Are Supercells Resistant to Entrainment because of Their Rotation?

This research investigates a hypothesis posed by Lilly (1986), which argued that the helical nature of the flow in supercell updrafts makes them more resistant to entrainment than nonsupercellular updrafts because of the suppressed turbulence in purely helical flows. He further supposed that this entrainment resistance contributes the the steadiness and longevity of supercell updrafts. A series of idealized large eddy simulations were run to address this idea, wherein the deep-layer shear and hodograph shape were varied, resulting in supercells in the strongly sheared runs, nonsupercells in the weakly sheared runs, and substantial low-to-mid level streamwise (crosswise) vorticity in runs with half-circle (straight) shaped hodographs. Fourier energy spectrum analyses show slopes of 5/3 within the inertial sub-ranges of all runs (e.g., consistent with Kolmogorov's law), which suggests that the percentages of streamwise and crosswise vorticity have little effect on turbulence in convective environments. Entrainment calculations and analyses of the distributions of passive tracers do indicate that supercells have smaller fractional entrainment rates than nonsupercells, but these differences are consistent with theoretical dependencies of entrainment on updraft width, and with supercells being wider than nonsupercell updrafts. Finally, a fourier spectrum analysis of entrainment suggests that entrainment in supercells is dominated by flow patterns that occur on the scale of the updraft itself (rather than by small-scale turbulence). It is concluded that, while supercells do experience reduced fractional entrainment rates and entrainment-driven dilution than nonsupercells, this "survival advantage" is entirely attributable to supercells having wider updrafts than ordinary convection, and has little to do with updraft rotation.