ICON-A: the atmospheric component of the ICON Earth System Model. Part II: Model evaluation

We evaluate the new icosahedral non-hydrostatic atmospheric (ICON-A) general circulation model of the Max Planck Institute for Meteorology that is flexible to be run at grid-spacings from a few tens of meters to hundreds of kilometers. A simulation with ICON-A at a low resolution (160 km) is compared to a not tuned four-fold higher resolution simulation (40 km). Simulations using the last release of the ECHAM climate model (ECHAM6.3) are also presented at two different resolutions. The ICON-A simulations provide a compelling representation of the climate and its variability. The climate of the low-resolution ICON-A is even slightly better than that of ECHAM6.3. Improvements are obtained in aspects that are sensitive to the representation of orography, including the representation of cloud-fields over eastern-boundary currents, the latitudinal distribution of cloud top heights, and the spatial distribution of convection over the Indian Ocean and the Maritime Continent. Precipitation over land is enhanced, in particular at high resolution ICON-A. The response of precipitation to El Ni\ {n}o SST variability is close to observations, particularly over the eastern Indian Ocean. Some parametrization changes lead to improvements, e.g. with respect to rain intensities and the representation of equatorial waves, but also imply a warmer troposphere, which we suggest leads to an unrealistic poleward mass shift. Many biases familiar to ECHAM6.3 are also evident in ICON-A, namely a too zonal South Pacific Convergence Zone, an inadequate representation of north hemispheric blocking, and a relatively poor representation of tropical intra-seasonal variability