Variations in Rainfall Timing and Changes in the Leading Edge of Hurricane Katrina (2005) During Gulf Coast Landfalls

Abstract

The leading edge of a tropical cyclone (TC) brings gusty winds and rainfall to locations many hours prior to the arrival of the storm’s eyewall, especially when the storm is large. Defining the leading edge of a TC’s rain field is a complex task as the storm begins to interact with the land surface and more dense continental air masses. This study measures the outer rainbands in Hurricane Katrina (2005) as detected by radar-based data around its Gulf Coast landfalls. The objectives are to 1) determine the hour when rainfall begins along the coast and relate that to storm position and the timing of warnings, and 2) track the position of the leading edge of the outermost rainband over the landfalling period. Rainfall started an average of 12 hours prior to Katrina’s landfall when the storm was 353km away from the 10 locations that were analyzed. Rainfall began prior to the arrival of gale-force winds at some locations. Five different rainbands containing 40dBZ reflectivity values were tracked over a 20h period. Earlier bands propagated outwards while later rainbands did not, yet they grew in length. The area occupied by the storm had at least 60mm of total precipitable water, with a 1mm decrease each 10km occurring on the western side of the storm where two bands dissipated. Future research should identify similar moisture gradients to better assess their role in enhancing or inhibiting rainband development and determine rainfall onset times for more TCs.