Diurnal (or sub-daily) variations are large in many surface and atmospheric fields such as solar radiation, surface latent and sensible heat fluxes, surface temperature and winds, atmospheric convection, and precipitation. These diurnal variations are especially important in air-land and air-sea interactions, which are highly non-linear and thus can not be resolved using daily mean values. Current regional and global climate models still have difficulties in simulating the diurnal variations correctly. Here we analyze the diurnal variations in surface air temperature and pressure, precipitation, and cloudiness simulated by the Community Climate System Model (CCSM), a state-of-art climate system model developed by a large number of scientists from the National Center for Atmospheric Research (NCAR) and partner institutions. The CCSM simulates well some aspects of diurnal variations such as the diurnal and semidiurnal pressure tides, and the diurnal cycle of temperature over land; but it has large biases in simulating diurnal cycles in cloud amount, moist convection, and precipitation. Diurnal variations over the oceans are too weak in the CCSM mainly because the surface ocean has no diurnal cycle in the model.