Sedimentation Effects in Subtropical Stratocumulus

Abstract

Stratocumulus clouds are climatically important because their widespread presence over subtropical oceans and their high albedo help to cool Earth's surface. Their structure and longevity are governed by turbulence dynamics, particularly the entrainment of warm, dry air from the free troposphere. Microphysical processes, such as droplet sedimentation, can modify these dynamics by reducing entrainment, though the magnitude of this effect remains uncertain. Representing these turbulence-entrainment interactions is particularly challenging due to the wide separation of length scales over which turbulence operates. This dissertation addresses this challenge by quantifying the effects of droplet sedimentation on turbulence and entrainment under varying environmental conditions, including those associated with climate change, using direct numerical simulations at meter-scale resolution that span the full boundary layer depth.

In my first study, I show that at a vertical grid spacing of 1.1 m (reference Reynolds number: Re=12500), sedimentation reduces the mean entrainment velocity by at least 20%. At this Reynolds number, sedimentation has a larger effect on entrainment than further increases in Reynolds number, suggesting that turbulence and sedimentation effects can be disentangled. Interestingly, turbulence intensity increases even as mean entrainment decreases. To reconcile this apparent contradiction, I use the flux-jump relation to decompose mean fluxes of the liquid-water static energy, showing that as sedimentation strength intensifies, the magnitude of the sedimentation flux grows faster than the turbulent flux, effectively compensating for the increase in turbulent flux. To explain the increase in turbulence intensity, I show that sedimentation enhances the contrast between descending dry, warm air in cloud holes and moist, cold air in cloud cores. This enhanced contrast intensifies evaporative cooling near cloud-hole edges, accelerates downdrafts, and redistributes moisture more evenly between the cloud and subcloud layers. These findings highlight the importance of resolving meter-scale turbulence in both cloud and subcloud layers to understand how moisture redistribution shapes turbulence organization.

In my second study, I explore how sedimentation interacts with low-cloud adjustment mechanisms in the context of warming-related environmental changes, specifically: (i) thermodynamic warming, (ii) reduced downwelling longwave radiation, and (iii) increased inversion strength. A moderate Reynolds number (Re=5000) simulation reproduces the sign and approximate magnitude of low-cloud responses reported in existing large-eddy simulation (LES) studies, demonstrating its potential as a turbulence-parameterization-free modeling tool. Sedimentation effects are comparable in magnitude to those of adjustment mechanisms, with the potential for amplifying and compensating interactions. The relative low-cloud thinning response is robust and independent of sedimentation strength, but the absolute cloud amount depends on sedimentation strength, indicating that droplet sedimentation is a non-negligible source of uncertainty in cloud feedbacks under warming scenarios.