Across much of the United States, the 24-h cycle of rainfall during the summer is dominated by diurnal and semi-diurnal modes. This is especially true in the Southeast, where airmass thunderstorms often develop in the afternoon, and in the western Great Plains, where mesoscale convective systems become organized in the evening and travel eastward overnight.

The authors will use these observed diurnal and semi-diurnal modes as a benchmark for an evaluation of the Weather Research and Forecasting (WRF) Model's skill at simulating the timing and frequency of rainfall. The authors will briefly summarize the modes apparent in data from the WSR-88D network and, based on these observations, will assess output from the WRF Model run with horizontal grid spacings of 22 and 10 km.

Such unconventional tests can be highly effective at revealing systematic weaknesses in model physics and at suggesting whether a model is realistically depicting specific diurnal and semi-diurnal phenomena (such as sea and land breezes) that force and organize cumulonimbi. The results of the research may point toward ways of improving quantitative precipitation forecasts (QPF) over short times and simulations of regional climates over long times.