The Influence of the Rocky Mountains on the 13–14 April 1986 Severe Weather Outbreak. Part II: Evolution of a Prefrontal Bore and Its Role in Triggering a Squall Line

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  • 1 Laboratory for Atmospheres NASA/Goddard Space Flight Center, Greenbelt, Maryland
  • | 2 Department of Marine, Earth and Atmospheric Science, North Carolina Stage University, Raleigh North Carolina
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Abstract

In this paper, Part II of a series, the evolution of a prefrontal bore on the leeside of the Rockies and its subsequent propagation and initiation of convection farther downstream over eastern Colorado and western Nebraska are investigated. The observational evidence for this sequence of events was obtained from combined analyses of high-resolution GOES satellite imagery and Program for Regional Observing and Forecasting Services mesonetwork data over the Colorado region for the severe weather event that occurred during 13–14 April 1986. A 2D nonhydrostatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger convection.

Analysis of satellite imagery and mesonet data indicated that an internal bore (ahead of a cold front), a moderate downslope windstorm, and a quasi-stationary hydraulic jump were generated within a few hours along the Iceslope as a Pacific cold front and its attendant upper-level jet streak advanced over the Rockies. The bore and the cold front then propagated eastward for several hours and interacted with a Ice cyclone, a dryline, and a warm front, initiating severe weather over Nebraska and Kansas. Wave-ducting analysis showed that favorable wave-trapping mechanisms such as a capping inversion above a neutral layer and wind curvature from a low-level jet, which appeared to he the most dominant ducting mechanism, existed across eastern Colorado and western Nebraska to maintain the bore strength. Numerical simulations of continuously stratified shear flow specified from upstream and downstream soundings suggested that the creation of a density current along the Ice slopes, a downstream inversion height lower than the upstream inversion height, and a strong curvature in the wind profile of the low-level jet are all needed to initiate and sustain the integrity of the propagating bore.

Based on the synthesis of observational analyses and 2D nonhydrostatic model simulations, a schematic illustration of the time evolution of the bore ahead of the Pacific cold front, the hydraulic jump associated with a mountain wave, and the arctic air intrusion from the north to the Ice of the Rockies are presented in the context of severe weather occurrence over western Nebraska and Kansas.

Abstract

In this paper, Part II of a series, the evolution of a prefrontal bore on the leeside of the Rockies and its subsequent propagation and initiation of convection farther downstream over eastern Colorado and western Nebraska are investigated. The observational evidence for this sequence of events was obtained from combined analyses of high-resolution GOES satellite imagery and Program for Regional Observing and Forecasting Services mesonetwork data over the Colorado region for the severe weather event that occurred during 13–14 April 1986. A 2D nonhydrostatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger convection.

Analysis of satellite imagery and mesonet data indicated that an internal bore (ahead of a cold front), a moderate downslope windstorm, and a quasi-stationary hydraulic jump were generated within a few hours along the Iceslope as a Pacific cold front and its attendant upper-level jet streak advanced over the Rockies. The bore and the cold front then propagated eastward for several hours and interacted with a Ice cyclone, a dryline, and a warm front, initiating severe weather over Nebraska and Kansas. Wave-ducting analysis showed that favorable wave-trapping mechanisms such as a capping inversion above a neutral layer and wind curvature from a low-level jet, which appeared to he the most dominant ducting mechanism, existed across eastern Colorado and western Nebraska to maintain the bore strength. Numerical simulations of continuously stratified shear flow specified from upstream and downstream soundings suggested that the creation of a density current along the Ice slopes, a downstream inversion height lower than the upstream inversion height, and a strong curvature in the wind profile of the low-level jet are all needed to initiate and sustain the integrity of the propagating bore.

Based on the synthesis of observational analyses and 2D nonhydrostatic model simulations, a schematic illustration of the time evolution of the bore ahead of the Pacific cold front, the hydraulic jump associated with a mountain wave, and the arctic air intrusion from the north to the Ice of the Rockies are presented in the context of severe weather occurrence over western Nebraska and Kansas.

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