In this paper we focus on the analysis of GCM-RCM coupling uncertainty considering the state-of-the-art ENSEMBLES dataset. We focus on precipitation over Spain and compare the performance of the GCM-driven control runs (20C3M) with the ERA40- driven ones (perfect boundary conditions) in a common period (1961-2000). Large deviations between both results indicate a large uncertainty/bias for the particular GCM-RCM combination and, hence, a small confidence for the corresponding transient simulations (or projections for a future period) due to the possible nonlinear amplification of biases. We find very large biases for some particular GCM-RCM couplings which can be considered clear outliers/errors of the ensemble. Moreover, we give some evidence of the nonlinear amplification of biases affecting the climate change signal (or "delta") for those defective simulations.
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