Decreasing Population Bias in Tornado Reports

Tornado-hazard assessment is hampered by a population bias in the available data.  We demonstrate a way to statistically quantify this bias using the ratio of city to country report densities. The expected report densities come from a model of the number of reports as a function of distance from nearest city center. On average since 1950 reports near cities with populations of at least 1000 in a 5.5 deg latitude by 5.5 deg longitude region centered on Russell, KS exceed those in the country by 70% (54%, 84%) [95% CI].

The model is applied to 10-year moving windows to show that the percentage is decreasing with time (see Figure).  Over the most recent period (2002-2011) the tornado report density in the city is slightly less than 3 reports per 100 square km per 100 years and this value is statistically indistinguishable from the report density in the country. On average the population bias is less pronounced for F0 tornadoes, but the bias disappears more quickly over time for the F1 and stronger tornadoes.  We show evidence that this decline could be related in part to an increase in the number of storm chasers. The population-bias model can enhance the usefulness of the Storm Prediction Center's tornado database and help create more meaningful spatial climatologies.

The research was done in collaboration with Laura E. Michaels, Kelsey N. Scheitlin, and Ian J. Elsner.  It will be published in the American Meteorological Society's Weather, Climate and Society journal later this year.