Propagating nocturnal convection within a 7–day WRF–model simulation

An explicit (convection-permitting) version of the Weather Research and Forecast (WRF) model is used to simulate a one-week episode (3-10 July 2003) of rainfall coherence that began several days prior to the end of the BAMEX field phase. Eastward propagating rainfall events that exhibit temporal and spatial coherence over scales greater than 12h and 1000 km, respectively, were a common feature of the simulations and observations during this period. In this presentation we focus on the nocturnal phase of organized deep convection, which represents the major portion of the lifecycle of individual events of rainfall coherence during the one-week period.

Five separate heavy precipitation episodes occurred in both the observations and the simulation in which convection initiated during the late afternoon or early evening over the western Plains of the United States (~102-98W), matured over the central Plains (~96-92W) overnight, and weakened near sunrise over the midwest (~90-86W), occasionally regenerating farther east during the following afternoon. Both the simulated mesoconvective structure and the structure of the larger-scale environment have been been composited relative to the leading edge of the five linearly-oriented mesoscale convective systems (which are the primary constituents of the coherent precipitation episodes) during their separate initiation, mature, and weakening stages. The composites indicate that the initial convection is rooted in the planetary boundary layer, but becomes based above the stable boundary layer at night within a frontogenetic lower-tropospheric region over the central Plains, where deep moisture is confined to a narrow latitudinal corridor. The strong, lower-tropospheric zone supplies both the necessary vertical shear and CAPE to support intense, linearly-oriented propagating convection through the night. The convection remains elevated during the early morning, but weakens as it moves eastward toward less favorable environmental conditions.