Opposing entrainment effects of cloud droplet sedimentation during the pre-breakup stage of the stratocumulus to cumulus transition

Cloud droplet sedimentation is known to influence the evolution of the stratocumulus-topped boundary layer by reducing entrainment. Although this mechanism is well studied regarding the early evolution of stratocumuli, its sustained effects over longer time scales remain largely unexplored. Here, we use large-eddy simulations to investigate how sedimentation influences stratocumulus development in the context of the stratocumulus to cumulus transition. We conduct 48 h long simulations of 10 transects in the Northeast Pacific, covering the full deepening stage before cloud breakup. All sedimentation cases show the previously reported initial reduction in entrainment, whereas the later stages reveal different effects depending on the cloud's liquid water path (LWP). While the more frequent precipitating, high-LWP (LWP >50gm-2) cases continue to exhibit weaker entrainment, the non-precipitating, low-LWP (LWP ≤50gm-2) cases reverse the initial effect and show stronger entrainment. In those radiatively unsaturated low-LWP clouds, the increase in LWP due to the initial entrainment reduction initiates a feedback chain that amplifies LWP, longwave cooling, and turbulent circulations in the boundary layer, ultimately leading to increased entrainment. Initial studies showed that droplet sedimentation reduces entrainment in short (≤6 h) simulations of low-LWP clouds, which has been extrapolated in the literature to all stratocumuli on much longer time scales. Our results suggest that this extrapolation is indeed correct in common high-LWP clouds, although it had previously been inferred from the rare low-LWP regime, where the opposite is found. Meanwhile, we find that cloud breakup remains largely unaffected across the transition.