Abstract. The scalability of the atmospheric model ECHAM6 at low resolution, as used in palaeoclimate simulations, suffers from the limited number of grid points. As a consequence, the potential of current high performance computing architectures cannot be used at full scale for such experiments, particularly within the available domain-decomposition approach. Radiation calculations are a relatively expensive part of the atmospheric simulations taking approximately up to over 50 % of the total

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... untime. This current level of cost is achieved by calculating the radiative transfer only once in every two simulation hours. In response, we propose to extend the available concurrency within the model further by running the radiation component in parallel with other atmospheric processes to improve scalability and performance. This paper introduces the concurrent radiation scheme in ECHAM6 and presents a thorough analysis of its impact on the performance of the model. It also evaluates the scientific results from such simulations. Our experiments show that ECHAM6 can achieve a speedup over 1.9x using the concurrent radiation scheme. This empirical study serves as a successful example that can stimulate research on other concurrent components in atmospheric modeing whenever scalability becomes challenging.