Composite Analysis of Bores Observed by Ground-Based Profilers during PECAN

Atmospheric bores are a type of gravity wave that form when a density current interacts with a stable boundary layer. Bores are thought to aid in nocturnal convective initiation by changing boundary layer stability via turbulent mixing and by mechanically lifting air parcels from the stable boundary layer up to the level of free convection. Because of their importance to nocturnal convective initiation, it is important to understand the thermodynamic and dynamic effects that bore passages may have on the stable boundary layer.

The Plains Elevated Convection At Night (PECAN) campaign took place from 1 June to 15 July 2015 in the Central Plains of the United States with the goal of improving the understanding and forecast accuracy of nocturnal convection. Using a network of fixed profiling sites and deploying a network of mobile vehicles, the campaign focused observations on nocturnal convective initiation, boundary layer waves, low-level jets, and mesoscale convective systems. Three fixed profiling sites and two mobile profiling systems were equipped with an Atmospheric Emitted Radiance Interferometer (AERI) and a Doppler Lidar. AERI measures downwelling infrared radiation from 520 to 3000 cm^-1 at a spectral resolution of about 1 cm^-1, from which high-temporal resolution temperature and moisture profiles can be retrieved. The Doppler Lidar uses a 1.5 micron laser to measure three-dimensional wind vectors in the boundary layer. Thus, each fixed profiling site and mobile profiling system is able to retrieve thermodynamic and kinematic variables of the boundary layer.

Observations from these five observing platforms during the PECAN campaign are used to assess the evolution of the boundary layer during bore passages. Given the near-continuous thermodynamic and kinematic profiling, high-temporal calculations of severe indices are possible. A composite bore analysis using data from all observing sites is used to identify characteristic changes of the boundary layer stability in the pre-and post-bore environment. This analysis will help characterize changes to the boundary layer induced by bore passages and provide insight into the role of bores in the convective initiation process. Preliminary results of composite potential temperature retrievals reveal quasi-permanent parcel displacements of 500 m in the vertical. The composite analysis still displays the bore structure, despite the averaging, giving confidence to the methodology. Additionally, a decrease in the level of free convection at the time of the bore has been observed in the composite, which supports the hypothesis that bores aid in the convective initiation process.