The Association of Cell Mergers with Tornado Occurrence

Cell mergers associated with supercells have been linked in the past to the occurrence of tornadoes. In this study (Rogers and Weiss 2008, submitted to MWR), cell mergers occurring over ten prolific tornado producing days in the West Texas region between 1999 and 2006 are analyzed using WSR-88D Level II and Level III 0.5° elevation base reflectivity and velocity data. A total of 63 mergers are found to be associated with tornado reports. Twenty null cases, involving intense thunderstorms that did not produce a tornado following a merger, are also documented. Reflectivity and rotation trends are determined for each event. Ancillary cell characteristics and the location of the merger within the primary cell are analyzed. A stepwise multiple regression model is developed to test the significance of potential predictors of tornado occurrence.

Results from this study indicate a strong relationship between tornadoes and cell mergers. Nearly 54% of all tornadoes over the ten day period are associated with a cell merger. Mergers that are associated with a tornado often produced a sustained increase in detected mesocyclone rotation. A position behind and to left of the primary updraft location (relative to the 850 to 300 hPa shear vector) is found to be favorable for mergers that produce tornadoes. A dearth of tornado cases is found when the ancillary cell merges directly with the principal updraft. The relative merits and impediments of updraft interaction with ancillary cell precipitation and outflow will be discussed.