Regional-scale variability and change in daily precipitation across the contiguous United States, 1949-2018

Recent research suggests that the characteristics of precipitation are changing with a warming global climate. This study uses two traditional measures of precipitation, amount and frequency, in addition to two metrics that have not been widely used in hydroclimatological research, the Gini coefficient (GC) and the Lorenz asymmetry coefficient (LAC), to analyse change and variability in precipitation characteristics across the United States from 1949 through 2018. The GC quantifies the inequity of an accumulating variable across individual contributors. For this study, the GC was used to quantify equity in the accumulating distribution of daily precipitation amounts through an annual timeframe. The LAC quantifies the relative magnitude of individual contributors which were primarily responsible for inequity in an accumulating distribution. For this analysis, the LAC assesses the relative magnitude of precipitation events (light, heavy) primarily responsible for occurrences of inequity in the temporal distribution of precipitation. Time series analysis of regionally averaged values of precipitation amount, precipitation frequency, GC, and LAC values suggest change in the hydroclimate for many regions across the United States, including a trend towards greater inequity in the temporal distribution of precipitation. The more recent trends and variability of the four precipitation characteristics were statistically examined for relationships with time series of two key atmospheric features - total column water vapour (moisture availability) and the 850 hPa-500 hPa lapse rate (static stability). Results of this study show that change and variability in the atmospheric characteristics likely help explain the observed trends and variability of several precipitation characteristics across the United States.