The Influence of Radiative Effects on the ITCZ position on an Aquaplanet

Aqua planet experiments performed with fixed sea surface temperatures (SST) using the ECHAM6 GCM are studied to understand interactions and feedbacks that influence the position of the inter tropical convergence zone (ITCZ). The position of the ITCZ is found to depend on a feedback loop process wherein convective heating drives pressure gradients and winds, which determine the rate of surface evaporation, which influences the boundary layer moist static energy magnitude, which finally couples back to the pattern of convective heating, as strong deep convection is closely bound to the highest boundary layer moist static energies (Möbis & Stevens 2012). This feedback loop is modulated by the convective heating, which in turn is sensitive to free tropospheric humidity in some convective schemes -- and in nature -- with lower environmental humidity leading to a higher updraft lapserate and a lower cloudtop, and less net heating. For the feedback loop to push the ITCZ towards the equator in ECHAM it proves necessary to moisten the ITCZ region and to dry other regions and suppress deep convection away from the ITCZ. Herewith the humidity mainly depends on the large scale vertical velocity, which itself can be approximated by the sum of convective heating and radiative cooling. For a single ITCZ to form on the equator, either the water vapor or the cloud radiative effect (CRE) must lead to a lower radiative cooling rate in the ITCZ than poleward of the ITCZ. To actually test the sensitivity of the ITCZ position to radiative cooling distribution, experiments with an idealized fixed cooling rate climatology instead of interactive radiation is used as well as idealized climatologies of water vapor and cloud as input for the radiative transfer calculation are used. Results show that the lateral cooling rate gradient in the lower troposphere dominates the ITCZ placement. Radiative heating within the ITCZ, through the CRE of the ice cloud anvils of the deep updrafts, are also shown to be essential for maintaining a single ITCZ.