Kinematic structure of a shallow bore in a low-shear environment

The initiation, evolution and structure of an unsteady, shallow bore observed over northern Alabama on 23 August 2013 during the Atmospheric Boundary Identification and Delineation Experiment (ABIDE) is documented. Observations are derived from WSR-88D radars, a short baseline (18 km) dual Doppler radar network utilizing the Mobile Alabama X-band dual polarization radar and the C-band ARMOR dual polarization radar, comprehensive observations from the Mobile Integrated Profiling System (MIPS, including a 915 MHz Doppler wind profiler (915), X-band Profiling Radar (XPR), 12-channel microwave profiling radiometer (MPR), and a Vaisla CL-31 lidar ceilometer), pre-bore balloon soundings, and surface data. Dual Doppler analyses are used to document the evolution 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. Based on surface pressure time series, the mesoscale outflow boundary appeared to morph into an incipient bore through the afternoon to evening transition 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 ABIDE network. As the bore passed over the MIPS around 2130 LST, weak updrafts were sampled over the lowest several hundred meters, and aerosol backscatter from the ceilometer 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 0.3-0.4 hPa at the MIPS location 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. • Additional complexities included the presence of longer period (10-15 min) waves located around the bore (but independent of the bore), which were characterized by positive/negative vertical motion throughout and above the depth of the residual layer, peaking near a stable layer located at 1.8 km AGL. • The wind shear was very weak, and the bore moved at a ground-relative speed of ~8 m/s in the direction of the 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 most shallow 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.