Multi-scale Processes Leading to the Supercells in the Landfalling Outer Rainbands of Hurricane Katrina (2005)

Shallow supercells are frequently observed within the outer rainbands of landfalling tropical cyclones (TCs), both inland and well offshore. These supercells can produce tornadoes along the coast even while the TC's center is still hundreds of kilometers from land, as was the case for at least three of the several dozen tornadoes spawned by Hurricane Katrina (2005). Besides radar and radiosonde observations, this study uses a high-resolution convection-permitting simulation with the innermost grid spacing of 1.5 km to investigate the multi-scale environmental conditions conducive to supercell activities in Katrina's landfalling outer rainbands. While the model produced a denser coverage of convection in the outer rainbands than observed (possibly from the absence of a dry air intrusion), phenomena larger than individual convective cells were well captured. There were strong heterogeneities in the distribution of thermodynamic and kinematic profiles across the TC and surrounding areas. Away from the inner core, the simulation developed a convergent baroclinic zone along the coastline. This front strongly influenced the distribution of helicity and CAPE, with the largest values being over land and water, respectively. The ability of coastal baroclinic boundaries to enhance low-level horizontal vorticity has obvious implications for the development and/or intensification of updraft rotation in cells that reach the coastline.