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Using multi-physics ensembles to explore sources of uncertainty in the climate system

The evaluation of the role of different sources of uncertainty in regional climate projections is one of the main goals of CORDEX. In particular, uncertainties arising from the different formulation of current Regional Climate Models (RCMs) are assessed by using an ensemble of simulations from different models. However, this approach makes difficult to determine which parts of the formulation are contributing the most to the uncertainty.

In this work, this uncertainty is explored within Euro-CORDEX by means of a single model run under different configuration. The Weather Research and Forecasting (WRF) model was used to produce an ensemble of 9 different configurations. These included not only physical parameterizations, but also other options. We isolated the effect of parameterizations by simulating a single year (2001) with all members, but changing only the physical schemes. We found that the differences among the Euro-CORDEX WRF members are mainly due to the selection of physical parameterizations.

The study focuses not only in temperature and precipitation, but also on other important variables such as soil moisture, cloud cover and turbulent fluxes. The largest sensitivity corresponds to the cumulus scheme, and occurs during summer. This sensitivity is amplified by a feedback between cloud cover and soil moisture.

Furthermore, another ensemble containing more than 200 different configurations was created to explore the extreme heat event of 2003. We found that only few physics combinations could reach the highest temperatures. Much of the variation is again due to the cumulus schemes, but also a large sensitivity is found to the choice of the radiation schemes, especially during the warmest days.

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