A theory on the expansion of Hurricane Katrina's wind field

Hurricane Katrina embarked upon two periods of rapid intensification (defined as a 30 kt or greater intensity increase in a 24-h period) between 26 and 28 August as it moved over the warm Gulf loop current. The first period involved an increase in the maximum sustained winds from 65 kt to 95 kt in a 24-h period ending 0600 UTC 27 August. An eye became clearly evident in infrared satellite imagery early on 27 August, and Katrina became a Category 3 hurricane with 100 kt winds at 1200 UTC about 365 n mi southeast of the mouth of the Mississippi River. During the remainder of the day, the inner eyewall deteriorated while a new, outer eyewall formed, and the intensity leveled off at 100 kt. Accompanying the intensification and the subsequent deterioration of the inner eyewall was a significant expansion of the wind field on 27 August. Katrina nearly doubled in size on 27 August, and by the end of that day tropical storm-force winds extended up to about 140 n mi from the center. A cursory examination of satellite imagery shows the possible influence of a trough or confluence zone to the north that may have contributed angular momentum to the intensifying cyclone.

Although the rapid intensification of Katrina was noteworthy, the expansion of the tropical storm-force winds is the key forecast issue. The devastation wrought by this storm upon landfall is attributable more to its size rather than its intensity, as it landed as a Category 3 hurricane. This large hurricane caused a record storm surge and exposed the coastal regions of Louisiana and Mississippi to hurricane-force winds for an extensive period of time.

Observations, as well as a WRF simulation, suggest that an influx of vorticity associated with a remnant front near north Florida contributed to the wind field expansion. Relative angular momentum (RAM) calculations were also conducted to examine if any RAM fluxes contributed to this expansion, and the results were negative. It appears the wind field expansion was due to mutually induced straining between the hurricane and the remnant front, which ultimately causes an attraction of PV.

Other tropical cyclones with different size expansions are also being investigated and will be summarized. The possibility of using either a vortex merger index or a critical capture radius in an operational setting will be discussed.