Difficulties in simulating the phase of diurnal water and energy cycles

The diurnal cycle is one of the Earth's fundamental cycles, producing strong variations in components of the water and energy budgets as observed at the surface. The diurnal solar forcing also produces strong features throughout the atmospheric column, leading to surface variations as well as three-dimensional structures that have a strong impact on regional behavior, including precipitation. This forcing has a particularly strong and direct impact on the energy cycle, but interacts with many aspects of the water cycle through complex diurnal structures that are poorly simulated by the NCEP/DOE Reanalysis-2 Global Spectral Model. The simulation of these features is improved over North America using the North American Regional Reanalysis, owing to the improved resolution both in grid point spacing and in surface topography, but significant difficulties still persist in accurately representing the diurnal cycle in the full atmospheric column. The dynamical structures that govern convergence of water vapor are particularly important in determining the hydrologic diurnal cycle, and until model resolution and parameterizations are improved the forecasts of diurnal variations and presumably, extreme events will be problematic.