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

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 ϳ20 m s Ϫ1 , 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 ϳ2-km-deep layer of high-e air helps sustain convection during its period of greatest organization overnight. Trajectory analysis for individual episodes reveals that the high-e 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.

The propagation and diurnal cycles of deep convection in northern tropical Africa

“…The development of organized, propagating convection with moderate or greater low-mid-level shear is consistent with theoretical studies (e.g. Weisman and Klemp, 1982;Rotunno et al, 1988), observations (Laing and Fritsch, 2000;Redelsperger et al, 2002) and numerical simulations (Lafore and Moncrieff, 1989;Trier et al, 2006). Future study will investigate other environmental influences, such as the role of the Saharan Air Layer (SAL), in modulating organized convection.…”

Comparison of the diurnal variations of warm-season precipitation for East Asia vs. North America downstream of the Tibetan Plateau vs. the Rocky Mountains

“…Carbone et al (2002) used a radar-based data set to demonstrate the delayed-phase diurnal signal of the precipitation from the western cordillera, and the zonal phasespeed range of rain streaks (∼ 1000 km and 20 h) propagating eastward from the eastern edge of the Rocky Mountains is between 7 and 30 ms −1 , with a median phase speed of 14 ms −1 . High-resolution convection-permitting simulations also show similar eastward propagation of precipitation over the US (Trier et al, 2006). Over the "three-step" terrain in East Asia, the summer precipitation on the TP maximizes in the afternoon to early evening, and nocturnal rainfall occurs over the Sichuan Basin (SCB) areas and precipitation phase delays over the low-lying plains (Tao, 1980;Ninomiya, 2000;Asai et al, 1998;Wang et al, 2004Wang et al, , 2005Yasunari and Miwa, 2006;Bao et al, 2011;Zipser, 2009, 2011;Sun et al, 2010;Yu et al, 2007Yu et al, , 2009).…”

“…Previous studies have noted that warm-season convection in the northern hemisphere often forms over the high mountains during the afternoon and subsequently propagates eastward across the lee-side plains overnight due to diurnal changes in diabatic heating between regions with different surface topography (Ding, 1993;Geng and Yamada, 2007;Carbone et al, 2002, hereafter, CAR02;Carbone and Turtle, 2008;Trier et al, 2006Trier et al, , 2010Wang et al, 2004Wang et al, , 2005He and Zhang, 2010, hereafter HZ10;Huang et al, 2010;Bao et al, 2011, hereafter BZS11). The terrains of both East Asia and North America feature a western plateau (mountainous region) and an eastern low-lying plain.…”

Sensitivity of WRF cloud microphysics to simulations of a severe thunderstorm event over Southeast India

“…With the growing computing power, it is anticipated that the horizontal grid space in operational NWP models, perhaps even global models will be a few kilometers within a decade (Liu and Moncrieff, 2007). The high-resolution 3-6 km grid spacing simulations of precipitation using convection allowing configuration showed good promise (Done et al, 2004;Liu et al, 2006;Kain et al, 2006Kain et al, , 2008Moncrieff and Liu, 2006;Trier et al, 2006;Lean et al, 2008). There are several reasons for better performance by the high-resolution models.…”