Observations of Boundary-Layer Convergence Lines and Associated Updrafts in the US Southern Great Plains

Boundary-layer convergence lines (CLs) are low-level lines or arcs of intense horizontal convergence that are associated with organized and long-lived planetary boundary-layer (PBL) updrafts. Observable by clear-air radar as narrow “fine lines” of enhanced reflectivity collocated with Doppler wind shifts, CLs are particularly effective at initiating deep convection in conditionally unstable flows. This finding suggests that the properties of CLs must differ from those of more widespread ordinary turbulent eddies in the unperturbed PBL. This study exploits long-term atmospheric observations at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) observatory in north Oklahoma to quantify CL properties and their relation to turbulent eddies in the unperturbed PBLs preceding them. Two independent methods for estimating CL properties, named the “radar” method and “surface” methods, are developed at the C-SAPR site and the SGP site respectively, using different combinations of instruments. Both methods rely on the assumption of a 2D CL circulation in the CL-normal plane. Mean daytime CL width (~2 km) and strength (~0.003 s^-1) are consistent for both methods, and mean daytime CL depth is around 0.75 km. The two methods disagree at night, with the surface method estimating wider and weaker CLs than the radar method. These difference may stem from the radar beam overshooting the shallow, highly stable nocturnal PBL. Although the width and depth of CL circulations are found to broadly scale with the PBL depth, the widest CL-related updrafts are much wider and stronger than the corresponding turbulent updrafts in the pre-CL flow. Consistently, turbulence length scales also notably increase during CL passage. Altogether, these findings suggest that the main difference between CL and PBL eddies lies in their updraft characteristics. The more coherent, wider, and stronger updrafts associated with CLs may explain their particularly high effectiveness at initiating deep convection.