Entrainment in the Development and Rotating Stages of Supercell Thunderstorms

Entrainment, the process by which turbulent clouds introduce dry air from outside the cloud inward via overturning eddies at the cloud edge, can decrease the cloud buoyancy, and the water and/or ice mass it contains, limiting both cloud and precipitation development. Numerous studies have shown that growing cumulus clouds entrain air primarily as a result of the overturning thermal circulation near their tops, but have focused upon cumuli in environments with minimal vertical wind shear. Little attention has been given to investigating the entrainment into developing thunderstorms growing in environments with strong vertical wind shear, or to how rotating updrafts in some thunderstorms (i.e. supercells; produced in environments with specific characteristics of the vertical wind shear) might alter the amount of entrainment they experience.

In the current study, we use idealized, 3D, high-resolution numerical simulations of supercell thunderstorms to evaluate entrainment and its effects during the developing and rotating stages of the storms. Entrainment is quantified using an algorithm that first estimates the sub-grid scale edge of the 3D cloud core, defined with specific condensate and vertical velocity thresholds, and then calculates the mass flux into that core. As entrainment proceeds in time, we track the resulting dilution of the core condensate, and precipitation development (and fallout). Multiple realizations in the same storm environment are created by altering the storm forcing type (heat flux versus warm “bubble”) and the horizontal area over which the forcing is applied.

In comparison to past calculations our group has performed with smaller cumulus clouds, initial results show that the amount of entrainment into the developing thunderstorms is often as much as five times greater, as expected from the stronger updrafts. When the storm forcing is applied over a smaller horizontal area, stronger updrafts result that increase the entrainment. The resulting dilution of the liquid and ice mass within the core is also quicker with the narrower forcing, as a result of the greater entrainment. Analysis of entrainment during the rotating stage of the thunderstorms will also be presented, as will the effects on overall precipitation production, for all cases.