The evolution of low-level rotation in the 29 May 1994 Newcastle-Graham, Texas, storm complex during VORTEX

This paper reports the results of an analysis of airflow evolution in the t ornadic Newcastle-Graham, Texas, storm complex of 29 May 1994. A series of seven pseudo-dual-Doppler analyses from 2242 to 2315 are performed from tai l radar observations by the National Oceanic and Atmospheric Administration P-3 aircraft. Subjective analyses of quasi-horizontal single-Doppler radar observations provide a detailed look at structure and evolution of the hoo k echo and the low-level Newcastle mesocyclone. Special emphasis is placed on the evolution of low-level [i.e., below 1 km above ground level (AGL)] r otation of the parent mesoscale circulation of the Newcastle tornado and th e origins of mesoscale rotation preceding tornadogenesis. The structure and evolution of the Newcastle and Graham mesocyclones are compared and contra sted. The airborne Doppler analyses reveal that the tornadic Newcastle cell had s upercell characteristics and that the Newcastle storm circulation could be classified as a mesocyclone based on commonly accepted criteria of circulat ion amplitude, spatial scale, and persistence. The Newcastle mesocyclone in itially developed downward from midlevels (i.e., 2-5 km AGL), then transiti oned into a subsequent period of rapid low-level stretching intensification and upward growth just prior to the development of an F3 tornado. Single-r adar analysis reveals the stretching contraction and intensification of the Newcastle mesocyclone and an embedded tornado cyclone prior to and after t ornadogenesis. In contrast, the nontornadic Graham mesocyclone ultimately b ecame rain-filled and transitioned from moderate stretching growth to negat ive stretching after the development of a central downdraft in low levels, possibly contributing to tornadogenesis failure. Using a hybrid, two-superc ell schematic diagram to depict the Newcastle-Graham storm complex, it was concluded that the Newcastle tornado occurred at the traditionally accepted location of a supercell tornado at the point of the warm sector occlusion in the westernmost cell. Computed trajectories based on a Lagrangian solution of the vertical vortic ity equation suggested that the midlevel Newcastle mesocyclone was formed b y a sequence of tilting of ambient horizontal vorticity followed by stretch ing intensification in the rotating updrafts. The air parcels that entered the low-level Newcastle mesocyclone initially possessed vertical vorticity of order 10(-3) s(-1), which was subsequently concentrated by stretching up on entering the Newcastle updraft to form the low-level mesocyclone. Though the vorticity dynamical origin of the weak ambient rotation could not be i dentified, the spatial origins of low-level trajectories that entered the N ewcastle mesocyclone were determined to be from a broad area of low-level r ainy easterly outflow from the Graham storm. The present findings were comp ared and contrasted with results of an earlier study of the Newcastle storm .