ICON-A, the Atmosphere Component of the ICON Earth System Model: I. Model Description

ICON-A is the new icosahedral nonhydrostatic (ICON) atmospheric general circulation model in a configuration using the Max Planck Institute physics package, which originates from the ECHAM6 general circulation model, and has been adapted to account for the changed dynamical core framework. The coupling scheme between dynamics and physics employs a sequential updating by dynamics and physics, and a fixed sequence of the physical processes similar to ECHAM6. To allow a meaningful initial comparison between ICON-A and the established ECHAM6-LR model, a setup with similar, low resolution in terms of number of grid points and levels is chosen. The ICON-A model is tuned on the base of the Atmospheric Model Intercomparison Project (AMIP) experiment aiming primarily at a well balanced top-of atmosphere energy budget to make the model suitable for coupled climate and Earth system modeling. The tuning addresses first the moisture and cloud distribution to achieve the top-of-atmosphere energy balance, followed by the tuning of the parameterized dynamic drag aiming at reduced wind errors in the troposphere. The resulting version of ICON-A has overall biases, which are comparable to those of ECHAM6. Problematic specific biases remain in the vertical distribution of clouds and in the stratospheric circulation, where the winter vortices are too weak. Biases in precipitable water and tropospheric temperature are, however, reduced compared to the ECHAM6. ICON-A will serve as the basis of further development and as the atmosphere component to the coupled model, ICON-Earth system model (ESM). Plain Language Summary ICON-A is a new atmospheric model as needed for research on the general circulation of the atmosphere, or as atmospheric component in an Earth system model, as used in climate research. This article describes the construction of the atmospheric model, in particular how two major parts are coupled to each other: "dynamics" and "physics." Dynamics is the part that solves the equations for the atmospheric motion, temperature, density, and concentrations of water vapor, cloud water, and cloud ice. Physics is the part that computes the changes in these fields related to processes like radiation, cloud condensation, or turbulence. These physical changes depend on the state of the atmosphere as computed by the dynamics, and the changes computed by physics force change in the dynamics. The article documents the details of this construction. Further, the article describes how the physics is tuned to obtain a good representation of the general circulation of the period 1979 to 1988 in comparison to observations. A more detailed evaluation of such simulations is presented in a companion article by Crueger et al.