Droplet dynamics and fine-scale structure in a shearless turbulent mixing layer with phase changes

Abstract

Three-dimensional direct numerical simulations of a shearless mixing layer in a small fraction of the cloud–clear air interface are performed to study the response of an ensemble of cloud water droplets to the turbulent entrainment of clear air into a cloud filament. The main goal of this work is to understand how mixing of cloudy and clear air evolves as turbulence and thermodynamics interact through phase changes, and how the cloud droplets respond. In the main simulation case, mixing proceeds between a higher level of turbulence in the cloudy filament and a lower level of turbulence in the clear air environment – the typical shearless mixing layer set-up. Fluid turbulence is driven solely by buoyancy, which incorporates feedbacks from the temperature, the vapour content and the liquid water content fields. Two different variations on the core set of shearless mixing layer simulations are discussed, a simulation in a larger domain and a simulation with the same turbulence level inside the filament and its environment. Overall, it is found that, as evaporation occurs for the droplets that enter subsaturated clear air regions, buoyancy comes to dominate the subsequent evolution of the mixing layer. The buoyancy feedback leads initially to downdraughts at the cloudy–clear air interface and to updraughts in the bulk regions. The strength of the turbulence after initial transients depends on the domain size, showing that the range of scales is an important parameter in the shearless mixing layer set-up. In contrast, the level of turbulence in the clear air is found to have little effect on the evolution of the mixing process. The distributions of cloud water droplet size, supersaturation at the droplet positions and vertical velocity are more sensitive to domain size than to the details of the turbulence profile, suggesting that the evolution of cloud microphysics is more sensitive to large-scale as opposed to small-scale properties of the flow.

Scale Dependence of Cloud Microphysical Response to Turbulent Entrainment and Mixing

“…The largest simulated volume, however, is over 4,000 times larger than the smallest volume, with the latter being typical of prior published DNS results. The large‐eddy times (calculated using , where k is the turbulence kinetic energy per unit mass) for the five simulations are T L = 1.75, 2.65, 4.08, 6.30, and 9.86 s. The phase relaxation time is calculated using , where = 0.0946 cm 2 /s (Götzfried et al, ). Prior to evaporation τ cloud ≈ 3.6 s, resulting in large‐eddy Damköhler numbers of D a L = 0.49, 0.74, 1.13, 1.75 and 2.74.…”

Investigation of Turbulent Entrainment‐Mixing Processes With a New Particle‐Resolved Direct Numerical Simulation Model

“…Second, like most previous DNS studies, this work investigates the turbulent entrainment‐mixing processes and their effects on cloud microphysics with a uniform level of fluid turbulence inside and outside the cloud. The more realistic configuration of stronger turbulence intensity inside the cloud slab, and a weaker turbulence intensity in the clear air region is worth pursuing (Götzfried et al, ; Tordella & Iovieno, ). Another important topic to be examined is the interaction between turbulence and cloud microphysics and the relative importance of turbulence and entrainment‐mixing processes in shaping droplet size distributions.…”

Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds

“…It can be determined by the Clausius-Clapeyron equation, which determines the change of e s with temperature T . Assuming constant latent heat L, e s is approximated as (Yau and Rogers, 1996;Götzfried et al, 2017)…”