Hurricane Isabel (2003): New Insights into the Physics of Intense Storms. Part 2. Extreme Localized Wind

On 13 September 2003, a dropwindsonde released along the inner edge of the eastern eyewall of Hurricane Isabel measured 107 and 25 m sexpn -1 horizontal and vertical winds, respectively, at about 1400 m above sea level. This is the strongest known horizontal wind directly measured in a tropical cyclone TC, and is in the upper 1 of measurements for the vertical wind Black et al. 1996. The behavior of the instrument suggests an eyewall misocyclone in a strong convective burst. This particular observation, along with concurrent observations of very fast wind from airborne Doppler radar and other airborne instruments, has important practical implications for emergency management planning, structural wind engineering, and scientific interests relating to TC potential intensity and intensity change. The relatively quiescent environment in which Hurricane Isabel persisted for 3 days low environmental shear, no interactions with midlatitude or tropical upper-tropospheric troughs, relatively uniform 27 deg C sea surface temperature allowed the TC to remain at or near category-5 status during that period. This environment and the observations taken during this time provide an unprecedented opportunity to gain important insight into eyewall misocyclones and maximum potential intensity. Persing and Montgomery 2003 found that in high-resolution axisymmetric TC simulations, storm intensity, as defined by the maximum sustained tangential wind speed at the top of the boundary layer, greatly exceeds currently understood upper bounds for maximum potential intensity of the steady, axisymmetric vortex Emanuel 1986, 1988, 1995. They termed this phenomenon superintensity and demonstrated that it occurs because of an enhancement of entropy at low levels in the hurricane eye. They suggested that the high-entropy air is mixed into the eyewall by mesocyclones at the interface between the eye and the eyewall.