Structure and Evolution of Hurricane Claudette on 7 September 1991 from Airborne Doppler Radar Observations. Part I: Kinematics

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  • 1 Centre d'étude des Environnements Terrestre et Planétaire (CNRS-UVSQ), Issy-Les-Moulineaux, France
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Abstract

On 7 September 1991, an experiment was conducted with the two National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft to investigate the inner-core region of Hurricane Claudette. Both aircraft carried airborne Doppler radar, and one of them (N43RF) was equipped with a French dual-beam antenna that allows velocity measurements in two different directions. The aircraft flew simultaneously a series of rotated radial penetrations of the eyewall, the upper one being oriented 90° clockwise from the lower one. This dataset provided an extensive survey of the storm inner region (radius less than 100 km) for a period of about 7 h.

The Doppler velocity data from the two aircraft were analyzed with an EVTD (extended velocity track display) method. The precipitation content and the tangential, radial, and vertical wind components were calculated at 6 hourly intervals, in 50 rings at 1–99-km radii and 25 levels at 0.5–12.5-km altitudes, as wavenumbers 0, 1, and 2 with respect to the storm-relative azimuth. The limited angular resolution, associated with an efficient time filtering, ensured that the small-scale convective and inertial perturbations were virtually eliminated in the obtained wind fields. Comparisons with independent flight-level measurements showed that the EVTD-derived velocity values were representative of the storm kinematic structure.

During the airborne observations, Hurricane Claudette underwent a partial “eyewall replacement cycle,” as an inner crescent-shaped zone of high-reflectivity values progressively shrank, while at larger radii a ring of strong echoes built up. The first kinematic characteristic of Hurricane Claudette was its motion at a speed smaller, although in a similar direction, than the mean horizontal winds. For this reason, the inflow was mainly from the rear (south-southeast) in the lower and upper levels. The symmetric vortex structure was characterized by a decreasing inner updraft at 5–25-km radii and, at larger radii, an intensifying updraft with downward motions on its inner and outer sides in the upper levels. The low-level inflow and upper-level outflow consequently intensified in the outer part of the domain, while there was a slight decrease of the tangential wind close to the radius of maximum wind and an increase in the low levels below the developing updraft.

In the inner region (15–25-km radii), the updraft and low-level inflow were located upwind of the reflectivity maximum and progressively decreased while the downdraft intensified on the downwind side. In the outer ring (25–50-km radii), upward motions built up from the east-southeast in the low levels to the west-southwest in the upper levels. The asymmetric part of the horizontal wind (total wind minus the mean horizontal component and the symmetric vortex) displayed a wavenumber 1 eddy couplet below 6-km altitude and a mass source—sink pair above. These features, which probably resulted from divergence patterns associated with the vertical motions and from interactions between the storm motion and the mean vortex, were perturbed by the evolution of the kinematic structure. The asymmetric flow changed substantially during the considered period, but the flow across the storm center remained approximately constant. This could be related to the nearly linear motion of the storm.

Abstract

On 7 September 1991, an experiment was conducted with the two National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft to investigate the inner-core region of Hurricane Claudette. Both aircraft carried airborne Doppler radar, and one of them (N43RF) was equipped with a French dual-beam antenna that allows velocity measurements in two different directions. The aircraft flew simultaneously a series of rotated radial penetrations of the eyewall, the upper one being oriented 90° clockwise from the lower one. This dataset provided an extensive survey of the storm inner region (radius less than 100 km) for a period of about 7 h.

The Doppler velocity data from the two aircraft were analyzed with an EVTD (extended velocity track display) method. The precipitation content and the tangential, radial, and vertical wind components were calculated at 6 hourly intervals, in 50 rings at 1–99-km radii and 25 levels at 0.5–12.5-km altitudes, as wavenumbers 0, 1, and 2 with respect to the storm-relative azimuth. The limited angular resolution, associated with an efficient time filtering, ensured that the small-scale convective and inertial perturbations were virtually eliminated in the obtained wind fields. Comparisons with independent flight-level measurements showed that the EVTD-derived velocity values were representative of the storm kinematic structure.

During the airborne observations, Hurricane Claudette underwent a partial “eyewall replacement cycle,” as an inner crescent-shaped zone of high-reflectivity values progressively shrank, while at larger radii a ring of strong echoes built up. The first kinematic characteristic of Hurricane Claudette was its motion at a speed smaller, although in a similar direction, than the mean horizontal winds. For this reason, the inflow was mainly from the rear (south-southeast) in the lower and upper levels. The symmetric vortex structure was characterized by a decreasing inner updraft at 5–25-km radii and, at larger radii, an intensifying updraft with downward motions on its inner and outer sides in the upper levels. The low-level inflow and upper-level outflow consequently intensified in the outer part of the domain, while there was a slight decrease of the tangential wind close to the radius of maximum wind and an increase in the low levels below the developing updraft.

In the inner region (15–25-km radii), the updraft and low-level inflow were located upwind of the reflectivity maximum and progressively decreased while the downdraft intensified on the downwind side. In the outer ring (25–50-km radii), upward motions built up from the east-southeast in the low levels to the west-southwest in the upper levels. The asymmetric part of the horizontal wind (total wind minus the mean horizontal component and the symmetric vortex) displayed a wavenumber 1 eddy couplet below 6-km altitude and a mass source—sink pair above. These features, which probably resulted from divergence patterns associated with the vertical motions and from interactions between the storm motion and the mean vortex, were perturbed by the evolution of the kinematic structure. The asymmetric flow changed substantially during the considered period, but the flow across the storm center remained approximately constant. This could be related to the nearly linear motion of the storm.

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