WIND FIELD AND REFLECTIVITY
STRUCTURE AND EVOLUTION OF HURRICANE OLIVIA ON 25 SEPTEMBER 1994
Principal Investigator:
John F. Gamache
Collaborating scientist(s):
Hugh
E. Willoughby
Michael
Black
Frank
D. Marks, Jr.
Peter
G. Black
Christopher Samsury/The Weather
Channel
Objective:
To document the structure of Hurricane Olivia during the NOAA aircraft
research mission conducted on 25 September 1994. To document the evolution
of the wind field during the 3+ hours of airborne Doppler observations.
Rationale : The two most important
forecast parameters for tropical cyclones are the future positions of the
hurricane (track) and the future strength of the winds (intensity). The
goal of the airborne mission was to document at 1/2 hour resolution the
changes in the tangential, radial and vertical winds, as well as the precipitation
structure and intensity, in the core of the storm.
During the on-station portion
of the 25 September mission, Hurricane Olivia underwent a dramatic reduction
in the intensity of the upper half of the storm, and the shear of the inner-core
mean wind greatly increased. We were very fortunate to arrive in the storm
while it was highly symmetric and to leave when it had become more asymmetric
than most hurricanes. Although the original intent of the eyewall evolution
experiment did not have this kind of storm in mind, it should help to illustrate
the roles of shear and changing sea-surface temperature.
Method: Perform the analysis
of wind speed and direction for each of the 7 "snapshots" of the inner
core wind field. Document the change with time in the wind field. Note
the relationship of wind and reflectivity to average shear of wind within
the storm core.
Results : The analyses show
that the overall reflectivity distribution changed dramatically over the
3+ hour period (Figure 1a and 1b).
The north-northeastward motion of the storm was carrying Hurricane Olivia
over a strong gradient in sea surface temperature, and toward the colder
water. The degree to which the increased shear in the storm results from
an approaching large scale shear zone, or it results from an internal response
to colder sea surface temperatures is unclear at this time. The change
in shear from the first of seven core wind analyses to the last one can
be seen in (Figure 2), .
The intensity of the radius-height
mean winds in the top half of the storm decreased greatly, while the bottom
half remained nearly unchanged, as shown in Figure
3.
Doppler radials for one pass
through Hurricane Olivia on 25 September were composited relative to a
moving cell in the eyewall. Motion of the reflectivity features was assumed
to be the tangential wind speed--a rough estimate for cell motion. Examples
of this vertical cross sections through the cell are shown in Figure
4.
Last modified: 02/05/00