Estimating statistics of European wet and dry spells and associated precipitation extremes - interannual variability and trends

Probability distributions of the durations of wet and dry spells were modeled by applying truncated geometric distribution. It has been also extended to the fractional truncated geometric distribution which allows for the discrimination between the roles of a changing number of wet days and of a regrouping of wet and dry days in forming synoptic structure of precipitation. Analyses were performed using 2 collections of daily rain gauge data namely ECA (about 1000 stations) and regional German DWD network (more than 6000 stations) for the period from 1950 to 2009. Wet spells exhibit a statistically significant lengthening over northern Europe and central European Russia, which is especially pronounced in winter when the mean duration of wet periods increased by 15%-20%. In summer wet spells become shorter over Scandinavia and northern Russia. The duration of dry spells decreases over Scandinavia and southern Europe in both winter and summer. Climate tendencies in extreme wet and dry spell durations may not necessarily follow those in mean characteristics. The changing numbers of wet days cannot explain the long-term variability in the duration of wet and dry periods. The observed changes are mainly due to the regrouping of wet and dry days. The tendencies in duration of wet and dry spells have been analyzed for a number of European areas. Over the Netherlands both wet and dry periods are extended in length during the cold and the warm season. A simultaneous shortening of wet and dry periods is found in southern Scandinavia in summer. Over France and central southern Europe during both winter and summer and over the Scandinavian Atlantic coast in summer, opposite tendencies in the duration of wet and dry spells were identified. Growing durations of wet spells are associated with more intense precipitation events while precipitation during shorter wet spells become weaker. Both analyses of relatively coarse resolution ECA data and high resolution DWD station network clearly show this tendency. This implies a mechanism for intensifying European flash floods associated with long rainy periods.