Precipitation Scenarios over Iberia: A Comparison between Direct GCM Output and Different Downscaling Techniques

The Iberian rainfall regime is characterized by a strong seasonal cycle and large interannual variability. Typically, frequency distributions of monthly precipitation present a large spread of values, implying frequent episodes of very wet or very dry years. Unfortunately, the most recent generation of general circulation models (GCMs) still has serious problems when modeling monthly precipitation over southern Europe. However, these models are able to reproduce the main patterns of atmospheric circulation, such as those derived from a principal component analysis of the sea level pressure anomaly field. Many downscaling techniques have been developed in recent years, all having in common the need to establish statistical links between the large-scale circulation and the observed precipitation at a local or regional scale. The final objective is usually the application of such transfer functions to GCM output. In this work, linear and nonlinear downscaling transfer functions are developed based on artificial neural networks (ANNs), to downscale monthly precipitation to nine grid boxes over the Iberian Peninsula. The nonlinear ANN models were run 20 times, with different initial conditions, in order to study the stability of the final results. All the models were developed on a seasonal basis, calibrated between 1901 and 1940 and validated between 1941 and 1990. It was found that linear or slightly nonlinear ANN models (with just one node in the first layer) were more capable of reproducing the observed precipitation than more complex nonlinear ANN models. GCM data from a greenhouse gas–plus-sulfates run from the Hadley Centre Model (HadCM2) were used to reproduce present-day precipitation over Iberia. It was found that the precipitation characteristics (mean, variance, and empirical distribution) were better reproduced by the downscaled results than by the GCM direct output. Precipitation scenarios constructed for the future (2041–90) reveal an increase of precipitation in winter and small decreases in most sectors of Iberia for the spring and autumn seasons. Such scenarios are in good agreement with those obtained by other researchers using different downscaling techniques with HadCM2 data.