The Diurnal Variability of Precipitation across the Central Rockies and Adjacent Great Plains

Abstract

The diurnal variation of precipitation across Wyoming, Colorado, South Dakota, Nebraska and Kansas has been studied by means of a harmonic analysis of 35 years of hourly precipitation data for 334 station and a regional probability of precipitation analysis for grouped stations. For all measurable (>0.25 mm) precipitation events, the phase of the diurnal cycle tends to peak between 0200?0600 LST in winter, 0000?0600 LST in spring; 1600?0400 LST in summer and 2100?0600 LST in autumn. The earlier times occur in the mountain regions and the later times near the eastern edge of the district. The well-known afternoon mountain and nocturnal Plains convective signal is seen for hourly precipitation intensities of 2.5 mm and 10.0 mm. New findings beyond previous work include: 1) a pervasive 0300 LST maximum for the precipitation category >0.25 mm that is most prominent during the cooler half of the year and partially masked in summer, 2) the transition from winter to spring (March/April) is accompanied by an increase in measurable precipitation frequency but a decrease in precipitation frequency for rainfall amounts >2.5 mm and 3) the summer rainfall regime is made up of distinct local and mountain generated signals. The summer heavier precipitation events tend to occur 1?4 hours earlier than all measurable rainfall events, particularly on the Plains east of 101°W. The implication of these results is that: 1) the winter regime is affected by large scale circulation features as the 0300 LST maximum is found elsewhere, e.g., the northeastern United States, 2) dynamically significant precipitation systems, although infrequent, do affect the five state region in winter, and 3) heavy summer nocturnal precipitation systems over the eastern Plains can not be explained solely by the eastward propagation of mountain generated systems from the previous afternoon.