Low-Level Mesocyclonic Concentration by Nonaxisymmetric Transport. Part II: Vorticity Dynamics

Abstract

An idealized supercell simulation using the Regional Atmospheric Modeling System (RAMS) produced an elongated low-level mesocyclone that subsequently collapsed into a concentrated vortex. Though vorticity continually increased in the mesocyclone due to horizontal convergence, the collapse phase was additionally characterized by rapidly decreasing pressure, closed streamlines, and the creation of a compact vorticity center isolated from the remaining vorticity. It was shown in Part I of this study that the concentration phase was not initiated by an increase in horizontal convergence, suggesting that the proximate cause resided elsewhere.In this study, the vortex concentration in Part I is examined from a vorticity dynamics perspective. It is shown that concentration occurs when inward radial velocity and vertical vorticity become more spatially correlated in the region surrounding the nascent vortex. It is also emphasized that the anisotropy of the horizontal convergence, which is nearly plane-convergent and of comparable magnitude to the mesocyclonic vorticity, is critical to an understanding of the process. The resultant evolution is intermediate between a state of purely two-dimensional nondivergent dynamics and one in which plane convergence confines vorticity to its axis of dilatation. This intermediate state produces a concentrated vortex more rapidly than either end state. The unsteady nature of the initial vorticity band also serves to increase the circulation and wind speed amplification of the final vortex. It is shown how conceptual models in the fluid dynamics literature can be applied to predicting the time and length scales of tornadic mesocyclone evolution.

Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell*

“…5). Only farther downstream does the vorticity sheet collapse into a coherent vortex, consistent with the analysis by Gaudet et al (2006). A possible ''shear instability'' mechanism for vortex genesis thus is not inconsistent with the baroclinic mechanism involving z production in downdrafts that we find here.…”

Relative sensitivities of simulated rainfall to fixed shape parameters and collection efficiencies

“…The sounding (adapted from Gaudet et al, ) used for the horizontally homogenous initialization for the model in all simulations. The red line denotes temperature (°C) and the green line denotes dew‐point (°C) [Colour figure can be viewed at wileyonlinelibrary.com].…”

Numerical Study of Precipitation Core-Gap Structure along Cold Fronts

“…7b is qualitatively different from that induced by the growth of HSI waves along a free shear layer or a density current interface. When a shear instability wave evolves along a uniform vortex sheet with positive (i.e., cyclonic) vorticity, a configuration is created where the vortex sheet's vorticity is advected toward the center on segments of the vortex sheet tilted upshear with respect to the mean tangential wind, while the vorticity is depleted at the center on segments tilted downshear (e.g., Batchelor 1967;Gaudet et al 2006). As the nonuniformity of the vorticity develops, the region around a vorticity maximum tends to undergo self-induced rotation, and the vortex sheet eventually breaks down (e.g., Corcos and Sherman 1984;Lee and Wilhelmson 1997).…”