Droplet Sedimentation Effects in Stratocumulus Clouds

Quantifying the effects of stratocumulus cloud feedbacks remains a key challenge, particularly due to the complex interactions between the boundary layer and the cloud top that occur at meter and submeter scales. Droplet sedimentation counteracts cloud-top entrainment by moving droplets away from the warm, dry free troposphere, thereby decreasing cloud-top evaporation and turbulence generation. Assessing how sedimentation influences entrainment and turbulence is essential for determining cloud lifetimes.

Previous works using large-eddy simulations (LES) with 5-10 m resolution demonstrated that sedimentation reduces the mean entrainment velocity (Ackerman 2004, Bretherton 2007, Hill 2009). However, the strength of this reduction remains uncertain because insufficient resolution introduces spurious upward fluxes that oppose the sedimentation flux. Local direct numerical simulations (DNS) studies, focused exclusively on the cloud layer at submeter-scale resolution, reported a reduction of the mean entrainment velocity due to sedimentation of up to 40%, or 3 times higher than LES results (de Lozar 2017, Schulz 2019). The question then remains: do these results also hold when considering the full vertical domain of the stratocumulus-topped boundary layer, spanning from the surface level to the free troposphere?

The novelty of this work lies in using DNS to simulate meter-scale processes at the cloud top, while encompassing the full vertical extent of the stratocumulus-topped boundary layer. We perform sensitivity experiments that involve changing the sedimentation strength and the Reynolds number. Consistent with previous studies, we find that sedimentation reduces the mean entrainment velocity by at least 20%, with the magnitude increasing for higher Reynolds numbers. Interestingly, the turbulence kinetic energy and the turbulent entrainment flux also increase with sedimentation.

To resolve this apparent contradiction, we quantify the mean fluxes of the liquid water static energy at the cloud-top region. Our results show that the magnitude of the sedimentation flux undergoes a more rapid growth than the turbulent flux with sedimentation, effectively compensating for the increase in the turbulent flux. Additionally, the contrast in the vertical velocity between updrafts and downdrafts in the subcloud layer becomes less extreme with sedimentation. The skewness in this region shifts from a predominantly positive profile to a more neutral one. This more balanced distribution of vertical motions results from increased liquid water availability for evaporation in the downdrafts, which in turn accelerates them. In summary, we show that sedimentation effects are as important as turbulent effects at meter-scale resolution. Moreover, sedimentation reshapes the vertical moisture distribution in both cloud and subcloud layers.