What Makes Boundary Layer Convergence Lines So Effective at Deep-Convection Initiation?

Abstract Boundary layer convergence lines (CLs) tend to be far more effective than ordinary boundary layer thermals at deep-convection initiation (DCI). Recent observations indicate that CL subcloud updrafts are slightly wider, deeper, and stronger than boundary layer thermals, which likely contributes to this trend but may not fully explain it. To determine the underlying physical mechanism(s), idealized large-eddy simulations are conducted of CLs propagating through turbulent, conditionally unstable flows. Two common CL types are simulated, one driven by surface differential heating (DH) and the other by a cold pool (CP). Consistent with observations, both CLs produce deeper and stronger moist convection, with wider and stronger subcloud updrafts, than the undisturbed “pre-CL” flow well ahead of the CL. However, despite their enhanced widths, the CL clouds exhibit similar dilution rates as the narrower pre-CL clouds, likely owing to enhanced turbulence in the denser CL cloud field. Nevertheless, CL clouds benefit from the superposition of subcloud turbulent and mesoscale CL updrafts, which enhances their widths and upward mass fluxes, rendering them better equipped to withstand cloud-layer detrainment. Moreover, the DH CL subcloud updrafts are moister than corresponding pre-CL updrafts because they predominantly lift the cooler, moister airstream behind the CL. The CP CL updrafts, by contrast, ascend through a moister midlevel environment that reduces the drying effects of entrainment due to lifting by both dynamic pressure forcing and a mesoscale gravity wave forced by the CL updraft. The resulting midlevel cloud buoyancy enhancements weaken detrainment and invigorate the CL-forced convection. Significance Statement Deep convection is more effectively initiated by boundary layer convergence lines (CLs) than by more widespread boundary layer turbulence. However, the underlying physical mechanisms that make CLs so effective at deep-convection initiation (DCI) are inadequately understood and require further investigation. To this end, idealized large-eddy simulations are conducted of DCI processes forced by two common CL types: one driven by surface differential heating and the other driven by cold pools. Both CLs propagate into turbulent convective boundary layers, generating deeper, wider, and more densely packed convection compared to that initiated by ordinary boundary layer turbulence. These deeper CL clouds benefit from enhanced cloud-base mass fluxes, greater cloud-core buoyancies, and lower cumulus detrainment rates.