Trends of heavy precipitation and wet and dry spells in Switzerland during the 20th century

The low-frequency variability of heavy precipitation and drought conditions is investigated for 104 rain-gauge stations in Switzerland covering the 20th century. This constitutes an exceptionally dense data set of centennial time series. The investigation is based on a wide range of daily and multi-day precipitation statistics encompassing basic characteristics, measures of heavy precipitation and indices of typical and extreme dry and wet spells. Two different methods of trend analysis and statistical testing are applied, depending on the data nature of the statistic. Linear regression is used for statistics with a continuous value range, and logistic regression is used for statistics with a discrete value range. The trends are calculated on a seasonal basis for the years 1901-2000. A clear trend signal is found for winter and autumn, with a high number of sites with a statistically significant trend. In winter, significant increases are found for all statistics related to precipitation strength and occurrence. The centennial increase is between 10 and 30% for the high quantiles and the seasonal 1 day to 10 day extremes. In autumn, statistically significant increases are found only for the statistics related to heavy precipitation, whereas precipitation frequency and spell-length statistics show little systematic change. Although the winter trend signal is strongest in northern and western Switzerland, the autumn trend signal is more uniform. In spring and summer, the heavy precipitation and the spell-duration statistics did not show statistically significant trends. Sensitivity tests indicate that the winter and autumn trends are robust with respect to inhomogeneities in the rain-gauge time series. Copyright (c) 2005 Royal Meteorological Society.