The association of thunderstorm cell mergers with tornado occurrence
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Cell mergers have been believed to be linked to the occurrence of tornadoes for quite some time. However, very few studies exist that have determined what types of mergers are favorable for tornadoes. The purpose of this study is to determine how cell mergers are related to the occurrence of tornadoes, and how frequently they occur. Ten prolific tornado producing days over the West Texas region between 1999 and 2006 are included in this study. Every tornado report that occurred during the ten days included in this study is analyzed using WSR-88D Level II and Level III base reflectivity and velocity data to determine its association with cell mergers. A total of 63 mergers are found to be associated with tornado reports. Thirty six null cases, involving intense thunderstorms that did not produce a tornado following, are also documented. A comparison is made between tornadic and null cases. Reflectivity and rotation trends are determined during the period surrounding mergers. In addition, merger characteristics and scenarios are categorically assigned to each merger event. The location within the primary cell where the merger occurs is recorded. Characteristics of the ancillary cell are also analyzed in each case. Two regression methods are used to determine the significance of variables associated with cell mergers. A stepwise regression model is developed that contains potential predictors of tornado occurrence during a cell merger. Two multiple regression models are also developed that predicted changes in rotation of the primary cell. Results from this study indicate a strong relationship between tornadoes and cell mergers. A striking 54\% of all tornadoes over a ten day period are associated with a cell merger. Mergers that are associated with a tornado often produced a sustained increase in rotation. A position to the west and north of the updraft center is found to be a favorable location for mergers to produce tornadoes. In addition, cases in which the ancillary cell initiates along the flanking line are found to be more common in mergers associated with tornadoes.