The Mysteries of Mammatus Clouds: Observations and Formation Mechanisms

Abstract

Mammatus clouds are an intriguing enigma of atmospheric fluid dynamics and cloud physics. Most commonly observed on the underside of cumulonimbus anvils, mammatus also occur on the underside of cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus, as well as in contrails from jet aircraft and pyrocumulus ash clouds from volcanic eruptions. Despite their aesthetic appearance, mammatus have been the subject of few quantitative research studies. Observations of mammatus have been obtained largely through serendipitous opportunities with a single observing system (e.g., aircraft penetrations, visual observations, lidar, radar) or tangential observations from field programs with other objectives. Theories describing mammatus remain untested, as adequate measurements for validation do not exist because of the small distance scales and short time scales of mammatus. Modeling studies of mammatus are virtually nonexistent. As a result, relatively little is known about the environment, formation mechanisms, properties, microphysics, and dynamics of mammatus.This paper presents a review of mammatus clouds that addresses these mysteries. Previous observations of mammatus and proposed formation mechanisms are discussed. These hypothesized mechanisms are anvil subsidence, subcloud evaporation/sublimation, melting, hydrometeor fallout, cloud-base detrainment instability, radiative effects, gravity waves, Kelvin-Helmholtz instability, Rayleigh-Taylor instability, and Rayleigh-Bénard-like convection. Other issues addressed in this paper include whether mammatus are composed of ice or liquid water hydrometeors, why mammatus are smooth, what controls the temporal and spatial scales and organization of individual mammatus lobes, and what are the properties of volcanic ash clouds that produce mammatus? The similarities and differences between mammatus, virga, stalactites, and reticular clouds are also discussed. Finally, because much still remains to be learned, research opportunities are described for using mammatus as a window into the microphysical, turbulent, and dynamical processes occurring on the underside of clouds.

Effects of linear, ambient wind shear on simulated mammatus‐like clouds

“…Such behavior could be consistent with the KH-type mammatus (e.g. Clarke, 1962;Stith, 1995;Schultz et al, 2006).…”

A review of volcanic ash aggregation

“…Ash-hydrometeor growth combined with cloud base sublimation results in mammatus cloud formation ( Fig. 6; Schultz et al, 2006). Aggregation efficiency increases in the presence of liquid water (Lawson et al, 1998) which allows fine ash aggregates to rapidly grow and sediment en masse.…”

Deposition and immersion-mode nucleation of ice by three distinct samples of volcanic ash

“…Fine volcanic ash can stay suspended in the atmosphere for 24 h and travel 100s to 1000s of km (Brown et al, 2012). Previous work has shown that, while initial plumes contain large concentrations of water, volcanic clouds can dry out markedly within hours of entering the atmosphere (Schultz et al, 2006). Further, very fine ash can stay suspended much longer than 24 h and ash fall deposits may remain in local environments for years to decades and can be re-suspended due to human activity (Horwell and Baxter, 2006).…”