Diurnal Variations in Precipitation and Thunderstorm Frequency over the Conterminous United States

Hourly data on the frequencies of all types of precipitation events, heavy precipitation events, trace precipitation events and thunderstorms for more than 100 stations in the United States were processed to generate statistics on the amplitude and phase of the diurnal and semidiurnal cycles at each station. Results are displayed on seasonal maps in a vectorial format that emphasizes the large scale geographical consistency of the diurnal variations. During summer each of the four parameters listed above displays a distinctive geographical pattern of diurnal variations. Thunderstorm frequency tends to be the most strongly modulated by the diurnal cycle; trace precipitation the least strongly modulated. Over the central United States the maximum frequency of severe convective storms occurs during the early evening; thunderstorms exhibit their maximum frequency around midnight, while most precipitation falls later in the night. These amplitude and phase differences offer some insight into the relative importance of thermodynamical and dynamical processes in controlling the frequency and intensity of convective activity. During winter heavy precipitation and thunderstorms are biased toward nighttime over much of the midwest and Atlantic seaboard. Trace precipitation exhibits a small but geographically consistent diurnal oscillation with a peak near or slightly after sunrise. It is suggested that this morning peak is associated with precipitation from low stratus decks. The semidiurnal cycle is generally smaller than the diurnal. Effects of the S2 pressure wave are clearly evident over much of the tropics, but over middle latitudes they are often obscured by regional and local influences.