Key parameters for droplet evaporation and mixing at the cloud edge

J Fries,G Sardina,G Svensson,B Mehlig

doi:10.1002/qj.4015

Abstract

AbstractThe 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.

Highlights

Clouds play a major role in regulating weather and climate on Earth, by modulating the incoming solar radiation
Kumar et al (2012; 2014) compute droplet size distributions with prominent exponential tails using direct numerical simulations (DNS)—some of them are seen in Figure 3—and connect these tails to corresponding exponential tails in the PDF of supersaturation at droplet positions
We derived a statistical model for evaporation and turbulent mixing at the cloud edge from first principles

Summary

Introduction

Clouds play a major role in regulating weather and climate on Earth, by modulating the incoming solar radiation. A key challenge is to understand how entrainment of dry air at the edges of ice-free clouds affects the size distribution and number density of droplets (Blyth, 1993). This is important because the amount and distribution of liquid water determine cloud optical properties (Kokhanovsky, 2004) and precipitation efficiency (Burnet and Brenguier, 2007). The optical properties of clouds are of crucial importance for the radiation balance of the Earth’s climate system (Dufresne and Bony, 2008; Caldwell et al, 2016). The size distribution and number density of droplets are key ingredients, because the light-extinction coefficient of the cloud is determined by the number of the droplets it contains times their average surface area (Kokhanovsky, 2004)

Methods

Results

Discussion

Conclusion