Diurnal variations of summer precipitation between wet and dry years in the central U.S

The diurnal variation of precipitation in summer has strong effects on applications such as surface hydrology, agriculture, and transportation. We have found that observed summertime precipitation in central U.S. has maximum daily rainfall in the early morning during wet years but in late afternoon during dry years. For example, in the 1993 flood daily rainfall peaked around 03 LST whereas in the 1988 drought rainfall peaked at about 17 LST. There is also evidence that the diurnal cycle tends to have dual weak peaks in dry years, one in the mid-late afternoon and the other in the early morning, but a single strong peak in the early morning in wet years.

In order to explore the physical processes responsible for the diurnal shift of rainfall peaks, a series of numerical experiments of flood/drought cases have been carried out using a nested mesoscale model (MM5). Particular emphasis in the diagnostic analysis is placed on phase relations among different atmospheric processes. Based on observational analysis and model simulations we found the following: (1) Large-scale forcing in wet years favors organization (upscale growth) of convection, which postpones precipitation to late night/early morning. On the other hand, in dry years large-scale forcing is weak and does not support organized convective systems. (2) In wet years major contributing components to precipitation, e.g., the low-level jet, and even atmospheric tides, peak about simultaneously in this region, producing a stronger diurnal cycle. In contrast, in dry years these components are out of diurnal phase, producing a weaker diurnal amplitude of precipitation. (3) Model horizontal grid resolution can artificially alter diurnal precipitation distribution by failing to resolve organized mesoscale convective systems.