National composites of radar reflectivity have permitted fine-scale analysis of the diurnal cycle of precipitation over the U.S. Previous studies focused on thunder observations and rain gage measurements to document favored times of day for precipitation over the U.S. However these methods do not have the sensitivity, spatial resolution, or blanket coverage that the WSR-88D network provides. More recently, the National Lightning Detection Network (NLDN) and satellite-based optical transient detector (OTD) have produced reasonable pictures of the diurnal climatology of lightning over both land and ocean. While these lightning sensors benefit from extended or even global coverage, the platforms still suffer from their own unique limitations.

On the continental scale, the findings based on the radar network are qualitatively similar to results based on rain gage or thunderstorm observations. However, regional and smaller-scale features observed in the phase and amplitude of the diurnal cycle are not documented in the literature. These features include a prominent minimum in the amplitude of the diurnal cycle in Texas, an apparent lake breeze influence southwest (upwind) of Lake Michigan and Superior, a sea/land breeze signal along the Gulf coast and Atlantic seaboard, and a tendency for rainfall to peak at progressively later times eastward from the Appalachian Mountains. The latter aspect echoes a better-known pattern seen on a larger scale in the lee of the Rocky Mountains. Systematic phase shifts apparently due to mountain-valley circulations are also evident within the Rocky Mountain cordillera itself. In addition to the diurnal cycle, the geographical distribution of the second harmonic (semi-diurnal) signal will also be presented.