Evolution of Low Level Mesocyclone Rotation: The 29 MAY 1994 Newcastle, Texas Storm During VORTEX

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). Flying a series of straight, low level legs along the southwest storm flank to within 7 km of the F-3 tornado-containing Newcastle cell and the neighboring Graham mesocyclone, Doppler velocity and reflectivity data are collected using the Fore-Aft Scan Technique (FAST) with tail Doppler radar data from the NOAA P-3 aircraft. Seven 3-D airflow/reflectivity analyses were performed with this data in the period 2242-2315. Since limited temporal and spatial resolution of the Doppler data and analyses preclude direct testing of various tornadogenesis hypotheses, we have concentrated instead on the morphological changes and vorticity dynamics of the low level Newcastle mesocyclone leading up to and through the time of observed tornadogenesis around 2309. The evolution of the tornadic mesocyclone is compared with the nontornadic Graham mesocyclone, to the southeast of the Newcastle circulation, and an elevated meso-anticyclone located between the mesocyclones. The vertical vorticity change following the motion of selected Lagrangian parcels entering the mesocyclones is evaluated by direct time integration of net forcing and compared to the corresponding evolution of observed vertical vorticity. The predominant low level inflow to the Newcastle mesocyclone is from rainy outflow which wraps around and moves westward from the Graham cell. Mesocyclone intensification is strongly forced by stretching of ambient vertical vorticity transported into the main updraft region in each of the three cells. The trajectories also document the contribution of tilting of ambient horizontal vorticity to amplify vertical vorticity prior to subsequent stretching upon entering the updraft. Both the Graham and Newcastle cells contain descending midlevel rotations, though the final intensification of the Newcastle circulation is dominated by intense stretching in the lowest 1-2 km. In contrast, the neighboring Graham mesocyclone decays after entering a period dominated by growth of an axial downdraft in low levels. Thus, the Newcastle mesocyclone appears to be a hybrid of two contrasting modes of development: 1) the classical descending midlevel mesocyclone; followed by 2) the final low level intensification with upward growth. A conceptual model of the Newcastle-Graham storm complex is derived by rotating the classical Lemon-Doswell (1979) and Davies-Jones (1986) conceptual supercells by 90 degrees to account for northwesterly flow and joining two conceptual supercells along their merged forward flank and RFD baroclinic zones.