The 19 april 1996 Illinois tornado outbreak. Part II: Cell mergers and associated tornado incidence

In the 19 April 1996 Illinois tornado outbreak, cell mergers played a very important role in the convective evolution. With a large number of cells forming within a short time period, the early stages of cell organization were marked by cell merger interactions and cell attrition that led to a pattern of isolated tornadic supercells as described in Part I of this study. Twenty-six mergers were documented and analyzed. Storm-rotation-induced differential cell propagation accounted for 58% of these 26 cell mergers while differing cell speeds prompted 27% of the mergers. Cell merger characterizations were utilized to describe the cell reflectivity coalescence morphology including aspects of new cell development, development along the periphery of an existing cell, or an upward pulse in the cell intensity of a dominant cell. In cases where the merging cells were of similar intensity, a rapidly developing cellular pulse “bridging” the two echoes was often observed. When the relationship between short-term cell intensity changes and cell mergers was examined, it was found that the maximum reflectivity tendency showed a bias toward higher reflectivity for the product storm. Depending upon the radar elevation angle utilized, 27%–44% of mergers were associated with an increase in peak reflectivity while 40%–58% of the storms realized little or no increase. With respect to short-term cell rotation changes, the merger signal was marked. Depending upon the length of the evaluation window, in 44%–60% of the mergers, there was evidence of a merger-associated increase in cell rotation. When considering the association between tornado occurrence and cell mergers, a striking 54% of the tornadoes occurred within 15 min before or after a cell merger. This high percentage is strongly suggestive of a physical relationship between storm mergers and tornadogenesis. A discussion is presented of potential merger scenarios and favorable ambient environmental conditions that may have been conducive to tornadogenesis in this event. Suggestions are presented to raise the awareness level of forecasters to key aspects of cell evolution and interaction in nowcasting severe convection.