Observed Climatological Relationships between Precipitable Water and Extreme Precipitation in the Contiguous United States

The foundation for the development of intensity-duration frequency relationships for extreme precipitation events is the annual maximum time series, that is, the annual daily maximum precipitation accumulations. An analysis of observed daily precipitation at over 3,000 stations in the contiguous U.S. shows that the magnitudes of the annual maximum values are positively correlated with precipitable water (PW) at virtually every station and 40% of the stations exhibit a statistically significant relationship. The relationship of the ratio of precipitation magnitude to precipitable water approaches a constant value in the limit of high PW values with a slight tendency toward higher values at the very highest PW values. The convergence of this ratio to a constant value at high PW values is consistent with Clausius-Clapeyron (CC) scaling. The tendency toward higher ratios at the highest PW values is suggestive of the possibility of super-CC scaling in the observational record. Other characteristic of the water vapor-extreme precipitation relationship found in the observation record include: (1) the number of stations exceeding the threshold for a 1-in-1yr recurrence is a strong function of PW; widespread occurrences of such extreme events occur only with very high values of PW; (2) the convergent value of the ratio of precipitation to PW does not exhibit strong spatial variability over the eastern U.S., but there is an east-west gradient with higher values in the western U.S.