12B.2 Evolution and Vertical Structure of the 20 June 2015 Bore Observed during PECAN

Thursday, 23 June 2016: 10:45 AM
Bryce (Sheraton Salt Lake City Hotel)
Dana M. Mueller, University of Wyoming, Laramie, WY; and B. Geerts, M. Deng, and Z. Wang

            Bores and accompanying solitary waves that emanated from MCS and squall line cold pools were observed during the Plains Elevated Convection at Night (PECAN) field campaign on June 20, July 11, and July 15, 2015. This study focuses primarily on the University of Wyoming King Air (UWKA) lidar and in-situ observations with supporting data from other mobile and fixed platforms. The UWKA employed two 355 nm wavelength lidars; the Wyoming Cloud Lidar (WCL) pointed upward while the Compact Raman Lidar (CRL) surveyed the boundary layer. The aircraft was in a unique position to capture the true evolution of the bore-soliton systems as opposed to a blend of distance and time evolution. These profile snapshots at different lifecycle stages show how the bore advanced out ahead of the parent thunderstorm density current, the subsequent addition of trailing solitary waves, and the final turbulent dissipation of the wave trains. The altered structure of the nocturnal boundary layer from the bore passages is evident in the lidar imagery (and other surface profiling instruments) as the top of the surface moist layer became elevated. The environmental stability and wave trapping ability were also analyzed. Wave characteristics and environmental observations were then compared to values calculated from hydraulic bore theory and solitary wave theory.

            The June 20 wave train occurred in Nebraska late in the night when a stable inversion was present. Two to three waves were sampled during 18 flight legs. This bore-soliton system was characterized by substantial roll clouds at wave crests, updrafts around 8 to 9 m s-1, and a maximum crest-to-trough amplitude of nearly 1.75 km. The initial stage of the phenomenon was captured well in this case with the UWKA first sampling a wave atop the cold pool (where a 6 oC drop in potential temperature was recorded.) The leading wave then propagated out ahead of the cold pool and temperature perturbations were significantly lower at flight level as cooling was then due to adiabatic ascent instead of advection. The leading wave was followed by a second wave still attached to the cold pool. For the next twenty minutes, the waves propagated ahead of the cold pool with approximately the same amplitude and updraft magnitudes. As the evolution progressed, the wavelength of the wave train increased, the leading wave amplitude decreased, but the second wave amplitude actually increased. During a flight track very close to the active MCS, the UWKA observed three waves where the second wave continued to have the highest amplitude. A surface tower and ground-based profiling instruments captured the feature a couple hours later, recording a 10 oC surface temperature inversion, a wind shift and pressure oscillations associated with the wave train passage. Water vapor layers were lofted over a kilometer due to the bore.

            The July 11 bore was a lower amplitude phenomenon observed in northern Kansas as a result of a small MCS. Again the evolution of the wave train was sampled, but this case highlights the decay stage. Initial flight legs revealed a deepening moist layer behind two waves. Initially, these waves were not accompanied by roll clouds. However, as the UWKA moved closer to the propagating MCS, the wave train appeared different with cloud-topped waves below flight level and wave signals in aerosol layers above flight level. The waves did not display vertical tilt with height, indicating trapping of wave energy. A mobile mesonet in the vicinity of the flight track reported a wind shift from southeasterly to northerly winds behind the bore and a surface pressure increase of approximately 2 mb. As the wave train decayed, the waves began to flatten, a continuous cloud deck formed aloft, and the waves atop the surface moist layer became turbulent. The sustained elevation of that layer provides evidence that the feature was a bore.

            The final wave train on July 15 triggered new convective cells ahead of a squall line. Despite its ability to initiate convection, this bore had lower amplitude waves than the June 20 bore and updrafts only reached around 2 m s-1 while pressure perturbations stayed below 1 mb. A very moist boundary layer was present this night and the waves were associated with cloud. During the decay stage of this bore-soliton system, the leading wave actually completely disappeared and waves were observed to tilt slightly with height.

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