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

Abstract. The double Inter-Tropical Convergence Zone (double-ITCZ) bias has been a persistent tropical precipitation bias over many climate model generations. This motivates investigations of whether increasing resolution and discarding parameterizations improves the representation of the large-scale atmospheric circulation and ITCZ. In this work, we study the double-ITCZ bias in an ICON XPP resolution hierarchy spanning from parameterized to explicitly described deep convection within a consistent framework. We demonstrate that the double ITCZ persists across horizontal resolutions from 160 to 5 km in specified sea-surface temperature simulations, independent of deep-convective and non-orographic gravity wave parameterizations. Changes in the treatment of near-surface wind speed within the turbulence parameterization can reduce the bias. However, we highlight that a key driver of the double-ITCZ bias seems to lie in the insufficient moisture transport from the subtropics to the inner tropics. The resulting dry bias in tropical near-surface moisture reduces deep convection over the Warm Pool, leading to a weakened Walker circulation. These biases ultimately culminate in the double-ITCZ feature. Increasing the near-surface wind speed limiter improves tropical near-surface moisture but exacerbates the bias in the moisture source, increasing the inner-tropical contribution at the expense of the subtropics. This degrades the representation of the global circulation, energy balance, and teleconnections. Additionally, we show that parameter adjustments at low resolution are informative of the response to the same parameter adjustments at high resolution. Our findings showcase the benefits of models supporting a range of resolutions and underline the importance of continuing the development of non-discardable parameterizations.