The hook echo has been recognized as a feature associated with tornadic supercells since the first radar observations of tornadoes in the early 1950s. Some have hypothesized that rain associated with the hook echo is a source of evaporative cooling and downdraft production that can tilt horizontal vorticity into the vertical near the ground and/or advect vertical vorticity toward the surface. The small temporal and spatial scale observations needed to link the hook echo to tornadogenesis have made detailed hook echo evolution studies a difficult task. Recent observations by mobile Doppler radars, when combined with broader-scale WSR-88D storm observations, have provided new opportunities for insightful study.

This study utilizes Doppler on Wheels (DOW) data, sometimes paired with WSR-88D data, to investigate the structure and evolution of the hook echo during tornadogenesis. At relatively close ranges, the WSR-88D samples the mesocyclone scale features in low, middle, and upper levels, while the DOW samples the mesocyclone and tornado-scale features, particularly in low levels. The DOW data show many different structures and evolutions of reflectivity in the hook echo. Sometimes, reflectivities where the tornado forms are much weaker than the reflectivities in the arm of the hook, while at other times the reflectivities are moderately strong from the arm of the hook to where the tornado forms. Thinning of the arm of the hook is sometimes observed. Reflectivity clusters, waves, and bands are observed and are related to velocity perturbations that appear to be important in tornadogenesis for the cases studied so far. Results will be presented that relate the structure and evolution of the hook echo to tornadogenesis in multiple cases.