We examine the evolution of low level rotation in the tornadic Newcastle, Texas storm (29 May 1994), observed during the Verification of the Origins of Rotation in Tornadoes EXperiment (VORTEX) with the tail Doppler radar and conventional lower-fuselage radar on the NOAA P-3 aircraft. Flying a series of straight, low level legs along the southwest storm flank to within 7 km of the tornadic region, Doppler velocity and reflectivity data are collected using the Fore-Aft Scan Technique (FAST). After correcting aliased radial velocities, reflectivities and radial velocities are spatially interpolated to a fixed, regular cartesian analysis grid employing a Barnes weighting function. The cartesian velocity field is derived using a iterative approach with upward integration of the mass continuity equation from a kinematic boundary condition of w=0 at ground level. The analysis is restricted to the lowest five kilometers to concentrate on vorticity changes in the region of inflow into the tornado's parent circulation. A total of seven analyses spanning the period from about 20 min prior to tornadogenesis until about 13 min after tornadogenesis are performed.
Using the vector (ie. 3-D) airflow and vorticity analyses and computed forcing terms from the vector vorticity equation, we will test aspects of the hypothesis that low-level mesocyclone intensification is due to the reorientation, or tilting, and subsequent stretching of low level horizontal vorticity transported into the main updraft. The vertical vorticity change following the motion of Lagrangian parcels will be evaluated by direct time integration of net forcing and compared to the corresponding observed evolution. Since limited temporal and spatial resolution of the Doppler data and analyses may preclude direct testing of the various tornadogenesis hypotheses, we will concentrate on the morphological changes and vorticity dynamics of the parent circulation ("tornado cyclone") near the time of observed tornadogenesis.