WIND FIELD AND REFLECTIVITY STRUCTURE AND EVOLUTION OF HURRICANE OLIVIA ON 25 SEPTEMBER 1994

Principal Investigator: John F. Gamache
Collaborating scientist(s):

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 will be the future positions of the hurricane (track) and how strong will the winds be (intensity). To 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.

Accomplishment: 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.

In FY1997, 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: 11/13/97