Mesoscale and Convective-Scale Characteristics of Mature Hurricanes. Part II. Inner Core Structure of Hurricane Allen (1980)

Abstract

The eyewall structure of Hurricane Alien is examined from analyses of multiple aircraft data on two days, 5 and 8 August 1980. These data sets are unique in that, for the first time, three instrumented aircraft executed coordinated radial penetrations of the eyewall at multiple levels. The data collected on 5 August illustrate the persistence of various features on horizontal scales > 10 km over several hours. Composite cross sections constructed from the 8 August data show similar structure, although the eye diameter had decreased to less than half that of 5 August. The convergence of air in the eyewall was highly two-dimensional. This convergence supported organized ascent that was along the inner edge of the high reflectivity region and displaced inward several kilometers from the radius of maximum wind (RMW). A mean eyewall updraft of 5–6 m s−1 is computed from integration of the two-dimensional continuity equation. Embedded within the two-dimensional eyewall were cores of high reflectivity that were 2–5 km in diameter, three-dimensional, and generally not traceable from pass to pass (∼20 min intervals). These convective-scale entities had highest updraft velocities of 7–9 m s−1. Upward mass flux in the eyewall was 4–5 times greater than that diagnosed by Zipser and others for a GATE slow-moving convective line. This greater mass flux was accomplished not through larger vertical velocities within convective cares, but by a greater area covered by active updrafts within the low-level convergence zone.