Observations of both hurricane intensification and intensity maintenance are described. On 2 and 3 August 1997, Hurricane Guillermo was investigated by the two NOAA WP-3D aircraft. Its center positions at 2100 UTC on 2 and 3 August were (13.3 N, 107.6 W) and (13.8 N, 112.3 W), respectively, as the storm moved mostly westward (heading 275 degrees) at 6.7 m/s over the intervening 24 hours. During a 5-hour period between the first and last successful fixes on 2 August the estimated central mean sea-level pressure (MSLP) dropped from 956 hPa to 944 hPa, as Guillermo was still intensifying rapidly. The mean rate of intensification during the time between missions was slightly more than 1 hPa/hour, and thus the estimated central MSLP at the beginning of the second mission on 3 August was 923 hPa. Throughout the mission on 3 August the hurricane maintained this intensity within 3 hPa, approximately the margin of error for a good center fix.
During the two-aircraft missions on each day, N42RF penetrated the storm 10 times at 3 km, while N43RF penetrated the storm 6 times at maximum safe altitude (~6-7 km, but descending as necessary to decrease aircraft charging). Each of the N43RF penetrations was coordinated with a N42RF penetration to obtain simultaneous airborne Doppler observations of the storm core, particularly within 60 km of the storm center. The radar on N43RF was continuously operated in fore/aft (FAST) scanning mode (Jorgensen et al., 1996), a mode that allows dual-Doppler observations to be collected with one aircraft, while on N42RF the radar scanned perpendicularly to the flight track during coordinated passes through the center, and in FAST when it penetrated the storm alone. The perpendicular scans were needed to permit a real time Velocity Track Display (VTD) wind analysis (Lee et al., 1994); however, since N43RF was operating in FAST, three semi-independent Doppler measurements were made in much of the core. Somewhat better and more complete Doppler analyses were produced for the coordinated two-aircraft passes than for the single-aircraft passes. During the 4 solo penetrations of N42RF, N43RF was monitoring the near periphery (within 300 km radius of storm center), and thus providing the best outer-core context ever for an inner-core Doppler wind set.
The maximum azimuthally averaged tangential wind at 1930 UTC on 2 August was 55 m/s at a radius of 30 km from storm center, while 4.5 hours later at 0003 UTC, it was 57 m/s at a radius of 28 km. When the aircraft re-entered the storm on 3 August (1910 UTC) the azimuthally averaged maximum was 68 m/s at a radius of 24 km. The three corresponding local wind maxima were 62, 63, and 76 m/s, respectively.
The mean hodograph at the eyewall indicates NW shear of ~1 m/s/km between 0 and 8 km and SE shear of ~2 m/s/km from 8 km to 14 km for 1930 UTC on 2 August. This pattern and intensity appears to continue through the beginning of the 3 August mission. The deep shear is weak; however there is a reversal of stronger shears in all three preliminary analyses. In the presence of such shear, but over warm Pacific water (climatological mean ~29 degrees Celsius), the hurricane is able to intensify rapidly. A more detailed description of wind-field evolution, mean inner-core hodographs, sea-surface temperatures, and storm environment will be shown at the conference.
Jorgensen, D. P., T. Matejka, and J. D. DuGranrut, 1996: Multi-beam techniques for deriving wind fields from airborne Doppler radars. J. Meteor. Atmos. Phys., 59, 83-104.
Lee, W.-C., F. D. Marks, Jr., and R. E. Carbone: Velocity Track Display--A Technique to extract real-time tropical cyclone circulations using a single airborne Doppler radar. J. Atmos. Oceanic Technol., 11, 337-356