Quality control and homogeneity of precipitation data in the southwest of Europe

A quality control process involving outliers processing, homogenization, an d interpolation has been applied to 95 monthly precipitation series in the Iberian Peninsula, southern France, and northern Africa during the period 1 899- 1989. A detailed description of the procedure results is provided and the impact of adjustments on trend estimation is discussed. Outliers have been censored by trimming extreme values. Homogeneity adjustm ents have been developed by applying the Standard Normal Homogeneity Test i n combination with an objective methodology to select reference series. The spatial distribution of outliers indicates that they are due to climate variability rather than measurement errors. After carrying out the homogen eity procedure, 40% of the series were found to be homogeneous, 49.5% becam e homogeneous after one adjustment, and 9.5% after two adjustments. About 3 0% of the inhomogeneities could be traced to information in the scarce hist ory files. It is shown that these data present severe homogeneity problems and that ap plying outliers and homogeneity adjustments greatly changes the patterns of trends for this area.