Multiple observations of the kinematic structure of a shallow bore in a low-shear environment

The initiation, evolution and late-mature structure of an unsteady, shallow bore observed over northern Alabama on 23 August 2013 is documented. Observations are derived from two scanning radars, two profiling systems (MIPS – Mobile Integrated Profiling System, and a Doppler Lidar), surface, and radiosonde data, are utilized to describe details of the bore structure and its environment. The Doppler lidar provided high-resolution vertical motion profiles at a location 24 km ENE of the MIPS. Single Doppler analyses from the scanning MAX radar, located 8 km north of the MIPS, are used to document the evolution and detailed vertical structure of the bore around the time of the detailed MIPS profiler observations. The bore was spawned by multiple thunderstorm outflow boundaries about 100 km north of the MIPS location. Surface pressure time series indicate that the mesoscale outflow boundary morphed into an incipient bore through the afternoon to evening transition about 30-60 min after sunset, during which time an episode of convective initiation events occurred along the bore. About 1-2 hr later, the weakening bore was sampled within the observational network. As the bore passed over the MIPS around 2130 LST, weak updrafts were sampled over the lowest several hundred meters, and aerosol backscatter showed a corresponding increase over the same height interval. Time-height sections of potential temperature and mixing ratio from the MPR both suggest a several hundred meter increase in the heights of isentropes and isohumes. The surface pressure increase of ~0.3 hPa at the MIPS, DWL, and ASOS locations is consistent with the shallow nature of the bore. In addition to the aforementioned fine scale measurements of the bore structure, other unique features include the following:

• The pre-bore surface-based nocturnal inversion was shallow (~200 m) and well defined.
• The time period of the pressure rise associated with the bore passage varied by a factor of 2, suggesting horizontal heterogeneity in bore structure.
• Additional complexities included the presence of longer period (10-20 min) waves located around the bore at both the MIPS and DWL locations. These were characterized by positive/negative vertical motion throughout and above the depth of the residual layer, peaking around a stable layer located near 1.8 km AGL.
• The wind shear was very weak (3 m s^-1 bulk wind difference over the 0-1 km layer), and the bore moved from the north at a ground-relative speed of ~6.5 m/s in the direction of, and much faster than the northerly low-level flow.
• Based on the radar fine line signature from the ARMOR, the bore persisted at least for another hour despite its shallow depth and unfavorable conditions for bore maintenance. Its total lifetime exceeded 5 hr.

This appears to be the shallowest bore documented by detailed radar and profiling measurements. These observations suggest that bores may persist in environments previously considered unfavorable for bore maintenance, and suggest that bores may be more common than previously realized.