Research Highlight: New Insights into Entrainment in Supercell Thunderstorms

Date

09/03/21

The study was conducted by Professor Sonia Lasher-Trapp, current graduate student Enoch Jo, and recent alumni Luke Allen and Bryan Engelsen.  It identifies and quantifies various mechanisms of entrainment, and their diluting effects, in the developing and mature stages of a simulated supercell thunderstorm. The two stages, differentiated by the lack or presence of a rotating updraft, are shown to entrain air by different, but related mechanisms that result from the strong vertical wind shear of the environment. The greatest entrainment rates in the developing stage result from the asymmetric overturning of large eddies near cloud top on the down shear side. These rates are greater than those published in the literature for cumuli developing in environments lacking strong shear. Although the entrainment rate increases exponentially in time throughout the developing stage, successive cloud turrets help to replenish some of the lost buoyancy and condensate, allowing the nascent storm to develop further. During the mature stage, the greatest entrainment rates occur via “ribbons” of horizontal vorticity wrapping around the rotating updraft that ascend in time. The smaller width of the ribbons in comparison to the wider storm core limits their dilutive effects. Passive tracers placed in the low-level air ingested by the mature storm indicate that on average 20% of the core contains some undiluted air from below the storm base, unaffected by any entrainment mechanism.

There’s an animation posted at https://slasher.web.illinois.edu.

This article was published in the AMS Journal of Atmospheric Sciences. To see the rest of this article, please visit https://doi.org/10.1175/JAS-D-20-0223.1.