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

Abstract. The double Inter-Tropical Convergence Zone (dITCZ) is a prominent precipitation bias persistent over several climate model generations. This motivates investigations of whether increasing resolution and discarding parametrizations can improve the representation of the large-scale atmospheric circulation. In this work, we use the unique possibility offered by the new ICON XPP model configuration to study the dITCZ bias in a resolution hierarchy spanning from parameterized to resolved convection within a consistent modeling framework. We demonstrate that the dITCZ persists from a horizontal resolution of 160 km up to 5 km in specified sea surface temperature simulations and is independent of the deep-convective and non-orographic gravity wave parametrization. Changes in the treatment of near-surface wind speed within the bulk flux formula can reduce the dITCZ bias over the resolution hierarchy. We however highlight that the root cause of the dITCZ lies in biased moisture transport from the subtropics to the inner tropics. The resulting low bias in tropical near-surface moisture substantially reduces deep convection over the Warm Pool, leading to a weakened Walker Circulation. These biases ultimately culminate in the dITCZ feature. An increase in near-surface wind speed addresses the low bias in near-surface moisture in the tropics, however it exacerbates a bias in the moisture source by increasing the inner tropical over the subtropical contribution. This could endanger the representation of the global circulation, energetic balance and teleconnections. Our findings showcase the benefits of models supporting a range of resolutions and underline the importance of continuing the development of non-discardable parametrizations.