Multiscale Processes Leading to Supercells in the Landfalling Outer Rainbands of Hurricane Katrina (2005)

Abstract Shallow supercells are frequently observed within the outer rainbands—both onshore and offshore—of landfalling tropical cyclones (TCs). Such supercells can produce tornadoes along the coast even when the center of the parent TC is hundreds of kilometers from land, as was the case with Hurricane Katrina (2005). A convection-permitting simulation with 1.5-km grid spacing in the innermost domain is used in conjunction with radar, radiosonde, and surface observations to investigate the multiscale conditions conducive to supercells in the landfalling outer rainbands of Katrina. Several hours before the eye of the TC made landfall, a baroclinic zone developed along the coast; this front strongly influenced the horizontal distributions of cell-relative helicity and CAPE such that the largest values of these parameters were located over land and water, respectively. An example of a tornadic supercell in the outer rainbands of Katrina is examined. This cell intensified just before landfall and spawned a tornado along the coast, demonstrating the ability of baroclinic boundaries to enhance low-level horizontal vorticity and subsequently intensify updraft rotation within passing cells. Farther inland, the tornadic cell weakened rapidly, suggesting the presence of a narrow coastal zone in which both shear and buoyancy are favorable for tornadogenesis.