Editorial Type:
Article
Article Type:
Research Article

The Evolution of Hurricane Danny (1997) at Landfall: Doppler-Observed Eyewall Replacement, Vortex Contraction/Intensification, and Low-Level Wind Maxima

Keith G. Blackwell Department of Geology, Geography, and Meteorology, University of South Alabama, Mobile, Alabama

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Print Publication:
01 Dec 2000
DOI:
https://doi.org/10.1175/1520-0493(2000)129<4002:TEOHDA>2.0.CO;2
Page(s):
4002–4016
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Abstract

Danny made landfall as a minimal hurricane on the Alabama coast on 19 July 1997 after drifting over Mobile Bay for over 10 h. Danny’s unusually close proximity to the Doppler radar (WSR-88D) in Mobile provided an unprecedented view of the storm’s complex and dramatic evolution during a prolonged landfall event over a 1-day period.

Base reflectivity and velocity products were combined with aircraft reconnaissance information to detail the formation of concentric eyewalls and complete evolution of an eyewall replacement cycle. This highly symmetric hurricane then underwent a rapid asymmetric transition in Mobile Bay during which a small eyewall mesovortex developed adjacent to intense convection in the western eyewall. Radar-estimated rainfall increased dramatically during the asymmetric phase. Rates exceeded 100 mm h−1 for nine consecutive hours west of the center while precipitation nearly vanished to the east. Changes in the distribution of precipitation corresponded with changes in the low-level wind velocity structure.

A 25-h temporal composite of WSR-88D base velocities displayed axisymmetric intensification and contraction of Danny’s core during the eyewall replacement cycle. Later, the asymmetric phase was dominated by further contraction and intensification on the west side only. In the western eyewall, a persistent boundary layer wind maximum evolved and contracted to a radius of only 10–13 km from the center. Concurrently, eastside boundary layer winds diminished as the maximum winds rose to 1–1.5-km altitude and the radius expanded. Danny reached maximum intensity during this asymmetric phase in Mobile Bay with base velocities >44 m s−1 (85 kt) at 600-m elevation. Eyewall contraction in the meandering storm, combined with climatologically elevated SSTs in shallow Mobile Bay, probably played significant roles in Danny’s continued intensification there.

Discrepancies arose in determining Danny’s structural patterns, intensity, and evolution during landfall. Interpretation varied depending on the type of platform used to observe the storm. The continual sampling of the storm by the nearby WSR-88D provided detail not available from aircraft data alone. Doppler base velocities in the intense westside convection were much stronger than measured at flight level. Yet, the opposite was true in the storm’s drier east side.

Doppler radar showed that the westside base velocity maxima were confined to the boundary layer (600–700 m) and represented a slightly conservative estimate of maximum surface gusts recorded at Dauphin Island during the passage of Danny’s convective eyewall. Thus, winds probably were even stronger at boundary layer levels below the WSR-88D’s lowest scan elevation. The shallowness and persistence of Danny’s boundary layer velocity maxima stressed the need for accurate wind information in the lowest few hundred meters of a tropical cyclone’s eyewall for a better indication of the storm’s true intensity and near-surface wind velocities.

Corresponding author address: Dr. Keith G. Blackwell, Department of Geology, Geography, and Meteorology, LSCB 136, University of South Alabama, Mobile, AL 36688.

Email: kblackwe@jaguar1.usouthal.edu.

Abstract

Danny made landfall as a minimal hurricane on the Alabama coast on 19 July 1997 after drifting over Mobile Bay for over 10 h. Danny’s unusually close proximity to the Doppler radar (WSR-88D) in Mobile provided an unprecedented view of the storm’s complex and dramatic evolution during a prolonged landfall event over a 1-day period.

Base reflectivity and velocity products were combined with aircraft reconnaissance information to detail the formation of concentric eyewalls and complete evolution of an eyewall replacement cycle. This highly symmetric hurricane then underwent a rapid asymmetric transition in Mobile Bay during which a small eyewall mesovortex developed adjacent to intense convection in the western eyewall. Radar-estimated rainfall increased dramatically during the asymmetric phase. Rates exceeded 100 mm h−1 for nine consecutive hours west of the center while precipitation nearly vanished to the east. Changes in the distribution of precipitation corresponded with changes in the low-level wind velocity structure.

A 25-h temporal composite of WSR-88D base velocities displayed axisymmetric intensification and contraction of Danny’s core during the eyewall replacement cycle. Later, the asymmetric phase was dominated by further contraction and intensification on the west side only. In the western eyewall, a persistent boundary layer wind maximum evolved and contracted to a radius of only 10–13 km from the center. Concurrently, eastside boundary layer winds diminished as the maximum winds rose to 1–1.5-km altitude and the radius expanded. Danny reached maximum intensity during this asymmetric phase in Mobile Bay with base velocities >44 m s−1 (85 kt) at 600-m elevation. Eyewall contraction in the meandering storm, combined with climatologically elevated SSTs in shallow Mobile Bay, probably played significant roles in Danny’s continued intensification there.

Discrepancies arose in determining Danny’s structural patterns, intensity, and evolution during landfall. Interpretation varied depending on the type of platform used to observe the storm. The continual sampling of the storm by the nearby WSR-88D provided detail not available from aircraft data alone. Doppler base velocities in the intense westside convection were much stronger than measured at flight level. Yet, the opposite was true in the storm’s drier east side.

Doppler radar showed that the westside base velocity maxima were confined to the boundary layer (600–700 m) and represented a slightly conservative estimate of maximum surface gusts recorded at Dauphin Island during the passage of Danny’s convective eyewall. Thus, winds probably were even stronger at boundary layer levels below the WSR-88D’s lowest scan elevation. The shallowness and persistence of Danny’s boundary layer velocity maxima stressed the need for accurate wind information in the lowest few hundred meters of a tropical cyclone’s eyewall for a better indication of the storm’s true intensity and near-surface wind velocities.

Corresponding author address: Dr. Keith G. Blackwell, Department of Geology, Geography, and Meteorology, LSCB 136, University of South Alabama, Mobile, AL 36688.

Email: kblackwe@jaguar1.usouthal.edu.

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