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

We analyzed diurnal variations in precipitation, surface pressure, and atmo spheric static energy over the United States from observations and NCAR reg ional climate model (RegCM) simulations. Consistent with previous studies, the mean (1963-1993) pattern of the diurnal cycle of summer U.S. precipitat ion is characterized by late afternoon maxima over the Southeast and the Ro cky Mountains and midnight maxima over the region east of the Rockies and t he adjacent plains. Diurnal variations of precipitation is weaker in other seasons, with early to late morning maxima over most of the United States i n winter. The diurnal cycle in precipitation frequency accounts for most of the diurnal variations, while the diurnal variations in precipitation inte nsity are small. The broad pattern of the diurnal cycle of summer precipita tion is fairly stable, but the interannual variability in the diurnal cycle of winter precipitation is large. The diurnal cycle of July convective ava ilable potential energy (CAPE) is dominated by a solar driven mach of a hig h-CAPE (2-4 kJ kg(-1)) tongue moving from the Southeast into the Northwest, with maximum values in the late afternoon to early evening over most of th e United States. The solar driven diurnal and semidiurnal cycles of surface pressure result in significant large-scale convergence over most of the we stern 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 e ast of the Rockies and the adjacent plains. The nocturnal maximum in the re gion east of the Rockies is also enhanced by the eastward propagation of la te afternoon thunderstorms generated over the Rockies. Over the Southeast a nd the Rockies, both the static instability and the surface convergence fav or afternoon moist convection in summer, resulting in very strong late afte rnoon maxima of precipitation over these regions. RegCM simulations of 1993 summer precipitation with three different cumulus convection schemes (Grel l, Kuo, CCM3) all had deficiencies in capturing the broad pattern of the di urnal cycle of precipitation over the United States. The model also overest imated precipitation frequency and underestimated precipitation intensity. The simulated diurnal cycles of surface pressure and CAFE were weak compare d to observations. All the schemes produced too much cloudiness over the So utheast for July 1993 which reduced surface solar radiation and thus daytim e peak warming at the surface. The model's criteria for onset of moist conv ection appear to be too weak, so moist convection in the model starts too e arly acid occurs too often with all the three schemes.