The Greenfield, Iowa, Tornado (2024): Wind-Damage Relationships, Novel LDR Dual-Polarization Analysis, and Integrated DOW/Video Analysis of Turbine Destruction

On 21-May-2024 an intense tornado passed through Greenfield, Iowa, destroying many structures, including several commercial wind turbines, killing four people, tracking over 70 km, and sometimes traveling at speeds exceeding 30 m s −1 . Two proximate dual-polarization fast-scanning Doppler On Wheels (DOW) radars collected unprecedented data as the tornado approached and crossed Greenfield. This provided the opportunity for the first-ever integration of ultra-fine-scale fast-scanning radar data with multi-indicator damage assessments, the first-ever mapping of Linear Depolarization Ratio (LDR) of tornado debris, and the first-ever analysis integrating radar wind mapping with real-time video documentation of the destruction of engineered wind turbines. Seven-second update DOW data over Greenfield, from as close as 780 m, revealed near-ground sub-second Doppler velocities as strong as 121 m s −1 , and ground-relative speeds of 136 m s −1 (over 300 mph), among the strongest wind speeds ever measured. DOW data allowed determination of time histories of speeds at individual structures. Damage surveys assessed peak wind speeds of 83-85 m s −1 (185-190 mph). These compare well with DOW-determined maximum wind speeds and time histories at ∼154 damaged structures, but DOW-determined peak speeds average 21% stronger. Differences in measurement techniques, spatiotemporal averaging, and altitude are discussed. Fine-scale LDR data were collected for the first time in a tornado, revealing extremely high depolarization, the highest ever measured, in likely large lofted debris, suggesting the potential that LDR in combination with copolar correlation coefficient, ρ hv , may be a useful discriminator for distinguishing intense versus weak tornadoes. Aerial and ground-based storm chaser videos reveal the precise timing and mechanisms of wind turbine failure. At least one turbine is destroyed well outside the visually-manifested condensation funnel. DOW-determined wind speed time histories are compared to the precise timing of video-documented turbine failures and damage-assessed speeds.