Gridded Climatic Monthly Frequencies of Precipitation Amount for 1-, 3-, and 6-h Periods over the Conterminous United States

Abstract

Gridded fields of monthly mean relative frequency for ≥0.10 (2.5), ≥0.25 (6.4), ≥0.50 (12.7), ≥1.00 (25.4), and ≥2.00 (50.8) in. (mm) of precipitation have been developed for 1-, 3-, and 6-h periods over the conterminous United States. The frequency fields are analyzed on a rectangular grid with a 20-km mesh. Raw (unsmoothed) frequencies at randomly spaced points were computed from 33 yr (1963?95) of hourly precipitation measurements from approximately 3000 stations composing the U.S. Climatic Hourly Precipitation Network. Initial grids of the raw frequencies were then obtained from objective analyses of the randomly spaced values. The final grids were obtained following the application of smoothing, which was applied spatially over the grid and temporally over consecutive months and consecutive time periods of the day. The smoothing applied for each precipitation category and accumulation period was minimized to retain as much coherent temporal and spatial detail as possible. The detail in the frequencies is greater than that for previous nationwide climatic precipitation analyses of this type. The database was developed for use as a climatological predictor input to a statistically based forecast model for the various categories of precipitation amount, but it should also have other operational or research applications. The monthly frequency fields exhibit known climatic features across the nation and features at small temporal and spatial scales that either have not been previously documented or that clarify those incompletely defined in the published literature. The discussions link both known and new climatic features to physiographic features, such as mountain ridges and valleys, large lakes, and ocean coasts, as well as to the diurnal heating?cooling cycle. Examples of clarifications of previous findings include the spatial and temporal properties of the eastward migration of summer afternoon precipitation frequency peaks from the Rocky Mountains to the high plains and corresponding aspects of the formation of the Great Plains nocturnal precipitation maximum. New discoveries include a secondary summertime nocturnal precipitation peak in the Texas panhandle, a persistent summer maximum for light precipitation southwest of Lake Superior, and a weak leeshore maximum (minimum) for summertime Lake Michigan precipitation during morning (afternoon) hours. These and other new findings resulted from the fine spatial and temporal scale of the analysis.