Conservation of Vertical Angular Momentum during Tornadogenesis

Vector angular moment is a conserved quantity. Angular momentum conservation in fluid mechanics is embodied in Kelvin's circulation theorem. In this study, Doppler radar observations of tornadic thunderstorms have been used to estimate a vertical angular momentum quantity during tornadogenesis. The velocity couplet V and radius R in the radial Doppler velocity field at every radar elevation angle of a mesocyclone-to-tornado event are extracted, and an approximate angular momentum quantity VxR is contoured and displayed in time-versus-height format, through the time of tornado appearance on the ground. These observations for a number of tornado events in Colorado, Florida, Oklahoma and Utah show an intensification of VxR in the upper (cold) portion of the storm, often coinciding with peak total lightning activity. This intensification phase is attributed to vortex stretching by the updraft. This phase is often followed by a diminishment of VxR aloft preceding and coincident with the formation of the tornado. Conservation of VxR requires that angular momentum in the tornado is drawn from the reservoir of angular momentum in the mesocyclone aloft, and therefore requires downward transport of vertical angular momentum from the mesocyclone to the surface. This downward transport is consistent with the partial collapse of the intracloud lightning activity many minutes prior to the tornado onset, as observed in other studies.