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ABSTRACT
A case study of a left-moving supercell with a rapid motion is presented to (i) elucidate differences in anvil orientations between left- and right-moving supercells and (ii) highlight the interaction of the left mover with a tornadic right mover. It is shown how anvil orientations, as viewed from satellite, may be used to assist in the identification of thunderstorms with differing motions and how this applies to splitting supercells. Additionally, the movement of the left mover into the forward flank of the right mover may have temporarily affected its tornadic circulation, as tornadoes occurred both before and after the merger, despite the structure of the right mover being interrupted during the merging process. Given the dearth of literature on thunderstorm mergers in general, and how mergers affect tornadic supercells in particular, this is an area that demands further research.
1. Introduction
Documentation of left-moving supercell thunderstorms has been relatively rare in the literature (Dostalek et al. 2004). Considerable attention has been given to right-moving supercells, primarily because they are much more common in the United States than their left-moving counterparts (Davies-Jones 1986; Bunkers 2002). Numerical modeling results in the early 1980s indicate symmetric splitting supercells can occur in a unidirectionally sheared environment. The left mover (right mover) is favored when the low-level shear vector turns counterclockwise (clockwise) with height (Weisman and Klemp 1984, 1986). A few observational studies of left-moving supercells support these findings (e.g., Nielsen-Gammon and Read 1995; Grasso and Hilgendorf 2001). Here we present a case study of a severe, left-moving supercell from 4 May 2003 that affected northeastern Oklahoma and extreme southwestern Missouri. Noteworthy aspects of this case study include (i) the storm's rapid motion and associated anvil orientation with respect to that of the right mover, and (ii) interaction/merger of the left mover with a tornadic right-moving supercell, which may have affected its tornadic circulation.
2. Case study
a. Synoptic setup
The storm of interest occurred in a large-scale environment that was favorable for severe storms. There was a deep trough over the western United States and a strong southerly flow of moist and unstable air at low levels across the southern and central plains. This day marked the beginning of an 8-day period of numerous tornado reports over...