The COPAT2 initiative of the CLM-Community: towards a recommended configuration of COSMO-CLM and ICON-CLM new model versions

<p>The structural uncertainty of a climate model is defined as the range of outcomes that can be obtained through different representations of physical processes of the climate system, the selection of different unconstrained parameter values and different choices for the numerical solution of underlying fundamental equations. Exploring the range of these outcomes with the goal of determining a model configuration that produces results in closer agreement with observational data is defined as model calibration or tuning.</p><p>In this study, the preliminary results of the Coordinated Parameter Testing 2 (COPAT2) initiative of the CLM-Community are presented. In COPAT2, volunteer members of the community join forces together, with the objective of testing and providing recommended configurations for the new and final version of COSMO-CLM (6.0), as well as for the newly released regional climate model ICON-CLM, for climate modeling applications over the European CORDEX domain.&#160;</p><p>A series of sensitivity tests is performed in which various configurations of the models are explored. The aspects that are tested have been carefully selected, based on expert judgment. In the case of COSMO-CLM 6.0, the primary focus is on newly introduced and recently updated parameterizations and physical schemes. For ICON-CLM, these&#160; tests are the first ever conducted with the climate version of the model and are based on the operational configuration and on information of experiments performed for the development of the NWP mode of ICON.</p><p>The simulations are conducted at a horizontal resolution of approximately 12 km over Europe, using ERA5 reanalysis data as boundary conditions. In a first step, a series of relatively short tests is conducted over a 7-year period, from 1979 to 1985. Successively, depending on the sensitivity of the model to the applied changes in its configuration, a sub-set of simulations is extended over a total period of 12 years. The results are systematically analyzed with an evaluation suite that has been further developed and extended for COPAT2. The standardized analysis and condensation of results in very few indices summarizing the models performance allow for an easy and fast comparison of the quality of the different simulations.&#160;&#160;</p><p>Beside introducing preliminary results of the conducted sensitivity tests, an overview of the calibration strategy followed in COPAT2 will be presented, including information on the selected metrics, employed observational data sets and further details inherent to the ranking of the different experiments.&#160;&#160;</p><p><br><br></p>