The influence of transient weather systems on warm-season midlatitude precipitation in convection-permitting models

In this study we examine the role of meso-synoptic weather systems on the diurnal cycle of midlatitude warm-season precipitation in convection-permitting models. We focus on a 12-day period (20-31 July 1998) during which an observed latitudinal corridor of heavy precipitation was present over the central United States. Both a 12-day control simulation and an idealized experiment, which uses an initial condition and diurnally varying lateral boundary conditions that are an average of the 12-day period, are discussed. The idealized experiment is designed to mitigate effects of transient features such as mobile cold fronts and midtropospheric shortwaves. Not surprisingly such features are found to enhance extreme precipitation events during the period. However, their overall effect is to reduce both local precipitation frequency and total precipitation amounts. In this sense, their role on helping to suppress precipitation for up to several days in their wake dominates their influence on precipitation enhancement in individual events, and has important possible implications for the regional water cycle. Supporting data illustrating the role of shortwaves on the diurnal cycle of propagating precipitation will also be discussed.