Numerical Simulation of Mammatus

Abstract

Mammatus are hanging lobes on the underside of clouds. Although many different mechanisms have been proposed for their formation, none have been rigorously tested. In this study, three-dimensional numerical simulations of mammatus on a portion of a cumulonimbus cirruslike anvil are performed to explore some of the dynamic and microphysical factors that affect mammatus formation and evolution. Initial conditions for the simulations are derived from observed thermodynamic soundings. Five observed soundings are chosen-four were associated with visually observed mammatus and one was not. Initial microphysical conditions in the simulations are consistent with in situ observations of cumulonimbus anvil and mammatus. Mammatus form in the four model simulations initialized with the soundings for which mammatus were observed, whereas mammatus do not form in the model simulation initialized with the no-mammatus sounding. Characteristics of the modeled mammatus compare favorably to previously published mammatus observations. Three hypothesized formation mechanisms for mammatus are tested: cloud-base detrainment instability, fallout of hydrometeors from cloud base, and sublimation of ice hydrometeors below cloud base. For the parameters considered, cloud-base detrainment instability is a necessary, but not sufficient, condition for mammatus formation. Mammatus can form without fallout, but not without sublimation. All the observed soundings for which mammatus were observed feature a dry-adiabatic subcloud layer of varying depth with low relative humidity, which supports the importance of sublimation to mammatus formation.

MU Radar and Lidar Observations of Clear-Air Turbulence underneath Cirrus

“…It is mainly function of virtual temperature, water vapor, ice and snow aggregate mixing ratios, and latent heat of sublimation. Using an idealized distribution of ice crystal ratio of 1 g kg 21 and snow aggregate of 0.75 g kg 21 relevant in the saturated layer from the bottom of cirrus clouds (Kanak et al 2008), we found that the CDI criterion (du iy /dz , 0) can be satisfied at the cloud base where du/dz . 0.…”

“…CDI can thus be an alternative to subcloud sublimation mechanism and KH instability. Kanak et al (2008) showed CDI was a necessary condition for the mammatus formation.…”

“…A similar correspondence between mamma and vertical motions was reported using millimeter wavelength radars (e.g., the review by Schultz et al 2006). Kanak et al (2008) reviewed estimates of vertical velocities (of the hydrometeors with respect to the air motions) in mammatus lobe areas published until then. A negative correlation is usually found between vertical velocity and reflectivity: large downdrafts (22.0 to 26.0 m s 21 ) in mammatus lobe areas (associated with high reflectivities) and weak updrafts (0-1.5 m s 21 ) in the drier air penetrating within the clouds (usually associated with low reflectivities).…”

“…A negative correlation is usually found between vertical velocity and reflectivity: large downdrafts (22.0 to 26.0 m s 21 ) in mammatus lobe areas (associated with high reflectivities) and weak updrafts (0-1.5 m s 21 ) in the drier air penetrating within the clouds (usually associated with low reflectivities). Because few estimates for cirrus mamma are available, the values reviewed by Kanak et al (2008) are not necessarily representative of the vertical velocities in cirrus mammatus areas. Also, because of the small hydrometeor size in cirrus, fall velocities cannot likely exceed a few tens of centimeters per second (e.g., Schultz et al 2006).…”

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

“…Two soundings S1 and S2 were used to initialize the model simulations (these two were also used in Kanak et al, 2008). These soundings were taken from…”