Life of a 6-hour hurricane

Hurricane Claudette (2003) provided an extreme example of intensity changes in the presence of large (>10ms-1) vertical wind shear. A hurricane-strength disturbance developed within an easterly wave in under six hours. Six hours later a reconnaissance aircraft at the same level could not find a center. We examine the mechanisms responsible for the growth and decay of this 6-hour hurricane. Extensive use of flight-level reconnaissance data collected by the US Air Force Reserve allows analysis of the evolution of the storm on high temporal and spatial scales.

Flight level observations from 700hPa at ~1400 UTC 10 July show Claudette exhibited classical hurricane structure in pressure, wind and equivalent potential temperature in every way but one: very dry air (dewpoint depressions near 7°C) extended from 100km upshear all the way to the edge of the eyewall. Flight-level observations only 90 minutes later (also at 700hPa) show a breakdown of the wind maximum on the left-of-shear side of the storm. Analysis of the radial flow at this time shows strong inflow collocated with the driest air left-of-shear, and outflow right-of-shear. A subsequent pass through Claudette at ~1700 UTC found a much weaker circulation. By ~0000 UTC 11 July little evidence of a circulation could be found at any flight level.

It is hypothesized that the rapid weakening of Claudette was forced by entrainment of the immediately-adjacent dry air into the eyewall. This process could have initiated cold downdrafts within the eyewall, which, on reaching the boundary layer, acted to cut off the storm's energy source. This resulted in a rapid weakening of the overturning circulation, and a spin-down of the storm's primary circulation. Alternatively, the weakening could have been hastened by precipitation falling out of the shear-tilted eyewall directly into the eye, resulting in rapid evaporative cooling. This could weaken the storm even if the induced downdrafts did not reach the surface.