Trends in U.S. hourly precipitation variance 1949-2009

Drawing from a NOAA database of hourly precipitation data from 5995 stations in the contiguous United States over the period 1949-2009, the authors investigate possible trends in the variance of the hourly precipitation, averaged over the diurnal and annual cycles and normalized by the square of the mean precipitation at that site. This normalized variance is a measure of storminess, distinct from increases in precipitation attributable to warming. For the 1722 stations surviving quality control with data on at least 80% of days in at least 30 years, the authors compute the rate of change of the logarithm of the normalized variance at each station and set bounds on its mean (over stations) trend. The logarithmic function weights the trends at calm stations equally to those at stormy stations and enhances the statistical power of the mean. The authors find a logarithmic rate of change of the mean normalized variance of (-28 ± 1.7) × 10^-4yr^-1 (± 1σ). The 2σ upper bounds on any continentally averaged trend, increasing or decreasing, are about 0.001 yr^-1 (doubling or halving times > 1000 years). It is found that the normalized variance in the Los Angeles basin has increased at a statistically significant rate. This may be attributable to a decrease in the number of aerosol condensation nuclei. Upper bounds are set on any effect of the 11-yr solar cycle.