Abstract
This paper reports the results of an analysis of airflow evolution in the tornadic Newcastle–Graham, Texas, storm complex of 29 May 1994. A series of seven pseudo-dual-Doppler analyses from 2242 to 2315 are performed from tail 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 hook 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)] rotation of the parent mesoscale circulation of the Newcastle tornado and the origins of mesoscale rotation preceding tornadogenesis. The structure and evolution of the Newcastle and Graham mesocyclones are compared and contrasted.
The airborne Doppler analyses reveal that the tornadic Newcastle cell had supercell characteristics and that the Newcastle storm circulation could be classified as a mesocyclone based on commonly accepted criteria of circulation amplitude, spatial scale, and persistence. The Newcastle mesocyclone initially developed downward from midlevels (i.e., 2–5 km AGL), then transitioned into a subsequent period of rapid low-level stretching intensification and upward growth just prior to the development of an F3 tornado. Single-radar analysis reveals the stretching contraction and intensification of the Newcastle mesocyclone and an embedded tornado cyclone prior to and after tornadogenesis. In contrast, the nontornadic Graham mesocyclone ultimately became rain-filled and transitioned from moderate stretching growth to negative stretching after the development of a central downdraft in low levels, possibly contributing to tornadogenesis failure. Using a hybrid, two-supercell 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 vorticity equation suggested that the midlevel Newcastle mesocyclone was formed by a sequence of tilting of ambient horizontal vorticity followed by stretching 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 upon entering the Newcastle updraft to form the low-level mesocyclone. Though the vorticity dynamical origin of the weak ambient rotation could not be identified, the spatial origins of low-level trajectories that entered the Newcastle mesocyclone were determined to be from a broad area of low-level rainy easterly outflow from the Graham storm. The present findings were compared and contrasted with results of an earlier study of the Newcastle storm.
Corresponding author address: Dr. Conrad L. Ziegler, National Severe Storms Laboratory, Mesoscale Research/Applications Division, 1313 Halley Circle, Norman, OK 73069.Email: Ziegler@nssl.noaa.gov
