A principal component and long‐term trend analysis of daily precipitation in Switzerland

Daily precipitation patterns over Switzerland are investigated by rotated and unrotated principal component analysis for the periods 1901–1990 (with 113 continuously operating rain-gauge sites) and 1961–1990 (with 304 sites). Empirical orthogonal functions are utilized to homogenize the precipitation series and to optimally transform the 113 series of the long-term record into a few variables. Several statistically significant linear trends are detected. This includes, in particular, a wintertime increase in precipitation by up to 30 per cent per 100 years in the western and northern parts of Switzerland. This trend is statistically significant at the 90 per cent level. In most parts of southeastern Switzerland, winter precipitation increased as well but by a smaller rate and at a slightly lower statistical significance level. For the period 1961–1990, precipitation amounts increased in most subregions and all seasons with the exception of summer. A statistical model is formulated in order to isolate the contributions to the observed precipitation trends that are related to changes in either the frequency or the precipitation activity of Alpine weather classes. It is demonstrated that the observed precipitation trends cannot be interpreted based upon changing frequencies of weather classes (the observed changes would in fact lead to drier wintertime conditions), but are rather dominated by the tendency of most rain-producing weather types to produce more rain. Some physical mechanisms that could account for this unexpected result are also discussed. © 1997 by the Royal Meteorological Society. Int. J. Climatol., 17: 1333–1356 (1997) (No. of Figs: 9. No. of Tables: 5. No. of References: 69)