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

It is known that cloud droplet sedimentation affects the development of stratocumulus-topped boundary layers by reducing entrainment. However, previous studies mainly focused on timescales below 6h covering the early stratocumulus stages, while later timescales remain largely unexplored. This study targets the impact of sedimentation on subtropical stratocumulus evolution in the context of the stratocumulus to cumulus transition (SCT) in the Northeast Pacific. To this end, we perform 48h long large-eddy simulations of 10 transects from the Marine ARM GPCI Investigation of Clouds ship campaign, capturing the full deepening phase prior to cloud breakup. 
In all cases of active droplet sedimentation, the previously reported reduction in entrainment is confirmed in the initial hours. However, the effects observed in the later stages differ depending on the cloud's liquid water path (LWP). The expected result of weaker boundary layer growth only continues to occur in the more frequent precipitating, high-LWP cases, whereas the opposite occurs in non-precipitating, low-LWP cases. Here, the initial effect is reversed and the cloud exhibits stronger entrainment, that can result in deeper boundary layers. The underlying reason is that low-LWP clouds are radiatively unsaturated, allowing the LWP increase associated with the initial reduction in entrainment to trigger a feedback chain, which amplifies LWP, longwave cooling and turbulence in the boundary layer. This counteracts the impact of the sedimenting droplets and ultimately yields increased entrainment. Previous process studies on droplet sedimentation have studied the low-LWP regime, where we actually find an opposite response over long time periods. Nonetheless, our results confirm the interpretation of the droplet sedimentation feedback in numerous aerosol-cloud interaction studies as these are applicable to the high-LWP regime where we show that also over long time scales droplet sedimentation decreases boundary layer deepening. Despite the substantial influence on boundary layer growth, the timing of cloud breakup remains largely unchanged across the transition.