How buoyancy-driven circulations shape momentum transport within the convective boundary layer: insights from Doppler lidar

Convective momentum transport emerges from interactions between buoyancy-driven circulations and vertical wind shear and adds to shear-driven turbulence in the dry and cloudy convective boundary layer (CBL). The objective of this study is to use observations to diagnose buoyancy-driven circulatory structures that recur across diverse wind and cloud regimes in the dry CBL and to quantify their impact on vertical momentum flux profiles. We present two summer months of high-resolution Doppler wind lidar and cloud radar observations from Cabauw, the Netherlands, capturing three-dimensional wind profiles from 100 m to the boundary-layer top ($z_i$) at 50 m vertical and $\sim$30 s temporal resolution, including periods with shallow clouds. Horizontal wind variance spans timescales shorter and longer than 10 min, with no clear spectral separation between convective and mesoscale circulations. While mesoscale circulations contribute little to momentum flux when averaged over all days, they can substantially amplify flux on individual days, highlighting regime dependence. At convective scales ($<$10 min), momentum flux profiles exhibit pronounced variability, including downgradient and countergradient transport and frequent sign reversals with height. On average, the convective-scale momentum flux is negative near the surface and becomes small or even positive above $z/z_i \sim 0.2$, reflecting a transition from passive transport of slow-moving air to active circulations that can produce net positive flux when asymmetric or tilted. Days with persistent positive fluxes even at the surface are characterized by stronger vertical velocities, weaker near-surface winds, and higher turbulent kinetic energy anisotropy, and larger boundary-layer wind shear. These dynamics are indicative of buoyancy-driven cellular convection leading to reduced mixing efficiency, which can maintain vertical wind shear rather than homogenizing it. Cloud presence does not fundamentally alter these behaviors, though clouds preferentially form atop stronger updrafts and coincide with more pronounced mesoscale circulations.