Intensity and temporal distributions of tornadoes from quasi-linear convective systems

Tornadoes are known to form in a variety of different parent storm types, such as high- and low-precipitation supercells, squall lines, bow echoes, and other convective systems. Reported tornadoes in the United States from 1998 – 2000 were classified by parent storm type: cell, quasi-linear convective system (QLCS), or other. Tornadoes from QLCSs, such as squall lines or bow echoes, composed 18% of the reported tornadoes. Historically, tornadoes from lines have been studied far less frequently than their cell-based counterparts, despite this relatively large percentage.

Previous studies have established the log-linear distribution of tornadoes by F-scale. During the period of study, cell tornadoes demonstrated the expected log-linearity while QLCS tornadoes displayed a somewhat different intensity distribution. Since there were many more tornadoes observed from cells than from QLCSs during the study period, it was difficult to compare these results. To compensate for the limited sample size, a Monte Carlo resampling method was used to evaluate the null hypothesis that the probability of a violent tornado, given a tornadic QLCS, was the same as the probability of a violent tornado, given a tornadic cell. Ten thousand years of simulated tornado activity were generated using a uniform random number generator. Results showed a significantly lower probability of F2-3 tornadoes, given a QLCS tornado, than the probability of F2-3 tornadoes, given a cell-based tornado, at the 99.9% confidence level. The tornado intensity distributions were indeed statistically different.

It was hypothesized that tornadoes from QLCSs occurred more frequently at night than those from cells. Consequently, nighttime weak tornado damage often may have been classified as straight-line winds, which accounts for the disproportionately small number of F0 QLCS tornadoes. There is a strong likelihood that the lowest-intensity tornadoes produced by linear convective storms were underreported. After examining the three-hour running mean tornado initiation times from cell- and line-based tornadoes, a single-peak diurnal cycle was evident only in the cell-type tornado distribution. To determine the statistical significance of the higher frequency of QLCS tornadoes observed in the nighttime hours, another Monte-Carlo resampling test was performed and showed results significant at the 99.9% confidence level. There was therefore a statistically significant difference between the observed daily tornado distributions from QLCSs and from cell-type storms.