Structure of the eye and eyewall of Hurricane Hugo (1989)

On 15 September 1989, one of NOAA's WP-3D research aircraft, N42RF (lower aircraft or LA), penetrated the eyewall of Hurricane Hugo. The aircraft had an engine fail in severe turbulence while passing the radius of maximum wind and before entering the eye at 450 m altitude. After the aircraft returned to controlled flight within the 7-km radius eye, it gained altitude gradually as it orbited in the eye. Observations taken during this period suggest that LA penetrated an intense cyclonic vorticity maximum adjacent to the strongest convection in the eyewall (eyewall vorticity maximum or EVM). This EVM was distinct from the vortex-scale cyclonic circulation observed to orbit within the eye three times during the 1 h that LA circled in the eye.

At the time, Hugo had been deepening rapidly for 12 h. The maximum flight-level tangential wind was 89 m s-1 at a radius of 12.5 km, however, the primary vortex peak tangential wind, derived from a 100-s filter of the flight-level data was estimated to be 70 m s-1, also at 12.5 km radius. The primary vortex tangential wind was in approximate gradient wind balance and characterized by a peak in angular velocity just inside the radius of maximum wind and had an annular vorticity structure slightly interior to the angular velocity maximum.

The EVM along the aircraft's track was roughly 1 km in diameter with a peak cyclonic vorticity of 1.25 10-1 s-1. The larger circulation center, with a diameter >15 km, was observed within the eye and exhibited an average orbital period of 19 minutes. This period is about the same as that of the angular velocity maximum of the axisymmetric mean vortex, and in reasonable agreement with recent theoretical and model predictions of a persistent trochoidal “wobble” of circulation centers in mature hurricane-like vortices. This study is the first with in-situ documentation of these vortical entities, which were recently hypothesized to be elements of a lower tropospheric eye/eyewall mixing mechanism that supports strong storms.