Trends in Selected Characteristics of Precipitation in the Cities above 100,000 Residents in Poland, 1966-2016

Contemporary climate change causes an increase in the sensitivity of urban areas to the effects of this change, manifested in the increased threat of periods of both shortage and surplus of water or a higher probability of flood or drought occurrence. That is why Polish Ministry of the Environment is currently implementing project aimed at developing in major Polish cities with more than 100,000 inhabitants Urban Adaptation Plans that would reduce the city's vulnerability to climate change. One of the elements of preparing such plans is the assessment of the city's exposure to the effects of changes in rainfall conditions. Therefore, it is extremely important to know the trends of these changes. The analysis of the variability of selected precipitation characteristics was carried out for 24 Polish cities with more than 100,000 inhabitants. The tendencies of annual and seasonal precipitation changes, the number of days with precipitation, the number of days with extreme precipitation (events exceeding the 19712000 95^th percentile of daily precipitation) as well as the fraction of annual total precipitation due to extreme precipitation have been characterized. Annual trends were calculated using standard linear regression method and its statistical significance at the 95% confidence level was assessed with the help of F Snedecor probability distribution. Changes in precipitation characteristics show mixed patterns. In the majority of analyzed cities, annual precipitation totals tend to decrease, however, there is no clear spatial pattern. In general, seasonal tendencies of summer and winter precipitation, and especially autumn precipitation, also show negative tendencies. However, the sums of precipitation in the spring season are generally characterized by positive tendencies. In the vast majority of analyzed cities, the number of days with precipitation decreases, as well as the number of days with extreme precipitation. Decreasing or stable precipitation together with increasing air temperature may result in water insecurity in many Polish cities.