Mammatus cloud formation by settling and evaporation

S Ravichandran,Rama Govindarajan,Eckart Meiburg

doi:10.1017/jfm.2020.439

Abstract

Highlights

Mammatus clouds are a fascinating meteorological phenomenon
The necessary condition we find for our instability-driven mechanism is in broad agreement with Kanak, Straka & Schultz (2008), who find from numerical simulations that the evaporation/sublimation of water droplets is crucial to the formation of mammatus clouds
Root of the imposed disturbance wavelength λ: In contrast, for the settling-driven instabilities we report here, both linear stability results and nonlinear simulations suggest that (a) the wavelength of the instability increases with the settling velocity; (b) the growth rates of the instabilities decrease with increasing settling velocity; and (c) the structures that the light and heavy fluids take as they penetrate one another are asymmetric, even in the Boussinesq limit; the fluid in the density overhang which is heavier than the dry ambient air forms bubbles, while the dry air penetrates the density overhang in spikes

Summary

Introduction

Mammatus clouds are a fascinating meteorological phenomenon Deriving their name from mamma, the Latin for ‘breast’, mammatus clouds are pendulous blobs, typically found hanging underneath cumulonimbus anvils (a photograph is reproduced in figure 1 and a schematic shown in figure 2a). A promising explanation is the fallout of ice particles or water droplets from the cumulus anvil into the subsaturated air below, suggested first almost a hundred years ago (Troeger 1921, cited by Schultz et al 2006), followed by the sublimation/evaporation of these hydrometeors This causes the layer of air just below the cloud (the ‘sub-cloud layer’) to become denser than the dry air below it. We find that the size of the water droplets matters more than the total amount of liquid water

Objectives

Findings

Discussion

Conclusion