High Resolution Real-data WRF Modeling and Verification of Tropical Cyclone Tornadoes Associated with Hurricane Ivan 2004

Abstract

This study used high-resolution real-data WRF simulations of Hurricane Ivan (2004) to document the structure of potentially tornadic supercells embedded in tropical cyclone (TC) rainbands. The simulated TC track and intensity matched well with observations post landfall, and the simulated TC structure closely replicated the observed shape and rainbands, as evident by an assessment of observed composite and simulated radar reflectivity. TC tornado surrogates (TCT-Ss) were identified and calibrated using thresholds based on percentile values of maximum updraft helicity (UHmax) and simulated radar reflectivity. Although the magnitude of UHmax generally decreased as the simulated TC moved inland, sensitivity testing revealed that a threshold based on the 99.95% percentile value of UHmax achieved optimal TCT-S coverage and agreement with observed TC tornado (TCT) tracks on domains with 3-km and 1-km grid spacings. Three cells were identified at three different stages of the TC inland evolution and were found to have hook appendages resembling supercells found in midlatitudes. The cells produced storm tops of 13 km, with rotating cores with depths of 4 km above ground level. Although results in this study are unique to Hurricane Ivan, the authors believe the use of convection-allowing models and UH-based surrogate methodologies can be applied to other TC cases.