Mesoscale and Convective-Scale Characteristics of Mature Hurricanes. Part I: General Observations by Research Aircraft

The analysis of instrumented aircraft data from four recent mature hurricanes reveals common mesoscale and convective-scale features. The structure and organization of the convection, precipitation, and wind flow are studied by combining the airborne observations, using basic instrumentation, with quantitative radar measurements. Three predominant precipitation features seen are 1) the eyewall, 2) a stratiform rain region adjacent to and outside the eyewall, and 3) rainbands that contain both convective and stratiform regions. The eyewall is so named because it surrounds the rain-free eye, but several additional features distinguish it from other convective bands in the hurricane. In particular the updraft, tangential wind maximum, and highest rainfall have preferred locations relative to each other, with the updraft being radially inward from the rain and wind maxima. The 10 dBZ radar reflectivity contour (indicative of the inner edge) slopes substantially outward with height, and there is evidence that the eyewall becomes more vertical with decreasing eye diameter. The inner eyewall edge is a region of cyclonic wind shear and substantial radial convergence. Of the four storms studied here, three had "double" eyewalls (concentric convective rings) during part of their lifetimes. The precipitation outside the eyewall is predominantly stratiform and is characterized by a "bright band" of enhanced radar reflectivity just below the melting level. Embedded within this uniform rain region an spiral and circular rainbands containing convective cells. Radar reflectivity is seen to decrease fairly rapidly with height above the 0°C level, even within the convective cores of the rainbands. Rainbands are distinguished from eyewalls by the absence of a pronounced tangential wind maximum, and the general lack of a consistently observed updraft location. Radar reflectivity maps of outer rainbands show a more "cellular" structure in the embedded convective elements when compared with eyewalls.