Mechanisms for the intensification and decay of tropical Storm Edouard (2002) in strong vertical shear

Many studies have shown that vertical shear in excess of ~12.5 ms-1 usually prevents tropical cyclone formation and intensification. However, Tropical Storm Edouard formed off the East Coast of Florida in early September 2002 and reached a peak intensity of 55 kt. amid 850-200 hPa vertical shear in excess of 13 ms-1. Despite environmental shear increasing with time to over 20 ms-1, Edouard was able to maintain tropical storm intensity for nearly 72 hours before making landfall.

The storm underwent a series of convective pulses in which deep convection formed near the center and then shifted more than 100 km downshear, all within a few hours. Visible and infrared satellite imagery as well as cloud-to-ground lightning locations from the National Lightning Detection Network (NLDN) were used to analyze these bursts of convection and determine their effect on the storm.

To further investigate the storm's structure and evolution, U.S. Air Force and NOAA reconnaissance flight data were examined. Horizontal cross-sections of storm relative tangential winds reveal a rapid spin-up of the storm while convection was near the center and subsequent abrupt spin-down as it moved away again. Dropsondes released into the center of circulation indicate the vortex was tilted downshear with height. Evidence for the existence of cold downdrafts will also be presented. The talk will address the mechanisms for the growth and maintenance of Tropical Storm Edouard in the presence of strong wind shear, and the reasons behind the oscillation of convection from downshear to within the storm core and back.