Dr. Nathan Dahl-December 1

Using High-Resolution Simulations to Quantify Errors in Radar Estimates of Tornado Intensity

The current practice of rating tornado intensity based on observed damage is rife with uncertainty, and there have been recent efforts to include mobile Doppler observations in the rating process. However, radar observations include error sources of their own, the impacts of which have not been quantified. To provide insight on this issue, we performed observation experiments in which synthetic mobile Doppler radar observations were obtained at close range (1-5 km) from high-resolution large eddy simulations of translating tornado-like vortices. The difference between time-averaged single-Doppler observations and the corresponding “true” 3 s gusts at 10 m height (S10-3s) is partitioned into various error sources arising from beam spreading and elevation, temporal sampling, and the deviation between the “true” wind direction and the radar beam. The median total error is ~10% for a “best-case” scenario but steadily increases as the radar placement becomes less optimal. As a result, the “true” maximum intensity for these vortices is substantially underrated by the radar observations, with the global maximum S10-3s underestimated by more than 25 m/s in some instances. However, the global maximum S10-3s estimated from the radar observations provides a reasonable approximation of the true “time-mean” intensity of the vortices.