Hurricane Debbie’s eye as seen from ESSA DC-6
On August 18th and 20th, 1969 Hurricane Debbie was subjected to a seeding experiment as part of Project STORMFURY. STORMFURY was a joint Navy and Environmental Science Services Administration (ESSA was the predecessor of NOAA) project to test the hypothesis that seeding hurricanes with silver iodide would disrupt their circulation and reduce their winds. Two previous attempts to seed hurricanes (Esther in 1961 and Beuhlah in 1963) had been inconclusive. The Debbie flights were the most extensive weather modification experiments carried out to date for the Project and the ones that yielded the most encouraging results. Although the experiments were covered in the press, the public’s attention was concentrated at that time on Hurricane Camille, which had just made landfall in Mississippi.
U.S. Navy Super Connie flying into Hurricane Debbie.
Notice ‘sharkfin’ radome housing vertical scanning radar on top.
Debbie was a classic Cape Verde hurricane, forming from an African easterly wave that had become a tropical storm on August 15th while still 1200 nautical miles east of the Antilles. It’s forecast track took it directly into the STORMFURY allowed target area to the east of the Bahamas, so the fleet of fifteen Navy, Air Force, and ESSA aircraft were put on notice by August 16th and deployed to Puerto Rico on the 17th. The next day, Debbie was within the allowed area but at the maximum distance at which the experiments could still be carried out. The first monitoring aircraft took off at 5 AM and the first seeding time (Tango) was set for 9 AM. The two ESSA DC-6 airplanes flew racetrack patterns in and out of the eye of the storm to monitor any changes due to the seeding, while the Navy Super Connies flew around the storm using their superior vertical scanning radars to document the storm’s structure and the Air Force C-130 monitored high-level cloud particles. Both the ESSA B-57 and Air Force WB-47 carried out high-altitude surveillance. The seeding was done by Navy A-6 Intruder jets deploying canisters of burning silver iodide compound into the outer part of the hurricane’s eyewall. The seeding continued at two-hour intervals until 3 PM, while the aircraft switched in and out of their assignments throughout the day. The last monitoring plane landed at 11 PM that night.
Location of Hurricane Debbie during seeding experiments
The STORMFURY team analyzed the results throughout the next day and were very encouraged when they saw evidence of a weakening of the storm’s peak winds from 98 knots to 68 knots. Debbie was forecast to still be within the STORMFURY target area the next day, so another round of seeding experiments was planned. Upon returning to the hurricane on August 20th, the monitoring planes found that Debbie had regained strength with maximum sustained winds of 99 knots. They also found that Debbie had formed a double eyewall during the 19th and that the outer eyewall had winds almost as strong as the inner. The Intruders seeded just outside of this inner eyewall, until the last round. By then the inner eyewall had all but disappeared so the seeding took place just outside the outer eyewall. The next day, Debbie had moved beyond reach of Puerto Rico and the day was spent analyzing the second round of seeding. There was another reduction of strength noted, from 99 knots to 84 knots. While less of a reduction than the previous experiment, many of the scientists involved were greatly encouraged.
Hurricane Debbie’s double eyewall on August 20th as seen on radar.
ESSA touted the results of these experiments at an end-of-season press conference, but the STORMFURY scientists were careful to note that this was only one set of experiments and that many more would be needed in order to distinguish naturally caused changes in the storms from those caused by the seeding. Unfortunately for the Project, only one other round of seedings were carried out in Ginger in 1971. The Project remained funded until 1982, but further candidate storms eluded the researchers. In the end, it was concluded that the changes in Debbie could well have been the result of an eyewall replacement cycle and that no positive conclusions could be made about the effect of the seeding.
Some papers by HRD scientists relating to Hurricane Debbie:
- Bergman, K. H., and T. N. Carlson, 1975: Objective analysis of aircraft data in tropical cyclones. Mon. Wea. Rev., 103, 431–444.
- Black, P. G., and R. A. Anthes, 1971: On the asymmetric structure of the tropical cyclone outflow layer. J. Atmos. Sci., 28, 1348–1366.
- Black, P. G., H. V. Senn, and C. L. Courtright, 1972: Airborne radar observations of eye configuration changes, bright band distribution, and precipitation tilt curing the 1969 multiple seeding experiments in Hurricane Debbie. Mon. Wea. Rev., 100, 208–217.
- Dorst, N. M., 2007: The National Hurricane Research Project: 50 Years of Research, Rough Rides, and Name Changes. Bull. Amer. Meteor. Soc., 88, 1566–1588.
- Gentry, R. C., 1970: Hurricane Debbie modification experiments, August 1969. Science, 168, 473-475.
- Hawkins, H. F., 1971: Comparison of results of the Hurricane Debbie (1969) modification experiments with those from Rosenthal’s numerical model simulation experiments. Mon. Wea. Rev., 99, 427–434.
- Lewis, B. M., and H. F. Hawkins, 1982: Polygonal eye walls and rainbands in hurricanes. Bull. Amer. Meteor. Soc., 63, 1294–1301.
- Sheets, R. C., 1973: Analysis of hurricane data using the variational optimization approach with a dynamic constraint. J. Appl. Meteor., 12, 963–976.
- Sheets, R. C., 1973: Analysis of Hurricane Debbie modification results using the variational optimization approach. Mon. Wea. Rev., 101, 663–684.
- Willoughby, H. E., D. P. Jorgensen, R. A. Black, and S. L. Rosenthal, 1985: Project STORMFURY, A Scientific Chronicle, 1962-1983, Bull. Amer. Meteor. Soc., 66, 505-514.