Characterizing the seasonal variability of the diurnal cycle of precipitation over the continental United States from radar reflectivity mosaics

The diurnal cycle of precipitation over the continental US is characterized from radar rainfall maps for several spring (April-May) and summer (July-August) seasons. The main interest is to determine the effect of different types of forcing (synoptic versus thermal) on the average daily variability of precipitation, and also to compare the interannual variability between the two seasons. A further interest is to evaluate the ability of a mesoscale Numerical Weather Prediction (NWP) model to depict both the seasonal and the interannual variability of the diurnal cycle of precipitation.

The results show that the main differences between the spring and summer diurnal cycles are in the propagation duration of precipitation systems developed over the Western Cordillera (springtime systems exhibiting longer durations) and in the frequency of occurrence of convective events in the Southeastern US (more frequent occurrence during summer). On the other hand, the timing of precipitation initiation over the Rockies during spring is still in phase with the cycle of solar warming, and precipitation events initiated in this region are consistent in terms of propagation characteristics. In addition, the spring diurnal cycle demonstrates more interannual variability than the diurnal cycle of summer rainfall. While the NWP model represents fairly well the timing of precipitation initiation along the Rockies during both seasons, it fails to depict well 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 the NWP model, indicating the different propagation mechanisms during the two seasons.