Early development and tuning of a global coupled cloud resolving model, and its fast response to increasing CO2

Thorsten Mauritsen, Rene Redler, Monika Esch, Bjorn Stevens, Cathy Hohenegger, Daniel Klocke, Renate Brokopf, Helmuth Haak, Leonidas Linardakis, Niklas Röber, Reiner Schnur

Since the dawn of functioning numerical dynamical atmosphere- and ocean models, their resolution has steadily increased, fed by an exponential growth in computational capabilities. The computationally limited resolution of models means that a number of mostly small-scale or micro-scale processes have to be parameterised -- in particular those of atmospheric moist convection and ocean eddies are problematic when scientists seek to interpret output from model experiments. Here we present the first coupled ocean-atmosphere model experiments with sufficient resolution to dispose of moist convection and ocean eddy parameterisations. We describe the early development and discuss the challenges associated with conducting the simulations with a focus on tuning the global mean radiation balance in order to limit drifts. A four-month experiment with quadrupled CO2 is then compared with a ten-member ensemble of low-resolution simulations using MPI-ESM1.2-LR. We find broad similarities of the response, albeit with a more diversified spatial response with both stronger and weaker regional warming, as well as a sharpening of precipitation in the inter tropical convergence zone. These early results demonstrate that it is already now possible to learn from such coupled model experiments, even if short by nature.