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David M. Loveless, Timothy J. Wagner, David D. Turner, Steven A. Ackerman, and Wayne F. Feltz

able to remove from the nocturnal boundary layer. This results in the layer with greatest instability being above the surface. One potential mechanism for initiating and maintaining elevated convection is atmospheric bores ( Parker 2008 ; French and Parker 2010 ). Bores are a type of gravity wave that form from the interaction of a density current with a stable fluid of lesser density. Bores will form in either a partially blocked system ( Rottman and Simpson 1989 ) or a completely blocked system

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Guo Lin, Coltin Grasmick, Bart Geerts, Zhien Wang, and Min Deng


This observational study documents the consequences of a collision between two converging shallow atmospheric boundaries over the central Great Plains on the evening of 7 June 2015. This study uses data from a profiling airborne Raman lidar (the Compact Raman Lidar, or CRL) and other airborne and ground-based data collected during the Plains Elevated Convection At Night (PECAN) field campaign to investigate the collision between a weak cold front and the outflow from a MCS. The collision between these boundaries led to the lofting of high-CAPE, low-CIN air, resulting in deep convection, as well as an undular bore. Both boundaries behaved as density currents prior to collision. Because the MCS outflow boundary was denser and less deep than the cold-frontal airmass, the bore propagated over the latter. This bore was tracked by the CRL for about three hours as it traveled north over the shallow cold-frontal surface and evolved into a soliton. This case study is unique by using the high temporal and spatial resolution of airborne Raman lidar measurements to describe the thermodynamic structure of interacting boundaries and a resulting bore.

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Stacey M. Hitchcock and Russ S. Schumacher

the heat sink in order to track the mass of the hypothetical downdraft as it interacts with the boundary layer. In the original 3D simulations, it is significantly more difficult to trace downdraft air in this way. Instead, tracer was placed in the lowest 1 km under the preface that since a density current transports mass, it should also replace mass. In other words, removal of tracer may act as a proxy for identifying convective outflow that does not present in typical surface cold-pool fashion

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Yun Lin, Jiwen Fan, Jong-Hoon Jeong, Yuwei Zhang, Cameron R. Homeyer, and Jingyu Wang

is currently coupled with the four-sector version of MOSAIC ( Fast et al. 2006 ; Zaveri et al. 2008 ). Besides improved representations of cloud microphysical processes compared with the original WRF-Chem with a two-moment bulk microphysics implementation, some aerosol processes are changed, including aerosol activation, resuspension, and in-cloud wet removal. With this model, both aerosol and cloud processes can be more realistically simulated, particularly under the condition of complicated

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Coltin Grasmick, Bart Geerts, David D. Turner, Zhien Wang, and T. M. Weckwerth

has no convective available potential energy (CAPE), or less CAPE than layers higher up, and the effective inflow layer feeding convective cells is elevated, this initiation is referred to as elevated CI ( Glickman 2000 ). Several observational studies have documented secondary CI along convective outflow boundaries of nocturnal MCSs. During the daytime, and often also at night, these boundaries behave as density currents (DCs; e.g., Weckwerth and Wakimoto 1992 ). These boundaries may evolve

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Kevin R. Haghi, Bart Geerts, Hristo G. Chipilski, Aaron Johnson, Samuel Degelia, David Imy, David B. Parsons, Rebecca D. Adams-Selin, David D. Turner, and Xuguang Wang

. First, bores are a challenge to observe in the field because the current network of vertical profilers is sparse. Second, surface observations, while relatively abundant, can only approximate the nature of a density current or bore/wave based on assumptions of hydrostatics. While current weather prediction models can supplement the lack of observations, the evolution of the bore in a numerical weather prediction model is very sensitive to the choice of the microphysics and boundary layer schemes

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Hristo G. Chipilski, Xuguang Wang, and David B. Parsons

1. Introduction Convectively generated outflow boundaries, such as density currents and bores, have an important contribution to the dynamics of mesoscale convective systems (MCSs). The theoretical importance of density currents is well established due to their critical role in the MCS evolution (e.g., Rotunno et al. 1988 ; Weisman and Rotunno 2004 ). On the other hand, atmospheric bores are still less familiar to the meteorological community, but these disturbances have received considerable

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Dana Mueller, Bart Geerts, Zhien Wang, Min Deng, and Coltin Grasmick

generally induced by a density current (e.g., a cold front or a thunderstorm outflow boundary) ( Koch et al. 1991 ). One main characteristic of bores is that they cause the stable boundary layer (SBL) to deepen and to remain deep following bore passage ( Koch et al. 2008b ). Thus, the hydrostatic pressure increases at the surface due to column cooling ( Koch et al. 1991 ). But unlike density currents, the passage of a bore does not result in any pronounced surface cooling. Surface warming may actually

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Tammy M. Weckwerth, John Hanesiak, James W. Wilson, Stanley B. Trier, Samuel K. Degelia, William A. Gallus Jr., Rita D. Roberts, and Xuguang Wang

/density current. An example from PECAN is shown at 0736 UTC 14 July 2015 ( Fig. 3e ). Pristine: An NCI event in this category occurred >100 km from any existing storms with no evidence of a boundary layer convergence zone, front, bore, density current, or LLJ that could have triggered the initiation. An example of pristine NCI during PECAN is shown at 0506 UTC 4 July 2015 ( Fig. 3f ). Frontal overrunning. Frontal overrunning, most commonly occurring with an LLJ, has been shown to initiate storms well

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David B. Parsons, Kevin R. Haghi, Kelton T. Halbert, Blake Elmer, and Junhong Wang

is treated as in a two-layer, two-dimensional, inviscid flow, similar to those studied by Long (1954) , Houghton and Kasahara (1968) , Baines (1984) , and Rottman and Simpson (1989) , with the lower layer representing the stable nocturnal boundary layer, while the second layer is neutral and infinitely deep. The flow regimes can be characterized by a Froude number (Fr) and a nondimensional height H , 1 given by Specifically, Fr was calculated from the ratio of the density current

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