensable parameterizations.