INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 23: 1577–1588 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.960 COMPARATIVE ANALYSIS OF SPATIAL AND SEASONAL VARIABILITY: AUSTRIAN PRECIPITATION DURING THE 20TH CENTURY CHRISTOPH MATULLA, a,b, * EDOUARD K. PENLAP, a,c PATRICK HAAS b and HERBERT FORMAYER b a Institute for Coastal Research, GKSS Research Centre, Max-Planck-Straße 1, Geesthacht, Germany b Institute of Meteorology and Physics, University of Natural Resources and Applied Life Sciences, T¨ urkenschanzstrasse 18, Vienna, Austria c Atmospheric Sciences Laboratory, Department of Physics, Faculty of Sciences, University of Yaounde I, Yaounde, Cameroon Received 26 March 2003 Revised 28 July 2003 Accepted 28 July 2003 ABSTRACT The purpose of this investigation is to demonstrate the usability of objective methods to study the variability of precipitation and hence to contribute to a better understanding of spatial and seasonal variability of Austria’s precipitation climate during the 20th century. This will be achieved by regionalizing the intra-annual variability of seasonal precipitation distributions during three non-overlapping 33 year samples (1901–33, 1934–66, 1967–99). Monthly precipitation totals were extracted at 31 Austrian stations from a homogenized long-term climate dataset provided by the Austrian weather service. Three statistical techniques, namely cluster analysis (CLA), rotated empirical orthogonal functions (REOFs) and an unsupervised learning procedure of artificial neural networks (ANNs), were utilized to find homogeneous precipitation regions. The results of summer (June, July, August (JJA)) and winter (December, January, February (DJF)) seasons are presented. The resulting homogeneous precipitation regions depend on season, period and method in this order. Hence, differences introduced by using different methods are small compared with those inferred by investigating different episodes and especially with those related to the seasons. During winter, three homogeneous precipitation regions are found, independent from the period considered. These regions can be assigned to different airflows dominating Austria’s climate and triggering precipitation events during the cold season. The situation during summer is more complicated. Thus, at least four clusters are necessary to record the circumstances, which are caused by spatially inhomogeneous convective events such as thunderstorms. Copyright  2003 Royal Meteorological Society. KEY WORDS: homogeneous regions; REOFs; cluster analysis; ANNs; Austria’s precipitation climate 1. INTRODUCTION Precipitation is one of the main climatic elements and its distribution can be highly variable in space and time, particularly over complex terrain. Long-term fluctuations of precipitation affect the composition of vegetation directly (Lexer et al., 2002). Hence, recording of past variations and simulation of future variations and changes of homogeneous precipitation regions are principal tasks of climate research. This work pursues two aims. First, to illustrate objective methods to detect homogeneous precipitation regions and thereby to contribute to a better understanding of the spatial and seasonal variability of precipitation climate in Austria. Second, to study precipitation behaviour during the 20th century. These goals appear to be of value, since Austria’s precipitation climate is marked by complicated patterns of spatial and seasonal variability (Auer, 1993). Nevertheless, the results of the study are limited by the difficulties of measuring precipitation in high mountains. * Correspondence to: Christoph Matulla, Institute for Coastal Research, GKSS Research Centre, Max-Planck-Straße 1, Geesthacht, Germany; e-mail: matulla@gkss.de Copyright  2003 Royal Meteorological Society