Spatial and temporal variability of extreme precipitation in Poland in the period 1951-2006

The study investigated spatial and temporal variability of extreme precipitation trend magnitudes and their directions. Daily precipitation from 48 synoptic stations in Poland for the period 1951-2006 were used. Five indices of precipitation extremes were chosen: the highest 5-day precipitation total, precipitation total from events >= 90(th) and 95(th) percentiles as well as number of days with precipitation >= 90(th) and 95(th) percentiles of daily precipitation amount. Trends in extreme precipitation indicators were analysed over semi-annual periods as well as over the standard climatological seasons. Trends were calculated for each of the 30-year moving periods within 1951-2006 using a simple linear regression method. Their significance was tested with the Mann-Kendall method. Decreasing trends in extreme precipitation indices dominated in both the warm and cool halves of the year and in the seasons. The greatest change was recorded in the least extreme precipitation indices. Summer was the season with the greatest number of statistically significant trends, mostly decreasing ones. Upward trends, having the greatest spatial extend in autumn, dominated the initial 30-year periods of the study period whereas the decreasing trends were common in the 30-years from 1956. In summer and winter, decreasing trends reached the greatest degrees of stability in southern Poland. In spring and autumn, a stable fall of extreme precipitation indices was mainly recorded in the south-western part of the country. Stable increasing trends were recorded sporadically. However, in spring trends towards an increase were more frequent than in other seasons. The strongest decreasing trends were observed in summer and winter, mainly in the south, while in autumn they also occurred in the west. Copyright (C) 2009 Royal Meteorological Society