INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 31: 2249–2271 (2011) Published online 7 October 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.2229 Regional and seasonal variability of extreme precipitation trends in southern Poland and central-eastern Germany 1951–2006 Ewa B. Lupikasza, a * Stephanie H¨ ansel b and J¨ org Matschullat b a Department of Climatology, University of Silesia, ul. B¸ edzi´ nska 60, 41-200 Sosnowiec, Poland b Interdisciplinary Environmental Research Centre, TU Bergakademie Freiberg, Brennhausgasse 14, 09599 Freiberg, Germany ABSTRACT: Spatial and temporal trend variability in extreme precipitation indices was studied for the meteorological seasons from 1951 to 2006. Eight indices were used, describing their frequency (90pNoD, 95pNoD), totals (1dayT, 5dayT, 90pT, 95pT) and intensity (90pInt, 95pInt). The following key challenges were addressed: (1) temporal variability in the relative number of stations with significant 30-year trends, (2) temporal stability and (3) average magnitude of the calculated 30-year trends. Temporal changes in the spatial extent of statistically significant extreme precipitation trends proceeded differently in each season. The trend direction, indicated by trend stability analysis, was consistent with that of averaged 30-year trend magnitudes at most stations. A distinct spatial differentiation emerged in the prevailing trend directions between the eastern and the western part of the study area. In all seasons, increasing trends in extreme precipitation dominated in central-eastern Germany, whereas opposite trends prevailed in southern Poland. This pattern was particularly prominent in winter. Similarities in the temporal variability of the percentage of significant trends between the eastern and the western sub-regions emerged in autumn only. Summer was characterized by the most pronounced temporal changes in the percentage of significant negative trends. Summer also showed the most stable extreme precipitation trends of all seasons and a higher trend magnitude than the transitional seasons. Spatial patterns of trend directions in spring, showing the most complex pattern of all seasons, vary depending on the index, particularly with respect to trend stability. Copyright  2010 Royal Meteorological Society KEY WORDS heavy precipitation; regional climate change; trend variability and stability; Central Eastern Europe Received 17 February 2010; Revised 6 July 2010; Accepted 25 August 2010 1. Introduction The character of changes in extreme weather events increasingly attracts attention of the scientific commu- nity due to its direct and indirect influences on environ- ment and economy (Henderson and Muller, 1997; Karl and Easterling, 1999; Parry, 2000; Subak et al., 2000; Kostopoulou and Jones, 2005). Continuous trend mon- itoring of extreme events is crucial to detect current changes in their frequency, amount and intensity. Related analytical results may be used to validate climate mod- els. Extreme precipitation is of particular interest, because precipitation is one of the most variable climate elements. Numerous studies reported increasing heavy precipi- tation trends in many regions of the world (Groisman et al., 1999; Easterling et al., 2000; Osborn et al., 2000; Frei and Sch¨ ar, 2001; Groisman et al., 2005; Alexander et al., 2006). Regional studies, based on a dense observa- tional network, indicate complex and non-uniform spatial pattern of extreme precipitation changes within the 20th century (Heino et al., 1999; Alpert et al., 2002; Brunetti * Correspondence to: Ewa B. Lupikasza, Department of Climatology, University of Silesia, ul. B¸ edzi´ nska 60, 41-200 Sosnowiec, Poland. E-mail: ewa.lupikasza@us.edu.pl et al., 2002; Frich et al., 2002; Fowler and Kilsby, 2003; Klein Tank and K¨ onnen, 2003; Kostopoulou and Jones, 2005; Moberg and Jones, 2005; Alexander et al., 2007). A Europe-wide analysis of annual extreme precipita- tion indices generally implied growing tendencies with low spatial coherence (Klein Tank and K¨ onnen, 2003) and robust trends at a few locations only (Zolina et al., 2005). Significant positive trends in annual precipita- tion extremes were detected for the Carpathian basin (Bartholy and Pongr´ acz, 2007), for north-eastern Spain (Ramos and Martinez-Casanovas, 2006) and for northern and central Italy (Brunetti et al., 2000a, 2001a, 2001b). In contrast, local-scale trend signals in Portugal (Costa and Soares, 2008) or in the central and eastern Mediterranean (Kostopoulou and Jones, 2005) were very heterogeneous. Zolina et al. (2005) showed a prominent seasonality in the trend pattern of precipitation extremes in Europe. Significant and robust increases in winter precipitation extremes were illustrated by many studies on continental and regional scales in Europe (Schmidli and Frei, 2005; Moberg et al., 2006; Ramos and Martinez-Casanovas, 2006; Maraun et al., 2008; H¨ ansel, 2009; Kysely, 2009), except for the Mediterranean area that showed decreases (Piervitali et al., 1998; Feidas et al., 2007; Martinez Copyright  2010 Royal Meteorological Society