RAPID SCAN AIRBORNE DOPPLER OBSERVATIONS OF HURRICANE
STRUCTURE AND EVOLUTION
Principal Investigator:
John F. Gamache
Collaborating
scientist(s):
H.E. Willoughby
Christopher Samsury
Michael Black
Frank D. Marks, Jr.
Peter G. Black
Objective:
Describe the various inner core kinematic and reflectivity structures, and
their relationships to environmental factors, such as wind shear and
sea-surface temperature
Rationale:
Hurricanes exhibit a variety of structures. The structure of the storm winds
and precipitation has an effect upon the distribution of damage, and may
affect the track and intensity changes. Thus, there is a need to document
the various types of structures seen in hurricanes and relate them to
variations in environmental conditions such as shear and sea-surface
temperature.
Method:
Fly coordinated missions of two NOAA WP-3D aircraft both carrying the now
standard Doppler radars that scan in a plane perpendicular to aircraft track.
Fly aircraft along simultaneous, but normal flight tracks through the storm
center, thus obtain winds out to 30-40 km radius from storm center within a
span of 10 minutes. Repeat approximately every 40 minutes. Analyze Doppler
data for each successive scan and note the core structure and its
changes.
The analysis method used in this study is a
three-dimensional variational scheme. Winds
are matched as closely as possible to the observed Doppler radial velocities
while satisying the three-dimensional continuity equation. Filtering is
also included within the cost functions used for the minimization. The wind
grid and its filtering may be either cartesian or cylindrical (usually used
for winds at the core of a hurricane).
Accomplishment:
Coordinated airborne Doppler missions have been flown into several hurricanes,
notably Hurricane Gustav in 1990, Hurricanes Claudette and Jimena in 1991,
Tropical Storm Tina in 1992, Hurricane Emily in 1993, Hurricane Olivia in
1994, Hurricanes Iris and Luis in 1995, and Hurricane Edouard in 1996.
The most extensive analyses of the Doppler data have been performed for the
dates: Hurricane Jimena on 23 September 1991, and Hurricane Olivia on
24 and
25 September 1994.
The hodographs obtained from the mean wind fields for all penetrations on
a given date are shown in Figure 1 for Jimena
on 23 September and in Figure 2
for Olivia on 24 and 25 September. The shear on 23 September 1991 in
Hurricane Jimena was east-southeasterly in the low-to-mid troposphere, while
on 24 September 1994 the shear was weaker in Hurricane Olivia. The mean
shear was no stronger on 25 September in Olivia than on 23 September in
Jimena, however it was westerly. Hurricane Jimena remained at a virtually
constant intensity for
24 hours, while during the flights of 24 and 25 September in Olivia, the
hurricane strengthened 1-2 mb/hour on the 24th and weakened by as much as
3 mb/hour on the 25th. Possible contributors to the various evolutions on
these three days must include not only the shear as manifested in the inner
core, but also the sea surface temperature. On 25 September Olivia was
traveling north across a sharp gradient of sea surface temperature, and this
probably contributed to its decreasing intensity, and to its evolution from
a very symmetric (Figure 3)
to a highly asymmetric structure (Figure 4).
Last modified: 10/9/96
