Observed and model-simulated diurnal cycles of precipitation
over the contiguous United States

Aiguo Dai, Filippo Giorgi,and  Kevin E. Trenberth

J. Geophys. Res., V.104,  6377-6402.


We analyzed diurnal variations in precipitation, surface pressure, and atmospheric static energy over the United States from observations and NCAR regional climate model (RegCM)
simulations. Consistent with previous studies, the mean (1963-1993) pattern of the diurnal cycle of summer U.S. precipitation is characterized by late afternoon maxima over the Southeast and the Rocky Mountains and midnight maxima over the region east of the Rockies and the adjacent plains. Diurnal variations of precipitation is weaker in other seasons, with early to late morning maxima over most of the United States in winter. The diurnal cycle in precipitation frequency accounts for most of the diurnal variations, while the
diurnal variations in precipitation intensity are small. The broad pattern of the diurnal cycle of summer precipitation is fairly stable, but the interannual variability in the diurnal cycle of winter precipitation is large.  The diurnal cycle of July convective available potential energy (CAPE) is dominated by a solar driven march of a high-CAPE (2-4 kJ/kg) tongue moving from the
Southeast into the Northwest, with maximum values in the late afternoon to early evening over most of the United States.  The solar driven diurnal and semidiurnal cycles of surface pressure result in significant large-scale convergence over most of the western United
States during the day and over the region east of the Rockies at night. The diurnal cycle of low-level large-scale convergence suppresses daytime convection and favors nighttime moist convection over the region east of the Rockies and the adjacent plains. The nocturnal maximum in the region east of the Rockies is also enhanced by the eastward propagation of late afternoon thunderstorms generated over the Rockies. Over the Southeast and the
Rockies, both the static instability and the surface convergence favor afternoon moist convection in summer, resulting in very strong late afternoon maxima of precipitation over these regions.  RegCM simulations of 1993 summer precipitation with three different cumulus convection schemes (Grell, Kuo, CCM3) all had deficiencies in capturing the broad pattern of the diurnal cycle of precipitation over the United States.  The model also overestimated precipitation frequency and underestimated precipitation intensity. The simulated diurnal cycles of surface pressure and CAPE were weak compared to observations. All the schemes produced too much cloudiness over the Southeast for July 1993 which reduced surface
solar radiation and thus daytime peak warming at the surface. The model's criteria for onset of moist convection appear to be too weak, so moist convection in the model starts too early and occurs too often with all the three schemes.