Abstract
Abstract Observations obtained from scanning radars, two profiling systems, surface, and radiosonde instruments are utilized to describe the weakening stages of a long-lived (>4 h), shallow bore that evolved within a low-shear environment on 23 August 2013 over northern Alabama. RHI scans from a scanning X-band radar provided details on the bore shape and surrounding mesoscale perturbations in airflow, while a Doppler lidar afforded high-resolution vertical motion profiles at a location 24 km away. During passage over both profiling sites, updrafts (~2 m s−1) were sampled over the lowest several hundred meters AGL, and aerosol backscatter showed upward displacements of 300-400 m near the 1.2 km AGL level. The surface pressure rise of ~0.3 hPa at multiple locations over the observational network corroborates the shallowness of both the bore and the surface-based inversion. Several other unique features were documented, including bore movement in the direction of the weak low-level flow; a gust front structure with a well-defined feeder flow trailing the bore gust front, both confined to the lowest 500 m AGL; the presence of waves preceding and following the bore disturbance, with a 12-21 min period within the 0.5-2.5 km layer above the bore at both profiler locations; and variations in horizontal flow in the form of a weak jet near 1.4 km AGL, with winds in the jet aligned in the same direction as bore movement, and located more than 10 km ahead of the bore gust front.
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