Hurricane Olivia
Inner Core Structure and Evolution Experiment
(940924I Aircraft 43RF)

Scientific crew
Chief Scientist F. Marks
Doppler Scientist H. Willoughby
Cloud Physics Scientist R. Black
Dropwindsondes J. Franklin
Observers Fremmel (Radian Corp.)
One IMAX individual
One Mexican AF Meteorologist

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

Mission Briefing

In the morning of 24 September Olivia was an intensifying hurricane ~680 nm WSW of Puerto Vallarta, tracking WNW at 5 m/s. Over the preceding 6 h the storm appeared to be tacking a more northerly track, indicating that the storm might be starting to rec urve. Unfortunately, the storm was too far west (location 15.2N, 117.3W) to be a candidate for the Vortex Motion and Evolution (VME) Experiment. Hence, we opted for the Inner Core Dynamics Experiment from the 1992 Operation Plan (a forerunner to VME), des igned to study the evolution of the inner core over 3-4 h. We briefed for a takeoff at 1030 local time (1630 UT). The intended flight tracks were the "rotating Figure 4" pattern with 50 nm radial legs about the storm center. N42RF would enter the pattern from the N and maintain 5 kft RA throughout the flight, while N43RF would enter the pattern from the E and maintain 10 kft RA. N43RF would drop one ODW along the ferry, one ODW at the end of the flight legs in each of the four cardinal locations around th e storm, and one ODW in the eye. If possible, each drop would include an ODW and a Radian LOD2 for intercomparison.

Mission Synopsis

Departure from Puerto Vallarta was delayed until 1652 UT. The approach to Olivia was from the E with an expected ETA of 1900 UT near 15.4N 118.4W, with an IP 50 nm E of that position. The ODW/LOD2 drop along the ferry was canceled because of a LOD2 system failure. At 1910 UT, ~200 nm from the IP, we fixed the center with the lower fuselage (LF) radar at 15.7N 117.9W, farther N and approximately 40 nm closer to Puerto Vallarta than expected. The LF indicated that there was a definite eye 15-20 nm in diamet er with relatively intense reflectivity in the S quadrant. There was a principal rainband spiraling from 50 nm NE of the center out to ~100 nm SW of the center. John Gamache and I decided to adjust the aircraft altitudes because of the small eye size and the intense reflectivities. N42RF would maintain 10 kft PA and N43RF 14 kft PA. PA was chosen because of a RA failure on N43RF during the ferry.

N43RF reached the IP (15.75N 117.1W) at 1933 UT, dropped an ODW just outside the principal rainband, and tracked W across the eye encountering 122 kts winds in the E eyewall. Both aircraft crossed the center of the eye 1944 UT, within 15 s of each other, fixing the center at 15.8N 117.9W. The eye was relatively clear below the aircraft altitude, but was covered by thin cirrus. The inner edge of the eyewall cloud, at a radius of ~9 nm from the center, was not very distinct, with a lot of cloud protruding into the eyewall above the aircraft altitude. The second ODW was dropped at the westernmost point in the first leg (2) in the midst of a number of weak rainbands embedded in an extensive stratiform rain region that extended ~200 nm S and W of the center.

The second leg was from a point 50 nm SW of the center (2), on a NE track through the center. In the SW portion of the eyewall we encountered a very broad (50 s) updraft >10 m/s, with a peak 1-s value of 21 m/s. The updraft was just upwind of the most in tense reflectivities in the S quadrant of the eyewall. We passed through the eye at 2019 UT (15.85N 118.0W), within 10 s of N42RF and encountered 126 kts winds in the NE eyewall. The radar system failed at 2032 UT during the downwind leg to a point 50 nm N of the center (5). We orbited at 5 until the radar system was restarted and then started our third leg at 2042 UT, dropping an ODW at 5. We passed through the eye at 2052 UT (15.9N 118.1W), within 15 s of N42RF. We encountered heavy rain, graupel and li ghtning as we passed through the intense reflectivity in the S eyewall, dropping another ODW at the point 50 nm S of the center (6). On leg 4, which started 50 nm SE of the center (7), we encountered heavy rain, graupel and lightning once again. We droppe d an ODW in the eye at 2124 UT (15.95N 118.15W) as we saw N42RF pass beneath us. The splash pressure was 950 mb.

We finished up the day with three legs oriented E-W across the center. The first E-W leg started at 2145 UT, 50 nm W of the center (2), passing through the eye at 2155 UT (16.05N, 118.2W). The second E-W leg started at 2209 UT 50 nm E of the center (IP), passing through the eye at 2223 UT (16.1N 118.3W), 20 s ahead of N42RF (the worst synchronization of the day). The final E-W leg started at 2238 UT at 2, passing through the eye at 2248 UT (16.15N 118.4W), 6 s behind N42RF (the best synchronization of th e day). At 2300 UT we ended the pattern at 1 and climbed to our ferry altitude back to Puerto Vallarta. The navigator computed a storm motion of 310 deg. at a speed of 8 kts over the three hours of the experiment.

Mission Evaluation and Problems

Fantastic flight!! Bob Burpee says that the mission was probably one of the 4-5 best missions HRD/AOC has flown in the last 15 years. The aircraft coordination was outstanding, with the AOC flight directors and navigators deserving the lion share of the c redit. Achieved seven complete mappings of the inner core within 50 nm radius of the storm center for true dual Doppler analyses over a 3.5 h period. The EVTD analyses performed on the aircraft showed that the storm was intensifying during the mission, wi th the azimuthally-averaged tangential winds increasing from 50-55 m/s on the first pass through the center, to over 60 m/s by the last pass. As the wind speed increased, the radius of maximum winds contracted, from 20 km radius on the first pass through the center to 16 km radius on the last pass. Six ODWs were dropped as planned, of which all were either good or recoverable. The ODW drop in the eye (950 mb central pressure) showed similar structure to those dropped in the eye of Hurricanes Gloria, Emily and Gilbert. There was a distinct inversion at ~875 mb, with relatively warm dry air above the inversion and nearly saturated air below.

This data set will be outstanding for studies of inner core dynamics and evolution. The storm's small size and intensity, were ideal for this experiment. The inner core out to at least three eye radii distance were within the domain mapped by the Doppler radars. The seven complete mappings over the 3.5 h (one analysis every 33 min) while the storm was intensifying should provide an excellent opportunity to study the temporal evolution of the primary and secondary circulations in the eyewall. These data sh ould also help improve our understanding of the interaction of the inner core and the environment as the storm starts to recurve.

N43RF also transmitted three radar composites and 5 of the 6 ODWs back to NHC via ASDL. All the radar composites were received and decoded for the hurricane specialists. They were very happy to receive all the information as it confirmed that the storm w as intensifying and starting to recurve.