A study of the pre-storm environment of tornadic quasi-linear convective systems

Tornadoes are known to evolve from a variety of different parent storm types, including supercells, squall lines, 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 during the period of study. Although much work has been done to determine the environments in which supercells may become tornadic, little effort has been made to determine the environmental conditions favoring QLCS tornadogenesis.

For comparison, the environments of pre-tornadic QLCSs, non-tornadic QLCSs, and tornadic cells were examined using NWS upper air soundings. The reported times and locations of tornado touchdown were used to locate proximity soundings that were characteristic of the pre-storm environment. Proximity thresholds were set to within one hour of nominal sounding times at 0000, 1200, and 1800 UTC and fewer than one hundred miles (161 km) from the launch location. In instances with multiple tornadoes near a particular sounding launch site, the highest F-scale ranking tornado was associated with the sounding. All soundings were manually evaluated in order to ensure that only those soundings displaying pre-storm environmental characteristics were retained for further analysis.

A total of 112 proximity soundings from pre-storm environments were gathered. Thermodynamic and kinematic parameters were derived and a discriminant analysis was performed to differentiate between pre-storm environments of tornadic cells, tornadic QLCSs, and non-tornadic QLCSs. The pre-tornadic cell and QLCS environments were characterized by similar levels of instability and deep-layer shear. Overall, pre-tornadic QLCS environments were characterized by higher levels of low-level bulk shear and CAPE than either non-tornadic QLCS environments or pre-tornadic cell environments. The differences between these parameters were greater between tornadic and non-tornadic QLCS environments than between tornadic QLCS and cell environments. Several other parameters showed slight differences in their average values between pre-tornadic cell and QLCS environments, but showed greater differences between tornadic and non-tornadic QLCS environments. A linear discriminant analysis, based on two-dimensional observations of sets of sounding parameters, was performed in order to statistically differentiate between pre-storm environments.