Sensitivities of the double-ITCZ rainfall bias in the Superparameterized Community Earth System Model

The transient development of tropical rainfall biases (including the double-ITCZ) is analyzed in the ocean-coupled, superparameterized (SP) version of Community Earth System Model (CESM). The double ITCZ bias is a first order problem in modern GCMs, and the role of convection parameterizations in its development is an ongoing area of research. Superparameterization—which avoids classic approximations of deep convection—provides a useful new perspective to understand the physics of the double ITCZ. We show that modern versions of SPCESM still produce a double-ITCZ bias and we document its transient development, and how it is altered by the configuration of the model's embedded cloud resolving models (CRMs), e.g. CRM dimensionality: domain size, grid resolution, 2D CRM orientation, and 2D vs. 3D. Implications for understanding the causes, and potential solutions, for the double ITCZ problem in GCMs are discussed.