Mission Summary
Danielle

Mission Briefing:

At mission planning time, Hurricane Danielle was a weak 1 hurricane, officially with 80 knot surface wind speeds, although indications were that actual strength might be decreasing slightly. The first morning visible pictures showed healthy convection near the storm center, and Danielle was still to the east of the axis of the sea-surface wake of Hurricane Bonnie, which had visited the area just to the west only five days before. At 1500 GMT on 29 August, the storm center was estimated to be at 25.1°N, 70.1°W, with a mean sea level central pressure of 988 mb.

Since the mission was to be launched from Opa Locka Airport in Dade County, Florida, and onstation time for the nominal pattern is approximately 5.5 to 6 hours, the mission was expected to be near the maximum duration of 10 hours, unless the pattern was abbreviated. The mission selection for the day was the Vortex Motion and Evolution experiment (VME), normally planned to be a two-NOAA-WP3D-aircraft mission. On 29 August, 1998, however, six planes would be coordinated to observe Danielle simultaneously: the two P3's, the NOAA Gulfstream IV (G-IV), the NASA DC-8 and ER-2, and during portions of the mission, a USAF WC-130 aircraft (whose task was to obtain official fixes and observations of the hurricane). N42RF was to fly at 12,000 ft PA, while N43RF would fly at 16,000 ft PA while near the center, and maximum safe altitude while outside a 50-nautical-mile (nary) radius from storm center. The NASA aircraft were to fly a pattern similar to N43RF, although rotated from N43RF, and close coordination between NASA and NOAA were not required except when dropwindsonde safety made it necessary. The G-IV flew an operational flight around the storm periphery, dropping sondes to determine the near environmental wind field.

N42RF's mission was to fly 5 rotating figure 4's, where the first, third and fifth would be coordinated with N43RF in an effort to maximize the usefulness of the airborne Doppler radar observations. During the second and fourth figure 4's, and before the first and after the fifth, N43RF would be conducting dropsonde observations 1-3° away from storm center.

Mission Synopsis:

Numerous sondes and AXBT's were dropped in the eyewall, or what could best be judged to be the eyeall. Although maximum flight-level winds at 12,000 ft were below 80 kts, an EVTD Doppler wind analysis showed maximum winds at 1 km to be about 80 kts, and at least one GPS dropsonde also reported 80 kts near the surface, thus confirming that Danielle, in spite of its less-than-impressive central appearance, was still a hurricane. The final center fix at 0147 UTC indicated storm center tobe at 26.1°N, 71.5°W, thus indicating Danielle's approximate motion during the mission was 14 kts toward azimuth 300. N42RF returned to Opa Locka airport at 034027 UTC N42RF departed Opa Locka at 183435 UTC, approximately 15 minutes after N43RF, in anticipation of the future flight tracks and the desire to pass through the storm center simultaneously. Descent to the flight-plan altitude began at 1954 UTC (26.1°N, 73.9°W), and the IP (130 nm from storm center at a bearing from storm center of 300) was reached at 200845 UTC (26.5°N, 73.0°W). There was some difficultly in pinpointing the storm center, since the central convection turned out to be assymetrically placed to the east of the wind center. The first fix was found to be at 25.7°N, 70.9°W, with a central MSLP of about 986 mb. The full mission of 5 figure 4's were performed,during which 10 hurricane penetrations were made, 24 GPS sondes were dropped and 18 AXBT's (Airborne eXpendable BathyThermographs) were dropped. The particulars concerning fixes, drops and flight legs are shown in the tables.

The tail radar was operated with the Fore/Aft Scanning Technique (FAST)through most of the mission. it was operated with flat scans normal to the flight track between points 10 and 11, and between points 12 and 13. A VTD analysis was performed and transmitted to NHC, using the flat-scan data. The NASA ER-2 was unable to participate in the mission.

Evaluation:

This mission represents the first of its kind, in which the structure and evolution of the hurricane core is detailed, while the region from the center to 160 nm from storm center is well mapped with sondes to a depth above 200 mb. Finally the near environment in all quadrants of the storm was mapped with sondes dropped from above 150 mb. The data thus hold great promise for understanding forecast track difficulty, and they should provide the best data set ever for computing the eddy angular momentum transport into the storm, as well as mapping the larger-scale shear of the near environment.

The initial structure of the storm core was somewhat disorganized (Fig. 1) as N42RF approached it, but it appeared to become better organized by the end of the mission (Fig. 2), and it was quite well organized on 30 August in a follow-up multi-aircraft mission. The data from the 29 and 30 August should provide insights into the role of the environment and sea surface in modulating storm structure and intensity.

Problems:

1. Radar data system went down briefly at 2220 UTC, and had to be reset.
2. Tail (Doppler) radar recording down from 2220 to 2235 UTC and from 0030 to 0038 UTC.
3. Dropsondes worked until the last three, the first two having little data, and the last was a failed drop.
4. The VTD analysis indicated too intense a hurricane showing winds above 100 knots on the side of the hurricane with little data. More care will be taken in the future to be sure the functional fits are not overextrapolating.

John Gamache
9 September 1998

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