Theory of Parcel Vorticity Evolution in Supercell-Like Flows

Abstract

Abstract In a prior paper, insights into tornadogenesis in supercell storms were gained by discovering analytical formulas for vorticity variations along streamlines in idealized, steady, frictionless, isentropic inflows of dry air imported from a horizontally uniform environment. This work is simplified and extended to the evolution of parcel vorticity in unsteady, nonisentropic flows by integrating the vorticity equation using nonorthogonal Lagrangian coordinates. The covariant basis vectors , , and are material line elements attached to each parcel. Initially they form an orthonormal set with in the direction of and left normal to the storm-relative wind at each level in the environment, and upward. The surface containing all parcels with the same initial height constitutes a material surface, within which initially streamwise and transverse material lines are reoriented and stretched or shrunk. The basis vectors propagate a parcel’s barotropic vorticity through time by factoring in the “frozen-field” effect. With a horizontally uniform environment, the barotropic vorticity of a parcel depends on its initial streamwise vorticity times its current plus its initial crosswise vorticity times its current . For baroclinic and frictional vorticity, each contravariant component is the integral from initial to current time of the corresponding contravariant component of the generation vector. The “river-bend” effect acting on all parts (baroclinic, frictional, and barotropic) of transverse vorticity produces streamwise vorticity (parallel to 3D wind). In left-turning steady flow, it arises from rotating toward . For steady, frictionless, dry isentropic flow, previous vorticity formulas are recovered. Significance Statement Air parcels rising in a tornado spin rapidly about their direction of motion. Theory herein describes the processes that can produce this streamwise spin in supercells. Cyclonic updraft rotation originates from strong low-level environmental storm-relative winds that turn clockwise with height. Parcels flowing into the updraft have initially large streamwise spins that are amplified by streamwise stretching. Rain curtains falling through the cyclonic updraft cause other parcels to descend and turn leftward. Buoyancy and frictional torques give them horizontal spin. Even if these spins are transverse to the flow initially, they are turned streamwise by secondary flow that develops in left-hand bends. As the parcels reach the ground and converge into the tornado, streamwise stretching greatly magnifies their streamwise spins.