Examining the Contribution of Deep-Atmosphere Dynamical Cores to Mitigating Double ITCZ Bias in Aquaplanets

Atmospheric dynamical cores solve the equations of motion that describe the fluid flow at the resolved scales. Commonly used dynamical cores, including the Department of Energy's (DOE) ‘Higher Order Methods Modeling Environment’ (HOMME) dynamical core, also known as a ‘Spectral Element’ (SE) dynamical core, and the National Center for Atmospheric Research's (NCAR’s) ‘Model for Prediction Across Scales’ (MPAS), contained two approximations of the fluid dynamics equations. They are called the Shallow-atmosphere and Traditional (SA+T) approximations. These approximations discard the so-called Nontraditional Coriolis terms. Omitting these terms significantly biases the dynamical response to diabatic heating, inducing up to 10% relative error in the wind field as linear model results suggested. The biases induced by these approximations are expected to be most severe in tropical regions, and to become more severe at finer grid spacings. We present preliminary evidence that removing these approximations may help mitigate the double Intertropical Convergence Zone (ITCZ) bias that is present in many climate models.

 

We present dynamical core intercomparisons between the shallow-atmosphere version of the HOMME and MPAS dynamical cores and their deep-atmosphere variants which remove the SA+T approximations. We present an overview of the modifications made to the HOMME and MPAS dynamical cores. Aquaplanet simulations made using NCAR’s Community Earth System Model (CESM) show that for sea surface temperature distributions that produce a double ITCZ, removing the SA+T approximations induces equatorward shifts in precipitation. We will examine sensitivities to physics-dynamics coupling strategies and parametric sensitivity to the deep convection scheme. We also examine the performance of MPAS, which collocates physics columns with dynamics columns, with HOMME, which simulates physics on a sparser finite-volume grid. Our simulations are done at a nominal 1º grid spacing, providing conclusive evidence that the SA+T approximations induce climatologically significant biases at the coarser grid spacings used in workhorse climate model configurations.