Dynamics and Structure of Supercells Induced by Hurricane Katrina (2005)

Miniature supercells were observed within the landfalling outer rainbands of Hurricane Katrina (2005); some of these cells were responsible for at least three reported tornadoes along the immediate coasts of Alabama and northern Florida. A cloud-resolving simulation of Katrina with the Weather Research and Forecasting (WRF) model, with grid spacing as small as 1.5 km and initialized with an ensemble Kalman filter assimilating airborne Doppler velocities, successfully captured the observed track, intensity, and structure of the hurricane. Moreover, the simulation also captured remarkably well the landfalling outer rainbands that included numerous miniature supercells.

A combination of observations such as from both airborne and ground-based Doppler radars and radiosondes, and the WRF simulation are used to examine the thermodynamic and dynamic structures and larger-scale environments of hurricane-related miniature supercells. The observational data were most useful for investigating the cell structure, whereas the model was most advantageous in the smallest scales of thunderstorm downdrafts and the largest scales of background thermodynamic parameters.

Our preliminary diagnostics show that the environment surrounding the miniature supercells is fairly buoyant with CAPE values approaching 2000 J kg-1. Furthermore, as Katrina approached land, a distinct baroclinic zone developed along the coast. Within this zone, high CAPE from the Gulf of Mexico overlapped with high shear that is enhanced by friction over land. Such baroclinic boundaries have been previously identified as focal points for tornadogenesis. In terms of individual cells, strong vertical shear of the horizontal wind resulted in an extremely tilted storm structure; features such as echo overhangs and bounded weak echo regions were also observed in the simulation. The life cycle of a miniature supercell in the model appears to have some similarities to those over the Great Plains. Current research focuses on more in-depth comparisons between the mid-latitude and hurricane-induced supercells, which will also be presented at the conference.