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Department of Atmospheric Science

Fri., July 22, 2:00, EN1045

Vertical Structure and Evolution of Bores Observed During PECAN

Dana Mueller

University of Wyoming


This study documents the evolution of an impressive undular bore triggered by an MCS-generated density current on 20 June 2015, observed as part of the Plains Elevated Convection at Night (PECAN) experiment. The University of Wyoming King Air with profiling nadir- and zenith-viewing lidars sampled the bore from the time the first wave emerged from the nocturnal convective cold pool and where updrafts over 10 m s-1 were encountered, through the early dissipative stage in which the leading wave began to lose amplitude and speed. Throughout the bore’s life cycle the second wave had the highest amplitude. Striking roll clouds formed in wave crests and wave energy was detected to about 4.5 km AGL.

Phase relationships observed at flight level are different from those at the surface. In particular, enhanced front-to-rear winds, cooler air, and negative pressure perturbations (p’) prevailed in crests. The latter probably are non-hydrostatic, caused by strong horizontal vorticity. Flight level winds above the bore fluid persistently are front-to-rear. Surface measurements made late in the bore’s life cycle, just after sunrise, indicate four waves with a northerly wind direction in crests and a maximum p’ = +2 mb. Ground-based profiling measurements show mixing and moistening of the boundary layer. A wave trapping analysis indicates a negative Scorer parameter region due to flow reversal at mid-levels, providing significant curvature above the stable boundary layer. The observed bore strength of 2.4-2.9 and speed of 15-16 m s-1 agree well with values predicted from hydraulic theory.

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