Instability driven by settling and evaporation in a shear flow: a model for asperitas clouds

TL;DR

This study uses numerical simulations to explore how shear influences cloud formation, revealing a transition from mammatus to asperitas clouds and highlighting the importance of three-dimensional effects and shear levels.

Contribution

It introduces a new model demonstrating how shear induces asperitas clouds and shows the significance of three-dimensional effects in cloud instability.

Findings

Shear transforms mammatus-like clouds into asperitas clouds.

Intermediate shear levels are critical for asperitas formation.

Three-dimensional simulations reveal qualitative differences from 2D results.

Abstract

We study, by direct numerical simulations in two and three dimensions, the instability caused by the settling and evaporation of water droplets out of a cloudy layer saturated with vapour into a dry sub-cloud ambient, under conditions where mammatus clouds were shown to form by [1], but with the addition of background shear. We show that shear changes the type of cloud formation qualitatively, from mammatus-like to a newly identified cloud type called asperitas. Intermediate levels of shear are shown to be needed. Shear suppresses the growth of small-scale perturbations, giving rise to smooth, long-lasting structures, and smaller rates of mixing. Three-dimensionality is shown to make a qualitative difference, unlike in mammatus clouds. We also show that under non-cloud-like conditions, the instability can be very different.