The Diurnal Cycle of Precipitation from Continental Radar Mosaics and Numerical Weather Prediction Models. Part I: Methodology and Seasonal Comparison

Abstract

The diurnal cycle of precipitation over the continental United States is characterized through the analysis of radar rainfall maps and is used as a measure of performance of the Global Environmental Multiscale (GEM) model during the spring (April?May) and summer (July?August) of 2008. The main interest is to determine the effects of different types of forcing (synoptic versus thermal) on the average daily variability of precipitation and on the model?s representation of it. A secondary objective is to study the interannual variability of the diurnal cycle. The investigation is based on the analysis of time?longitude diagrams of precipitation fields, of average statistics, and of model skill scores. The results show that the main differences between the spring and summer diurnal cycles are the duration of propagating systems, the frequency of convective events in the southeastern United States, and more interannual variability of the spring diurnal cycle. However, most interesting is that the timing of precipitation initiation over the Rockies is in phase with the cycle of solar warming for both seasons, despite the strong synoptic forcing during spring. Also, east of the Rockies, the diurnal cycle is mainly determined by transport mechanism and is consequently out of phase with the solar cycle. While GEM represents fairly well the timing of precipitation initiation along the Rockies during both seasons, it fails to correctly depict the propagation characteristics of these systems. During spring, the simulated systems show more variability in propagation paths than observed, while during summer, the observed propagation is simply not captured by GEM. This is probably a consequence of different propagation mechanisms acting in the model and in the atmosphere, and between spring and summer.