Dual-Aircraft Investigation of the Inner Core of Hurricane Norbert. Part I: Kinematic Structure

On 24–25 September 1984 air motions and physical processes in the eyewall region of Hurricane Norbert were investigated with two National Oceanic and Atmospheric Administration (NOAA) Aircraft Operations Center (AOC) WP-3D research aircraft. One aircraft, equipped with the airborne Doppler radar, flew repeated radial penetrations of the eyewall, mapping the three-dimensional wind field throughout a 40-km radius of the storm center. These data provide the first complete documentation of the three-dimensional wind field of the inner core of a hurricane on spatial scales of 10–25 km in the horizontal and 1 km in the vertical and temporal scales of 1–2 h. The wind field was asymmetric, and the asymmetry varied with altitude. As altitude increased the location of the maximum tangential wind sloped upwind, from left of the storm track at 1-km altitude to right of the storm track above 3 km. The radial wind at 1-km altitude was inward in front of the storm and outward behind. This pattern in the radial flow reversed above 3-km altitude, with inflow at the rear of the storm and outflow in the front. The maximum vertical velocity was located to the left of the storm track at all levels. This maximum sloped downwind, increasing in altitude from 2 km in front of and to the left of the track to 8 km behind and to the right of the track. To investigate the nature of the wind field asymmetry, the horizontal wind components were partitioned into a horizontal mean wind, varying with altitude and a perturbation wind. The cylindrical nature of the wind field permitted further partitioning of the perturbation wind into the mean vortex and a perturbation from the mean vortex. The mean wind derived from the partitioning agreed with synoptic analyses and was consistent with past studies—lying about 20° to the right of the storm motion. The symmetric vortex structure was consistent with past studies when it is taken into consideration that Norbert was a weakening storm. The asymmetric perturbation field had a distinct signature at both lower and upper levels. At 1 km, the asymmetric perturbation had the form of a source–sink field. The source and sink points were at the radius of maximum wind with the convergent sink on the leading, inflow side of the vortex and the divergent source on the trailing, outflow side of the vortex. At 3 km and higher, the asymmetric wind fields had the character of a vortex couplet with cyclonic and anticyclonic gyres in the right-front and left-rear quadrants of the storm, respectively, and stagnation points at the radius of maximum wind on the upwind, trailing and downwind, leading side of the storm.