Development of daily rainfall scenarios for southeast Spain using a circulation-type approach to downscaling

A method for downscaling from the relatively coarse General Circulation Model (GCM) spatial scale to the finer spatial scale required for impact assessment has been developed and tested in the Guadalentin Basin, southeast Spain. The method uses a circulation-type approach and relates large-scale patterns of a predictor variable, gridded sea level pressure, to local values of a surface climate variable (daily rainfall at six stations). The large-scale patterns are defined using an automated version of the Lamb Weather Type classification scheme, originally developed for the British Isles. It is demonstrated that this scheme can be successfully transferred to another region, southeast Spain. The 14 basic circulation types are combined into eight groups. These provide a legitimate basis for downscaling because each has a characteristic pressure pattern which produces the expected type and direction of flow over the study region. Furthermore, a set of consistent and distinct relationships is identified between these circulation types and daily rainfall in the Guadalentin Basin. The ability of the GCM to reproduce the observed circulation types is assessed before applying these relationships to control and perturbed-run GCM output using a statistical weather generator. The effects of the GCM's failure to reproduce the observed frequency of the circulation types are detectable in the weather generator output. The GCM changes in SLP and circulation-type frequency between the control and perturbed-runs are generally small. Nonetheless the weather generator results indicate significant changes in the number of rain days in spring and summer. These scenarios are presented as illustrative results rather than as reliable predictions. It is concluded that the circulation-type based approach to downscaling offers great potential.