Statistics of severe tornadoes and severe tornado outbreaks

Abstract

Abstract. The standard measures of the intensity of a tornado in the USA and many other countries are the Fujita and Enhanced Fujita scales. These scales are based on the damage that a tornado causes. Another measure of the strength of a tornado is its path length of touchdown, L. In this study we consider severe tornadoes, which we define as L ≥ 10 km, in the continental USA (USA Storm Prediction Center Severe Weather Database). We find that for the period 1982-2011, for individual severe tornadoes (L ≥ 10 km): (i) There is a strong linear scaling between the number of severe tornadoes in a year and their total path length in that year. (ii) The cumulative frequency path length data suggests that, not taking into account any changing trends over time, we would expect in a given year (on average) one severe tornado with a path length L ≥ 115 km and in a decade (on average) one severe tornado with a path length L ≥215 km. (iii) The noncumulative frequency-length statistics of severe tornado touchdown path lengths, 20 < L < 200 km, is well approximated by an inverse power-law relationship with exponent near 3.

Spatial–temporal clustering of tornadoes

“…This scale is based qualitatively on the damage caused by a tornado. An alternative measure of tornado intensity is the tornado path length L. Malamud and Turcotte (2012) showed that records of tornado path lengths from the 1990s to the present appear to be relatively complete for severe tornadoes (defined to be L ≥10 km) in the United States. They also showed that the number-length scaling of severe tornado touchdowns is well approximated by a power-law distribution.…”

“…Elsner et al (2014) showed that the distribution of daily tornado counts in the United States is also well approximated by a power-law relationship. Malamud and Turcotte (2012) also studied the statistics of recent severe tornado outbreaks. They quantified the strength of a severe tornado outbreak to be the total tornado path length L D of the severe tornadoes occurring during a convective day.…”

“…The Severe Weather Database that we use in our analyses list most tornadoes in Central Standard Time (CST), so we will consider tornadoes in a convective day as 06:00-06:00 CST. However, consistent with the studies of severe tornado outbreaks given by Malamud and Turcotte (2012), we will consider a severe tornado outbreak to include only those tornadoes with path lengths L ≥ 10 km. Elsner et al (2015) developed a method for separating distinct spatial clusters of tornado touchdowns during a convective day.…”

“…Brooks (2004) has provided a detailed study of the statistical correlations between the Fujita-scale intensities and the path length. Malamud and Turcotte (2012) extended these studies and defined a severe tornado to be a tornado with path length L ≥ 10 km. In the studies reported in this paper, we will retain this definition and consider only tornadoes with L ≥ 10 km.…”

“…In the studies reported in this paper, we will retain this definition and consider only tornadoes with L ≥ 10 km. A path length of L = 10 km corresponds roughly to an EF2 tornado (Malamud and Turcotte, 2012). Most severe tornadoes are generated by supercell thunderstorms (Doswell et al, 1993).…”