J. Climate, 27, 2983-2999.

An empirical relation between U.S. tornado activity and monthly environmental parameters

Michael K. Tippett
International Research Institute for Climate and Society, Columbia University, Palisades, NY

Adam H. Sobel
Department of Applied Physics and Applied Mathematics, Department of Earth and Environmental Sciences, and Lamont-Doherty Earth Observatory, Columbia University, New York, NY

Suzana J. Camargo
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY.

John T. Allen
International Research Institute for Climate and Society, Columbia University, Palisades, NY


In previous work the authors established an empirical relation between U.S. monthly tornado activity and monthly-averaged environmental parameters, in the form of an index. Here a detailed comparison is made between the index and reported tornado activity. The index is a function of two environmental parameters taken from the North American Regional Reanalysis: convective precipitation (cPrcp) and storm relative helicity (SRH). The contributions of the two environmental parameters to the index annual cycle and spatial distribution are examined. The spatial distribution of SRH establishes the central U.S. "tornado alley" of the index, while the spatial distribution of cPrcp enhances the index in the South and Southeast and surpresses it west of the Rockies and over elevation. At the scale of the NOAA climate regions, the largest deficiency of the index climatology occurs over the Central region where the index peak in spring is too low and where the late summer drop-off in the reported number of tornadoes is poorly captured. Increasing the index sensitivity to SRH improves the representation of the annual cycle in this region. The ability of the index to represent the interannual variability of the monthly number of U.S. tornadoes can be ascribed during most times of the year to interannual variations of cPrcp rather than of SRH. However, both factors are important during the peak spring period. Additionally, the index shows some skill in representing the interannual variability of monthly tornado numbers at the scale of NOAA climate regions.