Thursday, April 04, 2013

Predicting the distribution of violent tornadoes

Here we illustrate a statistical point process model that uses the spatial occurrence of non-violent tornadoes to predict the distribution of the rare, violent tornadoes during springtime across the U.S. central Great Plains. The average rate of non-violent tornadoes is 55 per 10000 square km per 62 years which compares with an average rate of only 1.5 violent tornadoes per 10000 square km over the same period (less than 3%). Violent tornado report density peaks at 2.6 per 10000 square km (62 yr) in the city to 0.7 per 10000 km in the countryside. 

The risk of a violent tornado is higher by a factor of 1.5, on average, in the vicinity of less violent tornadoes after accounting for the population bias. The model for the occurrence rate of violent tornadoes indicates that rates are lower by 10.3 (3.6, 16.5)% (95% CI) for every 1 km increase in distance from nearest non-violent tornado controlling for distance from nearest city. Model significance and distance-from-nearest non-violent tornado parameter are not sensitive to population threshold or definition of violent tornado. We show that the model is useful for generating a catalogue of touchdown points that can be used as a component to a tornado catastrophe model.

The research was done in collaboration with Richard Murnane, Thomas Jagger, and Holly Widen at Florida State University and will be published later this year in the journal Mathematical Geosciences.


Anonymous said...

Very interesting approach to the question! Why was a period of 62 years chosen? Is this a statistically significant fraction of an arbitrary period of time (ie out of 100 years, or a human lifespan issue, or a result of the time when statistically reliable/useful first began to be recorded?

James Elsner said...

The latter. To perform this analysis it is necessary to have a complete, consistent statistical database. The National Severe Storms Forecast Center (NSSFC) together with the U.S. Nuclear Regulatory Commission pursued newspaper accounts of all tornadoes reported over the contiguous U.S. starting in 1950 (Kelly et al. 1978). Data were compiled on each tornado's touchdown and retraction point in latitude/longitude. The database is updated annually by NOAA.