Hurricane Olivia
Inner Core Structure and Evolution Experiment
(940924H Aircraft 42RF)

Scientific crew
Chief Scientist J. Gamache
Doppler Scientist P. Dodge
Cloud Physics Scientist J. Roles (AOC)
J. Barr (AOC)
Dropwindsondes (listening) R. Burpee
Visiting Scientist J. Lawrence (UHouston)

This document is divided into 3 sections (Each section is written by the Chief Scientist):

Mission Briefing

Please refer to summary for N43RF for a description of storm motion, position and intensity. The priority mission for Hurricane Olivia was to fly the Vortex Motion and Evolution experiment. This was considered a high enou gh priority that the Inner Core Structure and Evolution experiment was removed from the Hurricane Field Program Plan in 1993. Since Olivia was so far from Puerto Vallarta, and VME requires 6 hours on station, we opted for the Inner Core Structure and Evo lution experiment. Repeated coordinated "alpha" patterns are flown by two aircraft, and the flight tracks are kept normal to each other. The experiment is conducted within 50 nm of the storm center, and a mapping every 35 minutes is made of the core wi nd fields by the airborne Doppler radars. It was intended that N42RF would fly at 5,000 ft RA while on station while N43RF flew at 10,000 ft. In coordination with N42RF, N43RF would drop ODW's and LOD2's.

Mission Synopsis

N42RF departed Puerto Vallarta at 1653 UTC. Ferry time was approximately 2 hours and 45 minutes, and the aircraft approached Hurricane Olivia from the N. At 1837, we began listening to an ODW drop from N43RF during its ferry. While enroute we heard fro m NHC that they now estimated that Hurricane Olivia had 100-kt winds, 955 mb pressure, and was moving toward 290 degrees. With this news, Frank Marks and John Gamache decided to move the flight altitudes of N43RF and N42RF to 14,000 and 10,000 ft PA, res pectively. Pressure altitude was chosen because of a failure of N43RF's radar altimeter.

We reached the IP (16.6 N, 117.9W), and proceeded S toward the eye. At 16.2N, 117.9W, the first AXBT was dropped, and was good. The winds on the N side of the eye had an approximately 100-kt maximum. The minimum extrapolated surface pressure was 954 m b, and the storm center was found near 15.7N, 117.9W. As we proceeded toward the southern eyewall, we saw an unbroken region of ~45 dBZ or more; however, the flight through the wall was not rough.

A wind maximum of ~110 kt was seen. At 1953 (15.2N, 118.0W), a second AXBT was dropped. We reached the southernmost point of leg 1 at 1957 and proceeded ENE to point 3. At 2005 (15.2N, 117.3W) we reached point 3 and turned NW. On the SE side of the st orm we found maximum winds of 120 kt, and then located the storm center near 15.9N, 118.0W. We reached the point farthest NW (point 4) at 2030 (16.5N, 118.8W), turned, and tracked 220 deg. to point 5, which we reached at 2040 (15.9N, 119.0W). During th is downwind leg the radar computer system was down from 2032-2035. Contrary to Murphy's law, this down time was during a downwind leg and was simultaneous with the down time on N43RF, and thus no data were lost during the radial legs. N42RF tracked E, an d found the center at 15.9N, 118.1W, with an extrapolated minimum surface pressure of 949 mb. Winds speeds of 130 kt were on the E side of the eye. We continued eastward, dropping an AXBT at 2059 (15.9N, 117.5W) that provided only a few data before fail ing, and reaching the eastmost point of the third leg at 2103 (15.9N, 117.2W). After travelling NNE to point 7 (16.4N, 117.4W at 2110), we dropped an AXBT that was good. We travelled SW, dropping another AXBT at 2119 (16.1N, 118.0N) that was bad. A win d maximum of 115 kt was found in the NE eyewall, while a 105-kt wind was found in the SW eyewall. We reached point 8 (15.4N, 118.8W) on the SE side at 2134 and point 9 (15.0N, 118.2W) on the S side at 2142. N42RF executed a 20 deg.-bank purl while coord inating, and turned directly N toward the center. The center was found on this leg at 16.0N, 118.3W, with an extrapolated minimum surface pressure of 947.5 mb. A 115-kt wind maximum was found in the N eyewall, and we arrived at point 1 (or the IP transl ated by the amount of motion of the storm) at 2209 (16.7N, 118.2W). We decided that we had time for two more passes through the storm, so we did a 90-270 turn and headed S again, finding a 117-kt wind maximum, and an extrapolated minimum pressure of 948. 5 mb at the center. We reached the S point (15.3N, 118.3W) at 2235 UTC, and turned around again to head N. During this last pass we found a 108-kt wind maximum in the S eyewall, a 119-kt wind maximum in the N eyewall, and a 948 mb center at ~16.2N, 118. 3W. The pattern ended at 2302 at 17.1N, 118.3W.

Mission Evaluation and Problems

The Inner-Core Structure and Evolution flights of 24 September 1994 were extremely successful. Although this plan had been removed from the Hurricane Field Program Plan in deference to the VME experiment, which provides a context for the inner-core data , Hurricane Olivia on 24 September was the kind of storm Frank Marks and I had hoped to perform the inner-core experiment on, i.e. a hurricane that was intensifying while the aircraft were measuring the three-dimensional wind structure with airborne Doppl er radar. Hurricane Gustav of 1990 was quite asymmetrical until 30 August. Unfortunately the 30 August flights were short, and the sensitivity of the Doppler radar was diminished. In 1991, data were obtained in Hurricane Jimena during four dual-aircraf t passes, followed by two complete circles by one aircraft, and INE was used to locate the Doppler radars. Also in 1991 was Hurricane Claudette. Radar sensitivity again affected the observation of the upper portions of the hurricane, and the data were l ocated using inertial navigation. In 1992 GPS navigation was available for three dual-aircraft passes through Hurricane Tina, but the system was still only a poorly organized tropical storm. The 24 September 1994 data set is as good as, or better than, any other Inner-Core Structure and Evolution Experiment data set in all the following categories:

  1. sensitivity of the radars
  2. evolution of the wind field
  3. navigation of the Doppler data
  4. reliability of the radars during the radial penetrations
  5. coordination of the passes of the two aircraft through the center
  6. supplementary ODW observations in the exterior and eye
  7. availability of surface wind speed and direction from stepped-frequency microwave radiometer and C-SCAT measurements, respectively.

A number of HRD researchers are interested in the data, including at least Hugh Willoughby, Peter Black, Jimmy Franklin, as well as Frank Marks and me. Improved description of structure and intensity change should result from the analyses of these data, and new and old ideas regarding eye dynamics and inner-core evolution should be evaluated.

The listening to the signals of ODW's dropped from N43RF was successful. This suggests that one aircraft will be able to back-up the recording of the other aircraft's ODW data during future VME's.