The influence of the cloud radiative effect on the double ITCZ bias

All state of the art global circulation models suffer from the double ITCZ model bias. This bias is one of the most longstanding model errors and it remains unsolved because we do not fully understand the mechanisms and feedbacks that cause the convective organization in the tropics.

Aqua-planet simulations with fixed SST are performed to analyze the convective organization on large scales. A heat and moisture budget analysis and correlations between cloud top height and free tropospheric humidity and thermal stability is used to analyze the convective organization. To test the influence of cloud radiative effects on the double ITCZ bias we either disable cloud-radiation interactions at certain levels in the atmosphere or we apply a prescribed cloud field as input to the radiative transfer calculation.

We show that the ITCZ behavior in the model is strongly influenced by cloud-radiation interactions. In particular, removing the effects of high-level clouds on radiation can lead to a transition from single a to a double ITCZ structure. This can be explained by the overall warming effect that cloud- radiation interactions exert on the active part of the Hadley circulation. This leads to an increased heating gradient between active and suppressed regions driving a stronger Hadley circulation, which in turn significantly affects the moisture distribution. We show that the potential counteracting effect of increased stability through cloud-radiation interaction is muted by the rapid redistribution of heat through gravity waves in the active regions of the Hadley circulation.

Given that upper-level clouds have such a strong influence on the first-order organization of the ITCZ, future research should focus on the link between cumulus convection and anvil properties in the tropics.