Multi-Doppler and multi-platform analysis of convergent boundary zones during the planetary boundary layer's afternoon to evening transition (AET) period

A recently published study documented over 140 cases of the planetary boundary layer's afternoon to evening transition (AET) period with a wide array of observations, including surface, ground-based radar, and profiling radiometer, Doppler radar, and Doppler lidar measurements. Results indicated several characteristics of this time frame that have been deemed important for the initiation and/or maintenance of convection, namely a rise in near-surface water vapor and increases in both the horizontal wind speed and horizontal wind convergence above the surface layer. The waning and eventual loss of surface heating causes the decay of turbulent eddies and decreases vertical heat and momentum fluxes, which the characterization study shows can be seen in the decline of surface wind speeds and the acceleration of horizontal flow above the surface layer, among other signatures. Motivated by these results, the current effort examines pre-existing convergent boundary zones (CBZs) observed during the AET period in an attempt to address the hypotheses that existing CBZs exhibit an increase (or at a minimum, the lack of a decrease) in convergence above the surface layer due to AET processes, and also that in response to the decay of large PBL eddies, CBZs should tend to contain less along-line variability. Effectively, these types of CBZ behaviors would render the initiation or maintenance of convective elements along a CBZ more favorable. In this study, a meso-γ scale network of multiple ground-based Doppler radars (S-, C-, and X-band), several profiling instruments (including a 915 MHz wind profiler, microwave radiometer, vertically pointing Doppler radar and Doppler lidar), and surface observations are used to evaluate the structural evolution of CBZs during the AET period through several analysis approaches, including single- and dual-Doppler retrievals of the CBZ flow field, kinematic and thermodynamic profile analysis, and a comparison of vertical motion measurements made at varied wavelengths (33 cm, 3.2 cm, and 1.5 μm). Results for several AET period CBZ cases will be considered to evaluate the possibility of an AET-effect on convective activity and vigor during this time frame, with the aim of improving our understanding of how the physics of the transitional PBL might impact future considerations for mesoscale forecasting.