Structural Changes and Variability of the ITCZ Induced By Radiation–Cloud–Convection–Circulation Interactions: Inferences from the Goddard Multi-Scale Modeling Framework (GMMF) Experiments

In this paper, we investigate the impact of radiation-cloud-convection-circulation interaction (RC3I) on structural changes and variability of the Inter-tropical Convergence Zone (ITCZ) using the Goddard Multi-scale Modeling Framework (GMMF), where cloud processes are super-parameterized, i.e., explicitly resolved with 2-D cloud resolving models embedded in each coarse grid of the host Goddard Earth Observing System-Version 5 (GEOS5) global climate model. Experiments have been conducted under prescribed sea surface temperature conditions for 10 years (2007-2016) with and without cloud radiation feedback, respectively. Diagnostic analyses separately for January and July show that RC3I leads to i) a quasi-uniform warming and moistening of the tropical atmosphere and a sharpening of the ITCZ with enhanced deep convection, more intense precipitation, and higher clouds, ii) a drying of the tropical marginal convective zones, and subtropical and extratropical troposphere, and iii) a cooling of the polar regions, with increased baroclinicity and midlatitude storm track activities. Computations based on the zonal mean thermodynamic energy balance equation show that the radiative warming and cooling are strongly balanced by local adiabatic processes associated with changes in large-scale vertical motions, as well as horizontal atmospheric heat transport. Cloud radiation and water vapor greenhouse effects play key roles, providing strong positive feedback to the warming, enhanced deep convection and heavy precipitation in the ITCZ core regions. Increased longwave cooling at cloud top, associated with enhanced drying by increased subsidence is found in tropical marginal convection zones, and subtropical subsidence zones. We also find a strong positive correlation between daily and pentad heavy rain in the ITCZ core, and expansion of the drier zones coupled to a contraction of the highly convective zones in the ITCZ, indicating a strong tendency for convective aggregation in tropical clouds, induced by RC3I.