The Role of the Cloud Radiative Effect in the Sensitivity of the ITCZ to Convective Mixing.

Understanding how clouds couple to circulation is recognised as a “Grand Challenge” by the World Climate Research Programme. Previous aquaplanet experiments illustrate that atmospheric models simulate a broad range of tropical precipitation mean states, given the same sea-surface temperature forcing. In particular, the Intertropical Convergence Zone (ITCZ) is sensitive to the treatment of sub-gridscale convection, associated with cloud-radiation interactions.

We use an atmospheric energy input framework to explore the influence of the cloud radiative effect (CRE) in modulating the sensitivity of the ITCZ to convective mixing in Met Office Unified Model aquaplanet simulations. Altering the convective mixing is related to changes in the ITCZ structure, which is associated with CRE feedbacks, with large convective mixing rates simulating a single ITCZ. We prescribe the CRE as either (a) zero or (b) a meridionally and diurnally varying climatology. Removing the CRE altogether is associated with a change from a single to a double ITCZ. Both methods of prescribing the CRE reduce, but do not eliminate, the sensitivity of the ITCZ to variations in convective mixing. The sensitivity of the ITCZ in prescribed CRE simulations to the convective mixing is associated with variations in the latent heat flux. Analysis of changes in the meridional transport of moist static energy illustrates that ITCZ shifts are associated with circulation changes rather than moist static energy profile variations, and it is possible to have an equatorward moist static energy transport at low latitudes even with equatorial ascent. The CRE is an important component of the sensitivity of the ITCZ to convective mixing, but the response of surface turbulent fluxes modulates this sensitivity through the large-scale circulation.