KINEMATIC VELOCITY STRUCTURE OF HURRICANE GILBERT (1988)
Dr. Frank D. Marks, Jr.
Dr. Robert W. Burpee
Dr. John Gamache
Analyze airborne Doppler radar data collected in Hurricane Gilbert
on 14 September, 1988, to estimate the axisymmetric components of the
three-dimensional windfield of this Category 5 hurricane.
Hurricane Gilbert of 1988 was the most intense tropical cyclone yet
observed in the Atlantic Basin. Data collected by a
NOAA research aircraft on 13 September
indicates that Gilbert's minimum sea-level pressure (MSLP) was 888 mb. A few
hours after this aircraft left the storm, a second NOAA aircraft monitored the
hurricane when its MSLP was ~ 895 mb. This aircraft had an airborne Doppler
radar and was the first to record Doppler data in a tropical cyclone with a
MSLP < 900 mb. The Doppler data allow us to estimate the kinematic windfield
in the inner core, and compare it with the winds in other less-intense
Doppler data from two orthogonal legs through the center of the
storm were interpolated to a storm-relative coordinate system, in a
pseudo-dual Doppler analysis. The three-dimensional wind field was then
retrieved from the Doppler observations (see Three-dimensional Variational Analysis of Airborne Doppler Observations).
Because the two legs
comprise an hours worth of data, the windfield does not resolve convective
features but does provide an estimate of the mean structure of the storm.
The reflectivity pattern from a single sweep of the lower-fuselage
radar shows Gilbert's mesoscale features (FIGURE
1). The inner eyewall is the
closed ring > 25 dBZ located 1-20 km from the center. Rainbands 60-90 km
from the center coalesced to form an outer eyewall.
In the inner core the tangential winds > 50 ms-1 extended
to 12 km, ~ 4 km higher than had been observed in previous
hurricanes. (FIGURE 2) The secondary circulation had
flow into the center up
to altitudes of 2.5 km, and quite weak outflow above. The region
between the inner and outer eyewalls was
characterized by lower reflectivities, reduced flight-level winds and vertical
velocities < 1 ms-1. The outer eyewall contained secondary
maxima of flight-level wind, radar reflectivity and vertical velocity.
The arrrangement of the rainbands composing the outer eyewall changed
thoughout the flight, giving the outer eye an oblate shape; these changes
may be related to track oscillations.
Doppler analysis of a small part of the outer eyewall
shows that there was a very shallow inflow layer, < 2 km deep.
The height of the radar bright-band decreased beyond
the outer eyewall, perhaps demarking the limit of subsidence warming.
A manuscript has been prepared that discusses the kinematic
structure of the inner core, a portion of the outer eyewall, and the regions
beyond, and compares these features with those observed in other hurricanes.
Dodge, P., R. W. Burpee, and F. D. Marks, Jr., 1991: Airborne
Doppler radar analyses of the core of Hurricane Gilbert. Preprints,
19th Conference on Hurricanes and Tropical Meteorology, Amer. Meteor.
Soc., Miami, 551-552.
Last modified: 9/13/96