Key parameters for droplet evaporation and mixing at the cloud edge

Abstract

The distribution of liquid water in ice-free clouds determines their radiative properties, a significant source of uncertainty in weather and climate models.Evaporation and turbulent mixing cause a cloud to display large variations in droplet number density, but quite small variations in droplet size (Beals et al., Science, 2015, vol. 350, pp. 87-90). However, direct numerical simulations of the joint effect of evaporation and mixing near the cloud edge predict quite different behaviours, and how to reconcile these results with the experimental findings remains an open question. To infer the history of mixing and evaporation from observational snapshots of droplets in clouds is challenging, because clouds are transient systems. We formulated a statistical model that provides a reliable description of the evaporation-mixing process as seen in direct numerical simulations and allows us to infer important aspects of the history of observed droplet populations, highlighting the key mechanisms at work and explaining the differences between observations and simulations. K E Y W O R D S cloud microphysics, droplet evaporation, Lagrangian droplet dynamics, turbulent mixingThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Connecting finite-time Lyapunov exponents with supersaturation and droplet dynamics in the bulk of a turbulent cloud

“…The inhomogeneous mixing in clouds causes also rapid changes of vapor and liquid water contents [16,25,30]; this has been also found recently in field measurements, e.g. in [31].…”

“…including condensation rate as a loss term, and (3) liquid water mixing ratio q l (x, t) including condensation rate as a gain term. The models of Sardina et al [10] and Fries et al [16] simplify this complex dynamics in three aspects. First, the liquid water content is represented by an ensemble of individual spherical point-like cloud water droplets.…”

“…This assumption is justified when the droplet radii remain small and the droplet number density n ∼ 100 cm −3 as in our study. The Eulerian equations of motion are then given by [10,16,17], see also [8] ∇ • u = 0, (3a)…”

“…In the appendix, we provide the detailed derivation of eq. (3c) for completeness, see also [10,16,32].…”

“…This simplification consists of a summary of the balance equations for the Eulerian fields of vapor mixing ratio and temperature, q v (x, t) and T (x, t), to an effective balance equation for the scalar supersaturation field, following refs. [10,16].…”