José Antonio Guijarro

Daily rainfall affinity areas in Mediterranean Spain

A subdivision of the Spanish Mediterranean region into daily rainfall affinity areas was conducted using a pluviometric data base of 410 gauges for the period 1964‐1993. Areas were derived using principal components analysis on the between-site correlation matrix. Two regionalizations defining 12 areas are offered: the first, non-overlapping, solution is derived by cluster analysis on the most important unrotated PCs extracted, while the second, with overlapping pluviometric regions, is obtained by mapping the obliquely rotated principal components. Both solutions are similar, and emphasize the dominant role exerted by the complex topography of the region on the spatial organization of rainfall through its interaction with the main rain-bearing airflows and systems. A higher-resolution regionalization comprising 20 non-overlapping areas is also included. South and southeast domains, where topography is very prominent and complex, as well as the Balearic Islands, contain most of the additional detail offered by this solution. Copyright

PREGRIDBAL 1.0: towards a high-resolution rainfall atlas for the Balearic Islands (1950–2009)

This work presents a catalog of daily precipitation fields in the Balearic Islands created with data from AEMET (State Meteorological Agency) assistant observations, including records from 1912. The original digital daily data file has been interpolated onto a regular 100 m-resolution grid (namely PREGRIDBAL), defined with the aim of becoming a valid standard for future methodological improvements and catalog upgrades. Daily precipitation amounts on each grid point are calculated using an analysis method based on ordinary kriging, using the daily anomaly with respect to the annual mean for all available observations each day. Due to quality concerns, the time span for products derived from the catalog is limited to the 1950–2009 period, when the number of operating stations reached 200. Therefore, from the time series of daily maps, monthly-, annual-, quinquennial-, and decadal-accumulations are produced. Similarly, the catalog allowed for quantification of climate trends in rainfall amounts in the Balearic Islands, with the significant advantage of minimizing the biases originated from heterogeneities in the spatial distribution of stations across the archipelago. Results show a general decrease in precipitation during the 1950–2009 period. From 1950 to 1979, the average annual precipitation across the islands was 624.3 mm, while from 1980 to 2009 it diminished to 555.36 mm. Changes in precipitation patterns, which vary among the different areas, are also detected. The most significant reductions are found in the northern half of the archipelago and especially in Mallorca, where the Tramuntana mountain range stands out. All seasonal trends show a decrease, with values ranging between 1 and 3 mm decade 1, with the exception of autumn, which reaches a positive trend up to 7 mm decade 1. October shows the most dramatic decrease ( 10.34 mm decade 1) and, conversely, September and November show an increase in precipitation (3.28 and 1.82 mm decade 1, respectively) with a statistical significance above 85 % across almost the entire archipelago, and even exceeding 95 % in Eivissa and Formentera.