Hurricane Olivia
Inner Core Structure and Evolution Experiment

(940925H 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 enoug h 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 Evol ution 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 win d fields by the airborne Doppler radars. It was decided that, due to storm intensity, N42RF would fly at 10,000 ft RA while on station while N43RF flew at 14,000 ft. In coordination with N42RF, N43RF would drop ODW's and LOD2's.

Mission Synopsis

N42RF departed Puerto Vallarta at 1732 UTC. Ferry time was approximately 2 hours and 50 minutes, and the aircraft approached Hurricane Olivia from the N.

At 2020 UTC, N42RF reached the IP (19.8N, 120.1W) at 10,000 ft RA. We attempted to listen to an ODW dropped by N43RF; however the ODW failed. At 2027 N42RF dropped an AXBT near 19.4N, 120.1W. The first penetrations of the N and S eyewalls measured 125 kts and 135 kts, respectively. A center was found at 2033 at 19.0N, 120.1W, while the extrapolated minimum surface pressure was 926 mb. The southernmost point (18.1N, 120.1W), point 2, was reached at 2045. Near this time an ODW was dropped by N43RF, an d N42RF received the signal. N42RF turned toward the ENE and reached point 3 (18.4N, 119.4W) at 2053. The second pass then began toward the NW. The circulation center was found at 210633 at 19.0N, 120.1W, and the minimum extrapolated surface central pr essure during the 2 days of flight, 925 mb, was measured. The maximum wind speed measured in the NW eyewall measured during this pass was 120 kts, at 19.1N, 120.2W. N42RF reached point 4 (19.6N, 120.7W) at 2118, and headed SSW to point 5 (19.0N, 121.0W) , which was reached at 2126.

During the third radial pass, only an approximate center was found at 19.1N, 120W. The maximum winds on the W and E sides of the eyewall were 115 kts and 125 kts, respectively. N42RF dropped an AXBT at 214630 at 19.1N, 119.5W. It had a good signal. Po int 6 (19.2N, 119.5W) was encountered at 2150 UTC. While N42RF was on the E side of the storm, N43RF launched an ODW that had a weak signal on the S side of Hurricane Olivia. N42RF turned NNW and reached point 7 (19.7N, 119.5W) at 2158, where it began a radial flight leg headed toward 225 degrees. Before reaching the NE eyewall N42RF launched another AXBT. The maximum winds in the NE and SW eyewalls were 115 and 110 kts, respectively. Point 8 (18.4N, 120.8N), to the SW of the storm center was attaine d at 2221 UTC. After then flying an ESE heading, N42RF reached point 9, S of the storm center, at 2233, and began orbiting while the N43RF's radar system was being repaired. At 2238 it began its fifth pass through the center, tracking N. The maximum wi nd speeds on the S and N sides of the eyewall were 110 and 125 kts, respectively. At 2252 (19.5N, 120.1W) graupel was encountered N of the storm center. N42RF reached point 1, N of the storm center, at 2301. N42RF turned WSW and arrived at point 4 (19.8 N, 120.6W), NW of storm center, at 2307. During the 6th radial pass toward the SE, the center was estimated at 19.3N, 120.0W, and 127 kts maximum wind were measured in the SE eyewall. At 2330 UTC, when N42RF was at point 3 (18.7N, 119.3W) to the SE of s torm center, the aircraft turned to toward 24 deg., and reached point 6 (19.3N, 119.1W), to the E of the storm. Two more passes were made through the storm center in an E-W orientation to minimize the time for both planes to return to Puerto Vallarta, wh ile allowing a seventh coordinated pass through the eye. The last pass, since it was not coordinated was to be used to fix the center, while hunting would not be done on the seventh radial pass. Amusingly, it was the first pass which provided a good fix . N42RF began the seventh pass at 2337 from point 6, and found an extrapolated minimum surface pressure of 931 mb, with a wind center at 19.4N, 120.0W, and wind maximum in the E eyewall of 125 kts. The westernmost point (19.4N, 120.8W) was reached at 00 00 UTC, and N42RF then turned around and headed eastward. On this last pass a center was found near 19.5N, 120.0W, with the lowest extrapolated surface pressure being 935 mb. The highest wind speed noticed for the whole day (135 kts) was then encounter ed in the E eyewall. The climb out the Puerto Vallarta began at 0019 UTC at 19.3N, 119.5W. N42RF returned to Puerto Vallarta at 0305 UTC after passing through an impressive line of cumulonimbi and lightning. An excellent job of threading the needle thr ough the line was done by a pair of pilots already taxed by the mission.

Mission Evaluation and Problems

The Inner-Core Structure and Evolution flights of 25 September 1994 were also extremely successful. A second case of great interest was documented. On this day a dramatic change in structure was documented, as the storm was transformed from very symmetr ical at the beginning of the flight, with moderate reflectivity, to highly asymmetrical at the end of the flight, with reflectivities low to the S of the center and approaching 60 dBZ to the N of the center. The central pressure appeared to be increasing by about 2 mb/h. Flight 940925H1 occurred shortly after recurvature, when a SW shear had developed over the hurricane. We will have wind fields, at 35-min resolution, to help explain the dramatic change in structure. The excellent coordination by AOC of both aircraft will permit the accurate construction of eyewall wind fields in three dimensions. The 24 and 25 September Doppler data sets are as good as any in:
  1. the sensitivity of the radars
  2. the evolution of the winds fields
  3. navigation of the Doppler data
  4. reliability of the radars during radial penetrations
  5. coordination of the passes of the two aircraft through the center
  6. supplementary ODW observations in the eye and the exterior of the inner core
  7. availability of surface wind speed and direction from stepped-frequency microwave radiometer, and C-SCAT scatterometer measurements, respectively.

In addition, stable-isotope measurements of water and water vapor in the eye and the rest of the storm may help to confirm or deny hypotheses concerning the eye dynamics and evolution.

As with the 24 September 1994 case, a number of HRD researchers have expressed interest in analyzing the data, including Hugh Willoughby, Peter Black, Frank Marks, Jimmy Franklin, and me. Improved description of structure and intensity changes should re sult from the analyses of these data, and new and old ideas regarding eye dynamics and core evolution should be evaluated.

As on the 24th, the listening by N42RF 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.