Physical processes controlling warm-season precipitation diurnal cycle in the central U.S

The diurnal cycle of precipitation is large over continents. It provides an excellent test bed for numerical parameterizations such as moist convection and planetary boundary layer schemes in atmospheric models. In contrast to later afternoon peaks over most land areas, warm-season precipitation in the central U.S. is characterized by a maximum from midnight to early morning, which is mainly a result of the diurnal cycle in precipitation frequency. Previous studies have suggested a number of processes that may contribute to the nocturnal precipitation maximum (NPM). However, the exact underlying processes leading to the NPM are not fully understood. As a result, the NPM is not captured by most global and regional weather and climate models. In this study, we investigate the physical processes that lead to the NPM and that suppress afternoon moist convection using data collected during the International H2O Project (IHOP) (May-June 2002) and the North American Regional Reanalysis (NARR) data. We focus on answering the following questions: (1) why the daytime solar heating does not result in an afternoon peak of moist convection in the central U.S., in contrast to most land areas; (2) what roles the diurnal cycle of the background, large-scale vertical motion plays in the diurnal timing of summer precipitation in the central U.S.; (3) how much the moist convections initiated locally and propagated from the Rocky Mountains contribute to the total precipitation in the central U.S.; (4) what roles the low level jet (LLJ) and bores play in the NPM; (5) why the initiation level of moist convection differs during nighttime (elevated) and daytime in the southern Great Plains; and (6) why most models fail to simulate the NPM. The preliminary results include: (1) The large-scale descending air during daytime prevents the development of moist convection in the central U.S. in the afternoon. (2) Southerly LLJ exists throughout the night and reaches the maximum wind speed at 500-600 m AGL around 2 am CST. The limited data show that rain often follows the end of LLJ events. (3) The bores observed during IHOP are ubiquitous at night, have a life time of ~3-4 hours, are often initiated by convection, and are frequently observed in the later stages of LLJ periods. Bores provide extremely strong lifting of moist air that results in elevated instability and subsequently elevated convective initiation. Extensive work is underway to fully understand processes controlling summer precipitation diurnal cycle in the central U.S. and explain why most models fail to simulate the NPM.