Mechanisms Supporting Long-Lived Episodes of Propagating Nocturnal Convection within a 7-Day WRF Model Simulation

Abstract

Abstract A large-domain explicit convection simulation is used to investigate the life cycle of nocturnal convection for a one-week period of successive zonally propagating heavy precipitation episodes occurring over the central United States. Similar to climatological studies of phase-coherent warm-season convection, the longest-lived precipitation episodes initiate during the late afternoon over the western Great Plains (105°–100°W), reach their greatest intensity at night over the central Great Plains (100°–95°W), and typically weaken around or slightly after sunrise over the Midwest (95°–85°W). The longest-lived episodes exhibit average zonal phase speeds of ∼20 m s−1, consistent with radar observations during the period. Composite analysis of the life cycle of five long-lived nocturnal precipitation episodes indicates that convection both develops and then propagates eastward along an east–west-oriented lower-tropospheric frontal zone. An elevated ∼2-km-deep layer of high-θe air helps sustain convection during its period of greatest organization overnight. Trajectory analysis for individual episodes reveals that the high-θe air originates both from within the frontal zone and to its south where, in this latter case, it is transported northward by the nocturnal low-level jet (LLJ). The mature (nocturnal) stage composite evinces a thermally direct cross-frontal circulation, within which the trajectories ascend 0.5–2 km to produce the elevated conditionally unstable layer. This transverse vertical circulation is forced by deformation frontogenesis, which itself is supported by the intensification of the nocturnal LLJ. The frontal zone also provides an environment of strong vertical shear, which helps organize the zonally propagating component of convection. Overnight the convection exhibits squall-line characteristics, where its phase speed is typically consistent with that which arises from deep convectively induced buoyancy perturbations combined with the opposing environmental surface flow. In a large majority of cases convection weakens as it reaches the Midwest around sunrise, where environmental thermodynamic stability is greater, and environmental vertical shear, frontogenesis, and vertical motions are weaker than those located farther west overnight.