Precipitation pattern analysis in the Tiber River basin (central Italy) using standardized indices

This study analyses the precipitation pattern in the Tiber River basin (central Italy) to determine trends, possible change points, periodicity and relationships with global climate indices, such as the North Atlantic Oscillation (NAO) index. The dataset is composed of daily rainfall time series, recorded at 21 to 102 different sampling stations, from 1952 to 2007. The following quantities were analysed at both an annual and seasonal scale: cumulative precipitation, number of rainy days, mean intensity, maximum daily precipitation and number of days needed to reach a given percentage of the cumulative precipitation. To compare the time series collected from different stations along with the different quantities, a standardization procedure based on the normal inverse function computed on the best-fit cumulative probability distribution was applied. The standardized time series from each station was regionalized by kriging. Trends were analysed using the Mann-Kendall test, and possible change points were determined using an algorithm based on the singular spectrum analysis. The periodicity of the signals was evaluated by cross-wavelet analysis. The results show a decreasing trend in annual precipitation (-8%) mainly because of decreasing winter precipitation (-16%). This trend appears to be related to a decrease in the number of rainy days. A significant change point in precipitation trends was identified in the mid-1980s. Conversely, those indices related to the intensity of precipitation (mean intensity, maximum daily precipitation) do not show significant trends. The cross-wavelet analysis performed on the annual/winter precipitation and NAO time series indicates the existence of four coherent periodic signals with periods of approximately 3, 4, 8 and 15 years. Copyright (c) 2012 Royal Meteorological Society